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Lecture 2

Chapter 7 Our Planetary System

Text Sections: 7.1 and 7.2

Specific Objectives - after studying this chapter you should be able to:

• describe the main features of the planetary orbits

• identify the differences between terrestrial and gas giant planets

Sun is a yellow main sequence G2 star. It contains 99.8% of the solar system's mass.•

All the planets lie in the ecliptic plane, to within 6 degrees.•

Solar System

Most moons go around primaries in the same direction○

Most moons spin the same direction as well○

Most planets spin in the same direction (Venus spins opposite way and Uranus has an

almost horizontal spin)

○

All the planets go around the Sun in the same direction•

Beyond the planetsHeliopause: (150 AU) the pressure of the solar winds = pressure of interstellar gas

Termination shock: (100AU) where the solar winds start slowing down

Bow Shock: (~300AU) possible area due to movement of sun through interstellar medium

Lecture 2: Solar System05 August 2013 12:43

The Solar System



Various people including Copernicus, Kepler and newton proved that the planets moved

around the sun.

•Orbits

The orbits of the planes are ellipses with the sun at one focus•

A line from a planet to the sun sweeps out equal areas in equal intervals of time•

A planets orbital period squared is proportional to its average distance from the sun cubed.

•

Periphelion means closest to the sun, aphelion is furthest away from the sun•

•

Kepler's Three Laws

Is in orbit around the suna.

Has sufficient mass for its self gravity to overcome rigid body forces so that it assumes

hydrostatic equilibrium shape (round)

b.

Has cleared the neighbourhood around its orbitc.

A planet is a celestial body that1.

A dwarf planet is a celestial body that2.(a) is in orbit around the Sun

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a

hydrostatic equilibrium (nearly round) shape

(c) has not cleared the neighbourhood around its orbit

Definition of Planet

The Solar System



(d) is not a satellite.

All other objects except satellites orbiting the sun shall be referred to collectively as "small

solar-system bodies"

3.

The terrestrial planetsThe terrestrial planets are the four planets of the inner solar system: Mercury, Venus, Earth, and

Mars. (Terrestrial means "Earth-like.") These planets are relatively small and dense, with rocky

surfaces and an abundance of metals in their cores. They have few moons, if any, and no rings.

The Jovian PlanetsThe Jovian planets are the four large planets of the outer solar system: Jupiter, Saturn, Uranus, and

Neptune. (Jovian means "Jupiter-like.") The Jovian planets are much larger in size and lower in

average density than the terrestrial planets. They have rings and numerous moons. Their

compositions are also quite different from those of the terrestrial worlds. They are made mostly of

hydrogen, helium, and hydrogen compounds-compounds containing hydrogen, such as water (H20),

ammonia (NH3), and methane (CH4).

The Solar System



Lecture 3

Chapter 8 Formation of the Solar System

Text Sections: 8.1 – 8.5

• identify the evidence for the age of the solar system

• identify the evidence related to the formation of the solar system

• describe the main stages in the formation of the solar system and formation of the planets

• describe the hypotheses explaining the vastly different compositions and masses of the terrestrial and

giant planets

Planetary and major satellite orbits close to circular path and in same plane•

Generally common direction of revolution and rotation•

Rocky-terrestrial planets○

Gaseous (low density0 gas and ice giants)○

Icy- moons, comets, KBOs○

3 types of objects•

Surface features- similarities differences○

Composition- similarities/differences•

Larger objects round and differentiated - heavy materials in core (e.g. Planets)•

Rings and moons around giant planets•

Asteroid Belt, Kuiper belt and Oort cloud○

Debris-asteroids, comets and meteors•

Common age?•

Only one star•

Characteristics of a Solar system which must be explained

Such as U238-->206Pb, with 1/2 life = 4.5 Gyr○

K-40 decaying to Argon-40 with half-life of 1.25 Gyr○

Can use radiometric dating•

How old is the solar system?: 4.5 billion years old

Making a solar system

The planets formed from 'blobs' of gas that were gravitationally pulled out from the sun by another star.•

Discarded because it did not account for observed orbital motions of the planets or the two categories of

planets

•

Close Encounter hypothesis (discarded)

Nebula hypothesis

Ingredients: Fragment of an interstellar cloud. H (71%), He (27%), rest (~2%) by mass. Initially cold gas1.

External trigger: Supernova? Gravitational disturbance? Causes collapse2.

Collapse: R ~ 5-10,000 AU to R ~ 700,000 km in 10^6 years. (only a million years to form the Protosun) Central

density increases fastest, so the centre collapses fastest. Thus Sun forms in centre, where density and

temperature is highest.

3.

Heating: Cloud collapse releases gravitational PE. Gas pressure eventually balances gravity.4.

Spin: Conservation of angular momentum. Velocity increases with collapse until it reaches orbital speeds

(neglecting viscous forces within the gas).

5.

Natural consequence of particles in a spinning clouda.

Flattening: From a cloud to a disk6.

Formation of the Sun and Structure of the Solar system

This process is seen in other currently forming solar systems through the observation of IR caused by heating. Other forming stars also appear to be ejecting jets outwards perpendicular to their disks.

Condensation: clumping of material.7.

Formation of the planets

Temperatures in different parts of the disk restrict what○

Lecture 3: How to form a Solar System05 August 2013 13:08

The Solar System



Condensation: clumping of material.7.

Collapse into the thin disk, while dust may not.○

Disk fragments→ small clouds. Lots of collisions, therefore only larger planetsimals could survive○

Coalesce in 100 km diameter objects and then protoplanets.○

Accretion of dust grains forms planetesimals km across.8.

Giant planets: When≥ 15 M(earth), gravitational collapse attracts MORE gas to form the most massive

planets. The giant planets create their own accretion disks. T~10 Myr, before sun blows away the nebula.

9.

Terrestrial planets f orm in ~30 Myr. Since temperatures are much hotter, most of the gases are expelled

(kinetic energy) The terrestrial planetismals collide and accrete resulting in the planets we see today

10.

As shown in the table above, only metals and rocks would condense in the hotter regions close to the sun. The

Jovian planets initially had metals rock and ice, and were much more massive. Unlike the smaller planets they

were able to capture Hydrogen and Helium

Hot in the centre due to collapse, hot enough for dense objects to sinka.

Differentiation: Driven by heat from collapse and radioactivity.11.

Gas that was trapped in the planets is released and captured through volcanic activitya.

Outgassing forms atmospheres.12.

Solar System complete in ~100 Myr

Clearing of Solar System by radiation pressure, solar wind, heavy bombardment, ejection. Because the sun

'turns on', blowing away lighter particles such as light gas and dust. The sun also loses its angular momentum

by ejection of particles

13.

Leads to craters because large objects are gathering massa.

Late Heavy Bombardment cleared much of the debrisb.

Heavy bombardment: Larger debris which is not blown away will be attracted to planets.14.

ormat on o t e p anets

It seems unlikley that Uranus and Neptume would form under this model, at their current distance•

Anomaly Explanations

Giant planets formed 15-20 AU with a disk of planetismals further out1.

Orbits Slowly Expand until after 900 Myr Saturn ends up in a 2:1 gravitational resonance with Jupiter and

pushes the other planets out.

2.

This can scatter planetismals towards inner solar system, producing late heavy bombardment 3.

Neptunes orbit may also govern Kuiper belt objects (causing orbit to expand)4.

Thus it means that it is most likely that the orbits change

This means that the outer planets may not have formed where they currently are.

Temperatures in different parts of the disk restrict what

materials will condense at which locations (and times)

○

Determines the make-up of objects in those regions.○

Between Mars and Jupiter is the Ice line: distance where

it is cool enough for ices to condense

○

The Solar System



Chapter 9 Planetary Geology: Earth and the other Terrestrial Worlds


• describe the process of differentiation• describe and explain the correlation between size of a planet and its present level of geologic activity

• describe the evidence that the Earth has a molten core

• relate cratering to the history of the solar system and the level of activity of a planetary surface

• describe the formation of the Moon and the evolution of the lunar surface

• compare the mean densities of the terrestrial planets

• describe and compare the surfaces and geological activity of the terrestrial planets

• describe the evidence for the existence of water on the surface of Mars

• describe and compare the evolution and current composition of the atmospheres of the terrestrial

planets

Geological Activity

Heat of Accretion1.

Heat from differentiation; as dense material sinks to the bottom friction causes heat2.

Heat from radioactive decay3.

Comes from the interior of the planet.

Large planets will keep this heat insulated for longer because the extra rock acts as insulation

Rigid thin crust broken into places•

Therefore not many craters seen○

Surface is young (average <100Myr) and shaped by plate tectonics, volcanism and weathering•

Mostly lowland basaltic plains (i.e. Ocean floors)•Water in all phases covering 75%•

The Earth (Direct observation)

Thus new crust being created.

Leads to mountain building, volcanic/tectonic activity○

Moving plates- continental drift•

Plates (Measurement of current motion and matching of rock types )

P and S waves are sent through the Earth, and will change in speed depending on the density of the

insides. Some waves will travel through liquid core and some won't.

•

Dense Fe-Ni core, partly liquid which is the origin of magnetic field•

Plastic (liquid) Si-O-Mg-Al mantle.•

Thin crust broken into plates•

Interiors (Information from seismology, measure the arrival of wave at different locations)

Weather is under 15 km○

Only 100-200 km thick•

LIFE on Earth drives oxygen and uses Carbon dioxide in the atmosphere○

Some oxygen is tied up in rocks○

O2 is unique in the atmosphere (compared to other planets) while nitrogen should be explained•

Lack of CO2 because it was fixed it in carbonate rocks and dissolved in ocean, thus mild greenhouse effect•

Oceans due to H2O which could condense in Earth temperatures (but Venus is too hot)•

Atmosphere (Direct observation and modelling)

Dynamo in liquid core generates magnetic field•

This magnetic field protect surface from solar wind •

Magnetosphere (direct observation and modelling)

Comparative Planetology

Lecture 4: Terrestrial (Rocky) Planets11 August 2013 11:06

The Solar System



Too small to retain heat of formation (therefore less volcanic activity)•

Tidally locked to the Earth•

This is because the near side has a thinner crust and is more easily punctured.○

The mare was flooded late and thus there is 'saturation cratering' on highlands○

The flooding of large impact basins caused by large impacts, puncturing the surface and producing mare.

(large flat lava plains)

•

The moon has similar composition to the outside rocky layer of Earth and does not contain any

vaporizable material such as water

○

Mars sized planetesimals collided with protoearth, thus creating the moon.•

The Moon

Can bounce back in the centre forming a central peak.○

A meteorite approaches and hits. It is deformed heated and vaporised, forming a round crater.•

Cratering:

More impacts form a few large craters, and about 3.8 bya lava flows into low regions○

Repeated lava flow covers most of the inner ring and merge with the other flows○

More impacts continue to form younger craters such as Copernicus○

An impact forms a large multi-ringed basin which continues to be impacted•

Mare Imbrium:

Causes steep cliffs○

Also too small to retain head of formation, thus the crust is winkled by shrinking during cooling•

Partially flooded Caloris basin○

No large dark flooded impact basins•

Tidally coupled to the sun, 3 Rotations for 2 orbits.•

Extreme surface temperatures because no atmosphere•

Lost geological activity after 1 billion years, similar to moon•

Relatively large metallic core, core shouldn't be liquid but produces weak magnetic field (unexplained)•

Mercury

Venus

Some mountains but mostly low lands•

Clear evidence of large volcanoes, due to larger size•

(Information from spacecraft down on the surface, and radar altimetry)

The Solar System



Dark grey basalts on the surface•

The water that would have initially formed would be too hot to condense, and instead by broken down by

UV radiation into Hydrogen which will escape.

•

Only slightly more craters than Earth, so approx. 300 Myr old•

Spins the opposite way and extremely slowly, possibly due to a large collision•

No Magnetic field (possibly due to slow rotation)•

These Northern lowlands have had their craters erased by geological processes○

Half ancient cratered terrain~2-3 Gyr old and half smoother flood plains with relatively few craters•

Large shield volcanoes supported by thick crust•

Polar caps of H2O and CO2•

Small core- no magnetic field•

Mars:

Tied up as permafrost under the surface

Atmosphere too thin at the moment to allow liquid water○

Gullies and channels carved by liquid flows○

Rounded pebbles○

Polar ice caps○

Increasing evidence for surface water in the past

Summary and TrendsInteriors

Mean Density: Earth>Venus>Mercury>Moon>Mars

Atmospheres

Mercury and moon have virtually no atmosphere

because they do not have the mass to hold an

atmosphere

Venus has 90x Earth's atmospheric density

where as Mars has only 0.01x Earth's

atmospheric density

•

Lots of CO2. Earth has a similar amount but it

is tied up in rocks

•

Little water as too hot for vapour to

condense, thus will eventually be broken

down and released as Hydrogen. A

consequence of runaway greenhouse effect

•

High H2SO4 Clouds, Severe Greenhouse effect•

Atmosphere rotates in only 4 days•

Venus

Carbon Dioxide outgassed by volcanism was

dissolved in large oceans, unlike other planets

•

Nitrogen produced by outgassing, since water

ended up in the ocean and carbon dioxide inrocks

•

Molecular Oxygen is a product of life•

Earth

Only 0.01x density of Earth's atmosphere•

Mars

The Solar System



Venus and Mars have very small magnetic fields due to their motion and size respectively•

Mercury has a noticeable magnetic field possibly due to its large metallic core•

Aging of the planet causes core to cool, reducing the magnetosphere and exposing it to solar wind○

This would allow UV light to penetrate and break down water molecules as welll○

If Mars was initially hotter, it would have had a magnetosphere which would allow for a thicker

atmosphere (because it is protected by solar winds.)

•

Magnetospheres

Only 0.01x density of Earth's atmosphere•

Mars

The Solar System



Chapter 11 Jovian Planet Systems

Text Sections: 11.1 - 11.3

Specific Objectives – emphasising the comparative properties and histories of the giant planets - after studying

• outline the formation of the gas giants, and contrast this with the formation of the terrestrial planets

• compare the sizes, masses and compositions of the giant planets

• identify the cloud layers in the giant planets’ atmospheres

• describe the belt and zone circulation in Jupiter's atmosphere

These planets initially all started beyond the ice line (where it was cold enough for hydrogen compounds to

condense) as ice-rich planetesimals about 10x the mass of Earth. They then accreted Hydrogen and Helium until

the solar winds blew the nebula away. The closer planets would be able to capture more.

Strong opposite winds to rotation•

Cloud tops rise in a zone (and are lighter) and drop in a belt (and are darker)○

Stable dark belts and bright zones•

Difference in colour is due to trace components within the hydrogen and helium such as S,P and C•

Long lived bright spots and ovals, as there are no landforms to dampen them.•

Great Red Spot has been observed for at least 300 years•

Gravitational compression makes Jupiter a higher density that expected•

Jupiter (Gas Giant)

More flattened than Jupiter•

Less colourful despite similar composition○

Stable belts and zones like Jupiter•

Clouds also form at same temperature but that is much deeper in Saturn's atmosphere•

Higher wind speeds•

Saturn (Gas Giant)

Small featureless blue-green disk•

Cold Hydrogen rich atmosphere contains methane that absorbs red photons•

Few high clouds of methane ice•

NO excess heat from core, therefore no atmospheric movement (don't know why)•

Inclination of 98 degrees•

Uranus (Ice Giant)

Extremely blue- partly explained by methane absorbing photons•

More active cloud formation than Uranus•Radiates twice as much heat as it gets from the sun-driving weather activity•

Fastest winds in the Solar System•

Neptune (Ice Giant)

Comparison

Jupiter has ammonia ice layer with water ice close under•

Ice is further down because it is colder○

Saturn has ammonia layer•

Uranus has ammonia layer but cannot see further down•

Neptune has ammonia layer•

Atmosphere comparison

Lecture 5: Giant Planets12 August 2013 14:19

The Solar System



Interiors

Since Jovian planets were bigger during formation of solar system, they can pull in the Hydrogen which escapesfrom smaller planets such as Earth.

Visible Clouds then gaseous, liquid and metallic hydrogen in Jupiter•

Hydrogen mantle is conductive material and will form the magnetic field•

Internal heat may be a result of ongoing contraction•

Jupiter

Visible Clouds then gaseous, liquid and metallic hydrogen as well•

Hydrogen mantle is conductive material and will form the magnetic field•

Saturn is not as big and thus there is not as much metallic hydrogen•

Internal heat may be a result of helium rain which can condense; thus differentiation•

Saturn

In Uranus and Neptune, not large enough to compress hydrogen down into a solid (metallic) state.•

Gaseous Hydrogen, Water with Rocky centre•

Less extreme temperatures allow interiors to differentiate•

Uranus and Neptune

The Solar System



Magnetic fields

Jupiter: enormous field and magnetosphere•

Saturn: 5% strength of Jupiter and aligned with rotation axis•

Uranus: titled by 59° and offset by 30% of radius•

Neptune: tilted by 47° and offset by at least 55% of radius•

Occurs on Jupiter and Saturn due to their magnetospheres•

Jupiter observed to emit radio waves due to charged particles hitting Jupiter and magnetic environment.•

Aurora

RingsRings structures are governed by moons and gravitational resonances.

Poorly understood- extremely thin (<1km)•

Dirtier rings are closer to Saturn○

Thin rings suggests small, icy objects (bright)•

Rings are not stable, lose angular momentum due to the small embedded moon•

Brightness suggests they are quite young, as solar winds will darken rings•

Saturn

Likely: Disruption by tidal forces, Moon goes too close to planet and gets ripped apart• Impact from comet and spraying material off •

Small moons losing material from the sun and occasionally being ripped apart constantly replenishes rings•

Origin

Caused by: Moon orbiting in the division; causing density waves•

Moon in orbital resonance•

Braided ring have Shepard moons on either side, causing particles of the ring to stay together in a tight ring•

Division in rings ; Governed by gravitational resonances

Dark and reddish rings (not bright) because it is too close to the sun to be an icy structure•

Densest near the inner moons, therefore probably blasted off these moons•

Jupiter

Extremely dark, due to methane ice being broken down•

Very narrow rings due to Shepard moons•

Not too old since particles are blasted from moons•

Clumpiness suggests too young to have reached steady state•

Uranus and Neptune

The Solar System



• describe the surfaces and compositions of the Galilean satellites in particular

• explain how the composition of moons can be inferred from mean density

• describe tidal heating of Io• identify the composition and dynamics of planetary rings

• describe formation scenarios for planetary ring systems

Jupiter: The Galilean moons

Inner planets form moons

through collisions. (Earth) Hence

such big moons

•

Small moons are often

captured asteroids which

lose energy due to friction

with accretion disks and

then are caught

○

Jovian planets form many moons,

due to excess debris. Excess

debris forms a disk, where objects

start forming

•

Large regular moons inprograde (same direction as

planet rotation) elliptical

orbits (due to their orbital

resonance)

○

Io, Europa and Ganymede locked in

an orbital resonance (8:4:2) and

Callisto will join in a few hundred

Myr

•

Lecture 6: Moons15 August 2013 15:25

The Solar System



Io

Tidal locking to Jupiter.•

Tidal force of Jupiter varies due to slightly elliptical orbit•

Extremely volcanic due to repeated flexing in different directions, melting some of the interior and

heating due to friction

•

Driven by sulfur, hence yellow appearance•

Virtually no craters, therefore relatively young•

Density suggests Fe/S core•

Europa

Largest moon in the solar system•

Moon like ice surface•

Ganymede

p ane ro a on e p ca

orbits (due to their orbital

resonance)

Small captured asteroids most in

retrograde orbits

•

Tidal heating effect exists but is much

smaller. It must exist because older

craters are erased

•

Smoothest object in the Solar System•

Ice crust hiding deep ocean, heated

by mantle

•

Criss-crossing bands 20-40 km wide

and thousands of km long are

fractures filled by water that froze,

coloured because of contaminants

•

Has a magnetic field which responds

to changes in Jupiter's. Thus Europa

must have a liquid layer of conducting

material.

•

Activity driven by tidal heating•

Most volcanically driven active object in

the solar system- due to resonancesthat cause orbit to be slightly elliptical

The Solar System



Some regions are dark and densely cratered while others are light coloured with very few

craters (indicating upwelling of liquid)

○

Limited tidal heating because far away from Jupiter, thus perhaps combination of tidal heating and

radioactive decay (larger mass) cause a water interior

•

Heavily cratered ice ball•

White spots are craters from large impacts which have blasted out 'clean' ice from underneath•

Callisto

62 moons and counting.•

Moonlets within rings, Has shepherd moons, co-orbitals, inner large moons and outer large moons•

Saturn: Icy Moons

Second largest moon in the Solar System•

Mostly Nitrogen, methane and traces of organic compounds

Thick smog denser than our atmosphere:○

Big and far enough from heat to have an atmosphere•

Titan

Two moons which have a Co-orbital: orbital radii differ by <50 km•

Due to gravitational attraction, moons trade orbits periodically•

Janus, Epimetheus

Neptune

Ice volcanoes, nitrogen geysers○

Very unusual terrain, intense geological activity despite being so far away•

Only major moon to have a retrograde orbit and at a high inclination to Neptune's equator•

One of two moons to have an atmosphere•

Triton

Possibly explains Neptune's odd spin and orbit after Triton was captured

Moons show enormous variety in their structure.

The Solar System



Chapter 12 Asteroids, Comets and Dwarf Planets: Their Nature, Orbits and Impacts



• identify the locations and nature of the asteroid belt, Kuiper belt and the Oort cloud• describe how meteorites are related to asteroids and comets

• sketch the structure of a comet

• describe their formation and roles in supplying long and short period comets to the inner solar system

• describe the nature of Pluto, Eris and other Kuiper Belt objects

• discuss the danger of asteroid impacts on Earth

Asteroid Belt

Bands of gaps which are in resonance with Jupiter.○

Preferred orbits governed by the gravitational influence of Jupiter.•

About 100,000km away from each other•

Most are extremely dark; Variety of compositions: C, S, Metal etc.•

33 main asteroid belts approx. 200km across•

The leftover of rocky planetesimals of the inner solar system.

Meteors and Meteorites

Flash of light when a particle burns up in the atmosphere•

Typically no larger than a pea or a grain•

Meteor

Meteorite: When it actually hits the ground•

Meteorite

Origins

Primitive: Asteroid appears to be condensed straight out of the solar nebular (with no differentiation)•

Processed: Different compositions because they have come from different places after differentiation. A

larger object which was differentiated has been broken down.

•

Mostly from asteroids, Two types; Primitive vs. Processed

Some from the moon and Some from Mars (ejected volcanically, or from giant impacts)

Easiest to find on pristine uniform background; Antarctica etc.

Kuiper Belt Objects

Belt past Neptune between 30 and 50 AU•

Forms short period comets•

Mostly constrained to the plane of the solar system•

First found through group of objects called Centaurs, after observing object with an eccentric orbit between Saturn

and Uranus. Ice-rich leftovers of planetesimals of the outer solar system

in resonance with Neptune (3 Neptune orbits to 2 Plutino orbits) have similar orbit to Pluto (35%)○

35% are Plutinos (or resonant KBOs) which form a band at 39 AU•

Most are between classical 42-48 AU•

Scattered KBOs form large donut orbits (3-4%)•

Gravitational Resonance: The inner and outer edge is sculpted by Neptune

Lecture 7: Asteroids, KBOs and Comets26 August 2013 13:07

The Solar System



Eris (another dwarf planet) may be bigger than Pluto and is an example of a scattered orbit•

Largest known trans-Neptunian objects

The asteroid belt and the Kuiper belt are protected regions from gravitational influence.•

Large objects such as Jupiter cause objects to be thrown out into scattered orbits○

This is why asteroids are not everywhere from the formation of the solar system•

10,000 to 100,000 AU (halfway to nearest star)○

Appear to come from all directions○

Unconstrained to the plane of the solar system, thought to be spherical, stretching extremely far out•

Produces long period comets•

The Oort Cloud and Comets

Comets (remember some comets are short period comets from Kuiper Belt)

Ion tail points away from sun due to radiation pressure○

Dust tail because it is heavier, is less affected by radiation pressure○

When they enter the inner solar system their icy layer starts melting forming the coma, which surrounds it.

The Solar System



Periodic visits every 75-76 years•

Halley's Comet

Big ball of ice mixed with some dust•

Marked on surface due to impacts•

Comet Nuclei

Comet broke up into sub comets and then collided with Jupiter•

Impacts on Jupiter caused chemical changes in its surface•

Comet Shoemaker Levy 9

The Solar System



Sun will become red giant and things may start warming up•

Earth has atmosphere and signs of an impact indicate the past when the moon was formed•

A sharp large shadow indicates a small planet, Jupiter and other planets are too big and too far

away from their moons

•

Uranus is facing side on however moons still orbit in the same plane•

Brown dwarfs will have a slight glow•

Lecture 8: Review on Planets26 August 2013 13:07

The Solar System



Lecture 9

Chapter 13 Other Planetary Systems: Their Nature, Orbits and Impacts



• identify the evidence for the existence of other planetary systems

• describe the differences between our solar system and other planetary systems

Stars are bright but Planets shine off reflected light•

Planets are much smaller •

Direct detection is very hard•

How do we detect Exoplanets?

A star will wobble back and forth as the star and planets orbit the common centre of mass•

Planets can block light from stars•Planets also gravitationally bend light •

Solutions

Methods

Star will orbit around the barycentre of a solar system•

The light will be red shifted and blue shifted by different amounts due to the effects of the planet.•

This tells us the periodic variation in velocity which indicates an unseen planet.•

Radial Velocity: The Doppler Effect

Transits and Eclipses

Slight dip in sunlight for systems which are edge on only•

When the planet is eclipsed there will be an even smaller dip•

As a planet transits across its parent star there will be a dip in starlight

Lecture 9: Exoplanets26 August 2013 13:07

The Solar System



Aiming to discover Earth Sized Planets•

Finding planets within the habitable zone•

Kepler- (Revolutionised the transit method)

When a planet is directly in front of light from a distant star it can bend the light and focus it.•

However can only happen once and does not occur regularly and thus is not as useful○

The light will be magnified first by the star in the solar system and then again by the planet•

Microlensing

Direct Imaging

Measuring changes in velocity of a pulsar•

Pulsar Timing

Detection BiasRadial velocity: favours close in, massive planets. (because they have a bigger effect on the star)•

Transit: favours large, close in, short period planets. (Larger planets will cause a bigger change in intensity)•

Typical observing campaigns favour shorter periods (days or months). (Because planets are not observed•

We are not necessarily seeing 'typical' solar systems

Hard to achieve○

Reducing the intensity until objects can be seen around

the star. Light is subtracted from an image

•

The Solar System



for years at a time)

Conclusions

More smaller planets (almost down to size of Earth) than planets such as Jupiter and Neptune•

Indicates same Nebula Hypothesis explanation of a flattened disc of gas and dust around the young

star

○

At least 500 million (1%) of planets in the habitable zone○

Many solar systems found where two more planets transit across star (thus in same plane)•

Three important results from Kepler

The Solar System



Lecture 10

Chapter 14 Our Star

Text Sections: 14.2 and 14.2

Specific Objectives - after studying this chapter you should be able to:• explain the balance between gravity and pressure

• describe the processes of energy transport in stars (radiation and convection)

• identify that energy in main sequence stars comes from nuclear reactions (proton-proton chain and

CNO

cycle)

• identify the relevant fundamental particles (proton, neutron, electron, positron, neutrino, gamma ray

photon)

H-Alpha- A particular signature colour of Hydrogen.

Radius = 696,000 km (109 x Earth) = 1R•

Mass=2 x 1030 kg (300,000 x Earth) = 1M•

Luminosity = 3.8 x 1026 Watts = 1¤•

Rotation = 25 days at equator (slow)•

Surface temperature = 5770 K (Sunspots = 4000 K, core = 15 million )•

Age: 4.6x10^9 Years•

Basic Properties

Inside the sun:

Core: High density core which provides energy through nuclear reactions•

Radiative zone: Where photons of light travel through radiation, photons are continually deflected

randomly and gradually move out through radiative diffusion

•

Convective Zone: Since gas is cooler here, photons can be absorbed. Thus it starts to boil and convection

begins. Lumps of hot gas start to rise up, like boiling water.

•

Lecture 10: The sun26 August 2013 13:33

The Stars



Granulation Pattern- The 'bubbling' layer on the surface of the sun.○

Sunspots- Areas that are cooler with intense magnetic fields. Indicate active regions. Occur in a 22

year cycle

○

Photosphere-Where the gas peters out. The place where on average, a photon will escape (visible layer)•

•

Chromosphere is where temperature increases with altitude and most of the UV light is emitted•

Corona is usually observed in Xrays and is the outer layer, most easily in seen in solar eclipses○

Shows material being thrown out from sun○

Not very dense but extremely hot○

Corona forms outer layer, which is significantly hotter than photosphere•

Sunspots and Magnetic Fields

This is found through studying the absorption spectrum and seeing splitting of spectral lines

(Zeeman effect)

○

Sunspots are regions with strong magnetic fields•

The strong magnetic field prevent surrounding plasma from entering the sunspot•

Tend to occur in pairs joined by a magnetic field. This creates prominences which occur when gas in the

sun's chromosphere gets trapped in these loops

•

Magnetic fields can undergo sudden change and cause solar flares•

Sunspots are relatively cooler regions on the sun's surface and in order for these to exist (and be cooler than

their surroundings) they must be magnetic fields preventing hot plasma from entering them.

Helioseismology (study of pressure waves)1.

Pressure waves are used to measure the properties inside the sun•

This information comes from:

Therefore Equator rotates faster that the poles•

The sun rotates on a slowly rotating inner cylinder and an outer cylinder (at equator) which rotates faster.

Solar maximums have high periods

of solar activity (and more sunspots)

and occur on an 11 year basis.

Furthermore the magnetic field will

flip every 11 years as well. Thus

forming a 22 year cycle. (to flip back

to normal)

The Stars



Mathematical Models2.

Solar Neutrinos3.

Initially not enough were detected, but then it was found that nuetrino's can decay into different

types

○

Predicted to be formed and also detected•

What makes the sun shine?Cannot be a chemical reaction because simply not enough energy

Nuclear Reactions: E=mc2

First step requires overcoming electrostatic repulsion (Thus high enough energy)○

Dominates in sun like stars○

The Proton-Proton Chain

Small amount of mass is lost, which is converted into energy.

Requires hotter conditions• Same net reaction, but different process with heavier element catalysts•

Dominant in much heavier stars•

The CNO cycle

Stars will eventually run out of Hydrogen.

The Stable SunFor the sun to shine steadily it must have a way of keeping its core hot and dense. It maintains these condition

through a natural balance between two competing forces; gravity pulling inwards and radiation pressure

pushing outwards. This is called gravitational equilibrium or hydrostatic equilibrium

Suppose the core temperature rose slightly and as a result, fusion rate also rose slightly. This would increase

the radiation pressure in the core and cause the sun to expand and col a little, thus causing the fusion rate todrop back down.

Requires more temperature•

This allows helium burning when Hydrogen is running low•

Triple Alpha Process: Burns helium into carbon

Heavier elements are successively burned till Iron. However after Iron and fusion REQUIRES energy.

Initially could not detect as many as predicted○

Extremely hard to detect, but expected to be produced by the sun•

Realised that neutrinos have mass, and can thus decay.•

Solar Neutrinos

The Stars



Lecture 11

Chapter 5 Light and Matter

Text Sections: 5.4


• describe and explain the formation of emission and absorption spectra

• describe the Doppler effect and its use for determining velocity

Chapter 15 Surveying the Stars

Text Sections: 15.1


• define luminosity and explain how it is determined

• define parallax and explain how it is used to determine distances to stars

• explain why the surface temperature of a star is related to its colour

• describe the blackbody radiation spectrum from a hot body and its relationship to thetemperature of the body

Brightness and size in a picture do not indicate the size of a star

Red glowing gas- Indicates hydrogen, gas cloud, and possibly large stars

Alpha Centauri- One of Two pointer stars to southern cross (actually is a binary star system)

Beta Centauri- 2nd Pointer star to southern cross (100x further away, 10000 times brighter)

Cluster- Stars that were made around the same time. (spacing tends to be much closer)

Planetary Nebula: Layers have puffed off the star, and continue to glow

Stars occur singly or in groups- Most stars are born in groups•

Groups of Stars

Stars that are gravitationally bound•

Binary/multiple systems

Young stars that are not gravitationally bound•

Stellar Associations

Star ClustersCan stay together for hundreds or millions of years (or more)

Open Clusters (galactic Star clusters)

Not gravitationally bound so disperse

over time

•

Less tightly packed•

Associated with the spiral arms and

disk of our galaxy

•

Lecture 11: Studying the Stars28 August 2013 12:45

The Stars



Globular star clusters

Measuring Distance

Uses baseline of the earth's orbit (6 months apart)○

Moves approx. 1 arc second○

Nearest star is 1.3pc away

D(pc)= 1/parallax (arc seconds)○

One method uses trigonometric parallax•

1 pc = 3.26 ly = 3.08x10^13 km•

Trigonometric Parallax

Methods based on motion•

E.g. In an eclipsing binary ○

Methods based on surface brightness of stars•

Spectroscopic parallax, uses properties of star and use HR diagram○

Methods based on stellar properties•

Using properties of galaxies•

Other methods of measuring distance

Apparent Magnitudes (m) are brightness's as seen from the earth measured relative to a set of

standard stars

•

mA−mB =−2.50log10 (fA/ fB)•

Sun's apparent magnitude: -26.7○

Correspondence•

Apparent Magnitudes

disk of our galaxy

The diagram below shows the

Pleiades cluster. The HR diagram

shows no O stars, but still has B. This

main sequence turn off point indicates

it is around 100 years old

Extremely old, densely packed•

Found in the halo around our galaxy•

Gravitationally bound and more spherical•

The main sequence turn off point of this

globular cluster indicate an age of about 10

billion years.

The Stars



•

Apparent magnitude if the star was placed at a distance of 10 pc•

Absolute Magnitudes

Other magnitudes

U – Effective λ 365 nm,•

– bandwidth Δλ ~ 70 nm

B – effective- λ 440 nm,•


V – effective λ 550 nm,•


Flux from a star will vary with wavelength

The Stars



• explain why the surface temperature of a star is related to its colour

• describe the blackbody radiation spectrum from a hot body and its relationship to the

temperature of thebody

The structure of Atoms

Dense positively charged nucleus, surrounded by electrons•

Accelerating charged particles should radiate energy and spiral into the nucleus•

Rutherford:

Only certain orbits are allowed for electrons•

Electrons can only gain or lose energy in discrete steps by jumping from/to these allowed orbitals•

Bohr Model

This is one of a series of emission lines which depend on the final level.○

Includes the H alpha line which is the transition from 3 to 2. This usually indicates the presence

of Hydrogen, but it is the pattern that confirms Hydrogen's presence

○

The Balmer series is a series of possible electron transitions for the hydrogen atom which emits

photons of light in the visible to UV range. Electrons end on the n=2 level.

•

The Balmer series

An atom can go from energy E2 to E1 by emitting a single photon of energy. Many of these photons

will produce an emission spectral lines

•

This will produce an absorption spectral line○

Similarly photons can be absorbed, and thus will to a higher energy orbital•

Absorption and Emission

Happens with hot, thin gas.

Peak shifts towards blue wavelengths (wiens)

Intensity Increases very fast. The total energy radiated per unit area, is given by

F=σT4(Stefan Boltzmann)

As the temperature increases:○

Blackbody/Plank curves depict ideal hot emitting objects. Any object that is hot enough can emit a

black body curve, usually relatively high density.

•

Continuum Emission

The 6000 K object will appear yellow to the

human eye, because it is more sensitive to

Lecture 12: SpectraMonday, 9 September 2013 12:20 PM

The Stars



What happens in a Star

Dense core of the star produces rainbow spectrum, but cooler less dense outer layers absorb certain

wavelengths of light.

The rainbow spectrum is produced because two hot atoms close together will interact with one another.

Thus instead of producing stronger spectral lines, they will become blurred.

Since energy levels are quantised, a given transition will always be a precise wavelength•

Different atoms/molecules will have different characteristic transitions•

Interpreting Spectra

What we see will depend on the background continuum emission. And thus we can get information about

stability of each energy level, density, temperature, velocities and strength of magnetic and electric fields.

The 6000 K object will appear yellow to the

human eye, because it is more sensitive to

those wavelengths.

The Stars



Chapter 15 Surveying the Stars

Text Sections: 15.1–

15.3Specific Objectives - after studying this chapter you should be able to:

• describe the classification of stars into spectral types

• describe how stellar spectra can provide information on surface temperature, density, composition

and velocity

• define luminosity classes

• describe the Hertzsprung-Russell (H-R) diagram and identify main sequence stars, red dwarfs, giants

and white dwarfs

• identify the frequency of stellar types

• explain how the ages of open clusters and globular clusters are determined from their H-R diagrams

Produced the Henry Draper Catalogue•

Spectral Catalogues

Too cool means not enough electrons would START in the number 2 level (all in number 1)

Too hot means not enough electrons would FINISH in the number 2 level (all at higher levels

or not in the atom)

The number of atoms with electrons available○

Strength of H lines is proportional to:•

Balmer lines of H

i.e. Even though the sun has lots of hydrogen. It is too cool for the optimum Hydrogen transitions.

Thus the temperature of the star determines what elements are going to show up strongly.

Line strength mainly reflects temperature sensitivity, not abundance.

i.e2. Titanium Oxide will break down in stars that are too hot but have lines in stars that are cooler

Spectral ClassesSince spectral lines depend on temperature. Stars began to be classified, depending on their temperature.

Brown dwarfs (core temp is too low to sustain H fusion) have new classes: ( L and T)

Lecture 13: Spectral and Luminosity Classes and the HR

diagram09 September 2013 14:38

The Stars



Lots of spectral lines represent lots of metal lines.•

Hotter stars tend to have spectral lines at lower wavelengths•

Spectral Line Widths

Natural Broadening: spectral lines are very thin•

Since the gas is hot, gas particles move and thus cause spectral broadening○

Thermal Doppler broadening•

Pulsation, Rotation, Orbital Motion (would cause shifting)○

Non thermal Doppler broadening•

In Supergiants, gas density is low thus less broadening. Stars like the sun have comparatively

high density, and thus will exhibit spectral broadening.

If tightly packed gas particles will interact with one another and broaden the line○

Collision/Pressure Broadening•

Magnetic fields cause spectral broadening, depending on the field strength○

Zeeman broadening•

Temperature mostly governs the strength of absorption lines, however line width is affected by different

factors.

Luminosity ClassesLuminosity classes are based on the width of spectral lines. (They indicate how big the star is)

This diagram indicates 3

stars of the same spectral

class (temperature).

However, they are all of

different luminosity.

1.The top star has thin

spectral lines. This isbecause it is giant, with a low

gas density. Thus limited

broadening

The Stars



Or Absolute Magnitude○

Plots Luminosity•

Or Spectral Type○

Or Colour Index○

Against Temperature•

HR Diagrams

On the main sequence , increasing temperature means increasing luminosity•

Properties

.

broadening

The 2nd and 3rd stars are

indicated by wider spectral

lines because they have a

higher density. This density

causes pressure broadening.

Thus stars can be

classified by spectral and

luminosity class.

i.e. Sun is G2V star

The Stars



Therefore for the main sequence, spectral classes also sequence in mass

M dwarfs are very common but are usually invisible•

O dwarfs are rare but are visible as naked eye dwarfs to 1000 pc•

Spectral effects

The Stars



Lecture 13

Chapter 16 Star Birth



• describe the main stages of star formation

• describe the concept of a pressure-temperature thermostat operating inside stars

These dark clouds are places where star formation is likely to occur.

Interstellar Dust

In dense gas clouds, blue light is scattered away by interstellar dust, until gradually all the light is blocked out.

(Thus stars on the border of the star will appear redder, called interstellar reddening) Since the dust absorbs

longer wavelengths, short wavelengths such as infrared can allow us to see through molecular clouds.

Molecular Cloud1.Giant molecular clouds are the primary place for star formation. Should be cold and dense. Cold

temperatures are due to interstellar dust which radiate away thermal energy

•

UV light cannot break up molecules deep inside the cloud (as they are protected by the outside)•

Cold temperatures are needed so that Gravity can overcome thermal pressure of gas clouds•

Gas particles in molecular clouds remove thermal energy by emitting photons•

Cloud Collapse2.

Nearby supernova or galactic spiral density wave○

If conditions are right the clouds may collapse•

The colder the cloud is, and the denser the cloud is; the more likely it is that it's going to collapse○

Clouds collapse under self-gravity if they exceed the jeans mass•

Collapse is uneven-> Core collapses faster than outer layers and approaches equilibrium while outer

layers still free fall inwards. This forms the protostar in the centre

•

Clouds occupy 1% of volume but contain 90% of mass of ISM (interstellar medium)

Lecture 14: Star Formation16 September 2013 14:06

The Stars



Protostar3.Density starts increasing and thus the radiation gets trapped and absorbed -> Protostar heats up•

Forms a pre-main sequence star

A pre main sequence star has had its surrounding gases blown away by solar winds and jets•

It will continue to contract and thus heat up till it can fuse hydrogen•

Pre-Main sequence stars

Rapid rotation of gas prevents it from going

straight into the star. Instead it may settle into a

protostellar disk, from which gas will gradually

spiral inwards.

Another effect of the protostars rotation are jets

which can come outwards of its axis.

Lower mass stars arrive lower down on the main

sequence, but also take a longer time to form than

higher mass stars.

•

The Stars



Beings very luminous but cool due to its large surface area○

After surface temperature rises to about 3000 convective contraction causes protostar to contract

without increasing in temperature, thus decreasing in luminosity.

○

Then radiative contraction increases temperature and luminosity slightly○

Explaining the curvy end of path on HR diagram:

Stellar Structure

Hydrostatic Equilibrium: Gravity vs. Pressure• Equation of state: Ideal Gas, gas molecules don't interact (How the gas behaves)•

Energy generation: Fusion•

Energy Transport: Radiation, Convection.•

Tells us about the lifetimes of Main Sequence stars

These things help us understand the lifetimes of stars on the main sequence.

The birth line marks the

emergence of the stars from

surrounding cloud. This is

the point when they become

visible.

Max~ 120-150 M

Min= 0.8 M

The Stars



Lower main sequence

→ entire star is convective & completely mixed (cannot use radiation)○→ all H eventually burns into He○

→ very slow evolution○

For M < 0.26 M

, star is cool & opacity of the gas is high•

Fusion is spread over a large region in the core○

Small temperature gradient means energy is transported in the core by radiation○

No mixing , therefore not all H will be consumed○

Outer layers are cooler and thus undergo convection○

For 0.26 M< M<1.5 M•

PP chain dominates in core.

For M > 1.5 M•

CNO cycle is dominant•

Fusion concentrated at core, making temperature gradient high (thus no radiation)•

Therefore core is mixed○

Convection is necessary for energy transport around the core•

Outer layers have low opacity; Radiative. (so no granulation patterns)•

Upper main sequence

The Stars



4 Hydrogen's are being converted into 1 helium and thus the core will contract. But the hydrostatic

equilibrium pushes up the fusion rate to counteract gravity

○

As Hydrogen fuel is used up temperature and density will increase to maintain fusion rate (due to the core

collapsing)

•

Radius and luminosity increase, surface T drops slightly•

•

The MS life of a star

When H abundance <1%, core begins to contract faster•

This heat ignites H shell around stable He core. This causes an increase in energy output and the

following effects.

○

Collapse heats core by release of gravitation potential energy•

Evolution to a Red Giant

Evidence for this is in star clusters. Young clusters have larger stars on the main sequence. Old clusters have no

large stars.


• identify that the evolution of a star is determined by its initial mass

• describe the evolution of a low-mass star into a red giant, a planetary nebula and a white

dwarf

• identify that fusion of elements heavier than hydrogen requires higher temperatures

MS star will move slowly up and to the right•i.e. The sun is currently 5% bigger and brighter 200

K warmer than ZAMS. This will accelerate over next

5 billion years

•

Lecture 15: Stars Life and Death16 September 2013 14:06

The Stars



Star size increases○

Luminosity Increases○

Surface Temperature falls○

Star will migrate on HR diagram because:•

Instead of the fusion being able to be regulated by expanding and cooling in a MS star, the new helium

causes further contraction.

a.

This causes the helium core to rise in temperatureb.

Helium begins to build in the core from hydrogen fusion in the shell.1.

When the temperature of the star is high enough (100million K) Helium burning begins in the core. (triple

alpha process He->C)

2.

The onset of helium fusion heats core rapidly without expansion (because degeneracy pressure, notthermal, was holding the inert helium core before)3.

This causes a massive increase in luminosity in core, called helium flash4.

Helium Flash

Triple alpha process: has extreme temperature dependence, and thus the star will expand andcontract, because it is unstable

○

Similar to first rise up the giant branch but with He and H burning shell•

Thermal pulses every few thousand years, until star develops a superwind, which blows in gusts and

removes the outer envelope to reveal the core as a white dwarf, surrounded by a planetary nebula.

•

Final Evolution of 1M Star

5. As temperature rises

due to He burning in

core and H burning in

shell: radius and

luminosity will drop

(even though two types

of fusion), but

temperature will

increase; thus changing

colour from red back to

yellow.

6. The weaker pull of

gravity will also cause

the release of mass in

stellar winds

It will now be on the

main sequence of He

burning

The Stars



Core is supported by electron degeneracy pressure•

They will also be smaller because greater gravity○

Larger white dwarfs will be made of heavier elements as they have fused more elements•

Maximum theoretical mass: 1.4 solar masses (Chandrasekhar limit)•

Shine by radiating thermal energy, no fusion•

White Dwarfs (size of earth, mass of the sun)

They accrete mass from their companion star to form a hot accretion disc (which would provide UV/X-rays)•

Temperature will rise in a shell surrounding the dwarf, blazing to life suddenly due to H burning on

accumulated shell.

•

This produces a very bright nova (100,000 times luminosity of sun)•

These supernovae can be identified by a lack of Hydrogen lines○

Some white dwarfs can keep increasing in mass past the 1.4Msun limit and their temperature rises enough

for Carbon fusion to begin. This causes an explosion called a white dwarf supernova (a class of type 1

supernovae)

•

White dwarfs in close binary systems undergo a few different mechanisms.

The super wind from the AGB (asymptotic giant branch) removes outer layers to form a 'planetary' nebula.•

Planetary nebulae (few tenths of mass of sun)

The radiation from the hot remnant core will ionise the gas in the expanding shell, making it hlow•

This nebula will disappear within a few million years•

Final Evolution of a 5M starThe evolution of a 5 solar mass star will be the same as the sun, just significantly quicker (<100 million years)

The Stars



Final evolution of Massive stars. (Greater than 8 solar masses)

In a massive star, elements will successively burn in the core till iron•

The iron core is created briefly, but is inert•

Stars below 8 Solar masses will blow away enough mass to be below the Chandrasekhar limit before they die.

Cannot fuse iron together, so gas pressure drops and gravity takes over•

Collapse rebounds from a superdense state and Blows off the outer layers as a supernova explosion

(10 billion times the luminosity of our sun)

○

Core Collapse in less than 1 second (electron degeneracy pressure cannot hold)•

This can produce a shockwave of fast moving gas which sweeps out from the supernova○

Ejecta produces a supernova remnant.•

Heavier elements are generated during a supernova via neutron capture•

Leaves behind a neutron star (which may collapse into a black hole)•

Death

These are expanding clouds of debris which are left after a supernova•

Supernova remnant

The Stars



The core remnant of massive stars after a supernova.

Mass of 1.5-3 solar masses. (but 10-20 km across)•

Supported by neutron degeneracy pressure•

Spinning very fast due to conservation of angular momentum, thus very strong magnetic field spinning

around, causes narrow beams of radio waves.

•

Neutron Stars

These pulsars have a rapid rotation, and produce a regular pulse.•

This can happen because the rotation axis is usually different to the magnetic axis•

Will eventually slow down due to loss of energy by emitting EMR•

Discovered as: PULSARS (magnetized rotating neutron stars)

When in a binary system accretion disks can form which are much more luminous than the accretion disc

around a white dwarf

•

These pulsars will speed up over time unlike normal•

Can result in X-Ray bursts after spontaneous fusion of material accreted from its companion star•

X-Ray Binaries

Pulsars allow for the probing of the interstellar medium, due to their regular sharp pulses. High frequency pulsars

will arrive faster than low frequency pulsars.

Core mass exceeds 3 solar masses•

Collapses to a singularity, surrounded by event horizon where not even light can escape from•

Event horizon lies at the Schwarzchild radius•Intense source of gravity. Have mass, spin and nothing else•

Black Holes

Know of binaries (also X-ray binaries) where one companion MUST be a black hole•

Evidence for super-massive black holes in cores of galaxies•

Proof



• describe the main features of white dwarfs, neutron stars and pulsars

Lecture 16: Pulsars and Black HolesMonday, 23 September 2013 12:54 PM

The Stars



Chapter 19 Our Galaxy



• describe the orbits of the stars in the Galactic disc and bulge• sketch the major structural components of the Galaxy

• describe the composition of interstellar gas

• identify the phases of the interstellar medium

• identify the effects of intervening dust on the properties of stars

• define HII regions, reflection nebulae and molecular clouds

The Milky Way appears in the sky as a faint band of light. Obscured by dusty gas clouds, which are part of the

interstellar medium. This cosmic dust mostly comes from supernovas and solar winds from red giants and is part of

interstellar medium.

The Components of the Galaxy

The main components of the galaxy. Are the bulge, the disc and the halo.

Orbits of stars in the Disc

X rays are observed from

hot gas above and belowthe disk.

•

21cm radio waves

emitted by atomic

Hydrogen shows gas

across the disc

•

Radio emission from CO

shows locations of

molecular clouds.

•

Long wavelength IR

emission shows where

young stars are heating

dust grains.

•

Gamma Rays shows

where cosmic rays from

supernovae collide with

atomic nuclei in gas

clouds.

•

If they go below the disc,

the gravitational pull of the

○

Stars in the disc, move around inthe plane of the disc. They have a

small up and down motion

•

Lecture 17: Milky WayMonday, 23 September 2013 4:50 PM

The Milky Way



The interstellar Medium

Gas: Mostly Hydrogen (70%), helium (28%) and CO (1%)• Dust: Carbon, Silicates•

Multiphase•

The interstellar medium has varying regions of density and temperature. Most of the ISM would be warm atomic gas.

Typical states of Gas in the Interstellar medium

Interstellar spectral absorption lines from the visible light from stars•

The 21 cm emission from H can be observed all across the galaxy.•

This arises from the spin of the electron and proton. When the spin flips, a 21 cm emission line will be emitted•

Proof of the ISM (how do we see this gas since it's not hot enough to be part of a star or emit xrays?)

Mapping HI can be done by telescopes such as the Parkes telescope. Since this is an emission line, velocities can also

be determined.

NebulaeCan range from small globules to Giant Molecular Clouds

If they go below the disc,

the gravitational pull of the

disc will pull it up, and vice

versa.

○

sma up an own mo on

Halo stars, bulge stars, globular

clusters; have chaotic orbits. If

disturbed, their orbits change

dramatically

•

The orbit of our sun (in the disc)

tells us the mass of the galaxy

within that orbit (1x10^11 Solar

masses)

In these star forming regions starlight is reflected from dust grains

produces a blue colour.

○

Reflection Nebulae:

UV light from hot young stars ionises hydrogen to form a HII

region- causing them to emit visible light (esp. red Hα line)

○

Are found near short-lived, high mass stars, signifying active

star formation.

○

Emission Nebulae:

The Milky Way



Dense clouds leading to reddening, with no starlight passing through○

Dark Nebulae:

OH and CO are easy to see at radio wavelengths○

These molecules are easily destroyed by UV photons from hot stars, and thus can only survive within

dense, dusty clouds where they are protected

○

H2 is hard to see at UV wavelengths so, CO is used as a tracer for H2 .○

Molecules

This is because blue is more easily scattered by interstellar dust

In these star forming regions starlight is reflected from dust grains

produces a blue colour.

○

The Milky Way




• sketch the rotation curve of the Galaxy and explain how it constitutes evidence for dark matter

• describe the global cycle of star formation and chemical enrichment of interstellar gas that occurs in the

Galaxy • identify the relative contributions of low and high mass stars to the total luminosity, mass and chemical

enrichment of the interstellar medium

• define metallicity, and explain how the metallicity of the interstellar medium evolves with time

• describe the trends in the spatial variation of metallicity within the Galaxy

• describe the various ways in which star formation may be triggered

• identify objects that trace spiral arms

• explain the winding problem for spiral arms

• describe the density wave theory of spiral structure

• define population I and II objects, and identify several examples of each

The Global Cycle of Star Formation + Enrichment

Lower mass stars return gas through stellar winds and planetary nebula○

High mass stars have strong stellar winds○

Stars: Make elements beyond hydrogen by nuclear fusion and return this gas to interstellar gas•

As supernova remnant will cool and begin to emit visible light as it expands.○

The new elements made by supernova will mix into the interstellar medium. (heavier elements are

made during explosions though neutron capture)

○

Multiple supernovae can create hot bubbles that can blow out of the disk of the galaxy. This

produces a galactic fountain. The gas will escape to a certain distance up, then cools and comes back.

○

Supernovas:•

As time goes on the ISM will become enriched with heavier elements. However with each new generation of stars,

despite recycling some gas will be tied up in brown dwarfs and stellar corpses. Thus there is only a finite amount of

gas

Lecture 18: Milky Way 226 September 2013 15:01

The Milky Way



Atomic hydrogen gas forms as hot gas cools○

Then molecular clouds form next and eventually gravity forms star out of the gas in molecular clouds○

When these stars start nuclear fusion they will break down the molecular dust○

The Formation of Stars:

Sites of star formationWe believe that the stars in the halo formed first then star formation stopped. Bulged stars formed early as well,

but then disc stars formed later and kept on forming

Disc stars: ionization nebulae, BLUE stars and 2% heavy elements•

Absorption at nearly all wavelengths due to chemical enrichment. (remember that stars will produce

elements as continue through their life)

○

Includes stars like our sun•

Youngest will be in the spiral arms•

Population I: Disc and Arm Stars

No evidence for new star formation in the halo•

No ionization nebulae (places where H gas is glowing), no blue stars○

Only old stars: 02% heavy elements○

Mostly old, RED stars•

Only a few weak absorption lines because it is not chemically enriched•

Population II: Bulge Stars

Proposed earliest generations of star with ZERO metal content (because even population 2 have SOME

metals such as Iron, which must have come from supernovae)

•

These exploded in supernovae•

Population III:

Lots of star formation in the disc happens in the spiral arms•

Contain lots of Emission Nebulae due to glowing hydrogen gas, and also lots of dust•

The spiral arms

Spiral Density waves- work by a similar mechanism to a Mexican wave, the stars themselves do not

move. Instead these waves cause gas to be more tightly packed, thus forming the spiral arms. This

wave will move through the galaxy without carrying the matter along with it

○

Star formation will stimulate more star formation○

Spiral arms are waves of star formation•

The spiral arms is simply a pattern, they DO NOT SPIN FASTER than stars further in (not like a spinning top)

The winding problem

It seems like spiral arms should move with the stars, but as seen above, this is not the case. The reason is that stars

near the centre of the galaxy complete an orbit in much less time than stars far away from the centre. Thus the

The Milky Way



inner stars would orbit in less time than the outer and this would result in the spiral arms winding or tightening.

(and we don't see this)

The Milky Way



Lecture 19

Chapter 19 Our Galaxy

Text Sections: 19.2


• sketch the rotation curve of the Galaxy and explain how it constitutes

evidence for dark matter

• identify objects that trace spiral arms

• explain the winding problem for spiral arms

• describe the density wave theory of spiral structureThe milky way is a barred spiral galaxy with diameter ~100,000 light years and a mass ~1012 solar masses, much

of it invisible!

Mapping the Milky Way

Selecting bright objects that are seen throughout the galaxy and trace their directions and distances. (like

candles which are further away)

•

They had no idea the milky way was full of dust○

Stars were much too faint○

Using a photographic plate to collect information from suns. The 1900s predicted an incorrect view of the

milky way. Because:

•

Strategies

Observe objects at other wavelengths, and catalogue their directions and distances (because of the

presence of dust)

•

Trace the orbital velocities of objects in their different directions relative to our position•

Approximately 200 known in the halo. Quite bright because so many stars•

Clustered around the centre of the galaxy•

Shows sun isn't the near the centre of the galaxy•

Globular Clusters

Groups of stars highlighted by O and B types. They formed together and move together•

OB associations

Lecture 19: Mapping the Milky WayMonday, 14 October 2013 12:23 PM

The Milky Way



Distances are found using Cepheid variables. These vary in brightness•

Cepheid Variables: Allow us to measure distances inside and beyond galaxy (provided an absence of dust)

Penetrating the Dust of the Milky Way

Infrared

Can use infrared light since dust absorbs visible but remits infrared.

Radio frequency allows the detection of neutral hydrogen (21 cm emission line) concentrated in SPIRAL

ARMS. Modern measurements also allow detection of CO

•

Overlap (due to our viewpoint) can be disentangled by radial velocity measurements. (doppler shift)

Multiple arms all in one plane with different velocities.

•

•

Gas density increases as you move inwards, peaking in a collection of clouds called the molecularring.

○

IR observations of stars and dust reveal a CENTRAL BAR across the bulge.•

Radio Observations

Change in brightness periodically (between

1-60 days)

•

Lie on instability strip on HR diagram•

Obey period luminosity law that allows the

distance of a Cepheid to be determined by

observing its period to determine the stars

absolute magnitude.

•

Henrietta Leavitt studied Large Magellenic

cloud (stars which are all approx. same

distance, without dust because different

galaxy). Realized that the brightest Cepheid's

have the longest periods and dimmest had

the shortest period.

•

The Milky Way



Dark Matter99% of the mass of the solar system is the Sun so orbital velocity decreases with distance as expected, however

this is not the case for galaxies.

Even though light is concentrated towards the centre; rotational velocity does NOT drop with distance in

the outer regions

•Rotation curve of Spiral Galaxies

Studying the rotational velocity of the gas shows that even extremely far out the velocity is still

increasing

○

Hydrogen gas beyond the stars (cannot see this normally) can be observed using the HI emission line.•

It shows that dark matter increases as the radius increases; there is a lot in the outskirts of the galaxy.•

HI gas as a probe of Dark Matter

Total mass in the disk is ~200 billion solar

masses, but requires ~ 1 trillion solar masses.

Most of the mass is not concentrated in the

center.

Therefore, most of the mass is invisible- dark

matter.This matter is thought to be in the halo

surrounding the milky way.

The Milky Way



The Centre of the Milky Way

Radio image of SgrA demonstrates a spiral surrounding SgrA*.•

IR image shows a high concentration of stars near the centre (because IR can reach us, unlike visiblelight)

•

Milky Way's black hole is believed to be about 4 million solar masses•

When stars fall in into a rotating black hole, they are torn apart and release X rays.•

Black Holes

Unlike most black holes, Sgr A* does not continually emit X-rays. Thus we can assume that it does not have

an accretion disk, possibly because matter falls into it in big chunks instead of a smooth swirling flow of anaccretion disk.

How did the galaxy form?Traditional: Oversimplified

Taken by Radio imaging because

dust blocks visible light

•

Strong jets of emission suggest

they are controlled by Magnetic

fields

•

Most other features are super

nova remnants or star forming

regions

•

The motion of stars can be measured and used

to make calculations

•

Thus we can do a calculation and know

that there must be an extremely

massive object in a tiny area of space

○

Stars accelerate when the come towards the

centre

•

Stars appear to be orbiting something massive but

invisible... Black Hole (4 million star masses)


• describe the evidence for a massive black hole at the Galactic centre

• outline a scenario for the formation of the Galaxy

Lecture 20: The Centre of the Galaxy14 October 2013 14:07

The Milky Way



Galaxy forms a giant gas cloud1.

Halo stars form first as gravity caused the cloud to contract2.

Remaining gas settles into a spinning disk3.

Stars continuously form in the disk as the galaxy grows older4.

This model explains many features of our galaxy, but upon further inspection it does not explain the heavy

element proportions

Halo stars formed in smaller clumps (protogalactic clouds) then later merged 1.

Accretion of smaller satellite galaxies continues2.Gas in the milky way's disk is replenished from outside3.

Current Picture

Dwarf galaxies leave behind a long filament of stars trailing in its path around the galaxies, which will

eventually merge with halo stars

•

This is something that continues today•

Accretion of Dwarf Galaxies (step 2)

The Milky Way



Proposed: The more remote a nebula is, the faster it appears to be moving away from the observer •

Introduced the concepts of elliptical and spiral galaxies•

Edwin Hubble

Types of GalaxiesClassified by appearance of stellar population. (Though sometimes there is no clear division)

Elliptical:

Dominant stellar population is old (with little or no recent star formation)•

Type 0-7 according to eccentricity •

Red-yellow colour suggests older population (though some may have regions of new)•

Both giants and dwarfs•

regular appearance, no obvious disk

Lenticular Galaxy:

Has disc shape but doesn't have dust or spiral arms•

Full of older stars•

Thought to be spiral galaxies that are not making new stars•Multiphase interstellar medium, mainly cold neutral gas in disk and spiral arms•

Intermediate between spiral and elliptical

Sombrero galaxy is in between elliptical and spiral. Spiral like shape is due to a ring of captured gas, around much older stars in

the centre. This may have happened when an initially spiral galaxy was dragged through the hot gas in the centre of a cluster;

stripping its own gaseous disk away.

describe the various types of galaxies•

contrast the stellar populations and gas content of irregular, spiral and elliptical Galaxies•

Specific Objectives

This particular galaxy is surrounded by 10,000

globular clusters. Thus thought that many of

these clusters were formed in cataclysmic event.

Especially since they are quite young

•

Interstellar gas is mainly hot, gaseous coronaoften seen in X-Rays. (therefore cannot form

gas)

•

Lecture 21: Galaxies14 October 2013 14:07

Extra-Galactic Astronomy



Spiral:

Old stars (Pop II) in halo, younger stars (Pop I) in disc•

Classified into a b and c according to tightness of arms, size of bulge and amount of gas•

Multiphase interstellar medium with cold neutral gas•

regular appearance, obvious disk, selection effect (70%) because of bright O/B stars

Has a bar of stars across the bulge. Probably a transition stage•

Barred spiral galaxy

Irregular:

Blue white colour indicates ongoing star formation (in the case of the magellanic cloud)○

Various origins; result of collisions and mergers○

Usually dwarf galaxy satellites○

Extremely faint○

Diverse population, some galaxies are rich in cold neutral gas others have zero○

irregular/asymmetric in appearance.

Spectroscopic Classification of Galaxies

Spiral galaxies are the brightest•

Elliptical are the most massive•




Specific Objectives - after studying this chapter you should be able to:• explain how to determine distances to galaxies using observations of distance indicators (Cepheids and type Ia

supernovae)• describe the Hubble law and how it can be used to determine distance

• identify the Local Group

• describe the expansion of the universe and the interpretation of redshift

Distance Ladder and the different methodsWe can use the distance ladder to help measure a distance as accurately as possible. There variety of different

rungs gives a method of checking each other.

Moving Cluster

Parallax- Using the

space motion of a

nearby cluster of stars

Cepheids using the

period luminosity

relationship to

determine their

brightness.

Type 1a supernovae

are the explosions of

white dwarfs in binary

systems and are very

bright.

Tully fisher- relates

rotation curve and

luminosity

Lecture 22: Extra Galactic Distances and Clusters of

Galaxies21 October 2013 14:07




Most of these methods are standard candle techniques, which figure out the true luminosity, compare it toapparent magnitude and work out the distance.

RedshiftRedshift is an observation which can be used to measure distances for galaxies extremely far away

The spectrum is shifted and the shifting of a particular wavelength is measured•

Quasars have emission lines which allow us to show distance•

Everything outside our local group is redshifted (with very large redshifts)○

Very close galaxies are not redshifted.•

More modern methods allow plotting of galaxies which are much further away○

By observing relatively nearby galaxies he plotted their apparent velocity (from redshift) against known

distance (from other methods)

•

Hubble's Law




This demonstrates a relationship between Velocity and distance•

H0 ~72km/s/Mpc○

Velocity=H0 x Distance•

This law extended over vast distances and was predicted by Einstein's theory of relativity•

The inverse of the Hubble constant tells us the age of the universe.•

This is cosmological redshift, and not Doppler redshift.•

Thus the universe will appear the same from any point○

It is caused by the expansion of space itself, and not due to the motion of galaxies through space•

Recession velocities can be greater than c. Since it is the expansion of space itself •

Cosmological Redshift

Redshift converts directly to a scaling factor of the universe•

Conversion to distance, look back time or recession velocity depends on model of the universe•

What Redshift means

Rich clusters- 1000 or more galaxies diameter of ~ 3 Mpc, condensed around a large, central galaxy•

Poor Clusters- Less than 1,000 galaxies, diameter of a few Mpc, generally not condensed towards the

center

•

Groups- Typically~20 galaxies•

Galaxy Group Classification

For example the Milky Way is a small group with only 3 MAIN spiral galaxies.•

Spiral galaxies are often found in small groups (up to a few dozen galaxies per group).

Elliptical galaxies are much more common in rich clusters of galaxies (hundreds to thousands of galaxies)

Galaxy Collisions

The galactic nuclei will merge•

Stars will not collide that often to due large distances, so essentially it is gas clouds colliding•

Immediate burst of intense star formation (starburst) then supernovae and stellar winds blow away

mass, leaving no new stars to form

•

Stars die out leading to the formation of a large elliptical galaxy•

Since the distances between galaxies is relatively small, galaxy collisions DO occur.

Super clusters and voids

Local groups such as ours form

part of larger clusters and super

clusters such as the Virgo super

cluster

•

Maps of galaxies reveal voids and

super clusters which probably

started off as regions of varying

density

•

Galaxies cluster together in

throughout the universe in a non

uniform fashion when you are

observing on a big scale

•




Histories of GalaxiesWhen looking at galaxies extremely far away, we see how galaxies are when they are young. Around this time

galaxy shapes were much less distinct. There must have been low level irregularity for the formation of galaxies.

Note: Before 300,000 years all we can see is the cosmic microwave background radiation. Light was scattered

during this time. We see low level structure, but at this point there were no galaxies.

Why do galaxies themselves differ?

Denser regions contracted, forming protogalactic clouds.•

These stars would have been more massive than present stars due to their different composition

(which we cannot achieve)

○

H and He gases in these clouds formed the first stars.•

Enriched the formation of new stars with metals○

Supernova explosions from first stars kept much of the gas from forming stars.•

Leftover gas settled into a spinning disk.•

Remember that the composition of gas in these galaxies would NOT contain any metals.

Spin: The initial angular momentum of the protogalactic cloud is thought to determine the size of the resulting

disc

This is because once a star forms it is able to keep its orbit, whereas gas would keep collapsing•

Density: Elliptical galaxies could have come from very dense protogalactic clouds that were able to cool and form

stars before gas settled into a disc.

Collisions: It has been observed that galaxies at great distances look violent disturbed, most likely due to

collisions.

Milky WayStar formation occurs at high redshift (early galaxies) in high mass galaxies in dense environments. At low

redshift (closer to present) these dense environments have little star formation activity.

Formation

The Milky Way is a typical spiral galaxy. It has evolved (as most spirals) in moderately over dense regions, but

Matter (mainly dark matter)

originally filled all of space almost

uniformly

•

Gravity of denser regions pulled in

surrounding matter

•

This picture shows the positions of

galaxies in the universe.

We assume:



• describe the models for the formation of galaxies


• explain what is meant by dark matter and dark energy

• describe methods to determine the masses of galaxies (rotation curve, velocities within a cluster)

• describe the evidence for dark matter in galaxies and clusters

• explain the gravitational lens effect

Lecture 23: Galaxy Formation and Dark MatterMonday, 28 October 2013 12:58 PM




Gravity caused tiny irregularities in dark matter distribution to grow including the one that now

surrounds the Milky Way (the roughly spherical halo of mass ~10^12 solar masses) and a multitude of sub

halos all the way down to the mass of Earth.

•

Inside the halo was a thin haze of primordial hydrogen and helium gas pulled along by the dark matter’s

gravity.

•

Contracting mass spun faster and flattened into a thin disk where gravitational interactions caused

the density waves that formed spiral arms

○

After a few hundred million years, this gas cooled enough to start forming stars•

The larger dark-matter sub-halos would have pulled in enough gas to form stars and become dwarf

galaxies.

•

Formation of galaxy continued with gas and dwarf galaxies swirling inwards towards an accumulating

mass of gas and stars at the dark-matter halo's centre:

•

not dense clusters.

Possibly extremely faint because they are dominated by dark matter•

The Milky Way should have thousands of dwarf galaxies in orbit around it but observers have found only ~20.

Continued star formation

Matter cycles back and forth between stars and interstellar gas•

They may be an outside reservoir: a halo of hot ionised hydrogen gas•

Since the milky way turns gas into stars at a few solar masses per year, it should have used up all the available

gas

Dark Matter

We expect orbital velocities of stars to reach a peak then decrease.1.

In Individual Galaxies

(as they do in the solar system)

In elliptical galaxies, the broadening of spectral lines tells us how fast the stars are orbiting (due to

Doppler shift). More broadening means broader lines. This high velocity is also explained by dark matter

2.

Orbits of galaxies in clusters: found that galaxies have much greater masses than their luminosity would

suggest

1.

85% dark matter○

13% hot gas○

2% stars○

Clusters contain large amounts of X-Ray emitting hot gas which fills the space between galaxies. The

temperature of hot gas tells us the cluster mass.

2.

Can also be seen through gravitational lensing. The bending of light rays by gravity can also be used to

determine a cluster's mass. This method is important as it uses a different method to using newton's law of

gravitation

3.

In clusters of Galaxies

The visible portion of a

galaxy lies deep in the

heart of a large halo ofdark matter. The

luminosity tells us how

many stars there are,

and using information

from the 21 cm H we

can determine velocity

to get a rotation curve.

Spiral Galaxies

tend to have flatrotation curves;

indicate large

amounts of dark

matter

Rotation curve

comes from red

and blue shift.

To get the amount of gravitational lensing




EvidenceAll three methods (velocities, hot gas and lensing) indicate similar amounts of dark matter.

Dark matter really exists and we are observing the effects of its gravitational attraction1.

Or something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of

dark matter

2.

Thus either:

Baryonic Matter (normal matter) - Things like brown dwarfs or planets etc. Could make up some of the

dark matter- collectively called massive compact halo objects (MACHOs).

•

Weakly interacting massive particles (WIMPS, the common view)- More exotic particles like axions•

Types of Dark Matter (candidates)

To get the amount of gravitational lensing

observed, more mass is required than can be

observed.

Once again this can only be explained by dark

matter.




Active Galactic NucleiIf the centre of a galaxy is unusually bright, we call it an active galactic nucleus. The brightest AGN are known as

quasars.

Things which look like a star , but are not•

Quasi Stellar Objects are extremely distant (highly red shifted) but luminous examples of Active Galactic

Nuclei

•

Quasars- Quasi Stellar Objects

Composite Quasar Spectrum: Formed by taking all quasar spectrum and adding together

Much brighter (and especially so in blue/ultraviolet light)•

The clear signal we receive energy comes from a region smaller than the solar system○

Their optical brightness often varies quite rapidly •

The difference between Quasars and Normal Galaxies

Something brighter than can be

explained by stars alone.

•

Every wavelength is emitted•

Often have jets coming out fromthe centre

•



• describe the observed properties of quasars and the evidence that they lie at great distances

• describe the observed properties of radio galaxies and Types I & II Seyfert galaxies

• describe the unified model of active galaxies and explain how it explains their observed

properties

Lecture 24: Active Galactic Nuclei28 October 2013 14:07




Their optical spectrum is dominated by lines from ionised gas (not stars, since the galaxy is far below the

bright light)

•

Brightness and distance imply luminosities that can be greater than 1012 Lsun•

Galaxies around quasars often show evidence of mergers/interactions•

•

Radio Galaxies

Radiate over a wide range of

wavelengths, indicating they

contain matter with a wide

range of temperatures

•

Quasars were more common at previous

times but not anymore.

•

(This is thought to be because the merger

rate was higher in the pass, thus boosting theaccretion rate on to the black hole.)

Examples of Optical Spectra-

Where most AGN showemission lines. Sometimes

emission lines are narrow and

some are broad. Broad lines

means gas that is moving at high

velocity, these are the ones that

vary.

Broad lines hold the clues about

what is going on in AGN




Radio galaxies were observed to be objects which emit unusually strong radio waves. (from pairs of radio

lobes). Quasars and radio galaxies are thought to be the same object viewed in different ways.

•

AGN shoots out jets of high energy particles directed by a twisting magnetic field. These strike the inter-

galactic medium to create the radio emitting lobes. Now these lobes give off radio waves

•

The radio waves are coming from relativistic electrons moving at near light speed (synchrotron emission)•

Radio Lobes

Trails of plasma can be radio imaged. Emissions from jets pointing towards us are enhanced compared to the jet

moving in the other direction.

This can reveal many things such as how AGN can shoot out blobs of plasma moving at nearly the speed of light (it

can appear as super luminal motion) Material in the jet is 'chasing' the light it emits.

Have active galactic nuclei which are less powerful•

Variability ~50% over a few months•

Probably what a Quasar looks like in its old age (much less luminous)•

Seyfert Galaxies

Properties and Explanation of AGN

The power source

The gravitational potential energy of matter falling into a black hole turns into kinetic energy.•

Friction in the accretion disk turns kinetic energy into thermal energy (heat).•

Heat produces thermal radiation (photons).•

Accretion of gas onto a supermassive black hole appears to be the only way to explain all the properties of AGN.

Radio galaxies don't appear as quasars because dusty gas clouds block our view of the accretion disk.

Black holes in galaxies

Remember that many nearby galaxies (perhaps all) have supermassive black holes in their centres- measured by the

These jets are thought to come

from the twisting of a magnetic

field in the inner part of the

accretion disk.

We must explain how Quasars

can produce so much luminosity,

and this can only be explained by

a supermassive black hole.




All galaxies may have passed through a quasar-like stage earlier in time•

The mass of a galaxy’s central black hole is closely related to the mass of its bulge - implying that the

development of a central black hole must somehow be related to galaxy evolution.

•

orbital speed and distance of gas orbiting them. These black holes may seem to be dormant active galactic nuclei.

The unified model for AGN

The unified model helps explain the properties of AGN by viewing angles. The black hole will create an accretion disk

and jets of material will be streamed out from this black hole at high velocities. However these broad emission lines

will only be seen at a certain angle, and when seen in the plane they are obscured by slower moving gas.

Some galaxies appear to

have broad and some

have narrow emission

lines. (hence two types)

The broad lines are

always there but

sometimes they are

obscured by slowly

moving gas. (when you

are looking through the

donut)

This is confirmed by

using infrared to look

through dust




The Location Principle: It unlikely that we occupy a special place in the universe.•

Universality: that all the laws of physics are the same everywhere in the universe•

Homogeneity: On the largest scales, the universe has the same physical properties everywhere○

Isotropy: On the largest scales, the local universe looks the same in any direction that one observes○

The Cosmological principle: Considering the largest scales in the universe, we make the following

fundamental properties

•

Assumptions in Cosmology

The Anthropic Principle

Fine tuning of gravity•

Smoothness of big bang•

Masses of sub atomic particles•

Strength of the strong nuclear force•

Magnitude of the cosmological constant•

Talks about how the universe is fine-tuned to our existence.

Olber's ParadoxWhy is the sky dark at night?

If the universe is infinite then every line of sight should end on the surface of a star at some point. (the night should

be as bright as the surface of stars)

Therefore this implies there is only a finite number of stars/galaxies that have light which had time to travel to us,

because the universe began at a particular moment.

Cosmology and General RelativityRelates mass energy and space time. The curvature of space-time, in turn, is determined by the distribution of mass


Specific Objectives - after studying this chapter you should be able to:• describe the assumptions of cosmology

• describe and explain Olbers' paradox

• describe and explain the primordial background radiation in the context of the big bang theory

• explain how the abundances of elements supports the big bang theory

• identify the main stages in the history of the universe according to standard big-bang theory of

cosmology, including recombination

Lecture 25: The Big BangTuesday, 29 October 2013 6:57 PM




and energy in the universe.

i.e. Negative curvature- infinite space○

Flat models - infinite space○

Positive Curvature- finite space○

General relativity predicts different possible histories of the universe: Cosmology studies which model we

live in

•

The Expansion of the Universe1.Initially the two theories to explain were this were the big bang or steady state theory. The steady state

theory stated that space is expanding but new atoms are being created in these gaps. This theory advocated

an infinitely old universe that continued to grow.

•

On a large scale galaxies are moving apart with velocity proportional to distance•

Space itself is expanding, carrying galaxies along (galaxies themselves are not expanding)•

The more distant objects we observe, the further back into the past we are looking•

The cosmic microwave background2.The radiation from the very early phase of the universe is detectable today as the Cosmic Microwave

Background.

•

This is the arrival of photons arriving at Earth from the end of the era of nuclei . It was at this point (380,000

years) that neutral atoms could remain stable and free electrons would not block photons.

•

This is a perfect thermal radiation spectrum which has been shifted into the microwave region•

This essentially ruled out the steady state theory, however it brought the problem of uniformity which will be

explained by the inflation theory next

•

The abundance of light elements3.The observed number of light elements such as deuterium matches the predicted amounts•

During the era of nucleosynthesis protons and neutrons were constantly being converted. However as the

temperature cooled down, the more favourable neutron to proton reaction was favoured. Thus there would

be more protons (7:1) which should lead to a 75% hydrogen 25% helium ratio; and this is what is observed.

•

Heavier elements were not produced because by the time stable helium nuclei formed, there was not enough

energy for more fusion

•

Evidence for the Big Bang

History of the Universe




The Epoch of Recombination

The Epoch of Recombination (dotted vertical line) is the time that after which photons could travel freely through

space. This is because protons and electrons recombine to form atoms. The wavelength of photons will be

stretched by cosmic expansion.

The Epoch of Reionisation

These stars will emit photons whichh ionise surrounding hydrogen (and it becomes more transparent)•

The formation of the first stars.




The expansion of the universe should be slowed down the mutual attraction of the galaxies.•

The fate of the universe should depend on the matter density in the universe.•

Thus there is effort to find the mass of the universe since we know the critical density which is just enough to

slow the cosmic expansion to a halt at infinity.

•

The fate of the universe

If our density is more

that the critical density,

the universe will

expand forever. If it

equals the density then

the universe will be flat.

If it's less than the

critical density, the

universe will collapse




It is also able to explain structure by inflation increasing the wavelength of quantum fluctuations, called

quantum ripples. This would cause density enhancements which would explain the structure of the

universe (density enhancements without inflation are too small)

•

It is also able to explain the flatness of the universe because the enormous expansion would have

flattened any curvature that the universe may have previously had

•

Dark Matter (another factor)

However even with dark matter observations show that the total density of matter is only about 25% of

the critical density

•

Is dark matter some kind of normal matter? However the density of baryonic matter is only 4% of the critical

density and thus most dark matter must be non baryonic.

By observing type I supernovae astronomers measured the Hubble relation at large distance.•The Accelerating Flat Universe

This is explained by the existence of dark energy.•

They expected that they would measure a deceleration of the universe, but they measured an increase.

Inflation is essentially able to explain

the uniformity . For example two

objects close to each before inflation

would be spread apart after inflation,

so they would be able to on opposite

sides in space but still have the same

characteristics.

•




Dark Energy

This dark energy Is thought to exert a repulsive force that causes the expansion of the universe to

accelerate with time

•

Dark energy can curve space time in the same way as dark matter. Calculations show that the amount of dark

energy required for acceleration of the universe is very similar to the amount needed for the observed flat

universe.




Unified model of the active galactic nucleus

-> Viewing angles on AGN

Spectral lines are dependent on temperature

Don't need to remember specific details like names of stars. Should know names of moons of

jupiter+ saturn.

Revision Lecture14 November 2013 14:08

Lectures



The Earth is turning and thus the Sky appears to moving around the South celestial pole•

Motion of the Sky

Special Lectures



Lines of Declination (horizontal)•

Lines of right ascension (vertical)•

The ecliptic crosses the celestial equator at the Vernal equinox (Sun spends 12 hours above

the horizon)

•

Fixed relative to the stars

Movement of Earth around Sun

As the Earth moves, the Sun appears to move against the background of stars. This traces out

a path around the celestial sphere called the ecliptic

•

Special Lectures



During Summer Solstice, Sunlight is spread over less area. Thus it is hotter as there is more

energy per square meter. It is also when the Sun appears directly overhead

•

Over a 26,000 period the Earth wobbles around and tilts its axis (changes completely every

13,000 years); called procession

•

Procession

The moon

It rotates once, in the same time that it orbits the Earth○

The moon is tidally locked to the earth. So the same point is always pointing towards the

earth.

•

Phases of the moon

Special Lectures



Tides

Gravitational force of the moon on the Earth varies on either side of the Earth. This causes atidal force, 'stretching' the Earth and explaining High Tide (like a rubber band)

•

Low Tides occurs at the places in between the tidal bulge•

Tidal bulges are slightly tilted in the direction of the Earth's rotation•

When the Earth moves in front of the moon•

Lunar Eclipses

Special Lectures



Solar EclipseThe shadow of the Earth on sun. Since the moon is moving further away this will not happen in the

distant future

Special Lectures



Special Lecture 3

Chapter 5 Light and Matter

Text Sections: 5.2


• describe light as part of the electromagnetic spectrum• identify that the wavelength of light determines the colour we see

• define photon and identify that light also behaves as a particle

Chapter 6 Telescopes: Portals of Discovery


• describe in general terms the principles behind the operation of a telescope

• describe the effect of the Earth's atmosphere on incoming radiation of different wavelengths

TelescopesMost telescopes at many wavelengths are basically similar

Configuration- Lens, mirror, paraboloids, prime focus, casse grain, grazing incidence•

Surface materials- glass, metal sheet, chicken wire•

The bigger the mirror the smaller the diffraction pattern is○

Radio telescopes such as the park telescope do not have to have as accurate a surface as a visible light

telescope

○

Surface accuracy- 'diffraction limited' optics that are too small for a large wavelength will be unable to

observe them. Needs to be correct down to 1/8th of the wavelength

•

Magnification is not very important•

Collecting area- light gathering power (sensitivity)•

Important factors are:

Sensitivity

Atmospheric transmission- must choose wavelength•

Is effected by

Special Lecture 3: TelescopesSunday, 17 November 2013 1:21 PM

Special Lectures



Collecting area•

System throughput- energy lost in the system, such as through an inefficient mirror•

Detector quantum efficiency•

Observing time•

Background - e.g. scattered light. As well as natural sources, man-made pollution is a major problem for

astronomy. At optical wavelengths for example….

•

ResolutionWe need to overcome the diffraction problem

In practice, this is limited (for optical, IR) by ‘seeing’ - practical limit is 0.3 ~ 1.0 arcsec.•

At visible light wavelengths the earth's atmosphere will blur the image•

At radio wavelengths, telescope size is the limiting factor.•

We can improve resolution

Using:

Adaptive and Active Optics-which sharpen up the image

And Interferometry- which combines telescopes working together to create a larger base diameter

The bigger the angle, the smaller the diffraction effect

phys1500 lecture notes astronomy

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