Stars & GalaxiesStars & Galaxies

Chapter 27Chapter 27Modern Earth ScienceModern Earth Science

Chapter 27, Section 1Chapter 27, Section 1

27.1 Characteristics of Stars27.1 Characteristics of Stars

How do astronomers How do astronomers determine the composition and determine the composition and surface temperature of a star?surface temperature of a star?

Composition & TemperatureComposition & Temperature Scientists use the following tools to study starsScientists use the following tools to study stars

Telescope ObservationTelescope Observation Spectral AnalysisSpectral Analysis

Hydrogen most common element in starsHydrogen most common element in stars Helium 2Helium 2ndnd most commonmost common Carbon, oxygen, nitrogen and calcium make up rest Carbon, oxygen, nitrogen and calcium make up rest

of starsof stars Surface Temperature indicated by colorSurface Temperature indicated by color

Blue Blue –– 35,000 Degrees Celsius35,000 Degrees Celsius Red Red –– 3,000 Degrees Celsius3,000 Degrees CelsiusOUR SUN OUR SUN –– 6,000 Degrees Celsius6,000 Degrees Celsius

Types & TempsTypes & Temps

Spectrum AnalysisSpectrum Analysis

Astronomers use a Astronomers use a spectrometerspectrometer to to analyze the light emitted by stars.analyze the light emitted by stars.

Spectrometers separate light into different Spectrometers separate light into different colors, or wavelengthscolors, or wavelengths

The wavelengths of light emitted by a star The wavelengths of light emitted by a star tell astronomers what elements the star is tell astronomers what elements the star is made from.made from.

Why do stars appear to move Why do stars appear to move to an observer on the Earth?to an observer on the Earth?

MotionMotion The The apparent motionapparent motion of stars is the motion of stars is the motion

visible to the unaided eye. Apparent motion is visible to the unaided eye. Apparent motion is caused by the movement of Earthcaused by the movement of Earth..

Most stars have several types of Most stars have several types of actual motionactual motion.. Stars can Stars can rotate on an axisrotate on an axis Stars can Stars can revolve aroundrevolve around another staranother star Stars can Stars can move towardmove toward and away from Earthand away from Earth

Star ROTATING on axisStar ROTATING on axis

Star moving TOWARD & AWAY from Star moving TOWARD & AWAY from EarthEarth

Star REVOLVING around another Star REVOLVING around another

The Doppler EffectThe Doppler Effect

Red ShiftRed Shift –– As a As a star star moves awaymoves awaythe Earth, the light the Earth, the light it emits appears to it emits appears to become redbecome red Most distant Most distant

galaxies, or large galaxies, or large groups of stars , groups of stars , have have red shiftedred shiftedspectraspectra, indicating , indicating that these galaxies that these galaxies are are moving awaymoving awayfrom the Earthfrom the Earth

Star moving Star moving AWAYAWAY from Earthfrom EarthRED SHIFTRED SHIFT

Star moving Star moving TOWARDTOWARD EarthEarthBLUE SHIFT BLUE SHIFT

Universal ExpansionUniversal Expansion

BlueBlue--shift & redshift & red--shiftshift

How do astronomers measure How do astronomers measure the distance from the Earth to the distance from the Earth to the stars?the stars?

Distance to the StarsDistance to the Stars

Distances in space are measured in lightDistances in space are measured in light--years.years.

Light yearLight year (ly)= the distance light travels (ly)= the distance light travels in one Earth year.in one Earth year.

Speed of light = 300,000 km/secSpeed of light = 300,000 km/sec 1 ly = 9,500,000,000 km1 ly = 9,500,000,000 km (1 ly = 9.5 trillion kilometers)(1 ly = 9.5 trillion kilometers)

Light YearsLight Years

ParallaxParallax---- The change in position of a nearby star The change in position of a nearby star compared with the position of a faraway compared with the position of a faraway star is called star is called parallaxparallax..

EARTH

SUNSUN

EARTH

Apparent Apparent Location of Location of

Star in Star in JanuaryJanuary

Actual Actual Location of Location of

StarStar

Earth In Earth In JulyJuly

Earth In Earth In JanuaryJanuary

Apparent Apparent Location of Location of Star in JulyStar in July

http://instruct1.cit.cornell.edu/courses/astro101/java/parallax/parallax.html#example

What is the difference between What is the difference between absolute magnitudeabsolute magnitude and and apparent magnitudeapparent magnitude??

Stellar MagnitudesStellar Magnitudes Astronomers use two scales to Astronomers use two scales to determine the brightness of a stardetermine the brightness of a star

The brightest stars have apparent The brightest stars have apparent magnitudes that are negative numbersmagnitudes that are negative numbers

Scale #1:How bright does a star appear to

be on Earth

Apparent Apparent MagnitudeMagnitude

Scale #2:How bright would a star be if lined up

next to all other stars

AbsoluteAbsoluteMagnitudeMagnitude

http://skyserver.fnal.gov/en/proj/advanced/hr/intro.asp

Actual & Apparent MagnitudeActual & Apparent Magnitude

Apparent MagnitudeApparent Magnitude

Classification of StarsClassification of Stars

The HertzsprungThe Hertzsprung--Russell Diagram shows Russell Diagram shows the relationship between the surface the relationship between the surface temperatures and the absolute temperatures and the absolute magnitudes (luminosity) of starsmagnitudes (luminosity) of stars

(also called the H(also called the H--R diagram)R diagram) See page 554.See page 554.

HH--R diagramR diagram

Another version of HAnother version of H--R diagramR diagram

GiantsGiants

White DwarfsWhite Dwarfs

Main Main SequenceSequence

StarsStars

SupergiantsSupergiants

Study QuestionsStudy Questions

How do astronomers determine the How do astronomers determine the composition and surface temperature composition and surface temperature of a star?of a star?

Why do stars appear to move to an Why do stars appear to move to an observer on the Earth?observer on the Earth?

How do astronomers measure the How do astronomers measure the distance from the Earth to the stars?distance from the Earth to the stars?

What is the difference between What is the difference between absolute magnitudeabsolute magnitude and and apparent apparent magnitudemagnitude??

27.2 Stellar Evolution27.2 Stellar Evolution

1. How does a protostar 1. How does a protostar develop into a star?develop into a star?

11stst step step -- NebulaNebula

nebulanebula a large cloud of gas and dust in a large cloud of gas and dust in interstellar space; a region in space where interstellar space; a region in space where stars are born.stars are born.

When the nebula is compressed, some of When the nebula is compressed, some of the particles move close to each other and the particles move close to each other and are pulled together by gravity. are pulled together by gravity.

22ndnd step step -- ProtostarProtostar

As gravity makes dense regions within a As gravity makes dense regions within a nebula more compact (smaller and more nebula more compact (smaller and more dense), these regions spin and shrink and dense), these regions spin and shrink and begin to form a flattened disk.begin to form a flattened disk.

The disk has a central concentration of The disk has a central concentration of matter called a matter called a protostarprotostar..

The Birth of a StarThe Birth of a Star

A protostarA protostar’’s temperature continually increases s temperature continually increases until it reaches about 10,000,000until it reaches about 10,000,000°°C.C.

At this temperature, nuclear fusion begins. At this temperature, nuclear fusion begins. Nuclear fusion starts by combining hydrogen to Nuclear fusion starts by combining hydrogen to form helium. The process releases enormous form helium. The process releases enormous amounts of energy.amounts of energy.

The start of nuclear fusion marks the birth of a The start of nuclear fusion marks the birth of a starstar. Once this process begins, it can continue . Once this process begins, it can continue for billions of years.for billions of years.

2. How does a main2. How does a main--sequence sequence star generate energy?star generate energy?

MainMain--Sequence StageSequence Stage The second and longest stage in the life of a star The second and longest stage in the life of a star

is the mainis the main--sequence stage. sequence stage.

During this stage, energy continues to be During this stage, energy continues to be generated in the core of the star as hydrogen generated in the core of the star as hydrogen fuses into helium. fuses into helium.

Scientists estimate that over a period of almost 5 Scientists estimate that over a period of almost 5 billion years, the sun has converted only 5% of billion years, the sun has converted only 5% of its original hydrogen nuclei into helium nuclei.its original hydrogen nuclei into helium nuclei.

A Delicate Balancing ActA Delicate Balancing Act As gravity increases the pressure on the matter As gravity increases the pressure on the matter

within the star, the rate of fusion increases. within the star, the rate of fusion increases.

Energy radiated from fusion heats the gas inside Energy radiated from fusion heats the gas inside the star. the star.

The outward pressures of the radiation and the The outward pressures of the radiation and the hot gas resist the inward pull of gravity. hot gas resist the inward pull of gravity.

This equilibrium makes the star stable in size.This equilibrium makes the star stable in size.

3. What can happen to a star 3. What can happen to a star during and after the giant during and after the giant stage?stage?

Giants & SupergiantsGiants & Supergiantsgiantgiant a very large and bright star whose hot core a very large and bright star whose hot core

has used most of its hydrogen enters its has used most of its hydrogen enters its third third stagestage (called the giant stage).(called the giant stage).

Loss of hydrogen = less fusion = less energy = Loss of hydrogen = less fusion = less energy = cooler temperature of the star.cooler temperature of the star.

As the gases in the outer shell become cooler, As the gases in the outer shell become cooler, they begin to glow with a reddish color.they begin to glow with a reddish color.

NormalNormal--sized stars become giants (sized stars become giants (our Sunour Sun).). Massive stars become supergiants (Massive stars become supergiants (RigelRigel).).

After the Giant StageAfter the Giant Stage……

Planetary nebulas (normalPlanetary nebulas (normal--size stars)size stars) White dwarf (normal)White dwarf (normal) Novas (white dwarf)Novas (white dwarf) Supernovas (massive stars)Supernovas (massive stars) Neutron stars (massive stars)Neutron stars (massive stars) Black holes (really massive stars)Black holes (really massive stars)

MUST be able to explain ONE of theseMUST be able to explain ONE of these……

Planetary NebulasPlanetary Nebulas

As the starAs the star’’s outer gases drift away, the s outer gases drift away, the remaining core heats these expanding remaining core heats these expanding gases. gases.

The gases appear as a planetary nebula, The gases appear as a planetary nebula, a cloud of gas that forms around a sunlike a cloud of gas that forms around a sunlike star that is dying.star that is dying.

White Dwarf StarsWhite Dwarf Stars

As a planetary nebula disperses, gravity As a planetary nebula disperses, gravity causes the remaining matter in the star to causes the remaining matter in the star to collapse inward.collapse inward.

The matter collapses until it cannot be The matter collapses until it cannot be pressed further together. pressed further together.

A hot, extremely dense core of matter is A hot, extremely dense core of matter is left. White dwarfs shine for billions of years left. White dwarfs shine for billions of years before they cool completely.before they cool completely.

Novas & SupernovasNovas & Supernovas

Novas are periodic explosions on white Novas are periodic explosions on white dwarfs that DO NOT destroy the star.dwarfs that DO NOT destroy the star.

Supernovas happen when a star has such Supernovas happen when a star has such a tremendous explosion that it blows itself a tremendous explosion that it blows itself apart.apart.

The explosions of supernovas completely The explosions of supernovas completely destroy the white dwarf star and may destroy the white dwarf star and may destroy much of the red giant.destroy much of the red giant.

Neutron StarsNeutron Stars

neutron starneutron star a star that has collapsed under a star that has collapsed under gravity to the point that the electrons and gravity to the point that the electrons and protons have smashed together to form protons have smashed together to form neutronsneutrons

Stars more massive than the sun do not Stars more massive than the sun do not become white dwarfs. become white dwarfs.

After a star explodes as a supernova, the After a star explodes as a supernova, the core may contract into a neutron star.core may contract into a neutron star.

Black HolesBlack Holes

black holeblack hole an object so massive and dense an object so massive and dense that even light cannot escape its gravitythat even light cannot escape its gravity

Some massive stars produce leftovers too Some massive stars produce leftovers too massive to become a stable neutron star. massive to become a stable neutron star.

These stars contract, and the force of the These stars contract, and the force of the contraction leaves a black hole. contraction leaves a black hole.

27.3 Star Groups27.3 Star Groups

Dividing Up the SkyDividing Up the Sky

constellationconstellation a group of stars organized in a group of stars organized in a recognizable patterna recognizable pattern

In 1930, astronomers around the world In 1930, astronomers around the world agreed upon a standard set of 88 agreed upon a standard set of 88 constellations.constellations.

Naming ConstellationsNaming Constellations

Many of the modern names we use for the Many of the modern names we use for the constellations come from Latin.constellations come from Latin.

Some constellations are named for real or Some constellations are named for real or imaginary animals, such as Ursa Major imaginary animals, such as Ursa Major (the great bear) or ancient gods or (the great bear) or ancient gods or legendary heroes, such as Hercules or legendary heroes, such as Hercules or Orion.Orion.

The Constellation OrionThe Constellation Orion

MultipleMultiple--Star SystemsStar Systems Over half of all observed stars form multipleOver half of all observed stars form multiple--star star

systems.systems.

Binary stars are pairs of stars that revolve Binary stars are pairs of stars that revolve around each other and are held together by around each other and are held together by gravity. The center of mass, or barycenter, is gravity. The center of mass, or barycenter, is somewhere between the two stars.somewhere between the two stars.

In star systems that have more than two stars, In star systems that have more than two stars, two stars may revolve rapidly around a common two stars may revolve rapidly around a common barycenter, while a third star revolves more barycenter, while a third star revolves more slowly at a greater distance from the pairslowly at a greater distance from the pair..

Star ClustersStar Clusters

Sometimes, nebulas collapse to form Sometimes, nebulas collapse to form groups of hundreds or thousands of stars groups of hundreds or thousands of stars called clusters.called clusters.

Globular clusters have a spherical shape Globular clusters have a spherical shape and can contain up to 100,000 stars. and can contain up to 100,000 stars.

An open cluster is loosely shaped and An open cluster is loosely shaped and rarely contains more than a few hundred rarely contains more than a few hundred stars.stars.

GalaxiesGalaxiesgalaxygalaxy a collection of stars, dust, and gas bound a collection of stars, dust, and gas bound

together by gravitytogether by gravity

Galaxies are the major building blocks of the Galaxies are the major building blocks of the universe. Astronomers estimate that the universe. Astronomers estimate that the universe contains hundreds of billions of universe contains hundreds of billions of galaxies.galaxies.

A typical galaxy, such as the Milky Way, has a A typical galaxy, such as the Milky Way, has a diameter of bout 100,000 lightdiameter of bout 100,000 light--years and may years and may contain more than 200 billion stars.contain more than 200 billion stars.

Distances to GalaxiesDistances to Galaxies Giant stars called Giant stars called Cepheid variablesCepheid variables brighten brighten

and fade in a regular pattern. Most Cepheids and fade in a regular pattern. Most Cepheids have regular cycles. The longer the cycle, the have regular cycles. The longer the cycle, the brighter the starbrighter the star’’s absolute magnitude.s absolute magnitude.

Scientists compare the CepheidScientists compare the Cepheid’’s absolute s absolute magnitude and the Cepheidmagnitude and the Cepheid’’s apparent s apparent magnitude to calculate the distance to the magnitude to calculate the distance to the Cepheid variable. Cepheid variable.

This distance tells scientists the distance to the This distance tells scientists the distance to the galaxy in which the Cepheid is located.galaxy in which the Cepheid is located.

Types of GalaxiesTypes of Galaxies Galaxies are classified by shape into three main Galaxies are classified by shape into three main

types.types.

A A spiral galaxyspiral galaxy has a nucleus of bright stars has a nucleus of bright stars and flattened arms that spiral around the and flattened arms that spiral around the nucleus.nucleus.

Elliptical galaxiesElliptical galaxies have various shapes and are have various shapes and are extremely bright in the center and do not have extremely bright in the center and do not have spiral arms.spiral arms.

An An irregular galaxyirregular galaxy has no particular shape, has no particular shape, and is fairly rich in dust and gas.and is fairly rich in dust and gas.

The Milky WayThe Milky Way

The galaxy in which we live, the Milky The galaxy in which we live, the Milky Way, is a spiral galaxy in which the sun is Way, is a spiral galaxy in which the sun is one of hundreds of billions of stars.one of hundreds of billions of stars.

Two irregular galaxies, the Large Two irregular galaxies, the Large Magellanic Cloud and Small Magellanic Magellanic Cloud and Small Magellanic Cloud, are our closest neighbors.Cloud, are our closest neighbors.

These three galaxies are called the These three galaxies are called the Local Local GroupGroup..

QuasarsQuasarsquasarquasar quasiquasi--stellar radio source; a very luminous stellar radio source; a very luminous

object that produces energy at a high rate.object that produces energy at a high rate.

Quasars appear as points of light, similar to Quasars appear as points of light, similar to stars.stars.

Quasars are located in the centers of galaxies Quasars are located in the centers of galaxies that are distant from Earth.that are distant from Earth.

Quasars are among the most distant objects that Quasars are among the most distant objects that have been observed from Earth.have been observed from Earth.

The Big Bang TheoryThe Big Bang Theory

Universal ExpansionUniversal Expansion