the sun is a star 1

7/28/2019 The sun is a star 1

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18-Apr-13

IESO

The Sun is a Star

Part 1


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Suns Energy Source

The Sun is the main source of light and heat in the

solar system.

Without the light (energy) from the Sun, therewould be no life on Earth.

The Sun closely approximates a blackbody with a

surface temp. of 5800K. Emits radiation of all wavelengths, with peak

emission in the visible region of the EM spectrum.


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Suns Energy Source

Suns size and its extremely high surface temperature

helps explain this tremendous output of energy.

Suns luminosity: L= 3.29 x 1026 watts

i.e. the Sun produces 3.26 x 1026 Joules of energy

per second

A typical reading bulb produces 100 watts, (i.e.

102 joules of energy per sec).


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Suns Energy Source

How does the Sun keep its surface so hot?

And how does it keep shining, day after day, year after

year, century after century?

what is the fundamental source of Suns energy?

For centuries, this was one the greatest mysteries in

science.

It was complicated by the discovery in the 19th century

that the Sun is at least 100 million yrs. old (current

data: Sun is 4.5 billion yrs. old)


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Suns Energy Source

1. Kelvin-Helmholtz contraction?

Suns high temperature is due to the compression

of its interior gases caused by the gravitationalcontraction.

Calculations show this is viable only if the Sun is

less than 25 million yrs. old This answer does not work!

Possible mechanisms:


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Suns Energy Source

2. Can we explain Suns energy as being produced by a

process similar to ordinary burning - i.e a chemical

reaction?

In this scheme the Sun will run out of stuff to burn

in less than 10,000 yrs.

This answer does not work either!

Possible mechanisms:

We need a burning process that produce much

more energy per atom!


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Suns Energy Source

1905: Albert Einstein discovered the key to

solving this century old mystery!

His special theory of relativity predicted that

matter can be converted to energy according to

the equation:

where m is the mass in kg and c = 3 x 108 m/s

is the speed of light in empty space.

E = m c2


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Suns Energy Source : Thermonuclear Fusion

What type of process will convert mass into energy?

Thermonuclear fusion: fusing together of two light

nuclei to form a heavier nuclei.

nucleus1 + nucleus2 nucleus3 + energy

In such a process: mass(nucleus1) + mass(nucleus2) > mass(nucleus3)

Missing mass is converted to energy according to Einsteins

mass-energy equation: E = m c2


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Thermonuclear Fusion

Thermonuclear fusion can take place only at extremelyhigh temperature and pressure:

Under these conditions atoms are completely ionized(i.e. stripped of all their electrons, and only thenucleus remain)

These conditions (high temp. and high press. arerequired for the positively charged nuclei toovercome the repulsive forces and fuse together.


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The proton-proton chain

Under the extreme conditions at the center of theSun, Hydrogen nuclei fuse together to form Helium

nuclei, and in the process convert a small amount ofmass into a large amount of energy.

Such extreme conditions exist at the Suns center.


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This nuclear reaction is called theproton-proton chain orHydrogen burning.

These reactions affect the nucleus of atoms -

hence the name nuclear reaction, as opposed tochemical reactions (ex: burning), that affect the

electrons of atoms.


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1H + 1H 2H + + (gamma ray photons)


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2H + 1H 3He + (gamma ray photons)


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3

He +3

He 4

He +1

H +1

H


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We can summarize the thermonuclear reaction of

hydrogen as follows:

4 H He + 2 neutrinos + gamma ray photons.

Neutrinos() are subatomic particles with no charge andlittle or no mass. (We will neglect the mass of the

neutrino).

Most of the energy released in the thermonuclear fusionis in the form ofgamma-ray photons.


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Amount of energy produced, (i.e. the energy of

the gamma ray photons produced) is given by:

E = m c2 (note: and photons are massless)

where m is the mass lost in one reaction:

m = mass of 4 H nuclei- mass of 1 He nucleus

mass lost in one reaction = 4.8 x 10-29 kg.

0.7% of the mass of the 4 H nuclei is lost

Energy produced E = 4.3 x 10-12 joule.

4 H He + 2 + -rays .


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Burning 1 kg of Hydrogen will produce 6.3 x

1014 joules of energy.

To produce the observed luminosity of the Sun

6 x 1011kg of Hydrogen is consumed per sec.

At this rate the Sun has enough Hydrogen to

keep burning for 5 billion years more.

The Sun has existed for 4.5 billion yrs.

The Sun is a middle aged star!


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A theoretical model of the Sun

For thermonuclear fusion to take place the temperature

has to be greater than 107 K (T >10 million

Kelvin).

The temp. of the Suns visible surface is 5800K. H. burning must take place in the interior.

Where does it take place?

How does the energy produced in the

interior make its way to the surface?


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A theoretical model of the Sun

To answer these questions we need to understand theconditions of the Suns interior.

Since we cannot send a probe into the Sun, astronomersuse laws of physics to construct theoretical models of the

Sun.

The main ingredient that go into building this model isthat - the Sun is not undergoing any Dramatic changes

it is not expanding, or collapsing.

nor is it significantly cooling or heating up.


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Pump it upHydrostatic equilibrium

The Sun has very strong gravity, but does not

collapse upon itself due to a balance of inward and

outward pressures. This balance is calledhydrostatic equilibrium.

inward: gravity

outward: pressure from being hot. heated gases expand.


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Pump it upHydrostatic equilibrium

Hydrostatic equilibrium also tells us that thedensity of the gas has to increase with depth.

From previous picture we can see that the

pressure must increase with increasing depth.

Also, since the pressure increases when you go

deeper into the interior, so does the temperature.

because when you compress a gas the

temperature tends to rise.


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Thermal equilibrium

This principle is called Thermal Equilibrium.

At a given depth the temperature is constant.

it does not change with time.

Since the Sun is in thermal equilibrium, then all the

energy generated in the interior must be transported

by some mechanism(s) to the surface, where it isemitted into space.

If too much or too little energy is transported, the Sun

will get either hotter or colder with time.


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Energy transport in the Sun

There are two mechanisms by which energy is

transported in the Sun: Convection: Circulation of gases (fluids) between hot

and cold regions.

Hot gases rises to the surface and the cooler gases

sink to the interior.


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Convection


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Energy transport in the Sun

Radiative diffusion: Photons created in the core diffuseoutwards.

In and near the core, the atoms are stripped off their

electrons because of extremely high temperature. They cant capture photons. The deep interior is

relatively transparent to radiation.

The result is a slow migration of the photonstowards the surface


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A Theoretical Model of the Sun

To develop a model of the Suns interior:

write down the physical ideas: hydrostatic

equilibrium, thermal equilibrium and energy

transport as a set of equations.Solve these equations using computer

simulations.

Check the answers with observed data (ex: Sunssurface temperature, luminosity, etc.) to test the

model.

Make other predictions.


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Core temp. greater than 107 KT.N. fusion can take

place


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Suns

Interior


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Inner parts of the Sun

Core - where energy is produced (Thermonuclearfusion). Temperature ~ 15 million kelvin.

Density ~ 160,000 kg/m3 ~ 14 times as dense as lead.

Pressure ~ 3.4 x 1011 atm ( 1atm = air pressure at sea level).

Suns energy is produced inside a region of 200,000 km (or 1/4th of

the radius).

Outside this region the temperature is too low for thermonuclear

fusion reactions to take place.


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Radiative zone

This region is comparatively transparent to EM radiation.

energy is carried away from core as electromagneticradiation (photons) by the radiative diffusion mechanism.

However light has a tough time traveling through this

region since the solar material in this region is very dense. Therefore, it takes light 170,000 years for the energy

created at the core to travel through the radiative zone(696,000 km) at a rate of 50cm per hour (20 times slowerthan a snails pace)


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Convective zone In this region the temperature is low enough for nuclei to

join with electrons and form hydrogen atoms, and these

absorb light very efficiently.

Gases are opaque to light, thus convection is thetransportation mechanism.

Therefore, radiative diffusion is not an efficient method

of energy transport in this region.

material(gas) convects energy (heat) to surface.

Hot gas goes up & cooler gas comes down.


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Methods of probing the interior of the Sun

Helioseismology: measuring vibrations of the Sun

as a whole. The Sun vibrates at a variety of frequencies like a ringing

bell.

These vibrations can be observed at the surface.

Studying these vibrations give scientists valuable

information about the Suns interior.


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Solar Neutrinos: The only direct evidence of the

thermonuclear reaction at the core.

Only the neutrino () survives the journey through the solar

interior.

The has energy but no charge an almost no mass.

Travels at the speed of light and interacts with nothing

goes right through the Earth.

With knowledge of neutrino physics scientists have built

neutrino detectors to study these particles.


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Neutrino telescope: Super

Kamiokande (Japan)

3000 tons of purified water in alarge underground tank.

1000 light detectors to detect

flashes of light that are emitted

during rare neutrino collisionswith electrons.


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Outer parts of the Sun:

The Solar Atmosphere

Photosphere - surface of Sun that we see. Radiates

energy as continuous spectrum (5800K)

Chromosphere - low density gases form

atmosphere - red color comes from hydrogen

emission line.

Corona - outer part of atmosphere - extremely hot .


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The Photosphere

Lowest of the

of the 3 layers.

All the visible

light that we

see is emittedby this layer.


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Photosphere shines(emit radiation) like a nearly

perfect blackbody at a temperature of 5800K.

The photosphere is heated from below by the energy

streaming out from the solar interior. Therefore, the temperature should decrease as you

go upwards in the photosphere.

Spectral studies show that the temperature

decreases to a cool 4800K.

The Solar AtmosphereThe Photosphere


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All the absorption lines in the Suns spectra areproduced by atoms in this relatively cool layerabsorbing photons with various wavelengths.

Photosphere consists of very low density gas,

primarily Hydrogen & Helium. Density ~10-4 kg/m3(.01% of Earths avg. density)

Although it is low density it is opaque to visiblelight.

We can only see 400km into the photosphere.

The Photosphere


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When observing with a telescope (fitted with a special filter)

we can see a blotchy pattern in the photosphere called

granulation.

Light colored granules surrounded by dark colored boundaries.

Caused by convection.

The Photosphere


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The Chromosphere has a density 1/10,000th that ofthe Photosphere

This is the reason why we cannot see it.

It can only be seen during a total Solar Eclipse, or byusing special filters, where the Photosphere is

blocked from view.

Unlike in the Photosphere the temperature rises withaltitude in the Chromosphere, from 4000K -25,000K.

The Chromosphere


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The Solar AtmosphereThe Chromosphere

Photograph takenduring a total

solar eclipse.

It shows theChromosphere as

a pinkish glowing

region around the

Sun.

Spicules: Stream

of gases pulled

upward.


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Unlike the photosphere, the chromosphere has aspectrum dominated by emission lines.

Emission lines are light emitted when electrons inatoms of thin hot gases fall to lower orbits.

The dominant emission line in the chromospheresspectrum is due to the single electron in Hydrogenatoms falling from the 3rd orbit to the 2nd orbit - H

emission line (656.2 nm - Red region).

Gives the characteristic pinkish color

The Chromosphere


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Outer most region of the Suns atmosphere.

Extends to several million kilometers and onemillionth as bright as the Photosphere

Can be seen only if we block the Photosphere Using filters or during a total solar eclipse

Corona is not a spherical shell of gas but numerousstreamers extending in different directions.

Displays emission line spectrum.

The Corona


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The Corona

Spectral studies show

that the temperature inthe Corona reaches 2

million kelvin.

However, it s notvery hot due to its low

density.


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Suns gravity keeps the atmosphere from escaping to space(just like on Earth)

To escape a body like the Sun, air molecules have to

acquire an escape velocity.

But, due to the Coronas high temperature, air moleculeshave extremely high speeds.

As a result some gas from the Corona gets ejected to space -

Solar Wind.

The Sun emits ~ a million tons of material to space everysecond.

The Solar Wind


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Solar wind consists mainly of electrons, Hydrogen ions, andHelium ions.

Solar wind causes the Aurora on Earth..

The

Aurora


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Mystery

Why the temperature

increases in the corona

and the chromosphere?

Astronomers have found

important clues in

Sunspots.

Due to Suns intense

magnetic field.


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Granules, Solar wind, etc. are continuous processes that areaspects of the quiet Sun.

There are also more dramatic features of the Sun that isperiodicalfeature of the active Sun.

One such feature is sunspots. These are irregular shape dark regions in the photosphere

Mostly found in groups.

Vary in size, typically of Earth size (few ten thousands kilometers indiameter).

These are not permanent feature, lasting anywhere between a fewhours to a few months.

Sunspots


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Sunspots have two regions: Dark central core named the umbra

And brighter border called the penumbra.

Sunspots are NOT shadows but regions in the photosphere

that are relatively low in temperature.

Sunspots


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The average temp. of the photosphere is 5800K whereas theumbra of a sunspot is at a cool 4300K and the penumbra is

somewhat hotter 5000K

Since these regions are cooler they emit less light than the

rest of the photosphere and thus look darker Galileo was the first to study sunspots.

He observed that he could determine the Suns rotation rate

by tracking sunspots.

He discovered that the Sun rotates once about every 4 weeks.

Sunspots


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However, the Sun does not rotate like a rigid body. The equatorial regions rotate more rapidly (once every 25 days) than

the polar regions (once every 35 days).

This type of rotation is called differential rotation.

The average number of sunspots vary in a predictable sunspotcycle.

The sunspot periodi.e time interval from sunspotmaximum to sunspot minimum back to a maximum is 11

years. Sunspot location also vary with this predictable 11 year

cycle.

Sunspots


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The solar atmosphere


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The Solar AtmosphereSunspots cycle

Sunspot

maximum

(1979, 1989,

2000)

Sunspot

minimum

(1976, 1986,

1996, 2007)


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why does the number of sunspots vary over a 11 year cycle ? Why do sunspots exist at all ?

In 1908 the American astronomer George Hale discoveredthat the sunspots are associated with the intense magnetic

field of the Sun. Magnetic field lines tend to deflect the hot plasma rising from

beneath the photosphere due to convection.

Where magnetic field lines are particularly strong theseforces push the plasma away.

Suns magnetic field


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The result is localized

regions where the gas is

relatively cool.

Cool gas emits less intense

light and we get sunspots.

Also sunspot pairs are

linked by magnetic field

lines

I.e. these pairs resemblegiant bar magnets.

Suns magnetic field


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Hale also discovered that the Suns polarity reverses every

11 years.

In fact, the 11-year sunspot cycle is only half of a 22-year

solar cyclewhere the Suns N-S polarity reverses and then

comes back to the starting configuration.

Suns magnetic field


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Much about sunspot & solar activity activity remains amystery.

Sunspots have vanished for years at a time in the past (1645 -1715).

Interestingly, this period seems to correspond to the little ice age,that chilled northern Europe.

There also had been periods of intense sunspot activity (11th& 12th century)

During this time Earth was warmer than today.

Variation in solar activity seem to affect climate on Earth.

Suns magnetic field


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Solar Activity There are other forms of solar

activity that is much more dramatic

and that also follows a 11-year cycle. Solar prominences are sheets or

loops of glowing gas ejected from anactive region in the Sun.

Instabilities in the intense magneticfield near sunspots causes these.

These loops are 10 times larger thanthe Earth

They last for weeks.

Solar prominences


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Solar Activity Solarflares occur in complex

sunspot groups.

Observed low in the Suns

atmosphere in the active region.

These are also due to instabilities in

the magnetic field.

Vast quantities of particles and

radiation are blasted into space.

Most energetic of these flares are

equal to 1014 nuclear bombs going

off simultaneously. Solar flares


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Solar Activity

Coronal mass ejections are much bigger than flares

Blasts a billion tons of hot coronal gas into space. Lasts for several hours.

Seems to be related to large-scale changes in the Suns

magnetic field.

Coronal Mass Ejection


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Solar Activity

All these activities seem to follow the 11-year cycle.

When solar flares & coronal mass ejections are aimed towards

Earth

A stream of high energy electrons & nuclei reaches us few days latter.

These interfere with satellites.

Poses a health hazard to astronomers in orbit.

Disrupt electronics & communication equipment.

the sun is a star 1

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