Chapter 11 Our Sun, Our Star

1
(No Transcript)
2
Objectives

• Source of the Suns energy.
• Internal structure of the Sun.
• How do we find out the properties of the Suns
  interior?
• Evidence for Thermonuclear reactions.
• What is solar wind?
• Sunspots and their relationship with magnetic
  field.
• Eruptions in the atmosphere of the Sun.

3
(No Transcript)
4
Structure of Sun

• Size about 100 times diameter of Earth

• Inner parts
• core
• radiative zone
• convective zone

• Outer parts
• photosphere
• chromosphere
• corona

• The Sun as a Star the Sun is a typical star, in
  terms of mass, size, surface temperature,
  chemical composition.

5
Suns Energy Source

• The Sun is the main source of light and heat in
  the solar system.
• Without the light (energy) from the Sun, there
  would 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.

6
Suns Energy Source

• 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).

• Suns size and its extremely high surface
  temperature helps explain this tremendous output
  of energy.

7
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)

8
Suns Energy Source

• Possible mechanisms

• Kelvin-Helmholtz contraction?
• Suns high temperature is due to the compression
  of its interior gases caused by the gravitational
  contraction.
• Calculations show this is viable only if the Sun
  is less than 25 million yrs. old
• This answer does not work!

9
Suns Energy Source

• Possible mechanisms

• 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!

We need a burning process that produce much
more energy per atom!
10
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.

11
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) gt mass(nucleus3)
• Missing mass is converted to energy according to
  Einsteins mass-energy equation E m c2

12
Thermonuclear Fusion

• Thermonuclear fusion can take place only at
  extremely high temperature and pressure
• Under these conditions atoms are completely
  ionized (i.e. stripped of all their electrons,
  and only the nucleus remain)
• These conditions (high temp. and high press. are
  required for the positively charged nuclei to
  overcome the repulsive forces and fuse together.

13
The proton-proton chain

• Such extreme conditions exist at the Suns
  center.

• Under the extreme conditions at the center of
  the Sun, Hydrogen nuclei fuse together to form
  Helium nuclei, and in the process convert a small
  amount of mass into a large amount of energy.

14
The proton-proton chain

• This nuclear reaction is called the proton-proton
  chain or Hydrogen burning.
• These reactions affect the nucleus of atoms -
  hence the name nuclear reaction, as opposed to
  chemical reactions (ex burning), that affect the
  electrons of atoms.

15
The proton-proton chain

• 1H 1H ? 2H ? ? (gamma ray photons)

16
The proton-proton chain

• 2H 1H ? 3He ? (gamma ray photons)

17
The proton-proton chain

• 3He 3He ? 4He 1H 1H

18
The proton-proton chain

• 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 and little or no mass. (We will neglect
  the mass of the neutrino).
• Most of the energy released in the thermonuclear
  fusion is in the form of gamma-ray photons.

19
The proton-proton chain
4 H ? He 2 ? ?-rays .

• 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.

20
The proton-proton chain

• 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!

21
A theoretical model of the Sun

• For thermonuclear fusion to take place the
  temperature has to be greater than 107 K
  (T gt10 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?

22
A theoretical model of the Sun

• To answer these questions we need to understand
  the conditions of the Suns interior.
• Since we cannot send a probe into the Sun,
  astronomers use laws of physics to construct
  theoretical models of the Sun.

• The main ingredient that go into building this
  model is that - the Sun is not undergoing any
  Dramatic changes
• it is not expanding, or collapsing.
• nor is it significantly cooling or heating up.

23
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 called
  hydrostatic equilibrium.
• inward gravity
• outward pressure from being hot.
• heated gases expand.

24
(No Transcript)
25
Pump it upHydrostatic equilibrium

• From previous picture we can see that the
  pressure must increase with increasing depth.

• Hydrostatic equilibrium also tells us that the
  density of the gas has to increase with 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.

26
Thermal equilibrium

• At a given depth the temperature is constant.
• it does not change with time.

• This principle is called Thermal Equilibrium.

• 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 is emitted into space.
• If too much or too little energy is transported,
  the Sun will get either hotter or colder with
  time.

27
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.

28
Convection
29
Energy transport in the Sun

• Radiative diffusion Photons created in the core
  diffuse outwards.
• 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 photons
  towards the surface

30
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 Suns
  surface temperature, luminosity, etc.) to test
  the model.
• Make other predictions.

31
Core temp. greater than 107 K ?T.N. fusion can
take place
32
Suns Interior
33
Inner parts of the Sun

• Core - where energy is produced (Thermonuclear
  fusion).
• 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.
  

34
Inner parts of the Sun

• Radiative zone
• This region is comparatively transparent to EM
  radiation.
• energy is carried away from core as
  electromagnetic radiation (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 slower than a snails pace)

35
Inner parts of the Sun

• 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 the
  transportation 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.

36
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.

37
Methods of probing the interior of the Sun

• 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.

38
Methods of probing the interior of the Sun

• Neutrino telescope Super Kamiokande (Japan)
• 3000 tons of purified water in a large
  underground tank.
• 1000 light detectors to detect flashes of light
  that are emitted during rare neutrino collisions
  with electrons.

39
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
  .

40
(No Transcript)
41
The Solar Atmosphere
The Photosphere

• Lowest of the of the 3 layers.
• All the visible light that we see is emitted by
  this layer.

42
The Solar Atmosphere
The Photosphere

• 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.

43
The Solar Atmosphere
The Photosphere

• All the absorption lines in the Suns spectra are
  produced by atoms in this relatively cool layer
  absorbing 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
  visible light.
• We can only see 400km into the photosphere.

44
The Solar Atmosphere
The Photosphere

• 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.

45
The Solar Atmosphere
The Chromosphere

• The Chromosphere has a density 1/10,000th that of
  the Photosphere
• This is the reason why we cannot see it.
• It can only be seen during a total Solar Eclipse,
  or by using special filters, where the
  Photosphere is blocked from view.
• Unlike in the Photosphere the temperature rises
  with altitude in the Chromosphere, from 4000K -
  25,000K.

46
The Solar Atmosphere
The Chromosphere

• Photograph taken during a total solar eclipse.
• It shows the Chromosphere as a pinkish glowing
  region around the Sun.
• Spicules Stream of gases pulled upward.

47
The Solar Atmosphere
The Chromosphere

• Unlike the photosphere, the chromosphere has a
  spectrum dominated by emission lines.
• Emission lines are light emitted when electrons
  in atoms of thin hot gases fall to lower orbits.
  
• The dominant emission line in the chromospheres
  spectrum is due to the single electron in
  Hydrogen atoms falling from the 3rd orbit to the
  2nd orbit - H? emission line (656.2 nm - Red
  region).
• Gives the characteristic pinkish color

48
The Solar Atmosphere
The Corona

• Outer most region of the Suns atmosphere.
• Extends to several million kilometers and one
  millionth 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
  numerous streamers extending in different
  directions.
• Displays emission line spectrum.

49
The Corona

• Spectral studies show that the temperature in the
  Corona reaches 2 million kelvin.
• However, it s not very hot due to its low
  density.

50
The Solar Atmosphere
The Solar Wind

• 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
  molecules have 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 every second.

51
The Aurora

• Solar wind consists mainly of electrons, Hydrogen
  ions, and Helium ions.
• Solar wind causes the Aurora on Earth..

52
Mystery

• Why the temperature increases in the corona and
  the chromosphere?
• Astronomers have found important clues in
  Sunspots.
• Due to Suns intense magnetic field.

53
The Solar Atmosphere
Sunspots

• Granules, Solar wind, etc. are continuous
  processes that are aspects of the quiet Sun.
• There are also more dramatic features of the Sun
  that is periodical feature 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 in diameter).
• These are not permanent feature, lasting anywhere
  between a few hours to a few months.

54
The Solar Atmosphere
Sunspots

• 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.

55
The Solar Atmosphere
Sunspots

• The average temp. of the photosphere is 5800K
  whereas the umbra 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.

56
The Solar Atmosphere
Sunspots

• 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 sunspot cycle.
• The sunspot period i.e time interval from
  sunspot maximum to sunspot minimum back to a
  maximum is 11 years.
• Sunspot location also vary with this predictable
  11 year cycle.

57
The solar atmosphere
58
The Solar Atmosphere
Sunspots cycle
Sunspot maximum (1979, 1989, 2000)
Sunspot minimum (1976, 1986, 1996, 2007)
59
The Solar Atmosphere
Suns magnetic field

• 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
  discovered that 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 these forces push the plasma away.

60
The Solar Atmosphere
Suns magnetic field

• 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 resemble giant bar magnets.

61
The Solar Atmosphere
Suns magnetic field

• 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 cycle where the Suns N-S
  polarity reverses and then comes back to the
  starting configuration.

62
The Solar Atmosphere
Suns magnetic field

• Much about sunspot solar activity activity
  remains a mystery.
• 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.

63
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 an active region in the Sun.
• Instabilities in the intense magnetic field near
  sunspots causes these.
• These loops are 10 times larger than the Earth
• They last for weeks.

Solar prominences
64
Solar Activity

• Solar flares 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
65
Solar Activity
Coronal Mass Ejection

• 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.

66
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.