ASTRO 101

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ASTRO 101

• Principles of Astronomy

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Instructor Jerome A. Orosz
(rhymes with boris)Contact

• Telephone [phone]
• E-mail orosz_at_sciences.sdsu.edu
• WWW http//mintaka.sdsu.edu/faculty/orosz/web/
• Office Physics 241, hours T TH 330-500

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Text Perspectives on Astronomy First
Editionby Michael A. Seeds Dana Milbank.
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Astronomy Help Room Hours

• Monday 1200-1300, 1700-1800
• Tuesday 1700-1800
• Wednesday 1200-1400, 1700-1800
• Thursday 1400-1800, 1700-1800
• Friday 900-1000, 1200-1400
• Help room is located in PA 215

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Coming Up

• Chapter 6 The family of stars
• Chapter 7 The Structure and Formation of Stars
• Chapter 8 The Deaths of Stars
• November 3 In-class review
• November 5 Exam 2
• November 10 Furlough day class cancelled

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Coming Up

• Homework due October 29 Question 15, Chapter 8
  (How are neutron stars and white dwarfs similar?
  How do they differ?)

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Questions from Before

• How are stars born? In clouds of gas and dust
• How do stars die? Coming up
• Can you tell how old a star is? Yes, if it is in
  a cluster

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Questions for Today

• What is a white dwarf?
• What is a neutron star?
• What is a black hole?

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Comparing Stellar Properties

• Sometimes in order to understand how stars work,
  it is useful to compare two or more stars.
• Note you can sometimes compare properties without
  knowing the actual values, as in The female
  rabbit of this species is larger than the male
  rabbit of the same species.
• A simple question to ask is Which star is more
  luminous than the others?

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Comparing Stellar Properties

• This large-area photograph shows the
  constellations of Orion, Canis Major, Canis Minor
  Taurus, and a few others.
• Which star is more luminous
• Rigel
• or
• Sirius

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Comparing Stellar Properties
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Comparing Stellar Properties

• Looking up the distances, we find
• Rigel
• d 240 pc
• L 66,000 Lo
• Sirius
• d 2.64 pc
• L 25.4 Lo
• The ratio of the fluxes is not the ratio of the
  luminosities since the distances are different.

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Comparing Stellar Properties
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Comparing Stellar Properties

• A cluster is a group of stars bound by their own
  gravity. The size of the cluster is small
  compared to its distance from Earth.
• Which star is more luminous
• Star A
• or
• Star B

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Comparing Stellar Properties
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Comparing Stellar Properties

• Comparing the apparent brightnesses does not help
  if the stars have different distances.

Figure from Michael Richmond (http//spiff.rit.edu
/classes/phys230/phys230.html)
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Comparing Stellar Properties

• Comparing the apparent brightnesses of stars in a
  cluster does help since each star in that cluster
  has the same distance from the Earth.
• The distance is still needed to compute the
  actual luminosities, and not just the relative
  ones.

Figure from Michael Richmond (http//spiff.rit.edu
/classes/phys230/phys230.html)
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Star Clusters

• Lets plot the stars from several different
  clusters on the diagram and draw tracks where
  the stars are to clean it up

Figure from Michael Richmond (http//spiff.rit.edu
/classes/phys230/phys230.html)
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Star Clusters

• The sequences occupied by cluster stars changes
  from cluster to cluster (within certain bounds).
  WHY????
• This is related to the life cycles of stars.

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The Life Cycles of Stars

• To understand why different star clusters have
  different tracks in the temperature-luminosity
  diagram, we must return to a result found from
  eclipsing binaries

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Mass-Luminosity Relation

• The luminosity of a star is related to its mass
  L Mp, where p is almost 4.

Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
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Mass-Luminosity Relation

• The luminosity of a star represents the amount of
  energy emitted per second. There must be a
  source of this energy, and it cannot last
  forever.
• The amount of fuel a star has is proportional
  to its initial mass.
• The length of time the fuel can be spent is equal
  to the amount of fuel divided by the consumption
  rate.
• Age mass/luminosity mass/(mass)41/(mass)3

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Mass-Age Relation

• Age 1/(mass)3 (age means time on the main
  sequence, mass means initial mass).
• More massive stars die much more quickly than
  less massive stars. For example, double the mass,
  and the age drops by a factor of 8.

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Mass-Age Relation

• Age 1/(mass)3 (age means time on the main
  sequence, mass means initial mass).
• More massive stars die much more quickly than
  less massive stars. For example, double the mass,
  and the age drops by a factor of 8.
• On the main sequence, O and B stars (the bluest
  ones) are the most massive. Their lifetimes are
  relatively short.

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Mass-Age Relation

• Detailed computations show

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Star Clusters

• Large radii
• Small radii
• High mass (main sequence)
• Low mass (main sequence)

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Star Clusters

• Some clusters have lost only the bluest main
  sequence stars.
• Others have lost main sequence stars down to type
  F.
• The differences in the tracks are due to age
  differences of the clusters!

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Star Clusters

• Here is a temperature luminosity diagram for the
  Hyades cluster.
• This one is relatively young.

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Star Clusters

• Here are the temperature luminosity diagrams for
  a three clusters.
• These diagrams and others can be used to make a
  movie on how stars evolve.

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Next Stellar Evolution

• Observational aspects
• Observations of clusters of stars
• Theory
• Outline of steps from birth to death

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Stellar Models
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Stellar Evolution

• There are several distinct phases in the life
  cycle of a star. The evolutionary path depends
  on the initial mass of the star.
• Although there is a continuous range of masses,
  we often talk about lightweight stars (masses
  similar to the Sun) and heavyweight stars
  (masses about about 10 solar masses).

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Stellar Evolution
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Stellar Evolution

• The basic steps are
• Gas cloud
• Main sequence
• Red giant
• Rapid mass loss (planetary nebula or supernova
  explosion)
• Remnant
• The length of time spent in each stage, and the
  details of what happens at the end depend on the
  initial mass.

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Points to Remember

• How to counter gravity
• Heat pressure from nuclear fusion in the core (no
  mass limit)
• Gas pressure proportional to the temperature.
• Electron degeneracy pressure (mass limit 1.4
  solar masses)
• Neutron degeneracy pressure (mass limit 3 solar
  masses)
• Stars experience rapid mass loss near the end of
  their lives, so the final mass can be much less
  than the initial mass.

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Points to Remember

• Sources of energy
• Nuclear fusion
• needs very high temperatures
• about 0.7 efficiency for hydrogen into helium.
• Gravitational accretion energy
• Drop matter from a high potential
• About 10 efficient when falling onto massive
  bodies with very small radii.

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Stellar Evolution
38
Stellar Evolution

• The basic steps are
• Gas cloud
• Main sequence
• Red giant
• Rapid mass loss (planetary nebula or supernova
  explosion)
• Remnant
• The length of time spent in each stage, and the
  details of what happens at the end depend on the
  initial mass.

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Star Formation

• The starting point is a giant molecular cloud.
  The gas is relatively dense and cool, and usually
  contains dust.
• A typical cloud is several light years across,
  and can contain up to one million solar masses of
  material.
• Thousands of clouds are known.

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Side Bar Observing Clouds

• Ways to see gas
• By reflection of a nearby light source. Blue
  light reflects better than red light, so
  reflection nebulae tend to look blue.
• By emission at discrete wavelengths. A common
  example is emission in the Balmer-alpha line of
  hydrogen, which appears red.

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Side Bar Observing Clouds

• Ways to see dust
• If the dust is warm (a few hundred degrees K)
  then it will emit light in the long-wavelength
  infrared region or in the short-wavelength radio.
• Dust will absorb light blue visible light is
  highly absorbed red visible light is less
  absorbed, and infrared light suffers from
  relatively little absorption. Dust causes
  reddening.

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Giant Molecular Clouds

• This nebula is in the belt of Orion. Dark dust
  lanes and also glowing gas are evident.

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Giant Molecular Clouds

• Interstellar dust makes stars appear redder.

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Gravity and Angular Momentum

• There are two important concepts to keep in mind
  when considering the fate of giant molecular
  clouds
• Gravity pulls things together
• Angular momentum a measure of the spin of an
  object or a collection of objects.

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Gravity

• There are giant clouds of gas and dust in the
  galaxy. They are roughly in equilibrium, where
  gas pressure balances gravity.

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Gravity

• There are giant clouds of gas and dust in the
  galaxy. They are roughly in equilibrium, where
  gas pressure balances gravity.
• Sometimes, an external disturbance can cause
  parts of the cloud to move closer together. In
  this case, the gravitational force may be
  stronger than the pressure force.

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Gravity

• Sometimes, an external disturbance can cause
  parts of the cloud to move closer together. In
  this case, the gravitational force may be
  stronger than the pressure force.
• As more matter is pulled in, the gravitational
  force increases, resulting in a runaway collapse.

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Angular Momentum

• Angular momentum is a measure of the spin of an
  object. It depends on the mass that is spinning,
  on the distance from the rotation axis, and on
  the rate of spin.
• I (mass).(radius).(spin rate)

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Angular Momentum

• Angular momentum is a measure of the spin of an
  object. It depends on the mass that is spinning,
  on the distance from the rotation axis, and on
  the rate of spin.
• For a given distance to the rotation axis, more
  mass means more angular momentum.

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Angular Momentum

• Angular momentum is a measure of the spin of an
  object. It depends on the mass that is spinning,
  on the distance from the rotation axis, and on
  the rate of spin.
• For a given distance to the rotation axis, more
  mass means more angular momentum.
• For a given mass, a larger distance means more
  angular momentum.

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Angular Momentum

• Angular momentum is a measure of the spin of an
  object. It depends on the mass that is spinning,
  on the distance from the rotation axis, and on
  the rate of spin.
• For a fixed mass and distance, a higher rate of
  spin means a larger angular momentum.

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Angular Momentum

• Angular momentum is a measure of the spin of an
  object. It depends on the mass that is spinning,
  on the distance from the rotation axis, and on
  the rate of spin.
• I (mass).(radius).(spin rate)
• The angular momentum in a system stays fixed,
  unless acted on by an outside force.

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Conservation of Angular Momentum

• An ice skater demonstrates the conservation of
  angular momentum

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Conservation of Angular Momentum

• An ice skater demonstrates the conservation of
  angular momentum
• Arms held in high rate of spin.
• Arms extended low rate of spin.
• I (mass).(radius).(spin rate) (angular momentum
  and mass are fixed here)

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Conservation of Angular Momentum

• If an interstellar cloud has some net rotation,
  then it cannot collapse to a point.

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Conservation of Angular Momentum

• If an interstellar cloud has some net rotation,
  then it cannot collapse to a point. Instead, the
  cloud collapses into a disk that is perpendicular
  to the rotation axis.

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Condensation Theory

• An interstellar cloud collapsed to a disk.
  Friction in the disk drives matter inward and
  outward (conserving angular momentum).
• Planets may eventually form in the disk.

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Condensation Theory
Image from Nick Strobels Astronomy Notes
(http//www.astromynotes.com)
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The Protostar

• This diagram shows the steps as computed using a
  computer model.

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The Protostar

• An external disturbance can cause the cloud to
  collapse
• The material collapses to a rotating disk, and
  friction drives material into the center, where
  it builds up.
• The central object heats up as the cloud
  collapses. Eventually, the temperature gets hot
  enough for nuclear fusion to occur.
• We are left with a newly born star surrounded by
  a disk of material.

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Young Star Systems

• There is strong evidence for a disk surrounding
  the star Beta Pictoris.

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Young Star Systems

• Many stars in the Orion nebula are surrounded by
  disks of material.

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Young Star Systems

• Many stars in the Orion nebula are surrounded by
  disks of material.

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Young Star Systems

• Many stars in the Orion nebula are surrounded by
  disks of material.

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The Protostar

• Infrared observations often reveal hundreds of
  newly-formed stars embedded in molecular clouds.

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The Protostar

• Newly-formed hot stars can alter their
  environment.

Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
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The Protostar

• A collapsing cloud can form hundreds of stars.
• Stars with small masses (less than a solar mass)
  are much more common than massive stars (stars
  more than about 15 to 20 solar masses).
• The highest mass stars are very hot and luminous,
  and can alter the cloud environment.

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The Protostar

• This diagram shows how a star moves through the
  temperature-luminosity diagram as it forms.

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Stellar Evolution
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Stellar Evolution

• The basic steps are
• Gas cloud
• Main sequence
• Red giant
• Rapid mass loss (planetary nebula or supernova
  explosion)
• Remnant
• The length of time spent in each stage, and the
  details of what happens at the end depend on the
  initial mass.

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The Main Sequence

• A star that is fusing hydrogen to helium in its
  core is said to be on the main sequence.
• A star spends most of its life on the main
  sequence the time spent is roughly proportional
  to 1/M3, where M is the initial mass.

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Hydrostatic Equilibrium

• The Sun (and other stars) does not collapse on
  itself, nor does it expand rapidly. Gravity and
  internal pressure balance. This is true at all
  layers of the Sun.
• The energy from fusion in the core ultimately
  provides the pressure needed to stabilize the
  star.

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Stellar Evolution
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Stellar Evolution

• The basic steps are
• Gas cloud
• Main sequence
• Red giant
• Rapid mass loss (planetary nebula or supernova
  explosion)
• Remnant
• The length of time spent in each stage, and the
  details of what happens at the end depend on the
  initial mass.

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After the Main Sequence

• On the main sequence, the star is in hydrostatic
  equilibrium where internal pressure supports the
  star against gravitational collapse. Nuclear
  fusion (hydrogen to helium) is the energy source.

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After the Main Sequence

• On the main sequence, the star is in hydrostatic
  equilibrium where internal pressure supports the
  star against gravitational collapse. Nuclear
  fusion (hydrogen to helium) is the energy source.
• What happens when all of the hydrogen in the core
  is converted to helium?

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After the Main Sequence

• On the main sequence, the star is in hydrostatic
  equilibrium where internal pressure supports the
  star against gravitational collapse. Nuclear
  fusion (hydrogen to helium) is the energy source.
• What happens when all of the hydrogen in the core
  is converted to helium? The details depend on the
  initial mass of the star

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Points to Remember

• Sources of energy
• Nuclear fusion
• needs very high temperatures
• about 0.7 efficiency for hydrogen into helium.
• Gravitational accretion energy
• Drop matter from a high potential
• About 10 efficient when falling onto massive
  bodies with very small radii.
• After a stage of nuclear fusion is complete in a
  stellar core, it will collapse and get hotter.

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More Nuclear Fusion

• Fusion of elements lighter than iron can release
  energy (leads to higher BEs).
• Fission of elements heaver than iron can release
  energy (leads to higher BEs).

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Nuclear Fusion

• Summary 4 hydrogen nuclei (which are protons)
  combine to form 1 helium nucleus (which has two
  protons and two neutrons).
• Extremely high temperatures and densities are
  needed (the Suns core is about 15,000,000 K).

Images from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
81
The 4 Forces of Nature

• Both gravity and the electromagnetic force are
  inverse square forces where the strength of the
  force depends on 1/d2.
• Fgrav product of masses divided by distance
  squared.
• Felec product of charges divided by distance
  squared. Higher concentrations of (like) charges
  need stronger forces to bring them together
  (recall like charges repel).

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More Nuclear Fusion

• Fusion of elements lighter than iron can release
  energy (leads to higher BEs).
• As you fuse heavier elements up to iron, higher
  and higher temperatures are needed since more and
  more electrical charge repulsion needs to be
  overcome.
• Hydrogen nuclei have 1 proton each temperature
  10,000,000 K
• Helium nuclei have 2 protons each
  temperature 100,000,000 K
• Carbon nuclei have 6 protons each temperature
  700,000,000 K
• ..
• After each stage of fusion is complete, the core
  collapses and heats up.
• More mass in the core --gt higher core temperature
  --gt fusion of heavier elements
• For a given core mass, there is a limit to how
  hot it can become.

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After the Main Sequence Low Mass

• After the core hydrogen is used up, internal
  pressure can no longer support the core, so it
  starts to collapse. This releases energy, and
  additional hydrogen can fuse outside the core.

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After the Main Sequence Low Mass

• After the core hydrogen is used up, internal
  pressure can no longer support the core, so it
  starts to collapse. This releases energy, and
  additional hydrogen can fuse outside the core.
• The excess energy causes the outer layers of the
  star to expand by a factor of 10 or more.

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After the Main Sequence Low Mass

• After the core hydrogen is used up, internal
  pressure can no longer support the core, so it
  starts to collapse. This releases energy, and
  additional hydrogen can fuse outside the core.
• The excess energy causes the outer layers of the
  star to expand by a factor of 10 or more. The
  star will be large and cool these are the red
  giants seen in the temperature-luminosity diagram.

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After the Main Sequence Low Mass

• The red giants are stars that just finished up
  fusing hydrogen in their cores.

Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
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After the Main Sequence Low Mass

• Some red giants are as large as the orbit of
  Jupiter!

Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
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After the Main Sequence Low Mass

• The excess energy causes the outer layers of the
  star to expand by a factor of 10 or more. The
  star will be large and cool these are the red
  giants seen in the temperature-luminosity
  diagram.

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After the Main Sequence Low Mass

• The excess energy causes the outer layers of the
  star to expand by a factor of 10 or more. The
  star will be large and cool these are the red
  giants seen in the temperature-luminosity
  diagram.
• The core continues to collapse, and helium can
  fuse into carbon for a short time. The star
  expands further.

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After the Main Sequence Low Mass

• The excess energy causes the outer layers of the
  star to expand by a factor of 10 or more. The
  star will be large and cool these are the red
  giants seen in the temperature-luminosity
  diagram.
• The core continues to collapse, and helium can
  fuse into carbon for a short time. The star
  expands further. The outer layers eventually may
  be ejected to form a planetary nebula.

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After the Main Sequence Low Mass

• After hydrogen fusion is completed, the core
  collapses, and the outer parts of the star
  expand.
• The core may fuse helium into carbon for a short
  time, after which the core collapses further.
• The outer parts of the star expand by large
  amounts, and eventually escape into space,
  forming a planetary nebula. Matter is recycled
  back into space.

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Planetary Nebulae

• These objects resembled planets in crude
  telescopes, hence the name planetary nebula.
• They are basically bubbles of glowing gas.

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Planetary Nebulae

• They are basically bubbles of glowing gas.
• The ring shape is a result of the viewing
  geometry.

Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
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Planetary Nebulae

• The red light is the Balmer alpha line of
  hydrogen, and the green line is due to oxygen.

Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
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Planetary Nebulae

• This HST image shows freshly ejected material
  interacting with previously ejected material.

Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
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Planetary Nebulae

• The outer layers of the star are ejected, thereby
  returning material to the interstellar medium.
  What about the core?

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The Remnant Low Mass

• After all of the helium in the core is used up, a
  low mass star cannot get hot enough to go to the
  next step of carbon fusion. There is no more
  energy source to support the core, so it
  collapses.

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The Remnant Low Mass

• After all of the helium in the core is used up, a
  low mass star cannot get hot enough to go to the
  next step of carbon fusion. There is no more
  energy source to support the core, so it
  collapses.
• To what?

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The Remnant Low Mass

• After all of the helium in the core is used up, a
  low mass star cannot get hot enough to go to the
  next step of carbon fusion. There is no more
  energy source to support the core, so it
  collapses.
• To what?
• But first a historical mystery involving the
  brightest star in the sky Sirius (the dog
  star).

100
Sirius

• This bright star is relatively close to the Sun.
  The spectral type is A1V, and its mass is about
  twice the Suns mass.
• In the 1830s it was discovered that Sirius moves
  in the plane of the sky (roughly 1 arcsecond per
  year).

101
Sirius

• This bright star is relatively close to the Sun.
  The spectral type is A1V, and its mass is about
  twice the Suns mass.
• In the 1830s it was discovered that Sirius moves
  in the plane of the sky (roughly 1 arcsecond per
  year). However, the motion was not in a straight
  line Sirius has a binary companion.

102
Sirius

• From the size of the wobble, it was estimated
  that the companion star had a mass roughly equal
  to the Suns mass.

103
Sirius

• From the size of the wobble, it was estimated
  that the companion star had a mass roughly equal
  to the Suns mass.
• However, this object was extremely faint, and
  observers tried for decades to spot it without
  success.

104
Sirius

• From the size of the wobble, it was estimated
  that the companion star had a mass roughly equal
  to the Suns mass.
• However, this object was extremely faint, and
  observers tried for decades to spot it without
  success.
• The famous telescope maker Clark spotted the
  faint companion in the 1870s when testing out his
  latest refracting telescope.

105
Sirius

• Clark discovered the faint companion was roughly
  10,000 times fainter than Sirius.

106
Sirius

• Clark discovered the faint companion was roughly
  10,000 times fainter than Sirius but bluer.
• Here is a modern image, early on it was
  relatively hard to study the faint star owing to
  the high contrast.

107
The Puzzle

• Sirius B has a mass roughly equal to the Suns
  mass, but it is about 10,000 times fainter than
  the Sun while being having a surface temperature
  about 10 times higher than the Suns.

108
The Puzzle

• Sirius B has a mass roughly equal to the Suns
  mass, but it is about 10,000 times fainter than
  the Sun while being having a surface temperature
  about 10 times higher than the Suns.
• To be so faint while being hot, the radius of
  Sirius B must be 1 of the Suns radius!

109
The Puzzle

• Sirius B has a mass roughly equal to the Suns
  mass, but it is about 10,000 times fainter than
  the Sun while being having a surface temperature
  about 10 times higher than the Suns.
• To be so faint while being hot, the radius of
  Sirius B must be 1 of the Suns radius!
• The density is roughly 1.4 million grams per
  cubic centimeter!

110
The Puzzle

• Sirius B has a mass roughly equal to the Suns
  mass, but it is about 10,000 times fainter than
  the Sun while being having a surface temperature
  about 10 times higher than the Suns.
• To be so faint while being hot, the radius of
  Sirius B must be 1 of the Suns radius!
• The density is roughly 1.4 million grams per
  cubic centimeter! ????

111
Degenerate Matter

• The nature of Sirius B was solved in the 1920s
  and 1930s. It has to do with what happens to the
  star when pressure can no longer support it

112
Degenerate Matter

• Once the internal pressure stops, the
  gravitational collapse begins.
• Eventually, the gas becomes supercompressed so
  that the particles are touching. The the gas is
  said to be degenerate, and acts more like a
  solid.
• For a star with an initial mass of less than
  about 8 solar masses, the final object has a
  radius of only about 1 of the solar radius, and
  is extremely hot (and therefore blue).

113
Degenerate Matter

• Once the internal pressure stops, the
  gravitational collapse begins.
• Eventually, the gas becomes supercompressed so
  that the particles are touching. The the gas is
  said to be degenerate, and acts more like a
  solid.
• For a star with an initial mass of less than
  about 8 solar masses, the final object has a
  radius of only about 1 of the solar radius, and
  is extremely hot (and therefore blue). These are
  the white dwarf stars.

114
After the Main Sequence Low Mass

• The red giants are stars that just finished up
  fusing hydrogen in their cores.
• The white dwarfs are the left over cores of red
  giants that have shed their mass in planetary
  nebulae.

Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
115
Planetary Nebulae and White Dwarfs

• The central star is a white dwarf.

116
Planetary Nebulae and White Dwarfs

• More central white dwarfs

Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
117
After the Main Sequence Low Mass

• The core collapses until the gas is degenerate,
  at which point it acts like a solid. It becomes
  a white dwarf
• The density is more than 1 million times that of
  water.
• The source of support is the electron
  degeneracy pressure. The maximum mass that can
  be supported is 1.4 solar masses.
• There is no internal source of energy, and the
  white dwarf cools down slowly over time.
  Initially, the white dwarf is relatively hot
  (several times the solar temperature).

118
Next

• Evolution of High Mass Stars

119
Stellar Evolution
120
Stellar Evolution

• The basic steps are
• Gas cloud
• Main sequence
• Red giant
• Rapid mass loss (planetary nebula or supernova
  explosion)
• Remnant
• The length of time spent in each stage, and the
  details of what happens at the end depend on the
  initial mass.

121
After the Main Sequence High Mass

• A massive star (more than about 10 to 15 solar
  masses) will use up its core hydrogen relatively
  quickly. The core will collapse.
• The core heats up, and helium is fused into
  carbon. After this, carbon and helium can fuse
  into oxygen since the high mass gives rise to
  very high temperatures.
• Eventually elements up to iron are formed in
  successive stages.

122
More Nuclear Fusion

• Fusion of elements lighter than iron can release
  energy (leads to higher BEs).
• Fission of elements heaver than iron can release
  energy (leads to higher BEs).
• Fission or fusion of iron does not give energy.

123
After the Main Sequence High Mass

• Eventually elements up to iron are formed in
  successive stages.
• Iron fusion does not produce energy, so there is
  no energy source to halt the gravitational
  collapse.

124
Points to Remember

• How to counter gravity
• Heat pressure from nuclear fusion in the core (no
  mass limit)
• Gas pressure proportional to the temperature.
• Electron degeneracy pressure (mass limit 1.4
  solar masses)
• Neutron degeneracy pressure (mass limit 3 solar
  masses)
• We have used up fusion, and there is a limit to
  how much mass electron degeneracy pressure can
  support.

125
After the Main Sequence High Mass

• Eventually elements up to iron are formed in
  successive stages.
• Iron fusion does not produce energy, so there is
  no energy source to halt the gravitational
  collapse.
• If the initial mass of the star is more than
  about 8 solar masses, the core will be too
  massive to form a white dwarf, since at that
  stage the gravity is stronger than the electron
  degeneracy pressure.

126
After the Main Sequence High Mass

• Eventually elements up to iron are formed in
  successive stages.
• Iron fusion does not produce energy, so there is
  no energy source to halt the gravitational
  collapse.
• If the initial mass of the star is more than
  about 8 solar masses, the core will be too
  massive to form a white dwarf, since at that
  stage the gravity is stronger than the electron
  degeneracy pressure. The collapse continues.

127
After the Main Sequence High Mass

• If the initial mass of the star is more than
  about 8 solar masses, the core will be too
  massive to form a white dwarf, since at that
  stage the gravity is stronger than the electron
  degeneracy pressure. The collapse continues.
• Protons and electrons are fused to form neutrons
  and neutrinos. The core collapses to a very tiny
  size, liberating a huge amount of energy. The
  outer layers are blown off in a supernova
  explosion.

128
Supernovae

• A supernova can be a billion times brighter than
  the Sun at its peak.

129
Supernovae

• Supernovae are rare events. One occurred in a
  relatively nearby galaxy in 1987.

130
Supernovae

• Supernovae are rare events. One occurred in a
  relatively nearby galaxy in 1987.
• It has been closely studied since with the Space
  Telescope and other telescopes.

131
Supernovae

• Several solar masses of material is ejected into
  space by the explosion.
• Many supernova remnants are known.

132
Supernovae

• Material is returned to the interstellar medium,
  to be recycled in the next generation of stars.
• Owing to the high temperatures, lots of exotic
  nuclear reactions occur, resulting in the
  production of various elements. All of the
  elements past helium were produced in supernovae.

133
Supernovae

• Material is returned to the interstellar medium,
  to be recycled in the next generation of stars.
• Owing to the high temperatures, lots of exotic
  nuclear reactions occur, resulting in the
  production of various elements. All of the
  elements past helium were produced in supernovae.
  
• Most of the atoms in your body came from a
  massive star!

134
The Remnant High Mass

• What happened to the core?

135
Next

• Neutron Stars
• Black Holes
• and
• A Bit on the Evolution of Binary Stars