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 today Question 15, Chapter 8 (How
  are neutron stars and white dwarfs similar? How
  do they differ?)
• No assigned question for next week.

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Questions from Before

• What is a white dwarf? The final point in the
  evolution of a low mass star.
• What is a neutron star? The final point in the
  evolution of stars with initial masses between
  about 8 and 30 times the mass of the Sun.
• What is a black hole? The final point in the
  evolution of the most massive stars. A black
  hole has a gravitational field so strong that
  nothing, not even light, can escape.

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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
10
Points to Remember

• 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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Points to Remember

• 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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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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Points to Remember

• 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
• 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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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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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.
• 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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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.
• 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

• 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).

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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.

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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.

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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.

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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! ????

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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

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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.

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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)
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Planetary Nebulae and White Dwarfs

• The central star is a white dwarf.

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Planetary Nebulae and White Dwarfs

• More central white dwarfs

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

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Next

• Evolution of High Mass Stars

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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 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.

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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).
• Fission or fusion of iron does not give energy.

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After the Main Sequence High Mass
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After the Main Sequence High Mass

• Eventually elements up to iron are formed in
  successive stages.
• The star develops an onion-like structure, where
  different elements fuse in different layers.
• Iron fusion does not produce energy, so there is
  no energy source in the core to halt the
  gravitational collapse.

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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)
• We have used up fusion, and there is a limit to
  how much mass electron degeneracy pressure can
  support.

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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.

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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.

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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.

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Supernovae

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

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Supernovae

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

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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.

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Supernovae

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

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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).
• Fission or fusion of iron does not give energy,
  although if you add energy, it can fuse

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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.

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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!

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The Remnant High Mass

• What happened to the core?

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Next

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

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The Evolution of Binary Stars

• In a binary system, the stars start to evolve
  independently the most massive star evolves
  first!
• If the separation between the stars is larger
  than the maximum size of each star, then no
  problem.
• If, however, the most massive star becomes bigger
  than the distance between the two stars, then the
  two stars will interact

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The Evolution of Binary Stars

• The dashed line represents the maximum size the
  star is allowed to be when inside the binary.
• Here is just one example of the many different
  possibilities (e.g. the stars move apart, or move
  closer, or merge).

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The Evolution of Binary Stars

• There are many known examples where a star loses
  mass onto a white dwarf. Lots of energy is
  liberated when the mass hits the white dwarf.

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Remnants of High Mass Stars

• In many cases, the remnants of high mass stars
  will appear in close binaries

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The Remnant High Mass

• What happened to the core?
• Gravity overcame the electron degeneracy
  pressure, so the collapse continued.
• Protons and electrons form neutrons, and the gas
  is compressed so that the neutrons become
  degenerate (i.e. they are basically touching).
• The resulting remnant has a radius of about 10
  km, and a typical mass of 1.4 solar masses. This
  is a neutron star.
• The density is 6.4 x 1014 grams/cc.
• The surface gravity is 1011 times that of Earth.

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Points to Remember

• How to counter gravity
• Heat pressure from nuclear fusion in the core (no
  mass limit)
• Electron degeneracy pressure (mass limit 1.4
  solar masses)
• Neutron degeneracy pressure (mass limit about 3
  solar masses)

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Neutron Stars

• According to model computations, a neutron star
  should be very small (radius of about 10 km), and
  very hot (temperatures more than 1 million
  degrees).
• Is there any hope of observing them?

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Neutron Stars

• According to model computations, a neutron star
  should be very small (radius of about 10 km), and
  very hot (temperatures more than 1 million
  degrees).
• Is there any hope of observing them?
• Yes there are some exotic phenomena that are
  best explained by neutron stars.

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Neutron Stars

• The best model for a radio pulsar is a rapidly
  rotating neutron star with a strong magnetic
  field.

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Neutron Stars

• The spinning neutron star acts like a light
  house, leading to pulsed radiation being
  observed on Earth.

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Neutron Stars

• If a neutron star is in a close binary, matter
  from the companion falls onto it, liberating a
  huge amount of energy, including pulsed X-ray
  beams in some cases.

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Neutron Stars and HST

• This object is relatively nearby (the parallax
  gives about 100 pc).
• Nevertheless, it is so faint it is at the HST
  detection threshold.
• However, its temperature is a few million
  degrees.
• ???

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Neutron Stars and HST

• The radius is only about 10 km.
• The temperature and radius are what one expects
  for a young neutron star.

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Where it Stops

• White dwarfs and neutron stars are pretty strange
  objects. Does it get any stranger?

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Where it Stops

• White dwarfs and neutron stars are pretty strange
  objects. Does it get any stranger?
• Yes consider the fate of the most massive stars
  (about 30 to 100 times the mass of the Sun).

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Black Holes
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Where it Stops

• For large masses (initial mass greater than about
  30 solar masses)
• The core ends up with a substantially more than
  1.4 solar masses. The temperature gets hot
  enough to fuse elements all the way up to iron.
• The fusion of iron takes energy rather than
  liberating it. The core collapses, but it is too
  massive to be supported by electron degeneracy
  pressure and neutron degeneracy pressure. No
  known force can halt the collapse, and the core
  collapses to a point. A black hole is born.

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A Black Hole

• At this point, the density, and hence the
  gravitational force, are quite large.
• Newtons gravitational theory no longer
  accurately describes gravity, one must use
  Einsteins more complex theory.

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Einsteins Theory

• In Newtons theory of gravity, gravity is a force
  between two objects.
• The force travels instantly through space by
  some unspecified mechanism.
• Space is the ordinary 3 dimensional Euclidean
  space.
• In Einsteins theory
• Nothing travels faster than light.
• Matter causes space to warp, and gravity is a
  manifestation of curved space.

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Einsteins Theory

• The curvature of space depends on the mass and
  density.
• The tendency of material and of light is to take
  the shortest path between two points.
• Large bodies can alter the path of light.

Image from Nick Strobels Astronomy Notes
(httpwww.astronomynotes.com)
89
Black Holes

• A black hole is an object with a gravitational
  field so strong that nothing, not even light, can
  escape.
• All of the matter is compressed to a point.
• There is no physical surface. However, one can
  define a radius within which nothing can escape
  this is called the event horizon or the
  Schwarzchild radius .
• Once matter or light crosses the event horizon,
  it is gone forever.

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Black Holes

• A black hole is an object with a gravitational
  field so strong that nothing, not even light, can
  escape.

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Black Holes

• Since it is so compact, the tidal force near a
  black hole is extremely strong matter is
  stretched lengthwise, and compressed in the
  perpendicular direction.

92
Black Holes

• A black hole is an object with a gravitational
  field so strong that nothing, not even light, can
  escape.
• Black holes have only three properties
• Mass
• Angular momentum (if it is spinning)
• Electric charge (not astrophysically important
  since macroscopic objects are neutral)
• Black holes cannot have magnetic fields, or a
  temperature, or a color, etc.

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Detecting a Black Hole

• If light cannot escape from a black hole, how do
  we detect them? By looking at material close to
  the black hole, before it disappears

94
Detecting a Black Hole

• If the black hole is close to another star, it
  can pull material off that star. As the matter
  falls into the black hole, it gets very hot, and
  emits X-rays.

Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com)
95
Detecting a Black Hole

• If the black hole is close to another star, it
  can pull material off that star. As the matter
  falls into the black hole, it gets very hot, and
  emits X-rays.

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The X-ray Sky from HEAO I

• There are a few hundred bright X-ray sources in
  the sky, and most are powered by accretion of
  matter onto a compact object.

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Whats Next?

• After the source is identified, what happens
  next?
• If the X-rays turn off, the companion star can
  be seen take and measure its radial velocity
  curve.
• Use Keplers laws to deduce mass limits. If the
  mass exceeds the maximum mass for a neutron star,
  the source must be a black hole.

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Recent Results from SDSU (and elsewhere)

• The Massive Black Hole in the Spiral Galaxy M33
• http//www.nature.com/nature/journal/v449/n7164/fu
  ll/nature06218.html

99

• The Massive Stellar Black Hole in M33

100
M33

• SA galaxy in Triangulum
• d 840 /- 20 kpc
• M33 X-7 discovered by Einstein in 1981

101
M33

• X-ray source localized with Chandra and optical
  counterpart found with HST by Pietsch et al.
  (2004)
• Pietsch et al. also showed that M33 X-7 is an
  eclipsing binary with P3.453014 days

102
M33

• Top Chandra X-ray light curve
• Bottom Radial velocity curve obtained from
  Gemini North 8.2m telescope.

103
M33

• The optical spectrum indicates the companion is
  an O-star with T35,000 K and a radius of R19.6
  solar radii

104
M33 X-7 Results

• Combine the radial velocity curve, the light
  curves, the eclipse width, the rotational
  velocity, and the radius (from temperature,
  apparent magnitude, and distance)
• MBH 15.65 /- 1.45 solar masses
• MSEC 70.0 /- 6.9 solar masses
• This is the most massive known stellar mass black
  hole.
• The secondary is among the most massive stars
  with a secure mass determination.

105
M33 X-7 Results

• Links to press releases
• http//chandra.harvard.edu/press/07_releases/press
  _101707.html
• http//advancement.sdsu.edu/marcomm/news/releases/
  fall2007/pr101707.html

106
Results

• There are 21 cases where there is good evidence
  that there is a black hole
• Strong X-ray sources (usually flares).
• Optically dark objects (that is, only one star is
  seen in the spectrum, and it is the mass-losing
  one).
• Masses too large to be a white dwarf or a neutron
  star.

107
Recap

• Before a massive star dies, it loses much of
  its initial mass
• If the initial mass is less than about 8 solar
  masses, the mass loss is in a gentle planetary
  nebula.
• If the initial mass is more than about 8 solar
  masses, the mass loss is in a violent explosion
  called a supernova.
• The universe started only with hydrogen and
  helium (more on that later). Thus all of the
  heavier elements were made in stars.

108
Recap

• When a star dies, it leaves behind a remnant
• A white dwarf if the initial mass is less than
  about 8 solar masses.
• A neutron star if the initial mass is between
  about 8 and 30 solar masses.
• A black hole if the initial mass is more than
  about 30 solar masses.
• Although white dwarfs, neutron stars, and black
  holes have strange properties, examples of each
  are observed.