Stars and the HR Diagram Dr. Matt Penn National Solar Observatory

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Stars and the HR DiagramDr. Matt Penn National
Solar Observatory

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Outline

• How do we form an HR diagram?
• Absolute brightness (Luminosity)
• Temperature (Spectral class)
• Where are most stars? Why?
• What happens when stars evolve over time?
• What does an HR diagram of a star cluster tell
  us?

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The basic problem

• When we try to understand the life of a star, we
  face a harder problem than a mosquito trying to
  understand a human life.

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The basic problem

• Human life 3,000 mosquito lives
• Stellar life 100,000,000 human lives
• We cannot sit back and watch we need a different
  approach
• We use the laws of physics and a few observable
  quantities to understand the lives of stars

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The basic problem

• Lets say the mosquito takes the same approach.
• The mosquito wants to measure two things for each
  person color of hair, and height of the person.
• The mosquito makes a graph of these two things
  and hopes to learn about the lives of people this
  way.

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The basic problem

• To measure the height of each person without
  flying there, what else must the mosquito know?

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The HR diagram

• The tool we use to study stars is called the
  Hertzsprung-Russell diagram.
• It plots two observable quantities the absolute
  brightness of a star and the temperature of a
  star.
• Combined with some laws of physics, the HR
  diagram provides a way to understand how stars
  evolve with time.

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The HR diagram

• Most stars lie along the Main Sequence
• Simple relationship between temperature and
  luminosity
• Stars spend most of their lives converting
  hydrogen to helium, and this is what occurs when
  the star is on the main sequence
• An HR diagram of the closest 16,000 stars shows
  most lie along MS

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The HR diagram

• Stars in the upper right are very large and stars
  in the lower left are very small.
• This defines only the SIZE of the star and not
  the MASS, since the density of stars can be very
  different.
• So the branch of stars to the upper right of the
  MS are giant and supergiant stars.

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Stellar Evolution 1 solar mass

• The job of a star is to balance the crushing
  force of gravity by producing an internal
  pressure by releasing energy from atomic fusion
  reactions.
• When the star can no longer balance gravity, or
  changes the way it makes internal pressure, we
  say that the star evolves.

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Stellar Evolution 1 solar mass

• Using physics and computer models, we can predict
  the evolution of stars. The changes which occur
  in a star even with the same mass as the Sun are
  profound.
• Inside, the core of the Sun will run out of
  hydrogen atoms and eventually turn to helium
  atoms for energy production.

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Stellar Evolution 1 solar mass

• Eventually the Sun can no longer produce internal
  pressure with fusion reactions the Sun runs out
  of energy.
• The envelope is ejected, and the core of the Sun
  forms a very dense, solid white dwarf star.
• A famous planetary nebula with a white dwarf in
  the center is M57

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Stellar Evolution 1 solar mass

• The evolution of a one solar mass star, from the
  main sequence through the giant phase to a white
  dwarf, can be traced on a HR diagram.

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Stellar Evolution 1 solar mass
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Stellar Evolution 2 to 5 solar mass

• The internal structure, and the evolution of a
  star varies depending on initial mass

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Stellar Evolution 2 to 5 solar mass

• Higher mass stars are much much hotter they use
  up their supply of hydrogen much faster than the
  Sun.
• A higher mass star can use helium for nuclear
  fusion, and with the higher temperatures

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Stellar Evolution 2 to 5 solar mass

• High mass stars can use heavy elements and can
  produce nuclei of carbon, oxygen and nitrogen in
  their core.
• The nuclei of all carbon, oxygen and nitrogen
  atoms in the Universe were produced inside the
  cores of massive stars at earlier times.

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Stellar Evolution 2 to 5 solar mass

• When a higher mass star can no longer produce
  internal pressure, it ejects the envelope in a
  violent explosion called a supernova.
• Supernova are so bright they can shine brighter
  than an entire galaxy, and they can be seen
  across the visible universe.

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Stellar Evolution 2 to 5 solar mass

• Gas thrown off during SN explosions forms remnant
  nebulae and glows for long times

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Stellar Evolution 2 to 5 solar mass

• The collapsing core of a high mass star forms a
  neutron star, usually in the form of a pulsar, a
  rapidly rotating stellar remnant which can appear
  to blink hundreds or thousands of times per
  second.
• The most famous pulsar is in the Crab nebula

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Stellar Evolution 5 solar mass

• Stars with initial masses greater than 5 solar
  masses or so produce violent supernova
  explosions.
• The cores of these stars are so massive that they
  continue collapsing past the neutron star phase
  and form black holes.

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Stellar Evolution 5 solar mass

• Since black-holes cannot be directly observed,
  the best support for their existence comes from
  observations of X-ray binaries.
• The high temperatures and small size of the X-ray
  emitters can only be found in the accretion disk
  surrounding a black hole.

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Globular Clusters and HR Diagram

• Stars in a globular cluster are all thought to
  form at roughly the same time.
• The stars in a globular have different initial
  masses, and so they will evolve at different
  rates.
• If we make an HR diagram of the stars in a
  cluster, we see stars in various stages of
  evolution.

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Globular Clusters and HR Diagram

• By looking at the turn-off point from the Main
  Sequence, we can estimate the age of the stars in
  the cluster.
• Turn-off point ? stellar mass ? age

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Summary

• Parallax and spectroscopy help us measure the
  luminosity and temperature of a star.
• Plotting the luminosity vs temperature gives us
  an HR diagram.
• The Main Sequence, where most stars fall in the
  HR diagram, is where stars convert hydrogen to
  helium.

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Summary

• We can estimate the radius and the mass of stars
  based on their position in the HR diagram
• Evolution of stars occurs as stars run out of
  fuel and this can be traced on the HR diagram
• HR diagrams of star clusters help us determine
  the age of the clusters.