Stellar Astronomy

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Stellar Astronomy

• Paul J. Thomas
• Department of Physics and Astronomy
• UW - Eau Claire

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Astronomical Magnitudes

• Measures apparent brightness of objects.
• The brighter the object, the smaller (or more
  negative) the magnitude.
• Each magnitude is 2.512 brighter than the next
  lowest step.
• 100 6 mag. stars 1 1 mag. star.
• 1011 28 mag. stars 1 1 mag. star.

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Examples of Magnitudes

• Sun -26.5
• Full Moon -12
• Venus (maximum) -4
• Sirius -1.5
• Naked eye limit 6
• Binocular limit 10
• 5 m telescope 25
• HST 28

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Intrinsic vs. Apparent Brightness

• Intrinsic brightness is the measurement of how
  luminous a star actually is.
• Apparent brightness is the measurement of how
  luminous a star appears to be. It is affected by
  the distance to a star.
• We use magnitudes to measure the brightness of a
  star.

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Intrinsic Brightness

• The intrinsic brightness of a star is given by
• Absolute magnitude (M) the magnitude a star
  would have at a distance of 10 parsecs (32.6
  light years)
• Luminosity (L?) brightness compared to the Sun.

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Intrinsic vs. Apparent Brightness
Vega
Altair
Deneb
Vega and Deneb are similar in apparent
brightness But Deneb is over 100 further
away! So Deneb must have a greater intrinsic
brightness!
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Intrinsic vs. Apparent Brightness

• Vega
• m 0.04
• M 0.5
• L 54 L?
• r 26.5 ly

• Deneb
• m 1.26
• M -7.1
• L 59,000 L?
• r 3000 ly

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Spectral Sequence

• O 28,000 K
• B 10,000 - 28,000 K
• A 7,500 - 10,000 K
• F 6,000 - 7,500 K
• G 5,000 - 6,000 K
• K 3,500 - 5,000 K
• M, R, N, S

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Spectral Sequence

• OBAFGKMRNS
• Oh, be a fine girl/guy kiss me, right now -
  smack!

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Sizes of stars

• Using parallax, we can determine the intrinsic
  brightness of stars.
• Stars that are hot and intrinsically dim are
  small (e.g. white dwarfs).
• Stars that are cool and intrinsically bright are
  large (e.g. red giants).

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The Hertzsprung-Russell Diagram

• Developed in 1912 by Ejnar Hertzsprung (Denmark)
  and Henry Norris Russell (USA).
• A statistical plot of luminosity vs. surface
  temperature for many stars.

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A Variety of Main Sequence Stars

• Temperature
• Hottest 50,000 K
• Sun 5,800 K
• Coldest 2,500 K

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A Variety of Main Sequence Stars

• Luminosity
• Most Luminous 800,000 L? (M -9.9)
• Sun 1 L? (M 4.8)
• Least Luminous 10-6 L? (M 20)

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A Variety of Main Sequence Stars

• Radius
• Biggest 20 R?
• Sun 1 R?
• Smallest 0.1 R?

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Determining the Mass of Stars

• 50 of all stars are in a binary system (2 or
  more stars orbiting around each other).
• By observing their separation and their period,
  we can determine their combined mass by Keplers
  Third Law

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A Variety of Main Sequence Stars

• Mass
• Most massive 60 M?
• Sun 1 M?
• Least massive 0.06 M?

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How does a star shine?

• Stars are balls of hot gas. They support
  themselves against gravity by the expansion of
  gas in their interiors.

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How does a star shine?

• The centers of stars must very hot. (The center
  of the Sun is about 15,000,000 K).
• At these temperatures, hydrogen can be converted
  to helium by nuclear fusion.

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Nuclear Fusion pp chain

• Mass of 4 protons 2 electrons
• 6.694 10-24 g
• Mass of 4He nucleus
• 6.644 10-24 g
• Difference 0.050 10-24 g
• Energy released (E mc2) 4.4 10-12 J
• 6 1014 g of H ? He per second!

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Energy from Nuclear Fusion

• 4.4 ? 10-12 J for each He nucleus produced
• (there are 1.5 ? 1023 He nuclei per g)
• 6.62 ? 1011 J for each g of He produced
• (6 ? 1014 g of He is produced every second)
• 3.97 ? 1026 W of power

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Stellar Evolution

• Stars form as groups in bright nebulae, e.g. the
  Orion Nebula.
• The mass of a star determines its evolution.
• The more massive a star, the shorter its Main
  Sequence lifetime.
• Stars generate energy by burning light elements
  to heavy elements nucleosynthesis.

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Life of a 0.1 M? Star

• We dont know much about these brown dwarfs.
• They are hard to see, and might be present in our
  galaxy in large numbers.
• MS lifetime probably 100 billion years.

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HST Views a Brown Dwarf
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Life of a 1.0 M? Star

• MS lifetime 1010 y.
• Burns H ? He in core on MS.
• Then becomes Red Giant and burns H ? He in shell.
• Starts to burn He ? C in core (helium flash).
• Becomes variable star.
• Final stage C white dwarf surrounded by
  planetary nebula.

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A 1.0 M? Star
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Planetary Nebula
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White Dwarfs

• R 7000 km, T 30,000 K.
• M
• ? 107 g/cm3.
• Composed of electron degenerate material.

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White Dwarfs
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Life of a 25 M? Star

• MS lifetime 7 My.
• Burns H ? He in core on MS.
• Then becomes Supergiant and burns He ? C, N, O ?
  Ne ? Mg ? Fe in shells and core.

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A 25 M? Star, Part 1
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A 25 M? Star, Part 2
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Just before the Supernova

• Burning in concentric shells of increasing
  nuclear mass.
• Fusion of Fe absorbs energy, and core collapses,
  triggering supernova.
• Core becomes neutron star or black hole.

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Eta Carinae Future Supernova
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Luminosity of Supernovae

• M -20
• 5 ? 109 solar luminosity
• Period of maximum brightness 20 days

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Supernova 1987a
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Supernova 1987a
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The Crab Nebula, a SNR
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Neutron Stars

• R 20 km.
• M 1.4 M? - 3 M?.
• ? 3 1014 g/cm3.
• Composed of neutron degenerate material.
• In a neutron star, the electrons and protons have
  been squeezed together, leaving only neutrons.

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Pulsars

• First discovered in 1967 by Jocelyn Bell. She
  detected radio pulses with a period of 1.3373011
  s.
• We think they are very rapidly rotating neutron
  stars.
• They radiate pulses at all wavelengths.

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Pulsars

• Pulsars are rapidly rotating neutron stars.
• The magnetic field of the neutron star
  accelerates charged particles in the SNR.
• Pulses are produced by synchrotron radiation.

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Planets around Pulsars

• Alex Wolszczan and co-workers have discovered
  three planets around pulsar PSR 125712.

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Black Holes

• Not even light can escape from a body if the
  escape speed exceeds the speed of light.

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Black Hole Size

• The Schwarzschild Radius is the radius of a black
  hole.

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Singularities and Event Horizons

• At the center of a black hole is a singularity a
  point of infinite density and zero size.
• We cant see this, as light cannot escape from
  the region around it. This region is bounded by
  the Event Horizon. The radius of the Event
  Horizon is Rs.

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Einsteins Theory of General Relativity

• A new way of looking at gravity in the universe.
• The three dimensions of space and the dimension
  of time form a surface of spacetime.
• This surface is distorted by the presence of mass.

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Albert Einstein

• One thing I have learned in a long life that
  all our science, measured against reality, is
  primitive and childlike -- and yet it is the most
  precious thing we have.

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Einsteins Theory of General Relativity

• Light moves in straight lines through spacetime,
  and so is deflected by the presence of mass,
  which distorts spacetime. This is gravitational
  lensing.
• Planets orbit around stars because they move in a
  spacetime distortion.
• This predicts the motions of planets better than
  Newtons laws.

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Gravitational Lensing
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Gravitational Lensing

• British astronomers observed a total eclipse in
  May 1919, from an island off West Africa and to
  Brazil. They succeeded in
  photographing stars near the eclipsed sun. The
  starlight had been deflected just as Einstein had
  predicted.

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Gravitational Lensing

• HST image of a cluster of galaxies. The
  concentrated mass of the cluster warps space
  around it, bending light from galaxies far beyond
  the cluster. The images of these galaxies appears
  as streaks and arcs in this image.

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General Relativity and Black Holes

• In General Relativity, a black hole is a tunnel
  in spacetime.
• An object falling into a black hole will appear
  to be
• greatly redshifted
• ageing more slowly.
• It will also be torn apart by tidal forces.

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Evidence for Black Holes

• X-ray binaries are stars where a normal star has
  an unseen companion. X-rays are observed coming
  from the system, probably from an accretion disk
  surrounding the unseen companion.
• The unseen companion is pulling material off the
  normal star. It must be a collapsar.

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Cygnus X-1

• The optical companion, HDE 226868, is an O9.7
  supergiant, with a mass 30 M?.
• The total mass of the system is 37 M ?.
• The unseen companion has a mass 7 M?.
• This exceeds the stability limit for a neutron
  star.

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Cygnus X-1
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Sagittarius A