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Astronomy 1020-H Stellar Astronomy Spring_2014 Day-29. Course Announcements. 1 st Quarter Observing – Mon. 4/7 @8:30pm Archwood parking lot OR atrium of SSB Rain, shine, sleet, snow … it’s on Lunar Eclipse … Mon-Tues. 4/14-15/2014

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course announcements
Course Announcements
  • 1st Quarter Observing – Mon. 4/7 @8:30pm
    • Archwood parking lot OR atrium of SSB
    • Rain, shine, sleet, snow … it’s on
  • Lunar Eclipse … Mon-Tues. 4/14-15/2014
    • IF CLEAR, we’ll be at the observatory from about midnight-ish on.
  • Dark night, 4/23/2014 (Wed.) weather dependent.
astronomy in the fall 2014
Astronomy in the Fall, 2014

Astr 1010 - Planetary Astronomy + Lab (H,R)

Astr 1020 - Stellar Astronomy + Lab (R)

Astr 2010 - Problems in Planet Astronomy

Astr 2011 - Intro. to Observational Astronomy

Astr 3005 - Observational Astronomy + Lab

Astr 4010 – Intro. to Stellar Astrophysics

Phys 3701 - Advanced Lab (this one will be astronomy based)


Stars are constantly radiating energy.

  • The energy available from fusion is very large, but finite.
  • Eventually, the fusion sources change, then run out.

The star’s luminosity, size, or temperature will change.

  • A star’s life depends on mass and composition.
  • Stars of different masses evolve differently.

The rates and types of fusion depend on the star’s mass.

  • Generally, stars with M < 3 M share many characteristics: low-mass stars.
  • Intermediate-mass stars: 3 M < M < 8 M
  • High-mass stars: M > 8 M

Higher temperature and pressure means faster nuclear fusion.

  • We can figure out main-sequence lifetimes:lifetime = (energy available) / (rate used).

More mass = more fuel available.

  • Rate energy used = luminosity.
  • More massive stars have much higher luminosity.
  • They use their fuel up more quickly and leave the MS faster.


  • Estimates can be made of star lifetimes, based on mass.
  • The mass-luminosity relationship:
  • The lifetime of a star depends on the amount of fuel (M) and how quickly it is used (L).
  • Can use this to compare other stars to the Sun:

Main-sequence stars fuse hydrogen to helium in their cores.

  • Eventually, much of the core H is converted to He.
  • A core of He ash is built up (does not fuse at this point).

Helium Core Is Degenerate

  • H fusion only takes place in a shell around the 100 percent He core: hydrogen shell burning.
  • If H fusion is not happening in the core, the star is no longer main sequence.
  • Since the He is not fusing, gravity begins to win over the pressure, crushing the He.
  • The core becomes more dense, and becomes electron-degenerate.
  • This means pressure is not from moving atoms, but from a quantum mechanical effect: There’s a limit to how tightly electrons can be packed together.

When the fuel runs out of the core, the luminosity increases. Why?

  • When the core shrinks, its gravitational pull gets stronger.
  • Weight of the outer layers increases.

This results in increased pressure: Fusion in the shell goes faster.

  • Faster nuclear reactions release more energy.
  • This leaves the star’s surface at a higher rate (higher luminosity).

Increase in pressure and luminosity results in increased size and decreased surface temperature: red giant.

  • H-R diagram: Star moves up and to the right.

He core is small, dense, electron-degenerate.

  • Outer envelope is greatly expanded, cooler.
  • Fusion of H in shell creates more He, making He nuclei in core denser and hotter.

Once hot enough, fusion of He begins in the degenerate core.

  • He fuses to carbon (C) via the triple-alpha process starts suddenly in the helium flash.
  • Star shrinks and heats up.

After the helium flash, the star is on the horizontal branch of the H-R diagram.

  • At first, He  C in the core, H  He in a shell around the core.
  • Star is smaller and hotter.

Helium is then used up in the core.

  • He fusion in an inner shell and H fusion in an outer shell all surrounding a C core.
  • Star gets more luminous and cool, and enters the asymptotic giant branch (AGB).

As an AGB star, the star expands even more than as a red giant, and cools.

  • H-R diagram: moves up and to the right again.
  • Dense, electron-degenerate carbon core.

After the AGB: Planetary Nebula

  • The star is very thinly spread.
  • Cannot hold on to the outer layers easily.
  • Outer layers are ejected into space, due to instabilities in the interior.

After the AGB: Planetary Nebula

  • The ejected material creates a planetary nebula.
  • The core shrinks and first gets very hot, but eventually cools into a compact white dwarf.

If the conditions are right, the star will ionize the gas in the expanding outer layers.

  • Will last for about 50,000 years before the gas expands too far and disperses.