Astronomy: H-R Diagram

Some stages in the life of a one-solar-mass star:

  1. Main sequence. Longest phase. 10 billion yr. Fuel: Core H fusion. Central temperature around 15 million K.
  2. Main sequence turn-off. Fuel: Core H fusion. He building up in core. Star brightens, gets slightly larger.
  3. Subgiant branch. Fuel: Core H fusion switches to "shell" geometry as He fills center. Luminosity stays about constant as the star grows in radius and cools in temperature.
  4. Red giant branch. Fuel: Shell H fusion. Shell gets larger and thinner in thickness. Star gets more luminous and cooler, and much larger.
  5. Helium Flash. Not a phase - it's a near-instantaneous event when helium fusion begins. Core temperature about 100 million K.
  6. Helium main sequence (also known as "red clump" if it hugs the giant branch, or "horizontal branch" if it spreads out to hotter temperatures). 2nd-longest phase. Roughly 0.4 billion yr. Fuel: Core He fusion. Star is a stable red giant.
  7. Asymptotic giant branch. Fuel: H and He fusion in shells. Star puffs to its largest size yet, and gets very luminous. At the end of AGB, the star sheds its outer layers and makes a dust cloud.
  8. Planetary Nebula. Shortest phase: 10,000 yr. No fusion. Latent heat powers nebula. The material previously shed gets lit up and ionized as the naked CO core is revealed.
  9. White dwarf. No fusion. What was once the core of the star is now a stellar remnant 1/100th of the size it was on the main sequence.
For all stages 1-4 and 6-7, the core of the star always shrinks, gets more dense, and gets hotter, even as the outer parts of the star are generally doing the opposite: getting puffier and cooler.
Last modified: Tue Mar 30 09:10:54 CST 1999