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Stars - Low Mass Stellar Evolution

Once the hydrogen fuel in the core of a low mass star is used up, this marks the Main-Sequence Turnoff Point. Hydrogen fusion is still occurring, but in a shell that surrounds the core. All that is left in the core is helium ash. In a nutshell, this is what happens next:
  • Hydrogen shell continues fusion
  • Helium ash from hydrogen shell collects at the core - the core has remnant helium ash from prior core fusion
  • Helium ash build as does pressure and temperature - helium flash then core helium fusion
  • hydrogen shell and helium core fusion continues - core helium fusion ends soon (more heat, faster burning)
  • Ash in core now carbon, helium shell burning begins, hydrogen shell burning continues
  • Carbon ash compresses as outer layers fluctuate
  • Outer layers shed in planetary nebula
  • Inert, compressed carbon core (White Dwarf) remains

Now for the details. Once off of the Main-Sequence, here is what the core of the star looks like:

Image credit: Brooks/Cole Thomson Learning

The exact stages of evolutions are:

  • Subgiant Branch (SGB) - hydrogen shell burning - outer layers swell
  • Red Giant Branch - helium ash core compresses - increased hydrogen shell burning
  • First Dredge Up - expanding atmosphere cools star - stirs carbon, nitrogen and oxygen upward - star heats up
  • Core Helium Flash - continued compression with added helium ash ignites helium - lots of neutrinos
  • Horizontal Branch - helium burning core - hydrogen burning shell
  • Pre AGB (Asymptotic Giant Branch) - outer layers expand cooling the star - hydrogen shell becomes dormant
  • AGB - re-ignited hydrogen shell burning (like a second Red Giant phase)

For stars under 4 Solar masses:

It is at this point that the outer layers of the star expand; however this time they are lost - expelled in the form of a planetary nebula.

( 2005 Russell Croman,

The end result of the thermal pulsing from the helium and hydrogen shell burning is a planetary nebula - like this image of the Ring nebula.

The remnant carbon core cools and contracts to form a White Dwarf. The star at the center of the Ring nebula (above) is a white dwarf.

Interestingly, there is a limit to how big a White Dwarf can be. The maximum mass a White Dwarf (WD) can have is 1.44 Solar masses. This is called the Chandrasekhar limit - named after Subrahmanyan Chandrasekhar, the astronomer who discovered this limit.

White dwarfs will eventually cool to the point to where they will no longer be visible. They will become Black Dwarfs.

(Image credit: Brooks/Cole Thomson Learning)

There are no known black dwarfs indicating this cool-off period can take millions of years.

If a white dwarf star is a part of a binary star system, some interesting things can happen.

(Image credit: Brooks/Cole Thomson Learning)
Because the white dwarf is pretty massive (the mass of the Sun in an object the diameter of Earth), the atmosphere of the companion star can be accreted onto the star.

This can only happen when the companion star enters its own red giant phase. Once this happens, material is pulled from the companion to the white dwarf.

Once this happens, there are two choices:

  • A nova
  • Type 1a supernova

If a thin layer of material collects in an accretion disk around the white dwarf - dense enough to initiate fusion - then the material will be explosive ejecting a thin hydrogen shell. The white dwarf remains to collect more material (the process repeats itself).

This is called a Nova:

(Image credit: Brooks/Cole Thomson Learning)

However, if material collects rapidly enough on the white dwarf, the mass of the white dwarf can exceed the Chandrasekhar limit. If this happens, there is only once choice: a complete annihilation of the white dwarf and the accreted material.

This is called a Type 1a supernova, and is a very dramatic event:

(Image credit: Brooks/Cole Thomson Learning)

What is unique about a Type 1a supernova is that is occurs just past the Chandrasekhar limit of the white dwarf. As such, the luminosity is standard and can be used an a distance ruler to distant galaxies - just like Cepheid variables are used to measure distance to closer objects.

Classifications of Supernova:

Type: Characteristics: Mechanism:
1a No H lines, strong Si II lines Thermonuclear runaway on white dwarf
1b No H lines, prominent He I lines Core collapse of massive star stripped of hydrogen envelope
1c No H, Si II or He I lines Core collapse of massive star stripped of helium (and hydrogen) envelope
II-P H lines - flat light curve Core collapse of massive star
II-L H lines - no flat light curve Core collapse of massive star


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