2. Goals
Explain why stars evolve
Explain how stars of different masses evolve
Describe two types of supernova
Explain where the heavier elements come from
Describe how star clusters support stellar evolution theories

3. Most stars spend a majority of their lives (~90%) on the main sequence (about 10 billion years for our Sun)
Virtually all the low mass stars ever formed still exist. None of them have left the main sequence.
On the other hand, massive O and B stars leave the main sequence after a few 10s of millions of years.
Most of the high mass stars that have ever existed perished a long time ago.
The stars in between are in the stages of evolving into…?

4. During the main sequence of a stars life the outward force of energy from burning Hydrogen is balanced by the inward pull of gravity (hydrostatic equilibrium).
As the star burns more Hydrogen eventually the reaction will slow down and the amount of energy released will diminish.
Gravity then begins to compress the star more.

5. The most important factor determining the fate of a star is its mass.
Low mass stars die gently while high mass stars die catastrophically.
The dividing line is about 8 solar masses.

6. Evolution of a Sun-like Star
As hydrogen is consumed in the core the Helium waste becomes concentrated.
Eventually, hydrogen becomes completely depleted at the center until the nuclear fires cease.
The hydrogen burning moves to higher levels.

7. As soon as the hydrogen becomes substantially depleted the helium core begins to shrink under the increased pressure of the unbalanced gravity.
The increased pressure and heat causes the hydrogen shell to burn even faster causing the star to get brighter as the helium core continues to shrink and heat up.
The star is becoming a red giant.
This stage takes 100 million years.

8. The change in energy production and radius causes the star to move off of the main sequence towards the red giant branch.
At the end of this phase the star’s luminosity is hundreds of times greater and its radius is 100 solar radii.

9. As a Solar mass star evolves it is massive enough to fuse Helium into carbon.

10. The simultaneous shrinking orf the core and expanding of the outer layers cannot continue forever.
A few 100 million years after a solar mass star leaves the main sequence, Helium begins to burn.
The initial burning of the helium is very fast and is called the helium flash and last for a few hours.
In response the star again begins to move on the HR diagram.

11. A star like our Sun is massive enough to burn helium and convert it into carbon.
A similar process of core surrounded by a helium shell develops just like it did when helium began to burn.
Now, however, the star swells to an even large size. It is now a red supergiant.
Our Sun will engulf the Earth.

12. If our Sun were massive enough it could burn carbon
If our Sun were massive enough it could burn carbon. But, it can’t, so now our Sun begins to die.
As the hydrogen and helium continue to burn in the outer shell the outer envelope becomes unstable and is ejected into space.
The remaining hot core ionizes the ejected atmosphere and we now have a planetary nebula.

13. The remaining core continues to evolve
The remaining core continues to evolve. It continues to shrink until the degenerate electrons prevent it from shrinking any more.
We now have a white dwarf star.
As the white dwarf continues to cool it eventually will become a cold, dense, burned-out ember in space, or a black dwarf.

14. Some white dwarf stars are part of a binary star system.
If the stars are close enough together then material from one star can be pulled off by the other star. The material then forms an accretion disk before the material falls to the surface.
If enough hydrogen gets dumped on a white dwarf star, then eventually the material will explosively ignite and we will have a nova.
Once a nova explodes it is ready to repeat the process and we get recurrent nova.