Model 3: the lifetime of a star

Lifetime of a star in list form!

1. Cool interstellar medium collides with other interstellar medium due to density waves in the spiral arms of our galaxy.

2. This blob of medium gains enough mass and begins to compress.

3. Spin of medium and compression combined with conservation of angular momentum cause the blob of gas to spin faster and faster as it shrinks. 

4. Pressure builds in the center of the blob as matter is pressed and condensed. 

~this entire process can take from 10,000 to 1,000,000 years depending on the mass of the star~

5. The rotation of the blob (protostar) and centrifugal motion cause an accretion disk (protoplanetary disk) to form around the protostar. 

6. The protostar continues to shrink and spin until the pressure and temperature inside its core are high enough to spark the fusion of hydrogen into helium. 

7. Jets form at the poles of the protostar along the magnetic field lines and heat up the surrounding gas and accretion disk. 

8. The accretion disk is blown away by the jets. 

~except for protoplanets and planetoids which remain due to their gravity~

9. The star is now on the main sequence, technically. (ZAMS)

10. Depending on the age of the star, it spends some years on the main sequence. 

11. The star cools and its hydrostatic equilibrium is put off balance. The radiation from the core stops pushing against the pressure of the outer layers and the core contracts. 

12. The outer layers cool and expand so the star because more luminous.

13. The core contracts until shell hydrogen fusion and helium fusion begin.

~this can occur in a "flash" for lower mass stars or in a gradual process for higher mass stars~

14. The star is now a giant star and spends time fusing helium.

15. If the star is very massive it will continue this shrinking and growing process as it fuses heavier and heavier elements. 

16. If the star is massive, when it finishes fusing iron, it will supernova and its remnant star will be a white dwarf (this white dwarf could turn into a neutron star or a black hole).

17. If the star is less massive, when it finishes fusing helium or carbon, it will simply cool off and its outer layers will expand creating a planetary nebula.