Astrophysics
What causes rapid rotation in more massive stars, compared to less massive stars?
Because massive stars lack convective envelopes which cause magnetic activity in the star, while less massive stars have convective envelopes which causes magnetic activity that causes magnetic braking, which slows the star's rotation over time.
Why do more massive stars expand into Red Giants, but Red dwarfs below 0.25-0.35 solar masses don't?
Because stars below this mass range are fully convective, and if they run out of fuel, and they won't have hydrogen in their envelopes to initiate hydrogen shell burning, which is primary reason why more massive stars expand once their hydrogen has been exhausted in the core.
Why do Asymptotic Giant Branch stars experience pulses?
Because of their internal structure, two burning shells, one hydrogen burning, and one helium burning, once the helium burning shells bloats the star past a certain point, the hydrogen shell stops fusing, and the helium shell undergoes a helium flash, temporarily shrinking the star while it ejects tons of mass away.
What causes extreme helium Blue Supergiants?
The most accepted theory is that two white dwarfs, one more massive, made of either carbon and oxygen or oxygen and neon, and one, less massive but bigger, made of helium collides, the more massive white dwarf shreds the helium white dwarf and the intense heat generated in the collision triggers helium fusion, causing the helium gas to expand. Turning it into a low mass helium Blue Supergiant.
Why can't extremely massive stars turn into Red Supergiants?
Because their luminosities are too high to ever reach a bloated and cool phase, once their envelopes expand past a certain point, the light pressure simply blows it off into space, which is why they can only evolve into Luminous Blue Variables, which are violently unstable and constantly blows off tons of material into space.