Stellar Evolution It all depends On Mass

The Role of Mass and Gravity The Mass of a Star determines how long it ‘lives” and its fate, because the mass determines its Gravity. Gravity determines the temperature and pressure in the core which in turn determines what elements can fuse in the core and the rate at which fusion occurs. The amount of gravity also determines the force that balances the nuclear forces making Hydrostatic Equilibrium

Main Sequence Stars Main Sequence stars fuse Hydrogen into Helium to make energy The “main-sequence life-span” of a star is how long a star is a main-sequence star Higher mass stars use hydrogen in their cores faster and thus have shorter main-sequence life-spans

Brown Dwarfs A Brown Dwarf is a ‘star’ with a mass of less than .1 (1/10) of the Sun. so little mass that no fusion can occur. Therefore they are never main-sequence stars. It glows with infrared light generated from its gravitational contraction like Jupiter does. They don’t “evolve” but stay brown dwarfs and slowly fade over 100s of billions of years

Red Dwarfs Stars with less than half the mass of the Sun have enough mass to fuse Hydrogen into Helium at a slow rate, generating enough energy to glow red. They are the coolest main-sequence stars … Red Dwarfs They take about 100 billion years or more to use up their hydrogen. As their gravity is too weak to create the heat and pressure needed to fuse helium into carbon When they run out of hydrogen in their core their nuclear fusion stops.

Lacking nuclear forces, gravity makes them shrink GPE becomes Thermal Energy They heat up becoming “white dwarfs” Gradually they cool…becoming Yellow orange, red and finally stop shining…a black dwarf.