What is stellar evolution?

Answer 1

Stellar Evolution [See Link] - or Life of a Star.

Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years (for the most massive) to trillions of years (for the less massive), considerably more than the age of the universe.

Birth:

Star formation begins with the gravitational collapse of a giant molecular cloud. As the gases coalesce, heat and pressure increase until they condense into a rotating sphere of superhot gases. This is known as a protostar.

Protostars with masses less than roughly 1.6×1029 kg never reach temperatures high enough for nuclear fusion and become brown dwarfs.

For larger protostars, the core temperature will eventually reach 10 megakelvins, and nuclear fusion will begin.. The onset of nuclear fusion leads relatively quickly to a hydrostatic equilibrium in which energy released by the core exerts a "radiation pressure" balancing the weight of the star's matter, preventing further gravitational collapse.

The star thus evolves rapidly to a stable state.

Main Sequence.

Small, relatively cold, low mass red dwarfs burn hydrogen slowly and will burn for hundreds of billions of years

Massive hot supergiants will live for just a few million years.

A mid-sized star like the Sun will remain on the main sequence for about 10 billion years.

Maturity:

After millions to billions of years, depending on the initial mass of the star, the continuous fusion of hydrogen into helium causes a build-up of helium in the core. Eventually, the core exhausts its supply of hydrogen. Depending on the mass of the star, the outcome can vary.

Low Mass will become red dwarfs, such as Proxima Centauri, some of which will live thousands of times longer than the Sun.

Mid Sized will become red giants, such as Aldebaran and Arcturus .

Massive Stars will become red supergiants, such as VY Canis Majoris, Betelguise and Antares.

Stellar Remnants (The End)

After a star has burned out its fuel supply, depending on its mass, one of three things can happen.

For a star with a mass similar to the Sun, it will turn into a white dwarf and radiate the remaining heat into space for billions of years. Finally ending it's life as a black dwarf. (Though none exist at the moment, as the universe is not old enough).

For larger stars, depending on the chemical composition and temperature, the star explodes as a supernova. and either collapses into a neutron star or, if the remaining mass is large enough, the pressure will be insufficient to stop the total collapse and the star will become a black hole.