Chairrex, 2019 (updated)
The stages of stellar evolution are mainly
affected by mass of the stars. They are also affected by metallicity,
that is, the percentage of elements heavier than Helium (e.g. Carbon, Oxygen, Iron) in stars. Metallicity affects the speed of nuclear processes and rapidity of mass loss in stars. Moreover, stellar evolution is altered in close binary and triple star systems. For simplicity, the discussions below were confined to single stars only.
The heavier stars have shorter life as they
“burn” their nuclear fuel much quicker and glow brighter. The majority of stars
in the universe are type ”M” stars (excluding brown
dwarfs which fail to ignite Hydrogen) – their life are very long compared to
the age of universe.
Proto-stars, mostly emitting infra-red radiation, are developed from gravitational collapse of molecular clouds. When the collapse triggers hydrogen fusion(or "burning"), stars are defined to be born successfully.
For failed stars (Brown dwarfs), nuclear
fusion is primitive - cannot supply enough energy generation against gravitional collapse, and the failed stars finally become degenerate and slowly cool down. However, nuclear fusion may still exist, for example, Deuterium burning and Lithium burning. However,
they gives only a little amount of energy for the
objects.
For low-mass main sequence stars
... (type “M”
/ “K” and “G”), hydrogen fusion is possible. It occurs via Proton-proton chain.
The reaction is rather slow. The Sun is an example of low-mass main sequence
stars. It is sometimes called “yellow dwarf” for its appearance. Its surface
temperature is around 6000K.
For medium-mass or high-mass main sequence
stars (type “F” / “A” / “B” and “O”), hydrogen fusion is carried out via
Carbon-Nitrogen-Oxygen cycle. The reaction is high. For “B” and “O” type stars,
mass loss is significant already at main sequence stage by stellar wind.
When stars use up the fuel ...,
stellar cores contract. For low-mass stars, the
cores often reach degenerate stage before the ash (Helium) could be ignited. The
cores cannot contract further. For stars with mass between 0.08 to 0.4 solar
mass, Helium fusion never occur, and the stars slowly become helium white
dwarfs. Some may briefly become red giant, but the evolution is rather gentle. White
dwarfs simply cool off slowly as no more energy sources are available.
For low-mass stars with mass between 0.4 to
~2 solar mass, Helium fusion (to carbon and oxygen) does occur, but under
degenerate condition when they reach red giant stage. As Helium is ignited
degenerately, it is uncontrolled. Helium flash occurs, marking an end to
Horizontal Branch. For stars with mass heavier than 2 solar mass, no helium
flash occurs. In some older context, for stars with very low metallicity, carbon flash may occur for stars with 6 to 8
solar mass which gives type 1.5 supernova, disrupting
red giants.
For masses heavier than 0.8 solar mass but lighter
than 8 solar mass, stars reach asymptotic giant branch (AGB) stage after horizontal
branch. Pulsations exist in such stars, so they are pulsating red giants. Separated into “Early AGB” and “Thermal Pulse AGB”, AGB
marks the repeating burning and extinguishing of Hydrogen and Helium shells. Mass
losses are significant that most of stellar mass are expelled, giving planetary
nebula and mostly carbon-oxygen white dwarfs. The white dwarfs are mostly with
mass of 0.6 solar mass, and they simply cools off. Occasionally, when hydrogen
burning shells still exist and are not extinguished, the proto-white dwarfs at
their early stage can temporally return to AGB stage with changing brightness.
For masses between 8 to 12 solar mass, the
stellar evolution after reaching red giant stage is less certain, and the
outcome can be changed for even slightest change of mass and metallicity. The common view is that the stars can burn
carbon degenerately, off-center after they reach red giant stage, transforming
the stars into Super-asympotic giant branch stars
with Oxygen-Neon core. If the core is lighter, planetary nebula will be the
outcome after more frequent thermal pulses, and Oxygen-Neon white dwarfs of
1.06 to 1.37 solar mass can be formed. The white dwarfs then cool down rather
rapidly than their lighter counterparts. If the Oxygen-Neon core is heavier and
still degenerate, electron capture of Neon-20 and Magneium-24 can occur,
resulting in collapse of core to neutron stars, and the expulsion of the
remaining matter in the process of electron-capture superonovae(ECSN).
Most neutron stars are thought to be
rapidly-rotating, namely Pulsars. The first Pulsar, the little green man, was
discovered in 1967.
For stars with mass between ~ 140 to 160
solar mass, pair-instability supernovae, resulting from uncontrolled oxygen
burning of stellar core, is thought to occur. No
neutron stars and black holes are left behind. For stars still more massive, photodisintegration is thought to play a role crushing the stars into black holes.
Currently, many stellar evolution programs are available. MESA is one of the notable programs widely used by astrophycists.