Stellar evolution

evolvedevolvingevolutionevolutionaryevolveevolved starevolution of starsevolutionary modelsevolutionary stageage of the star
Stellar evolution is the process by which a star changes over the course of time.wikipedia
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Protostar

protostellarprotostarsbirth of new stars
Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star. Observations from the Wide-field Infrared Survey Explorer (WISE) have been especially important for unveiling numerous Galactic protostars and their parent star clusters.
The protostellar phase is the earliest one in the process of stellar evolution.

Subgiant

subgiant staryellow subgiantsubgiant branch
This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red giant phase.
The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of a star.

Red giant

red giant starred giantsred giant stars
This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red giant phase.
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ) in a late phase of stellar evolution.

White dwarf

white dwarfswhite dwarf starcentral star
Once a star like the Sun has exhausted its nuclear fuel, its core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula.
White dwarfs are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star, that of about 10 solar masses.

Neutron star

neutron starsdying starneutron star formation
Stars with around ten or more times the mass of the Sun can explode in a supernova as their inert iron cores collapse into an extremely dense neutron star or black hole. If the mass of the core exceeds the Chandrasekhar limit, electron degeneracy pressure will be unable to support its weight against the force of gravity, and the core will undergo sudden, catastrophic collapse to form a neutron star or (in the case of cores that exceed the Tolman-Oppenheimer-Volkoff limit), a black hole.
They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.

Supernova

supernovaecore-collapse supernovasupernovas
Stars with around ten or more times the mass of the Sun can explode in a supernova as their inert iron cores collapse into an extremely dense neutron star or black hole.
In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy as a supernova.

Stellar structure

corehydrogen envelopeconvective
Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models.
Stars of different mass and age have varying internal structures. Stellar structure models describe the internal structure of a star in detail and make detailed predictions about the luminosity, the color and the future evolution of the star.

Sun

solarSolThe Sun
Later, as the preponderance of atoms at the core becomes helium, stars like the Sun begin to fuse hydrogen along a spherical shell surrounding the core.
The hydrogen and most of the helium in the Sun would have been produced by Big Bang nucleosynthesis in the first 20 minutes of the universe, and the heavier elements were produced by previous generations of stars before the Sun was formed, and spread into the interstellar medium during the final stages of stellar life and by events such as supernovae.

Gravitational collapse

collapsecollapsedcollapsed star
All stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds.
Depending on the mass during its lifetime, these stellar remnants can take one of three forms:

Astrophysics

astrophysicistastrophysicaltheoretical astrophysics
Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models.
Topics also studied by theoretical astrophysicists include Solar System formation and evolution; stellar dynamics and evolution; galaxy formation and evolution; magnetohydrodynamics; large-scale structure of matter in the universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics.

Brown dwarf

brown dwarfsbrown dwarvesPlanetar
These are known as brown dwarfs.
Early theories concerning the nature of the lowest-mass stars and the hydrogen-burning limit suggested that a population I object with a mass less than 0.07 solar masses or a population II object less than would never go through normal stellar evolution and would become a completely degenerate star.

Nebula

nebulaediffuse nebuladiffuse nebulae
All stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds.
Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of its stellar evolution.

Hertzsprung–Russell diagram

color-magnitude diagramHR diagramcolor magnitude diagram
A new star will sit at a specific point on the main sequence of the Hertzsprung–Russell diagram, with the main-sequence spectral type depending upon the mass of the star.
The diagram was created circa 1910 by Ejnar Hertzsprung and Henry Norris Russell and represents a major step towards an understanding of stellar evolution.

Star

starsmassive starstellar radius
The total mass of a star is the main factor that determines its evolution and eventual fate.

Star cluster

star clustersclusterC
Observations from the Wide-field Infrared Survey Explorer (WISE) have been especially important for unveiling numerous Galactic protostars and their parent star clusters.
Until the mid-1990s, globular clusters were the cause of a great mystery in astronomy, as theories of stellar evolution gave ages for the oldest members of globular clusters that were greater than the estimated age of the universe.

Planetary nebula

planetary nebulaeplanetaryNebula, planetary
Once a star like the Sun has exhausted its nuclear fuel, its core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula.
Planetary nebulae came to be understood as a final stage of stellar evolution.

Gravity

gravitationgravitationalgravitational force
Without the outward pressure generated by the fusion of hydrogen to counteract the force of gravity the core contracts until either electron degeneracy pressure becomes sufficient to oppose gravity or the core becomes hot enough (around 100 MK) for helium fusion to begin.
For example, gravity causes the Earth and the other planets to orbit the Sun, it also causes the Moon to orbit the Earth, and causes the formation of tides, the formation and evolution of the Solar System, stars and galaxies.

Asymptotic giant branch

AGBpost-AGBasymptotic-giant-branch
When hydrogen shell burning finishes, these stars move directly off the red giant branch like a post-asymptotic-giant-branch (AGB) star, but at lower luminosity, to become a white dwarf.
This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (0.6–10 solar masses) late in their lives.

Mira variable

MiraMira-type variable starMira-type
Another well known class of asymptotic-giant-branch stars are the Mira variables, which pulsate with well-defined periods of tens to hundreds of days and large amplitudes up to about 10 magnitudes (in the visual, total luminosity changes by a much smaller amount).
They are red giants in the very late stages of stellar evolution, on the asymptotic giant branch, that will expel their outer envelopes as planetary nebulae and become white dwarfs within a few million years.

Horizontal branch

horizontal-branchblue horizontal-branchHB
Between these two phases, stars spend a period on the horizontal branch with a helium-fusing core.
The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red giant branch in stars whose masses are similar to the Sun's.

Arcturus

ArcturiansArcturianArcturan
Examples include Aldebaran in the constellation Taurus and Arcturus in the constellation of Boötes.
Arcturus is an evolved red giant star with a stellar classification of K0 III.

Chandrasekhar limit

limit(Chandrasekhar) limitChandrasekhar's work on limits
If the mass of the core exceeds the Chandrasekhar limit, electron degeneracy pressure will be unable to support its weight against the force of gravity, and the core will undergo sudden, catastrophic collapse to form a neutron star or (in the case of cores that exceed the Tolman-Oppenheimer-Volkoff limit), a black hole.
Consequently, a white dwarf with a mass greater than the limit is subject to further gravitational collapse, evolving into a different type of stellar remnant, such as a neutron star or black hole.

Oxygen

OO 2 molecular oxygen
After a star has consumed the helium at the core, hydrogen and helium fusion continues in shells around a hot core of carbon and oxygen.
Most 18 O is produced when 14 N (made abundant from CNO burning) captures a 4 He nucleus, making 18 O common in the helium-rich zones of evolved, massive stars.

Stellar nucleosynthesis

hydrogen burningnucleosynthesisstellar fusion
Once the nucleosynthesis process arrives at iron-56, the continuation of this process consumes energy (the addition of fragments to nuclei releases less energy than required to break them off the parent nuclei).
Stars evolve because of changes in their composition (the abundance of their constituent elements) over their lifespans, first by burning hydrogen (main sequence star), then helium (red giant star), and progressively burning higher elements.

Red clump

clump giantred-clumpclump giants
Many of these helium-fusing stars cluster towards the cool end of the horizontal branch as K-type giants and are referred to as red clump giants.
This is again a very rapid phase of evolution, but stars such as OU Andromedae are found in the red clump region (5,500 K and ) even though it is thought to be a subgiant crossing the Hertzsprung gap.