Representative lifetimes of stars as a function of their masses
The life cycle of a Sun-like star
Artist's
depiction of the life cycle of a Sun-like star, starting as a
main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right
Complete chart of stellar evolution
Stellar evolution is the process by which a star
changes over the course of time. Depending on the mass of the star, its
lifetime can range from a few million years for the most massive to
trillions of years for the least massive, which is considerably longer
than the age of the universe. The table shows the lifetimes of stars as a function of their masses. All stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds. 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.
Nuclear fusion powers a star for most of its life. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star. 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.
This process causes the star to gradually grow in size, passing through
the subgiant stage until it reaches the red giant
phase. Stars with at least half the mass of the Sun can also begin to
generate energy through the fusion of helium at their core, whereas
more-massive stars can fuse heavier elements along a series of
concentric shells. 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. 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. Although the universe is not old enough for any of the smallest red dwarfs to have reached the end of their lives, stellar models suggest they will slowly become brighter and hotter before running out of hydrogen fuel and becoming low-mass white dwarfs.
Stellar evolution is not studied by observing the life of a
single star, as most stellar changes occur too slowly to be detected,
even over many centuries. 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.
Birth of a star
Simplistic representation of the stages of stellar evolution
Schematic of stellar evolution.
Protostar
Stellar evolution starts with the gravitational collapse of a giant molecular cloud. Typical giant molecular clouds are roughly 100 light-years (9.5×1014 km) across and contain up to 6,000,000 solar masses (1.2×1037 kg).
As it collapses, a giant molecular cloud breaks into smaller and
smaller pieces. In each of these fragments, the collapsing gas releases gravitational potential energy as heat. As its temperature and pressure increase, a fragment condenses into a rotating sphere of superhot gas known as a protostar.
A protostar continues to grow by accretion of gas and dust from the molecular cloud, becoming a pre-main-sequence star as it reaches its final mass. Further development is determined by its mass. Mass is typically compared to the mass of the Sun: 1.0 M☉ (2.0×1030 kg) means 1 solar mass.
Protostars are encompassed in dust, and are thus more readily visible at infrared wavelengths.
Observations from the Wide-field Infrared Survey Explorer (WISE) have been especially important for unveiling numerous Galactic protostars and their parent star clusters.
Brown dwarfs and sub-stellar objects
Protostars with masses less than roughly 0.08 M☉ (1.6×1029 kg) never reach temperatures high enough for nuclear fusion of hydrogen to begin. These are known as brown dwarfs. The International Astronomical Union defines brown dwarfs as stars massive enough to fuse deuterium at some point in their lives (13 Jupiter masses (MJ), 2.5 × 1028 kg, or 0.0125 M☉). Objects smaller than 13 MJ are classified as sub-brown dwarfs (but if they orbit around another stellar object they are classified as planets). Both types, deuterium-burning and not, shine dimly and die away slowly, cooling gradually over hundreds of millions of years.
A dense starfield in Sagittarius
Main sequence
For a more-massive protostar, the core temperature will eventually reach 10 million kelvin, initiating the proton–proton chain reaction and allowing hydrogen to fuse, first to deuterium and then to helium. In stars of slightly over 1 M☉ (2.0×1030 kg), the carbon–nitrogen–oxygen fusion reaction (CNO cycle) contributes a large portion of the energy generation. The onset of nuclear fusion leads relatively quickly to a hydrostatic equilibrium
in which energy released by the core maintains a high gas pressure,
balancing the weight of the star's matter and preventing further
gravitational collapse. The star thus evolves rapidly to a stable state,
beginning the main-sequence phase of its evolution.
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. Small, relatively cold, low-mass red dwarfs fuse hydrogen slowly and will remain on the main sequence for hundreds of billions of years or longer, whereas massive, hot O-type stars will leave the main sequence after just a few million years. A mid-sized yellow dwarf
star, like the Sun, will remain on the main sequence for about 10
billion years. The Sun is thought to be in the middle of its main
sequence lifespan.
The evolutionary tracks of stars with different initial masses on the Hertzsprung–Russell diagram. The tracks start once the star has evolved to the main sequence and stop when fusion stops (for massive stars) and at the end of the red giant branch (for stars 1 M☉ and less).
A yellow track is shown for the Sun, which will become a red giant after its main-sequence phase ends before expanding further along the asymptotic giant branch, which will be the last phase in which the Sun undergoes fusion.
Mature stars
Eventually the core exhausts its supply of hydrogen and the star begins to evolve off of the main sequence, 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. Which of these happens first depends upon the star's mass.
Low-mass stars
What happens after a low-mass star ceases to produce energy through fusion has not been directly observed; the universe
is around 13.8 billion years old, which is less time (by several orders
of magnitude, in some cases) than it takes for fusion to cease in such
stars.
Recent astrophysical models suggest that red dwarfs of 0.1 M☉ may stay on the main sequence for some six to twelve trillion years, gradually increasing in both temperature and luminosity, and take several hundred billion more to collapse, slowly, into a white dwarf.
Such stars will not become red giants as they are fully convective and
will not develop a degenerate helium core with a shell burning
hydrogen. Instead, hydrogen fusion will proceed until almost the whole
star is helium.
Internal structures of main-sequence stars, convection zones with arrowed cycles and radiative zones with red flashes. To the left a low-mass red dwarf, in the center a mid-sized yellow dwarf and at the right a massive blue-white main-sequence star.
Slightly more massive stars do expand into red giants,
but their helium cores are not massive enough to reach the temperatures
required for helium fusion so they never reach the tip of the red 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. A star with an initial mass above about 0.8 M☉
will be able to reach temperatures high enough to fuse helium, and
these "mid-sized" stars go on to further stages of evolution beyond the
red giant branch.
Mid-sized stars
The evolutionary track of a solar mass, solar metallicity, star from main sequence to post-AGB
Stars of roughly 0.8–10 M☉ become red giants, which are large non-main-sequence stars of stellar classification
K or M. Red giants lie along the right edge of the Hertzsprung–Russell
diagram due to their red color and large luminosity. Examples include Aldebaran in the constellation Taurus and Arcturus in the constellation of Boötes.
Mid-sized stars are red giants during two different phases of
their post-main-sequence evolution: red-giant-branch stars, with inert
cores made of helium and hydrogen-burning shells, and
asymptotic-giant-branch stars, with inert cores made of carbon and
helium-burning shells inside the hydrogen-burning shells. Between these two phases, stars spend a period on the horizontal branch
with a helium-fusing core. 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.
Subgiant phase
When a star exhausts the hydrogen in its core, it leaves the main
sequence and begins to fuse hydrogen in a shell outside the core. The
core increases in mass as the shell produces more helium. Depending on
the mass of the helium core, this continues for several million to one
or two billion years, with the star expanding and cooling at a similar
or slightly lower luminosity to its main sequence state. Eventually
either the core becomes degenerate, in stars around the mass of the sun,
or the outer layers cool sufficiently to become opaque, in more massive
stars. Either of these changes cause the hydrogen shell to increase in
temperature and the luminosity of the star to increase, at which point
the star expands onto the red giant branch.
Red-giant-branch phase
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Typical stellar evolution for 0.8-8 M☉
The expanding outer layers of the star are convective,
with the material being mixed by turbulence from near the fusing
regions up to the surface of the star. For all but the lowest-mass
stars, the fused material has remained deep in the stellar interior
prior to this point, so the convecting envelope makes fusion products
visible at the star's surface for the first time. At this stage of
evolution, the results are subtle, with the largest effects, alterations
to the isotopes of hydrogen and helium, being unobservable. The effects of the CNO cycle appear at the surface during the first dredge-up, with lower 12C/13C ratios and altered proportions of carbon and nitrogen. These are detectable with spectroscopy and have been measured for many evolved stars.
The helium core continues to grow on the red giant branch. It is
no longer in thermal equilibrium, either degenerate or above the Schoenberg-Chandrasekhar limit,
so it increases in temperature which causes the rate of fusion in the
hydrogen shell to increase. The star increases in luminosity towards
the tip of the red-giant branch.
Red giant branch stars with a degenerate helium core all reach the tip
with very similar core masses and very similar luminosities, although
the more massive of the red giants become hot enough to ignite helium
fusion before that point.
Horizontal branch
In the helium cores of stars in the 0.8 to 2.0 solar mass range, which are largely supported by electron degeneracy pressure, helium fusion will ignite on a timescale of days in a helium flash. In the nondegenerate cores of more massive stars, the ignition of helium fusion occurs relatively slowly with no flash. The nuclear power released during the helium flash is very large, on the order of 108 times the luminosity of the Sun for a few days and 1011 times the luminosity of the Sun (roughly the luminosity of the Milky Way Galaxy) for a few seconds.
However, the energy is consumed by the thermal expansion of the
initially degenerate core and thus cannot be seen from outside the star.
Due to the expansion of the core, the hydrogen fusion in the overlying
layers slows and total energy generation decreases. The star contracts,
although not all the way to the main sequence, and it migrates to the horizontal branch on the Hertzsprung–Russell diagram, gradually shrinking in radius and increasing its surface temperature.
Core helium flash stars evolve to the red end of the horizontal
branch but do not migrate to higher temperatures before they gain a
degenerate carbon-oxygen core and start helium shell burning. These
stars are often observed as a red clump
of stars in the colour-magnitude diagram of a cluster, hotter and less
luminous than the red giants. Higher-mass stars with larger helium cores
move along the horizontal branch to higher temperatures, some becoming
unstable pulsating stars in the yellow instability strip (RR Lyrae variables),
whereas some become even hotter and can form a blue tail or blue hook
to the horizontal branch. The morphology of the horizontal branch
depends on parameters such as metallicity, age, and helium content, but
the exact details are still being modelled.
Asymptotic-giant-branch phase
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. The star follows the asymptotic giant branch
on the Hertzsprung–Russell diagram, paralleling the original red giant
evolution, but with even faster energy generation (which lasts for a
shorter time).
Although helium is being burnt in a shell, the majority of the energy
is produced by hydrogen burning in a shell further from the core of the
star. Helium from these hydrogen burning shells drops towards the
center of the star and periodically the energy output from the helium
shell increases dramatically. This is known as a thermal pulse
and they occur towards the end of the asymptotic-giant-branch phase,
sometimes even into the post-asymptotic-giant-branch phase. Depending on
mass and composition, there may be several to hundreds of thermal
pulses.
There is a phase on the ascent of the asymptotic-giant-branch
where a deep convective zone forms and can bring carbon from the core to
the surface. This is known as the second dredge up, and in some stars
there may even be a third dredge up. In this way a carbon star
is formed, very cool and strongly reddened stars showing strong carbon
lines in their spectra. A process known as hot bottom burning may
convert carbon into oxygen and nitrogen before it can be dredged to the
surface, and the interaction between these processes determines the
observed luminosities and spectra of carbon stars in particular
clusters.
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). In more-massive stars the
stars become more luminous and the pulsation period is longer, leading
to enhanced mass loss, and the stars become heavily obscured at visual
wavelengths. These stars can be observed as OH/IR stars, pulsating in the infra-red and showing OH maser activity. These stars are clearly oxygen rich, in contrast to the carbon stars, but both must be produced by dredge ups.
Post-AGB
The Cat's Eye Nebula, a planetary nebula formed by the death of a star with about the same mass as the Sun
These mid-range stars ultimately reach the tip of the
asymptotic-giant-branch and run out of fuel for shell burning. They are
not sufficiently massive to start full-scale carbon fusion, so they
contract again, going through a period of post-asymptotic-giant-branch
superwind to produce a planetary nebula with an extremely hot central
star. The central star then cools to a white dwarf. The expelled gas is
relatively rich in heavy elements created within the star and may be
particularly oxygen or carbon enriched, depending on the type of the star. The gas builds up in an expanding shell called a circumstellar envelope and cools as it moves away from the star, allowing dust particles
and molecules to form. With the high infrared energy input from the
central star, ideal conditions are formed in these circumstellar
envelopes for maser excitation.
It is possible for thermal pulses to be produced once
post-asymptotic-giant-branch evolution has begun, producing a variety of
unusual and poorly understood stars known as born-again
asymptotic-giant-branch stars. These may result in extreme horizontal-branch stars (subdwarf B stars), hydrogen deficient post-asymptotic-giant-branch stars, variable planetary nebula central stars, and R Coronae Borealis variables.
Massive stars
Reconstructed image of Antares, a red supergiant
In massive stars, the core is already large enough at the onset of
the hydrogen burning shell that helium ignition will occur before
electron degeneracy pressure has a chance to become prevalent. Thus,
when these stars expand and cool, they do not brighten as much as
lower-mass stars; however, they were much brighter than lower-mass stars
to begin with, and are thus still brighter than the red giants formed
from less-massive stars. These stars are unlikely to survive as red supergiants; instead they will destroy themselves as type II supernovas.
Extremely massive stars (more than approximately 40 M☉),
which are very luminous and thus have very rapid stellar winds, lose
mass so rapidly due to radiation pressure that they tend to strip off
their own envelopes before they can expand to become red supergiants,
and thus retain extremely high surface temperatures (and blue-white
color) from their main-sequence time onwards. The largest stars of the
current generation are about 100-150 M☉ because
the outer layers would be expelled by the extreme radiation. Although
lower-mass stars normally do not burn off their outer layers so rapidly,
they can likewise avoid becoming red giants or red supergiants if they
are in binary systems close enough so that the companion star strips off
the envelope as it expands, or if they rotate rapidly enough so that
convection extends all the way from the core to the surface, resulting
in the absence of a separate core and envelope due to thorough mixing.
The core grows hotter and denser as it gains material from fusion
of hydrogen at the base of the envelope. In all massive stars, electron
degeneracy pressure is insufficient to halt collapse by itself, so as
each major element is consumed in the center, progressively heavier
elements ignite, temporarily halting collapse. If the core of the star
is not too massive (less than approximately 1.4 M☉,
taking into account mass loss that has occurred by this time), it may
then form a white dwarf (possibly surrounded by a planetary nebula) as
described above for less-massive stars, with the difference that the
white dwarf is composed chiefly of oxygen, neon, and magnesium.
The onion-like layers of a massive, evolved star just before core collapse. (Not to scale.)
Above a certain mass (estimated at approximately 2.5 M☉ and whose star's progenitor was around 10 M☉), the core will reach the temperature (approximately 1.1 gigakelvins) at which neon partially breaks down to form oxygen and helium, the latter of which immediately fuses with some of the remaining neon to form magnesium; then oxygen fuses to form sulfur, silicon, and smaller amounts of other elements. Finally, the temperature gets high enough that any nucleus can be partially broken down, most commonly releasing an alpha particle (helium nucleus) which immediately fuses with another nucleus,
so that several nuclei are effectively rearranged into a smaller number
of heavier nuclei, with net release of energy because the addition of
fragments to nuclei exceeds the energy required to break them off the
parent nuclei.
A star with a core mass too great to form a white dwarf but
insufficient to achieve sustained conversion of neon to oxygen and
magnesium, will undergo core collapse (due to electron capture) before achieving fusion of the heavier elements. Both heating and cooling caused by electron capture onto minor constituent elements (such as aluminum and sodium) prior to collapse may have a significant impact on total energy generation within the star shortly before collapse. This may produce a noticeable effect on the abundance of elements and isotopes ejected in the subsequent supernova.
Supernova
The Crab Nebula, the shattered remnants of a star which exploded as a supernova, the light of which reached Earth in 1054 AD
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). 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. Through a process that is not completely understood, some of the gravitational potential energy released by this core collapse is converted into a Type Ib, Type Ic, or Type II supernova. It is known that the core collapse produces a massive surge of neutrinos, as observed with supernova SN 1987A. The extremely energetic neutrinos fragment some nuclei; some of their energy is consumed in releasing nucleons, including neutrons, and some of their energy is transformed into heat and kinetic energy, thus augmenting the shock wave
started by rebound of some of the infalling material from the collapse
of the core. Electron capture in very dense parts of the infalling
matter may produce additional neutrons. Because some of the rebounding
matter is bombarded by the neutrons, some of its nuclei capture them,
creating a spectrum of heavier-than-iron material including the
radioactive elements up to (and likely beyond) uranium.
Although non-exploding red giants can produce significant quantities of
elements heavier than iron using neutrons released in side reactions of
earlier nuclear reactions, the abundance of elements heavier than iron
(and in particular, of certain isotopes of elements that have multiple
stable or long-lived isotopes) produced in such reactions is quite
different from that produced in a supernova. Neither abundance alone
matches that found in the Solar System,
so both supernovae and ejection of elements from red giants are
required to explain the observed abundance of heavy elements and isotopes thereof.
The energy transferred from collapse of the core to rebounding
material not only generates heavy elements, but provides for their
acceleration well beyond escape velocity,
thus causing a Type Ib, Type Ic, or Type II supernova. Note that
current understanding of this energy transfer is still not satisfactory;
although current computer models of Type Ib, Type Ic, and Type II
supernovae account for part of the energy transfer, they are not able to
account for enough energy transfer to produce the observed ejection of
material.
However, neutrino oscillations may play an important role in the energy
transfer problem as they not only affect the energy available in a
particular flavour of neutrinos but also through other
general-relativistic effects on neutrinos.
Some evidence gained from analysis of the mass and orbital
parameters of binary neutron stars (which require two such supernovae)
hints that the collapse of an oxygen-neon-magnesium core may produce a
supernova that differs observably (in ways other than size) from a
supernova produced by the collapse of an iron core.
The most massive stars that exist today may be completely destroyed by a supernova with an energy greatly exceeding its gravitational binding energy. This rare event, caused by pair-instability, leaves behind no black hole remnant.
In the past history of the universe, some stars were even larger than
the largest that exists today, and they would immediately collapse into a
black hole at the end of their lives, due to photodisintegration.
Stellar evolution of low-mass (left cycle) and high-mass (right cycle) stars, with examples in italics
Stellar remnants
After
a star has burned out its fuel supply, its remnants can take one of
three forms, depending on the mass during its lifetime.
White and black dwarfs
For a star of 1 M☉, the resulting white dwarf is of about 0.6 M☉,
compressed into approximately the volume of the Earth. White dwarfs are
stable because the inward pull of gravity is balanced by the degeneracy pressure of the star's electrons, a consequence of the Pauli exclusion principle.
Electron degeneracy pressure provides a rather soft limit against
further compression; therefore, for a given chemical composition, white
dwarfs of higher mass have a smaller volume. With no fuel left to burn,
the star radiates its remaining heat into space for billions of years.
A white dwarf is very hot when it first forms, more than 100,000 K
at the surface and even hotter in its interior. It is so hot that a lot
of its energy is lost in the form of neutrinos for the first 10 million
years of its existence, but will have lost most of its energy after a
billion years.
The chemical composition of the white dwarf depends upon its mass. A star of a few solar masses will ignite carbon fusion
to form magnesium, neon, and smaller amounts of other elements,
resulting in a white dwarf composed chiefly of oxygen, neon, and
magnesium, provided that it can lose enough mass to get below the Chandrasekhar limit, and provided that the ignition of carbon is not so violent as to blow the star apart in a supernova.
A star of mass on the order of magnitude of the Sun will be unable to
ignite carbon fusion, and will produce a white dwarf composed chiefly of
carbon and oxygen, and of mass too low to collapse unless matter is
added to it later (see below). A star of less than about half the mass
of the Sun will be unable to ignite helium fusion (as noted earlier),
and will produce a white dwarf composed chiefly of helium.
In the end, all that remains is a cold dark mass sometimes called a black dwarf. However, the universe is not old enough for any black dwarfs to exist yet.
If the white dwarf's mass increases above the Chandrasekhar limit, which is 1.4 M☉ for a white dwarf composed chiefly of carbon, oxygen, neon, and/or magnesium, then electron degeneracy pressure fails due to electron capture
and the star collapses. Depending upon the chemical composition and
pre-collapse temperature in the center, this will lead either to
collapse into a neutron star
or runaway ignition of carbon and oxygen. Heavier elements favor
continued core collapse, because they require a higher temperature to
ignite, because electron capture onto these elements and their fusion
products is easier; higher core temperatures favor runaway nuclear
reaction, which halts core collapse and leads to a Type Ia supernova.
These supernovae may be many times brighter than the Type II supernova
marking the death of a massive star, even though the latter has the
greater total energy release. This instability to collapse means that no
white dwarf more massive than approximately 1.4 M☉ can exist (with a possible minor exception for very rapidly spinning white dwarfs, whose centrifugal force due to rotation partially counteracts the weight of their matter). Mass transfer in a binary system may cause an initially stable white dwarf to surpass the Chandrasekhar limit.
If a white dwarf forms a close binary system with another star,
hydrogen from the larger companion may accrete around and onto a white
dwarf until it gets hot enough to fuse in a runaway reaction at its
surface, although the white dwarf remains below the Chandrasekhar limit.
Such an explosion is termed a nova.
Neutron stars
Bubble-like shock wave still expanding from a supernova explosion 15,000 years ago.
Ordinarily, atoms are mostly electron clouds by volume, with very
compact nuclei at the center (proportionally, if atoms were the size of a
football stadium, their nuclei would be the size of dust mites). When a
stellar core collapses, the pressure causes electrons and protons to
fuse by electron capture.
Without electrons, which keep nuclei apart, the neutrons collapse into a
dense ball (in some ways like a giant atomic nucleus), with a thin
overlying layer of degenerate matter
(chiefly iron unless matter of different composition is added later).
The neutrons resist further compression by the Pauli Exclusion
Principle, in a way analogous to electron degeneracy pressure, but
stronger.
These stars, known as neutron stars, are extremely small—on the
order of radius 10 km, no bigger than the size of a large city—and are
phenomenally dense. Their period of rotation shortens dramatically as
the stars shrink (due to conservation of angular momentum);
observed rotational periods of neutron stars range from about 1.5
milliseconds (over 600 revolutions per second) to several seconds.
When these rapidly rotating stars' magnetic poles are aligned with the
Earth, we detect a pulse of radiation each revolution. Such neutron
stars are called pulsars,
and were the first neutron stars to be discovered. Though
electromagnetic radiation detected from pulsars is most often in the
form of radio waves, pulsars have also been detected at visible, X-ray,
and gamma ray wavelengths.
Black holes
If the mass of the stellar remnant is high enough, the neutron
degeneracy pressure will be insufficient to prevent collapse below the Schwarzschild radius.
The stellar remnant thus becomes a black hole. The mass at which this
occurs is not known with certainty, but is currently estimated at
between 2 and 3 M☉.
Black holes are predicted by the theory of general relativity.
According to classical general relativity, no matter or information can
flow from the interior of a black hole to an outside observer, although
quantum effects
may allow deviations from this strict rule. The existence of black
holes in the universe is well supported, both theoretically and by
astronomical observation.
Because the core-collapse mechanism of a supernova is, at
present, only partially understood, it is still not known whether it is
possible for a star to collapse directly to a black hole without
producing a visible supernova, or whether some supernovae initially form
unstable neutron stars which then collapse into black holes; the exact
relation between the initial mass of the star and the final remnant is
also not completely certain. Resolution of these uncertainties requires
the analysis of more supernovae and supernova remnants.
Models
A stellar evolutionary model is a mathematical model
that can be used to compute the evolutionary phases of a star from its
formation until it becomes a remnant. The mass and chemical composition
of the star are used as the inputs, and the luminosity and surface
temperature are the only constraints. The model formulae are based upon
the physical understanding of the star, usually under the assumption of
hydrostatic equilibrium. Extensive computer calculations are then run to
determine the changing state of the star over time, yielding a table of
data that can be used to determine the evolutionary track of the star across the Hertzsprung–Russell diagram, along with other evolving properties.
Accurate models can be used to estimate the current age of a star by
comparing its physical properties with those of stars along a matching
evolutionary track.
