A star is type of astronomical object consisting
of a luminous spheroid of plasma held together
by its own gravity. The nearest star to Earth
is the Sun. Many other stars are visible to
the naked eye from Earth during the night,
appearing as a multitude of fixed luminous
points in the sky due to their immense distance
from Earth. Historically, the most prominent
stars were grouped into constellations and
asterisms, the brightest of which gained proper
names. Astronomers have assembled star catalogues
that identify the known stars and provide
standardized stellar designations. However,
most of the stars in the Universe, including
all stars outside our galaxy, the Milky Way,
are invisible to the naked eye from Earth.
Indeed, most are invisible from Earth even
through the most powerful telescopes.
For at least a portion of its life, a star
shines due to thermonuclear fusion of hydrogen
into helium in its core, releasing energy
that traverses the star's interior and then
radiates into outer space. Almost all naturally
occurring elements heavier than helium are
created by stellar nucleosynthesis during
the star's lifetime, and for some stars by
supernova nucleosynthesis when it explodes.
Near the end of its life, a star can also
contain degenerate matter. Astronomers can
determine the mass, age, metallicity (chemical
composition), and many other properties of
a star by observing its motion through space,
its luminosity, and spectrum respectively.
The total mass of a star is the main factor
that determines its evolution and eventual
fate. Other characteristics of a star, including
diameter and temperature, change over its
life, while the star's environment affects
its rotation and movement. A plot of the temperature
of many stars against their luminosities produces
a plot known as a Hertzsprung–Russell diagram
(H–R diagram). Plotting a particular star
on that diagram allows the age and evolutionary
state of that star to be determined.
A star's life begins with the gravitational
collapse of a gaseous nebula of material composed
primarily of hydrogen, along with helium and
trace amounts of heavier elements. When the
stellar core is sufficiently dense, hydrogen
becomes steadily converted into helium through
nuclear fusion, releasing energy in the process.
The remainder of the star's interior carries
energy away from the core through a combination
of radiative and convective heat transfer
processes. The star's internal pressure prevents
it from collapsing further under its own gravity.
A star with mass greater than 0.4 times the
Sun's will expand to become a red giant when
the hydrogen fuel in its core is exhausted.
In some cases, it will fuse heavier elements
at the core or in shells around the core.
As the star expands it throws a part of its
mass, enriched with those heavier elements,
into the interstellar environment, to be recycled
later as new stars. Meanwhile, the core becomes
a stellar remnant: a white dwarf, a neutron
star, or if it is sufficiently massive a black
hole.
Binary and multi-star systems consist of two
or more stars that are gravitationally bound
and generally move around each other in stable
orbits. When two such stars have a relatively
close orbit, their gravitational interaction
can have a significant impact on their evolution.
Stars can form part of a much larger gravitationally
bound structure, such as a star cluster or
a galaxy.
== Observation history ==
Historically, stars have been important to
civilizations throughout the world. They have
been part of religious practices and used
for celestial navigation and orientation.
Many ancient astronomers believed that stars
were permanently affixed to a heavenly sphere
and that they were immutable. By convention,
astronomers grouped stars into constellations
and used them to track the motions of the
planets and the inferred position of the Sun.
The motion of the Sun against the background
stars (and the horizon) was used to create
calendars, which could be used to regulate
agricultural practices. The Gregorian calendar,
currently used nearly everywhere in the world,
is a solar calendar based on the angle of
the Earth's rotational axis relative to its
local star, the Sun.
The oldest accurately dated star chart was
the result of ancient Egyptian astronomy in
1534 BC. The earliest known star catalogues
were compiled by the ancient Babylonian astronomers
of Mesopotamia in the late 2nd millennium
BC, during the Kassite Period (ca. 1531–1155
BC).The first star catalogue in Greek astronomy
was created by Aristillus in approximately
300 BC, with the help of Timocharis. The star
catalog of Hipparchus (2nd century BC) included
1020 stars, and was used to assemble Ptolemy's
star catalogue. Hipparchus is known for the
discovery of the first recorded nova (new
star). Many of the constellations and star
names in use today derive from Greek astronomy.
In spite of the apparent immutability of the
heavens, Chinese astronomers were aware that
new stars could appear. In 185 AD, they were
the first to observe and write about a supernova,
now known as the SN 185. The brightest stellar
event in recorded history was the SN 1006
supernova, which was observed in 1006 and
written about by the Egyptian astronomer Ali
ibn Ridwan and several Chinese astronomers.
The SN 1054 supernova, which gave birth to
the Crab Nebula, was also observed by Chinese
and Islamic astronomers.Medieval Islamic astronomers
gave Arabic names to many stars that are still
used today and they invented numerous astronomical
instruments that could compute the positions
of the stars. They built the first large observatory
research institutes, mainly for the purpose
of producing Zij star catalogues. Among these,
the Book of Fixed Stars (964) was written
by the Persian astronomer Abd al-Rahman al-Sufi,
who observed a number of stars, star clusters
(including the Omicron Velorum and Brocchi's
Clusters) and galaxies (including the Andromeda
Galaxy). According to A. Zahoor, in the 11th
century, the Persian polymath scholar Abu
Rayhan Biruni described the Milky Way galaxy
as a multitude of fragments having the properties
of nebulous stars, and also gave the latitudes
of various stars during a lunar eclipse in
1019.According to Josep Puig, the Andalusian
astronomer Ibn Bajjah proposed that the Milky
Way was made up of many stars that almost
touched one another and appeared to be a continuous
image due to the effect of refraction from
sublunary material, citing his observation
of the conjunction of Jupiter and Mars on
500 AH (1106/1107 AD) as evidence.
Early European astronomers such as Tycho Brahe
identified new stars in the night sky (later
termed novae), suggesting that the heavens
were not immutable. In 1584, Giordano Bruno
suggested that the stars were like the Sun,
and may have other planets, possibly even
Earth-like, in orbit around them, an idea
that had been suggested earlier by the ancient
Greek philosophers, Democritus and Epicurus,
and by medieval Islamic cosmologists such
as Fakhr al-Din al-Razi. By the following
century, the idea of the stars being the same
as the Sun was reaching a consensus among
astronomers. To explain why these stars exerted
no net gravitational pull on the Solar System,
Isaac Newton suggested that the stars were
equally distributed in every direction, an
idea prompted by the theologian Richard Bentley.The
Italian astronomer Geminiano Montanari recorded
observing variations in luminosity of the
star Algol in 1667. Edmond Halley published
the first measurements of the proper motion
of a pair of nearby "fixed" stars, demonstrating
that they had changed positions since the
time of the ancient Greek astronomers Ptolemy
and Hipparchus.William Herschel was the first
astronomer to attempt to determine the distribution
of stars in the sky. During the 1780s, he
established a series of gauges in 600 directions
and counted the stars observed along each
line of sight. From this he deduced that the
number of stars steadily increased toward
one side of the sky, in the direction of the
Milky Way core. His son John Herschel repeated
this study in the southern hemisphere and
found a corresponding increase in the same
direction. In addition to his other accomplishments,
William Herschel is also noted for his discovery
that some stars do not merely lie along the
same line of sight, but are also physical
companions that form binary star systems.
The science of stellar spectroscopy was pioneered
by Joseph von Fraunhofer and Angelo Secchi.
By comparing the spectra of stars such as
Sirius to the Sun, they found differences
in the strength and number of their absorption
lines—the dark lines in stellar spectra
caused by the atmosphere's absorption of specific
frequencies. In 1865, Secchi began classifying
stars into spectral types. However, the modern
version of the stellar classification scheme
was developed by Annie J. Cannon during the
1900s.
The first direct measurement of the distance
to a star (61 Cygni at 11.4 light-years) was
made in 1838 by Friedrich Bessel using the
parallax technique. Parallax measurements
demonstrated the vast separation of the stars
in the heavens. Observation of double stars
gained increasing importance during the 19th
century. In 1834, Friedrich Bessel observed
changes in the proper motion of the star Sirius
and inferred a hidden companion. Edward Pickering
discovered the first spectroscopic binary
in 1899 when he observed the periodic splitting
of the spectral lines of the star Mizar in
a 104-day period. Detailed observations of
many binary star systems were collected by
astronomers such as Friedrich Georg Wilhelm
von Struve and S. W. Burnham, allowing the
masses of stars to be determined from computation
of orbital elements. The first solution to
the problem of deriving an orbit of binary
stars from telescope observations was made
by Felix Savary in 1827.
The twentieth century saw increasingly rapid
advances in the scientific study of stars.
The photograph became a valuable astronomical
tool. Karl Schwarzschild discovered that the
color of a star and, hence, its temperature,
could be determined by comparing the visual
magnitude against the photographic magnitude.
The development of the photoelectric photometer
allowed precise measurements of magnitude
at multiple wavelength intervals. In 1921
Albert A. Michelson made the first measurements
of a stellar diameter using an interferometer
on the Hooker telescope at Mount Wilson Observatory.Important
theoretical work on the physical structure
of stars occurred during the first decades
of the twentieth century. In 1913, the Hertzsprung-Russell
diagram was developed, propelling the astrophysical
study of stars. Successful models were developed
to explain the interiors of stars and stellar
evolution. Cecilia Payne-Gaposchkin first
proposed that stars were made primarily of
hydrogen and helium in her 1925 PhD thesis.
The spectra of stars were further understood
through advances in quantum physics. This
allowed the chemical composition of the stellar
atmosphere to be determined.
With the exception of supernovae, individual
stars have primarily been observed in the
Local Group, and especially in the visible
part of the Milky Way (as demonstrated by
the detailed star catalogues available for
our
galaxy). But some stars have been observed
in the M100 galaxy of the Virgo Cluster, about
100 million light years from the Earth.
In the Local Supercluster it is possible to
see star clusters, and current telescopes
could in principle observe faint individual
stars in the Local Group (see Cepheids). However,
outside the Local Supercluster of galaxies,
neither individual stars nor clusters of stars
have been observed. The only exception is
a faint image of a large star cluster containing
hundreds of thousands of stars located at
a distance of one billion light years—ten
times further than the most distant star cluster
previously observed.
In February 2018, astronomers reported, for
the first time, a signal of the reionization
epoch, an indirect detection of light from
the earliest stars formed - about 180 million
years after the Big Bang.In April, 2018, astronomers
reported the detection of the most distant
"ordinary" (i.e., main sequence) star, named
Icarus (formally, MACS J1149 Lensed Star 1),
at 9 billion light-years away from Earth.In
May 2018, astronomers reported the detection
of the most distant oxygen ever detected in
the Universe - and the most distant galaxy
ever observed by Atacama Large Millimeter
Array or the Very Large Telescope - with the
team inferring that the signal was emitted
13.3 billion years ago (or 500 million years
after the Big Bang. They found that the observed
brightness of the galaxy is well-explained
by a model where the onset of star formation
corresponds to only 250 million years after
the Universe began, corresponding to a redshift
of about 15.
== Designations ==
The concept of a constellation was known to
exist during the Babylonian period. Ancient
sky watchers imagined that prominent arrangements
of stars formed patterns, and they associated
these with particular aspects of nature or
their myths. Twelve of these formations lay
along the band of the ecliptic and these became
the basis of astrology. Many of the more prominent
individual stars were also given names, particularly
with Arabic or Latin designations.
As well as certain constellations and the
Sun itself, individual stars have their own
myths. To the Ancient Greeks, some "stars",
known as planets (Greek πλανήτης (planētēs),
meaning "wanderer"), represented various important
deities, from which the names of the planets
Mercury, Venus, Mars, Jupiter and Saturn were
taken. (Uranus and Neptune were also Greek
and Roman gods, but neither planet was known
in Antiquity because of their low brightness.
Their names were assigned by later astronomers.)
Circa 1600, the names of the constellations
were used to name the stars in the corresponding
regions of the sky. The German astronomer
Johann Bayer created a series of star maps
and applied Greek letters as designations
to the stars in each constellation. Later
a numbering system based on the star's right
ascension was invented and added to John Flamsteed's
star catalogue in his book "Historia coelestis
Britannica" (the 1712 edition), whereby this
numbering system came to be called Flamsteed
designation or Flamsteed numbering.The only
internationally recognized authority for naming
celestial bodies is the International Astronomical
Union (IAU). The International Astronomical
Union maintains the Working Group on Star
Names (WGSN) which catalogs and standardizes
proper names for stars. A number of private
companies sell names of stars, which the British
Library calls an unregulated commercial enterprise.
The IAU has disassociated itself from this
commercial practice, and these names are neither
recognized by the IAU, professional astronomers,
nor the amateur astronomy community. One such
star-naming company is the International Star
Registry, which, during the 1980s, was accused
of deceptive practice for making it appear
that the assigned name was official. This
now-discontinued ISR practice was informally
labeled a scam and a fraud, and the New York
City Department of Consumer Affairs issued
a violation against ISR for engaging in a
deceptive trade practice.
== Units of measurement ==
Although stellar parameters can be expressed
in SI units or CGS units, it is often most
convenient to express mass, luminosity, and
radii in solar units, based on the characteristics
of the Sun. In 2015, the IAU defined a set
of nominal solar values (defined as SI constants,
without uncertainties) which can be used for
quoting stellar parameters:
The solar mass M⊙ was not explicitly defined
by the IAU due to the large relative uncertainty
(10−4) of the Newtonian gravitational constant
G. However, since the product of the Newtonian
gravitational constant and solar mass
together (GM⊙) has been determined to much
greater precision, the IAU defined the nominal
solar mass parameter to be:
However, one can combine the nominal solar
mass parameter with the most recent (2014)
CODATA estimate of the Newtonian gravitational
constant G to derive the solar mass to be
approximately 1.9885 × 1030 kg. Although
the exact values for the luminosity, radius,
mass parameter, and mass may vary slightly
in the future due to observational uncertainties,
the 2015 IAU nominal constants will remain
the same SI values as they remain useful measures
for quoting stellar parameters.
Large lengths, such as the radius of a giant
star or the semi-major axis of a binary star
system, are often expressed in terms of the
astronomical unit — approximately equal
to the mean distance between the Earth and
the Sun (150 million km or approximately 93
million miles). In 2012, the IAU defined the
astronomical constant to be an exact length
in meters: 149,597,870,700 m.
== Formation and evolution ==
Stars condense from regions of space of higher
matter density, yet those regions are less
dense than within a vacuum chamber. These
regions – known as molecular clouds – consist
mostly of hydrogen, with about 23 to 28 percent
helium and a few percent heavier elements.
One example of such a star-forming region
is the Orion Nebula. Most stars form in groups
of dozens to hundreds of thousands of stars.Massive
stars in these groups may powerfully illuminate
those clouds, ionizing the hydrogen, and creating
H II regions. Such feedback effects, from
star formation, may ultimately disrupt the
cloud and prevent further star formation.
All stars spend the majority of their existence
as main sequence stars, fueled primarily by
the nuclear fusion of hydrogen into helium
within their cores. However, stars of different
masses have markedly different properties
at various stages of their development. The
ultimate fate of more massive stars differs
from that of less massive stars, as do their
luminosities and the impact they have on their
environment. Accordingly, astronomers often
group stars by their mass:
Very low mass stars, with masses below 0.5
M☉, are fully convective and distribute
helium evenly throughout the whole star while
on the main sequence. Therefore, they never
undergo shell burning, never become red giants,
which cease fusing and become helium white
dwarfs and slowly cool after exhausting their
hydrogen. However, as the lifetime of 0.5
M☉ stars is longer than the age of the universe,
no such star has yet reached the white dwarf
stage.
Low mass stars (including the Sun), with a
mass between 0.5 M☉ and 1.8–2.5 M☉ depending
on composition, do become red giants as their
core hydrogen is depleted and they begin to
burn helium in core in a helium flash; they
develop a degenerate carbon-oxygen core later
on the asymptotic giant branch; they finally
blow off their outer shell as a planetary
nebula and leave behind their core in the
form of a white dwarf.
Intermediate-mass stars, between 1.8–2.5
M☉ and 5–10 M☉, pass through evolutionary
stages similar to low mass stars, but after
a relatively short period on the red giant
branch they ignite helium without a flash
and spend an extended period in the red clump
before forming a degenerate carbon-oxygen
core.
Massive stars generally have a minimum mass
of 7–10 M☉ (possibly as low as 5–6 M☉).
After exhausting the hydrogen at the core
these stars become supergiants and go on to
fuse elements heavier than helium. They end
their lives when their cores collapse and
they explode as supernovae.
=== Star formation ===
The formation of a star begins with gravitational
instability within a molecular cloud, caused
by regions of higher density – often triggered
by compression of clouds by radiation from
massive stars, expanding bubbles in the interstellar
medium, the collision of different molecular
clouds, or the collision of galaxies (as in
a starburst galaxy). When a region reaches
a sufficient density of matter to satisfy
the criteria for Jeans instability, it begins
to collapse under its own gravitational force.
As the cloud collapses, individual conglomerations
of dense dust and gas form "Bok globules".
As a globule collapses and the density increases,
the gravitational energy converts into heat
and the temperature rises. When the protostellar
cloud has approximately reached the stable
condition of hydrostatic equilibrium, a protostar
forms at the core. These pre-main-sequence
stars are often surrounded by a protoplanetary
disk and powered mainly by the conversion
of gravitational energy. The period of gravitational
contraction lasts about 10 to 15 million years.
Early stars of less than 2 M☉ are called
T Tauri stars, while those with greater mass
are Herbig Ae/Be stars. These newly formed
stars emit jets of gas along their axis of
rotation, which may reduce the angular momentum
of the collapsing star and result in small
patches of nebulosity known as Herbig–Haro
objects.
These jets, in combination with radiation
from nearby massive stars, may help to drive
away the surrounding cloud from which the
star was formed.Early in their development,
T Tauri stars follow the Hayashi track—they
contract and decrease in luminosity while
remaining at roughly the same temperature.
Less massive T Tauri stars follow this track
to the main sequence, while more massive stars
turn onto the Henyey track.
Most stars are observed to be members of binary
star systems, and the properties of those
binaries are the result of the conditions
in which they formed. A gas cloud must lose
its angular momentum in order to collapse
and form a star. The fragmentation of the
cloud into multiple stars distributes some
of that angular momentum. The primordial binaries
transfer some angular momentum by gravitational
interactions during close encounters with
other stars in young stellar clusters. These
interactions tend to split apart more widely
separated (soft) binaries while causing hard
binaries to become more tightly bound. This
produces the separation of binaries into their
two observed populations distributions.
=== Main sequence ===
Stars spend about 90% of their existence fusing
hydrogen into helium in high-temperature and
high-pressure reactions near the core. Such
stars are said to be on the main sequence,
and are called dwarf stars. Starting at zero-age
main sequence, the proportion of helium in
a star's core will steadily increase, the
rate of nuclear fusion at the core will slowly
increase, as will the star's temperature and
luminosity.
The Sun, for example, is estimated to have
increased in luminosity by about 40% since
it reached the main sequence 4.6 billion (4.6
× 109) years ago.Every star generates a stellar
wind of particles that causes a continual
outflow of gas into space. For most stars,
the mass lost is negligible. The Sun loses
10−14 M☉ every year, or about 0.01% of
its total mass over its entire lifespan. However,
very massive stars can lose 10−7 to 10−5
M☉ each year, significantly affecting their
evolution. Stars that begin with more than
50 M☉ can lose over half their total mass
while on the main sequence.
The time a star spends on the main sequence
depends primarily on the amount of fuel it
has and the rate at which it fuses it. The
Sun is expected to live 10 billion (1010)
years. Massive stars consume their fuel very
rapidly and are short-lived. Low mass stars
consume their fuel very slowly. Stars less
massive than 0.25 M☉, called red dwarfs,
are able to fuse nearly all of their mass
while stars of about 1 M☉ can only fuse
about 10% of their mass. The combination of
their slow fuel-consumption and relatively
large usable fuel supply allows low mass stars
to last about one trillion (1012) years; the
most extreme of 0.08 M☉) will last for about
12 trillion years. Red dwarfs become hotter
and more luminous as they accumulate helium.
When they eventually run out of hydrogen,
they contract into a white dwarf and decline
in temperature. However, since the lifespan
of such stars is greater than the current
age of the universe (13.8 billion years),
no stars under about 0.85 M☉ are expected
to have moved off the main sequence.
Besides mass, the elements heavier than helium
can play a significant role in the evolution
of stars. Astronomers label all elements heavier
than helium "metals", and call the chemical
concentration of these elements in a star,
its metallicity. A star's metallicity can
influence the time the star takes to burn
its fuel, and controls the formation of its
magnetic fields, which affects the strength
of its stellar wind. Older, population II
stars have substantially less metallicity
than the younger, population I stars due to
the composition of the molecular clouds from
which they formed. Over time, such clouds
become increasingly enriched in heavier elements
as older stars die and shed portions of their
atmospheres.
=== Post–main sequence ===
As stars of at least 0.4 M☉ exhaust their
supply of hydrogen at their core, they start
to fuse hydrogen in a shell outside the helium
core. Their outer layers expand and cool greatly
as they form a red giant. In about 5 billion
years, when the Sun enters the helium burning
phase, it will expand to a maximum radius
of roughly 1 astronomical unit (150 million
kilometres), 250 times its present size, and
lose 30% of its current mass.As the hydrogen
shell burning produces more helium, the core
increases in mass and temperature. In a red
giant of up to 2.25 M☉, the mass of the
helium core becomes degenerate prior to helium
fusion. Finally, when the temperature increases
sufficiently, helium fusion begins explosively
in what is called a helium flash, and the
star rapidly shrinks in radius, increases
its surface temperature, and moves to the
horizontal branch of the HR diagram. For more
massive stars, helium core fusion starts before
the core becomes degenerate, and the star
spends some time in the red clump, slowly
burning helium, before the outer convective
envelope collapses and the star then moves
to the horizontal branch.After the star has
fused the helium of its core, the carbon product
fuses producing a hot core with an outer shell
of fusing helium. The star then follows an
evolutionary path called the asymptotic giant
branch (AGB) that parallels the other described
red giant phase, but with a higher luminosity.
The more massive AGB stars may undergo a brief
period of carbon fusion before the core becomes
degenerate.
==== Massive stars ====
During their helium-burning phase, a star
of more than 9 solar masses expands to form
first a blue and then a red supergiant. Particularly
massive stars may evolve to a Wolf-Rayet star,
characterised by spectra dominated by emission
lines of elements heavier than hydrogen, which
have reached the surface due to strong convection
and intense mass loss.
When helium is exhausted at the core of a
massive star, the core contracts and the temperature
and pressure rises enough to fuse carbon (see
Carbon-burning process). This process continues,
with the successive stages being fueled by
neon (see neon-burning process), oxygen (see
oxygen-burning process), and silicon (see
silicon-burning process). Near the end of
the star's life, fusion continues along a
series of onion-layer shells within a massive
star. Each shell fuses a different element,
with the outermost shell fusing hydrogen;
the next shell fusing helium, and so forth.The
final stage occurs when a massive star begins
producing iron. Since iron nuclei are more
tightly bound than any heavier nuclei, any
fusion beyond iron does not produce a net
release of energy. To a very limited degree
such a process proceeds, but it consumes energy.
Likewise, since they are more tightly bound
than all lighter nuclei, such energy cannot
be released by fission.
==== Collapse ====
As a star's core shrinks, the intensity of
radiation from that surface increases, creating
such radiation pressure on the outer shell
of gas that it will push those layers away,
forming a planetary nebula. If what remains
after the outer atmosphere has been shed is
less than 1.4 M☉, it shrinks to a relatively
tiny object about the size of Earth, known
as a white dwarf. White dwarfs lack the mass
for further gravitational compression to take
place. The electron-degenerate matter inside
a white dwarf is no longer a plasma, even
though stars are generally referred to as
being spheres of plasma. Eventually, white
dwarfs fade into black dwarfs over a very
long period of time.
In massive stars, fusion continues until the
iron core has grown so large (more than 1.4
M☉) that it can no longer support its own
mass. This core will suddenly collapse as
its electrons are driven into its protons,
forming neutrons, neutrinos, and gamma rays
in a burst of electron capture and inverse
beta decay. The shockwave formed by this sudden
collapse causes the rest of the star to explode
in a supernova. Supernovae become so bright
that they may briefly outshine the star's
entire home galaxy. When they occur within
the Milky Way, supernovae have historically
been observed by naked-eye observers as "new
stars" where none seemingly existed before.A
supernova explosion blows away the star's
outer layers, leaving a remnant such as the
Crab Nebula. The core is compressed into a
neutron star, which sometimes manifests itself
as a pulsar or X-ray burster. In the case
of the largest stars, the remnant is a black
hole greater than 4 M☉. In a neutron star
the matter is in a state known as neutron-degenerate
matter, with a more exotic form of degenerate
matter, QCD matter, possibly present in the
core. Within a black hole, the matter is in
a state that is not currently understood.
The blown-off outer layers of dying stars
include heavy elements, which may be recycled
during the formation of new stars. These heavy
elements allow the formation of rocky planets.
The outflow from supernovae and the stellar
wind of large stars play an important part
in shaping the interstellar medium.
==== Binary stars ====
The post–main-sequence evolution of binary
stars may be significantly different from
the evolution of single stars of the same
mass. If stars in a binary system are sufficiently
close, when one of the stars expands to become
a red giant it may overflow its Roche lobe,
the region around a star where material is
gravitationally bound to that star, leading
to transfer of material to the other. When
the Roche lobe is violated, a variety of phenomena
can result, including contact binaries, common-envelope
binaries, cataclysmic variables, and type
Ia supernovae.
== Distribution ==
Stars are not spread uniformly across the
universe, but are normally grouped into galaxies
along with interstellar gas and dust. A typical
galaxy contains hundreds of billions of stars,
and there are more than 100 billion (1011)
galaxies in the observable universe. In 2010,
one estimate of the number of stars in the
observable universe was 300 sextillion (3
× 1023). While it is often believed that
stars only exist within galaxies, intergalactic
stars have been discovered.A multi-star system
consists of two or more gravitationally bound
stars that orbit each other. The simplest
and most common multi-star system is a binary
star, but systems of three or more stars are
also found. For reasons of orbital stability,
such multi-star systems are often organized
into hierarchical sets of binary stars. Larger
groups called star clusters also exist. These
range from loose stellar associations with
only a few stars, up to enormous globular
clusters with hundreds of thousands of stars.
Such systems orbit their host galaxy.
It has been a long-held assumption that the
majority of stars occur in gravitationally
bound, multiple-star systems. This is particularly
true for very massive O and B class stars,
where 80% of the stars are believed to be
part of multiple-star systems. The proportion
of single star systems increases with decreasing
star mass, so that only 25% of red dwarfs
are known to have stellar companions. As 85%
of all stars are red dwarfs, most stars in
the Milky Way are likely single from birth.
The nearest star to the Earth, apart from
the Sun, is Proxima Centauri, which is 39.9
trillion kilometres, or 4.2 light-years. Travelling
at the orbital speed of the Space Shuttle
(8 kilometres per second—almost 30,000 kilometres
per hour), it would take about 150,000 years
to arrive. This is typical of stellar separations
in galactic discs. Stars can be much closer
to each other in the centres of galaxies and
in globular clusters, or much farther apart
in galactic halos.
Due to the relatively vast distances between
stars outside the galactic nucleus, collisions
between stars are thought to be rare. In denser
regions such as the core of globular clusters
or the galactic center, collisions can be
more common. Such collisions can produce what
are known as blue stragglers. These abnormal
stars have a higher surface temperature than
the other main sequence stars with the same
luminosity of the cluster to which it belongs.
== Characteristics ==
Almost everything about a star is determined
by its initial mass, including such characteristics
as luminosity, size, evolution, lifespan,
and its eventual fate.
=== Age ===
Most stars are between 1 billion and 10 billion
years old. Some stars may even be close to
13.8 billion years old—the observed age
of the universe. The oldest star yet discovered,
HD 140283, nicknamed Methuselah star, is an
estimated 14.46 ± 0.8 billion years old.
(Due to the uncertainty in the value, this
age for the star does not conflict with the
age of the Universe, determined by the Planck
satellite as 13.799 ± 0.021).The more massive
the star, the shorter its lifespan, primarily
because massive stars have greater pressure
on their cores, causing them to burn hydrogen
more rapidly. The most massive stars last
an average of a few million years, while stars
of minimum mass (red dwarfs) burn their fuel
very slowly and can last tens to hundreds
of billions of years.
=== Chemical composition ===
When stars form in the present Milky Way galaxy
they are composed of about 71% hydrogen and
27% helium, as measured by mass, with a small
fraction of heavier elements. Typically the
portion of heavy elements is measured in terms
of the iron content of the stellar atmosphere,
as iron is a common element and its absorption
lines are relatively easy to measure. The
portion of heavier elements may be an indicator
of the likelihood that the star has a planetary
system.The star with the lowest iron content
ever measured is the dwarf HE1327-2326, with
only 1/200,000th the iron content of the Sun.
By contrast, the super-metal-rich star μ
Leonis has nearly double the abundance of
iron as the Sun, while the planet-bearing
star 14 Herculis has nearly triple the iron.
There also exist chemically peculiar stars
that show unusual abundances of certain elements
in their spectrum; especially chromium and
rare earth elements. Stars with cooler outer
atmospheres, including the Sun, can form various
diatomic and polyatomic molecules.
=== Diameter ===
Due to their great distance from the Earth,
all stars except the Sun appear to the unaided
eye as shining points in the night sky that
twinkle because of the effect of the Earth's
atmosphere. The Sun is also a star, but it
is close enough to the Earth to appear as
a disk instead, and to provide daylight. Other
than the Sun, the star with the largest apparent
size is R Doradus, with an angular diameter
of only 0.057 arcseconds.The disks of most
stars are much too small in angular size to
be observed with current ground-based optical
telescopes, and so interferometer telescopes
are required to produce images of these objects.
Another technique for measuring the angular
size of stars is through occultation. By precisely
measuring the drop in brightness of a star
as it is occulted by the Moon (or the rise
in brightness when it reappears), the star's
angular diameter can be computed.Stars range
in size from neutron stars, which vary anywhere
from 20 to 40 km (25 mi) in diameter, to supergiants
like Betelgeuse in the Orion constellation,
which has a diameter about 1,000 times that
of our sun. Betelgeuse, however, has a much
lower density than the Sun.
=== Kinematics ===
The motion of a star relative to the Sun can
provide useful information about the origin
and age of a star, as well as the structure
and evolution of the surrounding galaxy. The
components of motion of a star consist of
the radial velocity toward or away from the
Sun, and the traverse angular movement, which
is called its proper motion.
Radial velocity is measured by the doppler
shift of the star's spectral lines, and is
given in units of km/s. The proper motion
of a star, its parallax, is determined by
precise astrometric measurements in units
of milli-arc seconds (mas) per year. With
knowledge of the star's parallax and its distance,
the proper motion velocity can be calculated.
Together with the radial velocity, the total
velocity can be calculated. Stars with high
rates of proper motion are likely to be relatively
close to the Sun, making them good candidates
for parallax measurements.When both rates
of movement are known, the space velocity
of the star relative to the Sun or the galaxy
can be computed. Among nearby stars, it has
been found that younger population I stars
have generally lower velocities than older,
population II stars. The latter have elliptical
orbits that are inclined to the plane of the
galaxy. A comparison of the kinematics of
nearby stars has allowed astronomers to trace
their origin to common points in giant molecular
clouds, and are referred to as stellar associations.
=== Magnetic field ===
The magnetic field of a star is generated
within regions of the interior where convective
circulation occurs. This movement of conductive
plasma functions like a dynamo, wherein the
movement of electrical charges induce magnetic
fields, as does a mechanical dynamo. Those
magnetic fields have a great range that extend
throughout and beyond the star. The strength
of the magnetic field varies with the mass
and composition of the star, and the amount
of magnetic surface activity depends upon
the star's rate of rotation. This surface
activity produces starspots, which are regions
of strong magnetic fields and lower than normal
surface temperatures. Coronal loops are arching
magnetic field flux lines that rise from a
star's surface into the star's outer atmosphere,
its corona. The coronal loops can be seen
due to the plasma they conduct along their
length. Stellar flares are bursts of high-energy
particles that are emitted due to the same
magnetic activity.Young, rapidly rotating
stars tend to have high levels of surface
activity because of their magnetic field.
The magnetic field can act upon a star's stellar
wind, functioning as a brake to gradually
slow the rate of rotation with time. Thus,
older stars such as the Sun have a much slower
rate of rotation and a lower level of surface
activity. The activity levels of slowly rotating
stars tend to vary in a cyclical manner and
can shut down altogether for periods of time.
During
the Maunder Minimum, for example, the Sun
underwent a
70-year period with almost no sunspot activity.
=== Mass ===
One of the most massive stars known is Eta
Carinae, which,
with 100–150 times as much mass as the Sun,
will have a lifespan of only several million
years. Studies of the most massive open clusters
suggests 150 M☉ as an upper limit for stars
in the current era of the universe. This
represents an empirical value for the theoretical
limit on the mass of forming stars due to
increasing radiation pressure on the accreting
gas cloud. Several stars in the R136 cluster
in the Large Magellanic Cloud have been measured
with larger masses, but
it has been determined that they could have
been created through the collision and merger
of massive stars in close binary systems,
sidestepping the 150 M☉ limit on massive
star formation.
The first stars to form after the Big Bang
may have been larger, up to 300 M☉, due
to the complete absence of elements heavier
than lithium in their composition. This generation
of supermassive population III stars is likely
to have existed in the very early universe
(i.e., they are observed to have a high redshift),
and may have started the production of chemical
elements heavier than hydrogen that are needed
for the later formation of planets and life.
In June 2015, astronomers reported evidence
for Population III stars in the Cosmos Redshift
7 galaxy at z = 6.60.With a mass only 80 times
that of Jupiter (MJ), 2MASS J0523-1403 is
the smallest known star undergoing nuclear
fusion in its core. For
stars with metallicity similar to the Sun,
the theoretical minimum mass the star can
have and still undergo fusion at the core,
is estimated to be about 75 MJ. When the metallicity
is very low, however, the minimum star size
seems to be about 8.3% of the solar mass,
or about 87 MJ. Smaller bodies called brown
dwarfs, occupy a poorly defined grey area
between stars and gas giants.
The combination of the radius and the mass
of a star determines its surface gravity.
Giant stars have a much lower surface gravity
than do main sequence stars, while the opposite
is the case for degenerate, compact stars
such as white dwarfs. The surface gravity
can influence the appearance of a star's spectrum,
with higher gravity causing a broadening of
the absorption lines.
=== Rotation ===
The rotation rate of stars can be determined
through spectroscopic measurement, or more
exactly determined by tracking their starspots.
Young stars can have a rotation greater than
100 km/s at the equator. The B-class star
Achernar, for example, has an equatorial velocity
of about 225 km/s or greater, causing its
equator to bulge outward and giving it an
equatorial diameter that is more than 50%
greater than between the poles. This rate
of rotation is just below the critical velocity
of 300 km/s at which speed the star would
break apart. By contrast, the Sun rotates
once every 25–35 days depending on latitude,
with an equatorial velocity of 1.93 km/s.
A main sequence star's magnetic field and
the stellar wind serve to slow its rotation
by a significant amount as it evolves on the
main sequence.Degenerate stars have contracted
into a compact mass, resulting in a rapid
rate of rotation. However they have relatively
low rates of rotation compared to what would
be expected by conservation of angular momentum—the
tendency of a rotating body to compensate
for a contraction in size by increasing its
rate of spin. A large portion of the star's
angular momentum is dissipated as a result
of mass loss through the stellar wind. In
spite of this, the rate of rotation for a
pulsar can be very rapid. The pulsar at the
heart of the Crab nebula, for example, rotates
30 times per second. The rotation rate of
the pulsar will gradually slow due to the
emission of radiation.
=== Temperature ===
The surface temperature of a main sequence
star is determined by the rate of energy production
of its core and by its radius, and is often
estimated from the star's color index. The
temperature is normally given in terms of
an effective temperature, which is the temperature
of an idealized black body that radiates its
energy at the same luminosity per surface
area as the star. Note that the effective
temperature is only a representative of the
surface, as the temperature increases toward
the core. The temperature in the core region
of a star is several million kelvins.The stellar
temperature will determine the rate of ionization
of various elements, resulting in characteristic
absorption lines in the spectrum. The surface
temperature of a star, along with its visual
absolute magnitude and absorption features,
is used to classify a star (see classification
below).Massive main sequence stars can have
surface temperatures of 50,000 K. Smaller
stars such as the Sun have surface temperatures
of a few thousand K. Red giants have relatively
low surface temperatures of about 3,600 K;
but they also have a high luminosity due to
their large exterior surface area.
== Radiation ==
The energy produced by stars, a product of
nuclear fusion, radiates to space as both
electromagnetic radiation and particle radiation.
The particle radiation emitted by a star is
manifested as the stellar wind, which
streams from the outer layers as electrically
charged protons and alpha and beta particles.
Although almost massless, there also exists
a steady stream of neutrinos emanating from
the star's core.
The production of energy at the core is the
reason stars shine so brightly: every time
two or more atomic nuclei fuse together to
form a single atomic nucleus of a new heavier
element, gamma ray photons are released from
the nuclear fusion product. This energy is
converted to other forms of electromagnetic
energy of lower frequency, such as visible
light, by the time it reaches the star's outer
layers.
The color of a star, as determined by the
most intense frequency of the visible light,
depends on the temperature of the star's outer
layers, including its photosphere. Besides
visible light, stars also emit forms of electromagnetic
radiation that are invisible to the human
eye. In fact, stellar electromagnetic radiation
spans the entire electromagnetic spectrum,
from the longest wavelengths of radio waves
through infrared, visible light, ultraviolet,
to the shortest of X-rays, and gamma rays.
From the standpoint of total energy emitted
by a star, not all components of stellar electromagnetic
radiation are significant, but all frequencies
provide insight into the star's physics.
Using the stellar spectrum, astronomers can
also determine the surface temperature, surface
gravity, metallicity and rotational velocity
of a star. If the distance of the star is
found, such as by measuring the parallax,
then the luminosity of the star can be derived.
The mass, radius, surface gravity, and rotation
period can then be estimated based on stellar
models. (Mass can be calculated for stars
in binary systems by measuring their orbital
velocities and distances. Gravitational microlensing
has been used to measure the mass of a single
star.) With these parameters, astronomers
can also estimate the age of the star.
=== Luminosity ===
The luminosity of a star is the amount of
light and other forms of radiant energy it
radiates per unit of time. It has units of
power. The luminosity of a star is determined
by its radius and surface temperature. Many
stars do not radiate uniformly across their
entire surface. The rapidly rotating star
Vega, for example, has a higher energy flux
(power per unit area) at its poles than along
its equator.Patches of the star's surface
with a lower temperature and luminosity than
average are known as starspots. Small, dwarf
stars such as our Sun generally have essentially
featureless disks with only small starspots.
Giant stars have much larger, more obvious
starspots, and
they also exhibit strong stellar limb darkening.
That is, the brightness decreases towards
the edge of the stellar disk. Red
dwarf flare stars such as UV Ceti may also
possess prominent starspot features.
=== Magnitude ===
The apparent brightness of a star is expressed
in terms of its apparent magnitude. It is
a function of the star's luminosity, its distance
from Earth, the extinction effect of interstellar
dust and gas, and the altering of the star's
light as it passes through Earth's atmosphere.
Intrinsic or absolute magnitude is directly
related to a star's luminosity, and is what
the apparent magnitude a star would be if
the distance between the Earth and the star
were 10 parsecs (32.6 light-years).
Both the apparent and absolute magnitude scales
are logarithmic units: one whole number difference
in magnitude is equal to a brightness variation
of about 2.5 times (the 5th root of 100 or
approximately 2.512). This means that a first
magnitude star (+1.00) is about 2.5 times
brighter than a second magnitude (+2.00) star,
and about 100 times brighter than a sixth
magnitude star (+6.00). The faintest stars
visible to the naked eye under good seeing
conditions are about magnitude +6.
On both apparent and absolute magnitude scales,
the smaller the magnitude number, the brighter
the star; the larger the magnitude number,
the fainter the star. The brightest stars,
on either scale, have negative magnitude numbers.
The variation in brightness (ΔL) between
two stars is calculated by subtracting the
magnitude number of the brighter star (mb)
from the magnitude number of the fainter star
(mf), then using the difference as an exponent
for the base number 2.512; that is to say:
Δ
m
=
m
f
−
m
b
{\displaystyle \Delta {m}=m_{\mathrm {f} }-m_{\mathrm
{b} }}
2.512
Δ
m
=
Δ
L
{\displaystyle 2.512^{\Delta {m}}=\Delta {L}}
Relative to both luminosity and distance from
Earth, a star's absolute magnitude (M) and
apparent magnitude (m) are not equivalent;
for example, the bright star Sirius has an
apparent magnitude of −1.44, but it has
an absolute magnitude of +1.41.
The Sun has an apparent magnitude of −26.7,
but its absolute magnitude is only +4.83.
Sirius, the brightest star in the night sky
as seen from Earth, is approximately 23 times
more luminous than the Sun, while Canopus,
the second brightest star in the night sky
with an absolute magnitude of −5.53, is
approximately 14,000 times more luminous than
the Sun. Despite Canopus being vastly more
luminous than Sirius, however, Sirius appears
brighter than Canopus. This is because Sirius
is merely 8.6 light-years from the Earth,
while Canopus is much farther away at a distance
of 310 light-years.
As of 2006, the star with the highest known
absolute magnitude is LBV 1806-20, with a
magnitude of −14.2. This star is at least
5,000,000 times more luminous than the Sun.
The least luminous stars that are currently
known are located in the NGC 6397 cluster.
The faintest red dwarfs in the cluster were
magnitude 26, while a 28th magnitude white
dwarf was also discovered. These faint stars
are so dim that their light is as bright as
a birthday candle on the Moon when viewed
from the Earth.
== Classification ==
The current stellar classification system
originated in the early 20th century, when
stars were classified from A to Q based on
the strength of the hydrogen line. It was
thought that the hydrogen line strength was
a simple linear function of temperature. Instead,
it was more complicated: it strengthened with
increasing temperature, peaked near 9000 K,
and then declined at greater temperatures.
The classifications were since reordered by
temperature, on which the modern scheme is
based.Stars are given a single-letter classification
according to their spectra, ranging from type
O, which are very hot, to M, which are so
cool that molecules may form in their atmospheres.
The main classifications in order of decreasing
surface temperature are: O, B, A, F, G, K,
and M. A variety of rare spectral types are
given special classifications. The most common
of these are types L and T, which classify
the coldest low-mass stars and brown dwarfs.
Each letter has 10 sub-divisions, numbered
from 0 to 9, in order of decreasing temperature.
However, this system breaks down at extreme
high temperatures as classes O0 and O1 may
not exist.In addition, stars may be classified
by the luminosity effects found in their spectral
lines, which correspond to their spatial size
and is determined by their surface gravity.
These range from 0 (hypergiants) through III
(giants) to V (main sequence dwarfs); some
authors add VII (white dwarfs). Main sequence
stars fall along a narrow, diagonal band when
graphed according to their absolute magnitude
and spectral type. The Sun is a main sequence
G2V yellow dwarf of intermediate temperature
and ordinary size.
Additional nomenclature, in the form of lower-case
letters added to the end of the spectral type
to indicate peculiar features of the spectrum.
For example, an "e" can indicate the presence
of emission lines; "m" represents unusually
strong levels of metals, and "var" can mean
variations in the spectral type.White dwarf
stars have their own class that begins with
the letter D. This is further sub-divided
into the classes DA, DB, DC, DO, DZ, and DQ,
depending on the types of prominent lines
found in the spectrum. This is followed by
a numerical value that indicates the temperature.
== Variable stars ==
Variable stars have periodic or random changes
in luminosity because of intrinsic or extrinsic
properties. Of the intrinsically variable
stars, the primary types can be subdivided
into three principal groups.
During their stellar evolution, some stars
pass through phases where they can become
pulsating variables. Pulsating variable stars
vary in radius and luminosity over time, expanding
and contracting with periods ranging from
minutes to years, depending on the size of
the star. This category includes Cepheid and
Cepheid-like stars, and long-period variables
such as Mira.Eruptive variables are stars
that experience sudden increases in luminosity
because of flares or mass ejection events.
This group includes protostars, Wolf-Rayet
stars, and flare stars, as well as giant and
supergiant stars.
Cataclysmic or explosive variable stars are
those that undergo a dramatic change in their
properties. This group includes novae and
supernovae. A binary star system that includes
a nearby white dwarf can produce certain types
of these spectacular stellar explosions, including
the nova and a Type 1a supernova. The explosion
is created when the white dwarf accretes hydrogen
from the companion star, building up mass
until the hydrogen undergoes fusion. Some
novae are also recurrent, having periodic
outbursts of moderate amplitude.Stars can
also vary in luminosity because of extrinsic
factors, such as eclipsing binaries, as well
as rotating stars that produce extreme starspots.
A notable example of an eclipsing binary is
Algol, which regularly varies in magnitude
from 2.1 to 3.4 over a period of 2.87 days.
== Structure ==
The interior of a stable star is in a state
of hydrostatic equilibrium: the forces on
any small volume almost exactly counterbalance
each other. The balanced forces are inward
gravitational force and an outward force due
to the pressure gradient within the star.
The pressure gradient is established by the
temperature gradient of the plasma; the outer
part of the star is cooler than the core.
The temperature at the core of a main sequence
or giant star is at least on the order of
107 K. The resulting temperature and pressure
at the hydrogen-burning core of a main sequence
star are sufficient for nuclear fusion to
occur and for sufficient energy to be produced
to prevent further collapse of the star.As
atomic nuclei are fused in the core, they
emit energy in the form of gamma rays. These
photons interact with the surrounding plasma,
adding to the thermal energy at the core.
Stars on the main sequence convert hydrogen
into helium, creating a slowly but steadily
increasing proportion of helium in the core.
Eventually the helium content becomes predominant,
and energy production ceases at the core.
Instead, for stars of more than 0.4 M☉,
fusion occurs in a slowly expanding shell
around the degenerate helium core.In addition
to hydrostatic equilibrium, the interior of
a stable star will also maintain an energy
balance of thermal equilibrium. There is a
radial temperature gradient throughout the
interior that results in a flux of energy
flowing toward the exterior. The outgoing
flux of energy leaving any layer within the
star will exactly match the incoming flux
from below.
The radiation zone is the region of the stellar
interior where the flux of energy outward
is dependent on radiative heat transfer, since
convective heat transfer is inefficient in
that zone. In this region the plasma will
not be perturbed, and any mass motions will
die out. If this is not the case, however,
then the plasma becomes unstable and convection
will occur, forming a convection zone. This
can occur, for example, in regions where very
high energy fluxes occur, such as near the
core or in areas with high opacity (making
radiatative heat transfer inefficient) as
in the outer envelope.The occurrence of convection
in the outer envelope of a main sequence star
depends on the star's mass. Stars with several
times the mass of the Sun have a convection
zone deep within the interior and a radiative
zone in the outer layers. Smaller stars such
as the Sun are just the opposite, with the
convective zone located in the outer layers.
Red dwarf stars with less than 0.4 M☉ are
convective throughout, which prevents the
accumulation of a helium core. For most stars
the convective zones will also vary over time
as the star ages and the constitution of the
interior is modified.
The photosphere is that portion of a star
that is visible to an observer. This is the
layer at which the plasma of the star becomes
transparent to photons of light. From here,
the energy generated at the core becomes free
to propagate into space. It is within the
photosphere that sun spots, regions of lower
than average temperature, appear.
Above the level of the photosphere is the
stellar atmosphere. In a main sequence star
such as the Sun, the lowest level of the atmosphere,
just above the photosphere, is the thin chromosphere
region, where spicules appear and stellar
flares begin. Above this is the transition
region, where the temperature rapidly increases
within a distance of only 100 km (62 mi).
Beyond this is the corona, a volume of super-heated
plasma that can extend outward to several
million kilometres. The existence of a corona
appears to be dependent on a convective zone
in the outer layers of the star. Despite its
high temperature, and the corona emits very
little light, due to its low gas density.
The corona region of the Sun is normally only
visible during a solar eclipse.
From the corona, a stellar wind of plasma
particles expands outward from the star, until
it interacts with the interstellar medium.
For the Sun, the influence of its solar wind
extends throughout a bubble-shaped region
called the heliosphere.
== Nuclear fusion reaction pathways ==
A variety of nuclear fusion reactions take
place in the cores of stars, that depend upon
their mass and composition. When nuclei fuse,
the mass of the fused product is less than
the mass of the original parts. This lost
mass is converted to electromagnetic energy,
according to the mass–energy equivalence
relationship E = mc2.The hydrogen fusion process
is temperature-sensitive, so a moderate increase
in the core temperature will result in a significant
increase in the fusion rate. As a result,
the core temperature of main sequence stars
only varies from 4 million kelvin for a small
M-class star to 40 million kelvin for a massive
O-class star.In the Sun, with a 10-million-kelvin
core, hydrogen fuses to form helium in the
proton–proton chain reaction:
41H → 22H + 2e+ + 2νe(2 x 0.4 MeV)
2e+ + 2e− → 2γ (2 x 1.0 MeV)
21H + 22H → 23He + 2γ (2 x 5.5 MeV)
23He → 4He + 21H (12.9 MeV)These reactions
result in the overall reaction:
41H → 4He + 2e+ + 2γ + 2νe (26.7 MeV)where
e+ is a positron, γ is a gamma ray photon,
νe is a neutrino, and H and He are isotopes
of hydrogen and helium, respectively. The
energy released by this reaction is in millions
of electron volts, which is actually only
a tiny amount of energy. However enormous
numbers of these reactions occur constantly,
producing all the energy necessary to sustain
the star's radiation output. In comparison,
the combustion of two hydrogen gas molecules
with one oxygen gas molecule releases only
5.7 eV.
In more massive stars, helium is produced
in a cycle of reactions catalyzed by carbon
called the carbon-nitrogen-oxygen cycle.In
evolved stars with cores at 100 million kelvin
and masses between 0.5 and 10 M☉, helium
can be transformed into carbon in the triple-alpha
process that uses the intermediate element
beryllium:
4He + 4He + 92 keV → 8*Be
4He + 8*Be + 67 keV → 12*C
12*C → 12C + γ + 7.4 MeVFor an overall
reaction of:
34He → 12C + γ + 7.2 MeVIn massive stars,
heavier elements can also be burned in a contracting
core through the neon-burning process and
oxygen-burning process. The final stage in
the stellar nucleosynthesis process is the
silicon-burning process that results in the
production of the stable isotope iron-56,
an endothermic process that consumes energy,
and so further energy can only be produced
through gravitational collapse.The example
below shows the amount of time required for
a star of 20 M☉ to consume all of its nuclear
fuel. As an O-class main sequence star, it
would be 8 times the solar radius and 62,000
times the Sun's luminosity.
== See also
