[guide.chat] all about stars
- From: vanessa <qwerty1234567a@xxxxxxxxx>
- To: "GUIDE CHAT" <guide.chat@xxxxxxxxxxxxx>
- Date: Tue, 14 Aug 2012 12:34:45 +0100
A star is a massive, luminous sphere of plasma held together by gravity. At the
end of its lifetime, a star can also contain a proportion of degenerate matter.
The nearest star to Earth is the Sun, which is the source of most of the energy
on Earth. Other stars are visible from Earth during the night, when they are
not obscured by atmospheric phenomena, appearing as a multitude of fixed
luminous points because of their immense distance. Historically, the most
prominent stars on the celestial sphere were grouped together into
constellations and asterisms, and the brightest stars gained proper names.
Extensive catalogues of stars have been assembled by astronomers, which provide
standardized star designations.
For at least a portion of its life, a star shines due to thermonuclear fusion
of hydrogen 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 were created by stars, either via stellar nucleosynthesis during
their lifetimes or by supernova nucleosynthesis when stars explode. Astronomers
can determine the mass, age, chemical composition and many other properties of
a star by observing its spectrum, luminosity and motion through space. The
total mass of a star is the principal determinant in its evolution and eventual
fate. Other characteristics of a star are determined by its evolutionary
history, including diameter, rotation, movement and temperature. A plot of the
temperature of many stars against their luminosities, known as a
Hertzsprung?Russell diagram (H?R diagram), allows the age and evolutionary
state of a star to be determined.
A star begins as a collapsing cloud of material composed primarily of hydrogen,
along with helium and trace amounts of heavier elements. Once the stellar core
is sufficiently dense, some of the hydrogen is steadily converted into helium
through the process of nuclear fusion.[1] The remainder of the star's interior
carries energy away from the core through a combination of radiative and
convective processes. The star's internal pressure prevents it from collapsing
further under its own gravity. Once the hydrogen fuel at the core is exhausted,
a star with at least 0.4 times the mass of the Sun[2] expands to become a red
giant, in some cases fusing heavier elements at the core or in shells around
the core. The star then evolves into a degenerate form, recycling a portion of
the matter into the interstellar environment, where it will form a new
generation of stars with a higher proportion of heavy elements.[3]
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.[4] Stars can form
part of a much larger gravitationally bound structure, such as a cluster or a
galaxy.
People have seen patterns in the stars since ancient times.[5] This 1690
depiction of the constellation of Leo, the lion, is by Johannes Hevelius.[6]
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.[5] 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.[7] 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 appeared in ancient Egyptian astronomy
in 1534 BC.[8] 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).[9]
The first star catalogue in Greek astronomy was created by Aristillus in
approximately 300 BC, with the help of Timocharis.[10] The star catalog of
Hipparchus (2nd century BC) included 1020 stars and was used to assemble
Ptolemy's star catalogue.[11] Hipparchus is known for the discovery of the
first recorded nova (new star).[12] 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.[13] In 185 AD, they were the first to
observe and write about a supernova, now known as the SN 185.[14] 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.[15] The SN 1054 supernova, which gave birth to the Crab
Nebula, was also observed by Chinese and Islamic astronomers.[16][17][18]
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.[19] 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).[20] 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.[21]
According to Josep Puig, the Andalusian astronomer Ibn Bajjah proposed that the
Milky Way was made up of many stars which 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.[22]
Andromeda as depicted in Urania's Mirror, set of constellation cards published
in London c.1825
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,[23] an idea that
had been suggested earlier by the ancient Greek philosophers, Democritus and
Epicurus,[24] and by medieval Islamic cosmologists[25] such as Fakhr al-Din
al-Razi.[26] 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.[27]
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 from the time of the ancient
Greek astronomers Ptolemy and Hipparchus. 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.[23]
William Herschel was the first astronomer to attempt to determine the
distribution of stars in the sky. During the 1780s, he performed 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.[28] 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 a stellar spectra due to the absorption of specific frequencies
by the atmosphere. In 1865 Secchi began classifying stars into spectral
types.[29] However, the modern version of the stellar classification scheme was
developed by Annie J. Cannon during the 1900s.
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 William
Struve and S. W. Burnham, allowing the masses of stars to be determined from
computation of the 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.[30] 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
very 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.[31]
Important conceptual work on the physical basis 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.
The spectra of stars were also successfully explained through advances in
quantum physics. This allowed the chemical composition of the stellar
atmosphere to be determined.[32]
With the exception of supernovae, individual stars have primarily been observed
in our Local Group of galaxies,[33] and especially in the visible part of the
Milky Way (as demonstrated by the detailed star catalogues available for our
galaxy).[34] But some stars have been observed in the M100 galaxy of the Virgo
Cluster, about 100 million light years from the Earth.[35] In the Local
Supercluster it is possible to see star clusters, and current telescopes could
in principle observe faint individual stars in the Local Cluster?the most
distant stars resolved have up to hundred million light years away[36] (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 one billion light years away[37]?ten times the distance of the
most distant star cluster previously observed.
Designations
Main articles: Star designation, Astronomical naming conventions, and Star
catalogue
The concept of the 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.[38] 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, stars as a whole have
their own myths.[39] To the Ancient Greeks, some "stars", known as planets
(Greek p?a??t?? (planetes), meaning "wanderer"), represented various important
deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and
Saturn were taken.[39] (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.[40][41]
Under space law, the only internationally recognized authority for naming
celestial bodies is the International Astronomical Union (IAU).[42] A number of
private companies sell names of stars, which the British Library calls an
unregulated commercial enterprise.[43][44] However, the IAU has disassociated
itself from this commercial practice, and these names are neither recognized by
the IAU nor used by them.[45] 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,[46][47][48][49] and the New
York City Department of Consumer Affairs issued a violation against ISR for
engaging in a deceptive trade practice.[50][51]
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:
solar mass: M? = 1.9891 × 1030 kg[52]
solar luminosity: L? = 3.827 × 1026 watts[52]
solar radius R? = 6.960 × 108 m[53]
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
(AU)?approximately the mean distance between the Earth and the Sun (150 million
km or 93 million miles).
Formation and evolution
Main article: Stellar evolution
Stars are formed within extended regions of higher density in the interstellar
medium, although the density is still lower than the inside of an earthly
vacuum chamber. These regions are called molecular clouds and consist mostly of
hydrogen, with about 23?28% helium and a few percent heavier elements. One
example of such a star-forming region is the Orion Nebula.[54] As massive stars
are formed from molecular clouds, they powerfully illuminate those clouds. They
also ionize the hydrogen, creating an H II region.
Protostar formation
Main article: Star formation
The formation of a star begins with gravitational instability within a
molecular cloud, caused by regions of higher density often triggered by shock
waves from supernovae (massive stellar explosions), the collision of different
molecular clouds, or the collision of galaxies (as in a starburst galaxy). Once
a region reaches a sufficient density of matter to satisfy the criteria for
Jeans instability, it begins to collapse under its own gravitational force.[55]
As the cloud collapses, individual conglomerations of dense dust and gas form
what are known as Bok globules. As a globule collapses and the density
increases, the gravitational energy is converted 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.[56] These
pre?main sequence stars are often surrounded by a protoplanetary disk. The
period of gravitational contraction lasts for about 10?15 million years.
Early stars of less than 2 solar masses are called T Tauri stars, while those
with greater mass are Herbig Ae/Be stars. These newly born 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.[57][58] These jets, in combination with radiation from nearby massive
stars, may help to drive away the surrounding cloud in which the star was
formed.[59]
Main sequence
Main article: Main sequence
Stars spend about 90% of their lifetime fusing hydrogen to produce 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. As a consequence, in order to maintain the required rate of nuclear
fusion at the core, the star will slowly increase in temperature and
luminosity[60]?the Sun, for example, is estimated to have increased in
luminosity by about 40% since it reached the main sequence 4.6 billion years
ago.[61]
Every star generates a stellar wind of particles that causes a continual
outflow of gas into space. For most stars, the amount of mass lost is
negligible. The Sun loses 10-14 solar masses every year,[62] or about 0.01% of
its total mass over its entire lifespan. However very massive stars can lose
10-7 to 10-5 solar masses each year, significantly affecting their
evolution.[63] Stars that begin with more than 50 solar masses can lose over
half their total mass while they remain on the main sequence.[64]
An example of a Hertzsprung?Russell diagram for a set of stars that includes
the Sun (center). (See "Classification" below.)
The duration that a star spends on the main sequence depends primarily on the
amount of fuel it has to fuse and the rate at which it fuses that fuel, i.e.
its initial mass and its luminosity. For the Sun, this is estimated to be about
1010 years. Large stars consume their fuel very rapidly and are short-lived.
Small stars (called red dwarfs) consume their fuel very slowly and last tens to
hundreds of billions of years. At the end of their lives, they simply become
dimmer and dimmer.[2] However, since the lifespan of such stars is greater than
the current age of the universe (13.7 billion years), no stars under about 85%
of solar mass,[65] including all red dwarfs, are expected to have moved off of
the main sequence.
Besides mass, the portion of elements heavier than helium can play a
significant role in the evolution of stars. In astronomy all elements heavier
than helium are considered a "metal", and the chemical concentration of these
elements is called the metallicity. The metallicity can influence the duration
that a star will burn its fuel, control the formation of magnetic fields[66]
and modify the strength of the stellar wind.[67] 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
these clouds become increasingly enriched in heavier elements as older stars
die and shed portions of their atmospheres.)
Post-main sequence
Main article: Red giant
As stars of at least 0.4 solar masses[2] exhaust their supply of hydrogen at
their core, their outer layers expand greatly and cool to form a red giant. For
example, in about 5 billion years, when the Sun is a red giant, it will expand
out to a maximum radius of roughly 1 astronomical unit (150 million
kilometres), 250 times its present size. As a giant, the Sun will lose roughly
30% of its current mass.[61][68]
In a red giant of up to 2.25 solar masses, hydrogen fusion proceeds in a
shell-layer surrounding the core.[69] Eventually the core is compressed enough
to start helium fusion, and the star now gradually shrinks in radius and
increases its surface temperature. For larger stars, the core region
transitions directly from fusing hydrogen to fusing helium.[4]
After the star has consumed the helium at the core, fusion continues in a shell
around a hot core of carbon and oxygen. The star then follows an evolutionary
path that parallels the original red giant phase, but at a higher surface
temperature.
Massive stars
Main article: Red supergiant
Betelgeuse is a red supergiant star approaching the end of its life cycle.
During their helium-burning phase, very high mass stars with more than nine
solar masses expand to form red supergiants. Once this fuel is exhausted at the
core, they can continue to fuse elements heavier than helium.
The core contracts until the temperature and pressure are sufficient 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 can occur along a series of onion-layer shells
within the star. Each shell fuses a different element, with the outermost shell
fusing hydrogen; the next shell fusing helium, and so forth.[70]
The final stage is reached when the star begins producing iron. Since iron
nuclei are more tightly bound than any heavier nuclei, if they are fused they
do not release energy?the process would, on the contrary, consume energy.
Likewise, since they are more tightly bound than all lighter nuclei, energy
cannot be released by fission.[69] In relatively old, very massive stars, a
large core of inert iron will accumulate in the center of the star. The heavier
elements in these stars can work their way up to the surface, forming evolved
objects known as Wolf-Rayet stars that have a dense stellar wind which sheds
the outer atmosphere.
Collapse
An evolved, average-size star will now shed its outer layers as a planetary
nebula. If what remains after the outer atmosphere has been shed is less than
1.4 solar masses, it shrinks to a relatively tiny object (about the size of
Earth) that is not massive enough for further compression to take place, known
as a white dwarf.[71] 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. White dwarfs will eventually fade into black dwarfs over a very long
stretch of time.
The Crab Nebula, remnants of a supernova that was first observed around 1050 AD
In larger stars, fusion continues until the iron core has grown so large (more
than 1.4 solar masses) that it can no longer support its own mass. This core
will suddenly collapse as its electrons are driven into its protons, forming
neutrons and neutrinos in a burst of inverse beta decay, or electron capture.
The shockwave formed by this sudden collapse causes the rest of the star to
explode in a supernova. Supernovae are 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 existed before.[72]
Most of the matter in the star is blown away by the supernova explosion
(forming nebulae such as the Crab Nebula)[72] and what remains will be a
neutron star (which sometimes manifests itself as a pulsar or X-ray burster)
or, in the case of the largest stars (large enough to leave a stellar remnant
greater than roughly 4 solar masses), a black hole.[73] 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 new star formation. 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.[72]
Distribution
A white dwarf star in orbit around Sirius (artist's impression). NASA image
In addition to isolated stars, a multi-star system can consist of two or more
gravitationally bound stars that orbit around each other. The 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 co-orbiting binary stars.[74] 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.
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 systems are believed to be
multiple. However the proportion of single star systems increases for smaller
stars, 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.[75]
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.[76] A 2010 star count estimate was 300
sextillion (3 × 1023) in the observable universe.[77] While it is often
believed that stars only exist within galaxies, intergalactic stars have been
discovered.[78]
The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which
is 39.9 trillion kilometres, or 4.2 light-years away. 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 get there.[79] Distances like
this are typical inside galactic discs, including in the vicinity of the solar
system.[80] 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.[81] 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 in the cluster .[82]
Characteristics
The Sun is the nearest star to Earth.
Almost everything about a star is determined by its initial mass, including
essential characteristics such as luminosity and size, as well as the star's
evolution, lifespan, and eventual fate.
Age
Most stars are between 1 billion and 10 billion years old. Some stars may even
be close to 13.7 billion years old?the observed age of the universe. The oldest
star yet discovered, HE 1523-0901, is an estimated 13.2 billion years
old.[83][84]
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 last tens to
hundreds of billions of years.[85][86]
Chemical composition
See also: Metallicity
When stars form in the present Milky Way galaxy they are composed of about 71%
hydrogen and 27% helium,[87] 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. Because the molecular
clouds where stars form are steadily enriched by heavier elements from
supernovae explosions, a measurement of the chemical composition of a star can
be used to infer its age.[88] The portion of heavier elements may also be an
indicator of the likelihood that the star has a planetary system.[89]
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.[90] 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.[91]
There also exist chemically peculiar stars that show unusual abundances of
certain elements in their spectrum; especially chromium and rare earth
elements.[92]
Diameter
Stars vary widely in size. In each image in the sequence, the right-most object
appears as the left-most object in the next panel. The Earth appears at right
in panel 1 and the Sun is second from the right in panel 3.
Due to their great distance from the Earth, all stars except the Sun appear to
the human 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.[93]
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.[94]
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 approximately 650 times larger than the Sun?about
900,000,000 km (560,000,000 mi). However, Betelgeuse has a much lower density
than the Sun.[95]
Kinematics
Main article: Stellar kinematics
The Pleiades, an open cluster of stars in the constellation of Taurus. These
stars share a common motion through space.[96] NASA photo
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 is determined by
precise astrometric measurements in units of milli-arc seconds (mas) per year.
By determining the parallax of a star, the proper motion can then be converted
into units of velocity. Stars with high rates of proper motion are likely to be
relatively close to the Sun, making them good candidates for parallax
measurements.[97]
Once 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 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.[98] Comparison of the kinematics of nearby stars has also led to
the identification of stellar associations. These are most likely groups of
stars that share a common point of origin in giant molecular clouds.[99]
Magnetic field
Main article: Stellar magnetic field
Surface magnetic field of SU Aur (a young star of T Tauri type), reconstructed
by means of Zeeman-Doppler imaging
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, generating magnetic fields that extend throughout 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 fields that reach out into the corona from active
regions. Stellar flares are bursts of high-energy particles that are emitted
due to the same magnetic activity.[100]
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, however, functioning as a brake to gradually slow the rate of
rotation as the star grows older. 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.[101] During the Maunder minimum, for example, the
Sun underwent a 70-year period with almost no sunspot activity.
Mass
Main article: Stellar mass
One of the most massive stars known is Eta Carinae,[102] with 100?150 times as
much mass as the Sun; its lifespan is very short?only several million years at
most. A study of the Arches cluster suggests that 150 solar masses is the upper
limit for stars in the current era of the universe.[103] The reason for this
limit is not precisely known, but it is partially due to the Eddington
luminosity which defines the maximum amount of luminosity that can pass through
the atmosphere of a star without ejecting the gases into space. However, a star
named R136a1 in the RMC 136a star cluster has been measured at 265 solar
masses, putting this limit into question.[104]
The reflection nebula NGC 1999 is brilliantly illuminated by V380 Orionis
(center), a variable star with about 3.5 times the mass of the Sun. The black
patch of sky is a vast hole of empty space and not a dark nebula as previously
thought. NASA image
The first stars to form after the Big Bang may have been larger, up to 300
solar masses or more,[105] due to the complete absence of elements heavier than
lithium in their composition. This generation of supermassive, population III
stars is long extinct, however, and currently only theoretical.
With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB
Doradus A, is the smallest known star undergoing nuclear fusion in its
core.[106] For stars with similar metallicity to the Sun, the theoretical
minimum mass the star can have, and still undergo fusion at the core, is
estimated to be about 75 times the mass of Jupiter.[107][108] When the
metallicity is very low, however, a recent study of the faintest stars found
that the minimum star size seems to be about 8.3% of the solar mass, or about
87 times the mass of Jupiter.[108][109] Smaller bodies are called brown dwarfs,
which occupy a poorly defined grey area between stars and gas giants.
The combination of the radius and the mass of a star determines the surface
gravity. Giant stars have a much lower surface gravity than 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.[32]
Stars are sometimes grouped by mass based upon their evolutionary behavior as
they approach the end of their nuclear fusion lifetimes. Very low mass stars
with masses below 0.5 solar masses do not enter the asymptotic giant branch
(AGB) but evolve directly into white dwarfs. Low mass stars with a mass below
about 1.8?2.2 solar masses (depending on composition) do enter the AGB, where
they develop a degenerate helium core. Intermediate-mass stars undergo helium
fusion and develop a degenerate carbon-oxygen core. Massive stars have a
minimum mass of 7?10 solar masses, but this may be as low as 5?6 solar masses.
These stars undergo carbon fusion, with their lives ending in a core-collapse
supernova explosion.[110]
Rotation
Main article: Stellar rotation
The rotation rate of stars can be approximated through spectroscopic
measurement, or more exactly determined by tracking the rotation rate of
starspots. Young stars can have a rapid rate of rotation greater than 100 km/s
at the equator. The B-class star Achernar, for example, has an equatorial
rotation velocity of about 225 km/s or greater, giving it an equatorial
diameter that is more than 50% larger than the distance between the poles. This
rate of rotation is just below the critical velocity of 300 km/s where the star
would break apart.[111] By contrast, the Sun only rotates once every 25 ? 35
days, with an equatorial velocity of 1.994 km/s. The star's magnetic field and
the stellar wind serve to slow down a main sequence star's rate of rotation by
a significant amount as it evolves on the main sequence.[112]
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.[113] 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.[114] 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 at the core and the radius of the star and is often estimated
from the star's color index.[115] It is normally given as the 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 value, however, as stars
actually have a temperature gradient that decreases with increasing distance
from the core.[116] The temperature in the core region of a star is several
million kelvins.[117]
The stellar temperature will determine the rate of energization or ionization
of different 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).[32]
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.[118]
Radiation
The energy produced by stars, as a by-product of nuclear fusion, radiates into
space as both electromagnetic radiation and particle radiation. The particle
radiation emitted by a star is manifested as the stellar wind[119] (which
exists as a steady stream of electrically charged particles, such as free
protons, alpha particles, and beta particles, emanating from the star's outer
layers) and as a steady stream of neutrinos emanating from the star's core.
The production of energy at the core is the reason why stars shine so brightly:
every time two or more atomic nuclei of one element fuse together to form an
atomic nucleus of a new heavier element, gamma ray photons are released from
the nuclear fusion reaction. This energy is converted to other forms of
electromagnetic energy, including visible light, by the time it reaches the
star's outer layers.
The color of a star, as determined by the peak frequency of the visible light,
depends on the temperature of the star's outer layers, including its
photosphere.[120] 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 and infrared to the shortest wavelengths of
ultraviolet, X-rays, and gamma rays. All components of stellar electromagnetic
radiation, both visible and invisible, are typically significant.
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 known, 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
measured directly for stars in binary systems. The technique of gravitational
microlensing will also yield the mass of a star)[121] With these parameters,
astronomers can also estimate the age of the star.[122]
Luminosity
In astronomy, luminosity is the amount of light, and other forms of radiant
energy, a star radiates per unit of time. The luminosity of a star is
determined by the radius and the surface temperature. However, many stars do
not radiate a uniform flux?the amount of energy radiated per unit area?across
their entire surface. The rapidly rotating star Vega, for example, has a higher
energy flux at its poles than along its equator.[123]
Surface patches with a lower temperature and luminosity than average are known
as starspots. Small, dwarf stars such as the Sun generally have essentially
featureless disks with only small starspots. Larger, giant stars have much
bigger, much more obvious starspots,[124] and they also exhibit strong stellar
limb darkening. That is, the brightness decreases towards the edge of the
stellar disk.[125] Red dwarf flare stars such as UV Ceti may also possess
prominent starspot features.[126]
Magnitude
Main articles: Apparent magnitude and Absolute magnitude
The apparent brightness of a star is measured by its apparent magnitude, which
is the brightness of a star with respect to the star's luminosity, distance
from Earth, 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).
Number of stars brighter than magnitude
Apparent
magnitude Number
of Stars[127]
0 4
1 15
2 48
3 171
4 513
5 1,602
6 4,800
7 14,000
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[128] (the 5th root of 100 or approximately 2.512). This means
that a first magnitude (+1.00) star is about 2.5 times brighter than a second
magnitude (+2.00) star, and approximately 100 times brighter than a sixth
magnitude (+6.00) star. 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 brightest stars, on either scale, have negative magnitude numbers. The
variation in brightness (?L) 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:
Relative to both luminosity and distance from Earth, absolute magnitude (M) and
apparent magnitude (m) are not equivalent for an individual star;[128] 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.[129] 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.[130]
Classification
Surface Temperature Ranges for
Different Stellar Classes[131]
Class Temperature Sample star
O 33,000 K or more Zeta Ophiuchi
B 10,500?30,000 K Rigel
A 7,500?10,000 K Altair
F 6,000?7,200 K Procyon A
G 5,500?6,000 K Sun
K 4,000?5,250 K Epsilon Indi
M 2,600?3,850 K Proxima Centauri
Main article: Stellar 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.[132] It was not known at the time that the major influence on the line
strength was temperature; the hydrogen line strength reaches a peak at over
9000 K, and is weaker at both hotter and cooler temperatures. When the
classifications were reordered by temperature, it more closely resembled the
modern scheme.[133]
There are different single-letter classifications of stars 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 have 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:
class O0 and O1 stars may not exist.[134]
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 the
surface gravity. These range from 0 (hypergiants) through III (giants) to V
(main sequence dwarfs); some authors add VII (white dwarfs). Most stars belong
to the main sequence, which consists of ordinary hydrogen-burning stars. These
fall along a narrow, diagonal band when graphed according to their absolute
magnitude and spectral type.[134] The Sun is a main sequence G2V yellow dwarf,
being of intermediate temperature and ordinary size.
Additional nomenclature, in the form of lower-case letters, can follow 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.[134]
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 index.[135]
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.[136]
Eruptive variables are stars that experience sudden increases in luminosity
because of flares or mass ejection events.[136] This group includes protostars,
Wolf-Rayet stars, and Flare stars, as well as giant and supergiant stars.
Cataclysmic or explosive variables 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.[4] The
explosion is created when the white dwarf accretes hydrogen from the companion
star, building up mass until the hydrogen undergoes fusion.[137] Some novae are
also recurrent, having periodic outbursts of moderate amplitude.[136]
Stars can also vary in luminosity because of extrinsic factors, such as
eclipsing binaries, as well as rotating stars that produce extreme
starspots.[136] A notable example of an eclipsing binary is Algol, which
regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days.
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.[138][139]
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 solar masses, fusion occurs in
a slowly expanding shell around the degenerate helium core.[140]
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 within the stellar interior where radiative
transfer is sufficiently efficient to maintain the flux of energy. 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 as in the outer envelope.[139]
The occurrence of convection in the outer envelope of a main sequence star
depends on the 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.[141] Red dwarf stars with less
than 0.4 solar masses are convective throughout, which prevents the
accumulation of a helium core.[2] For most stars the convective zones will also
vary over time as the star ages and the constitution of the interior is
modified.[139]
The portion of a star that is visible to an observer is called the photosphere.
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 out into space. It is within the photosphere that sun spots, or
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 is the thin
chromosphere region, where spicules appear and stellar flares begin. This is
surrounded by a 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.[142] The existence of a corona appears to be dependent on a
convective zone in the outer layers of the star.[141] Despite its high
temperature, the corona emits very little light. 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, propagating until it interacts with the interstellar medium. For the Sun,
the influence of its solar wind extends throughout the bubble-shaped region of
the heliosphere.[143]
The carbon-nitrogen-oxygen cycle
A variety of different nuclear fusion reactions take place inside the cores of
stars, depending upon their mass and composition, as part of stellar
nucleosynthesis. The net mass of the fused atomic nuclei is smaller than the
sum of the constituents. This lost mass is released as electromagnetic energy,
according to the mass-energy equivalence relationship E = mc2.[1]
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.[117]
In the Sun, with a 10-million-kelvin core, hydrogen fuses to form helium in the
proton-proton chain reaction:[144]
41H ? 22H + 2e+ + 2?e (4.0 MeV + 1.0 MeV)
21H + 22H ? 23He + 2? (5.5 MeV)
23He ? 4He + 21H (12.9 MeV)
These reactions result in the overall reaction:
41H ? 4He + 2e+ + 2? + 2?e (26.7 MeV)
where e+ is a positron, ? is a gamma ray photon, ?e 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.
Minimum stellar mass required for fusion
Element Solar
masses
Hydrogen 0.01
Helium 0.4
Carbon 5[145]
Neon 8
In more massive stars, helium is produced in a cycle of reactions catalyzed by
carbon?the carbon-nitrogen-oxygen cycle.[144]
In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10
solar masses, helium can be transformed into carbon in the triple-alpha process
that uses the intermediate element beryllium:[144]
4He + 4He + 92 keV ? 8*Be
4He + 8*Be + 67 keV ? 12*C
12*C ? 12C + ? + 7.4 MeV
For an overall reaction of:
34He ? 12C + ? + 7.2 MeV
In 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. Fusion can not proceed any
further except through an endothermic process, and so further energy can only
be produced through gravitational collapse
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