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Star

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)


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   The Pleiades, an open cluster of stars in the constellation of Taurus.
   NASA photo
   Enlarge
   The Pleiades, an open cluster of stars in the constellation of Taurus.
   NASA photo

   A star is a massive, luminous ball of plasma. Stars group together to
   form galaxies, and they dominate the visible universe. The nearest star
   is the Sun, which is the source of most of the energy on Earth,
   including daylight. Other stars are visible in the night sky, when they
   are not outshone by the Sun. A star shines because nuclear fusion in
   its core releases energy which traverses the star's interior and then
   radiates into outer space. Without stars, life on Earth and most atomic
   elements would not exist.

   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 that are determined by its evolutionary history include the
   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 current age and
   evolutionary state of a particular star to be determined.

   A star begins as a collapsing cloud of material that is composed
   primarily of hydrogen along with some helium and heavier trace
   elements. Once the stellar core is sufficiently dense, some of the
   hydrogen is steadily converted into helium through the process of
   nuclear fusion. The remainder of the star's interior carries energy
   away from the core through a combination of radiation and convective
   processes. These processes keep the star from collapsing upon itself
   and the energy generates a stellar wind at the surface and radiation
   into outer space.

   Once the hydrogen fuel at the core is exhausted, a star of at least 0.4
   times the mass of the Sun expands to become a red giant, fusing heavier
   elements at the core, or in shells around the core. It 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.

   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.

Observation history

   Stars have been important to every culture. They have been used in
   religious practices and for celestial navigation and orientation. The
   Gregorian calendar, used nearly everywhere in the world, is a solar
   calendar based on the angle of the Earth's rotational axis relative to
   the nearest star, the Sun.

   Early 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
   actually other suns, and may have other planets, possibly even
   Earth-like, in orbit around them, an idea that had been suggested
   earlier by such ancient Greek philosophers as Democritus and Epicurus.
   By the following century the idea of the stars as distant suns 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
   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.

   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. 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. 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.

   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 colour 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.

   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.

Star designations

   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. 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. They were thought to be the souls of the dead or
   gods. An example is the star Algol, which was thought to represent the
   eye of the Gorgon Medusa.

   To the Ancient Greeks, some "stars," later identified as planets,
   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 the English
   astronomer John Flamsteed came up with a system using numbers, which
   would later be known as the Flamsteed designation. Numerous additional
   systems have since been created as star catalogues have appeared.

   The only body which has been recognized by the scientific community as
   having the authority to name stars or other celestial bodies is the
   International Astronomical Union (IAU). A number of private companies
   (for instance, the " International Star Registry") purport to sell
   names to stars; however, these names are neither recognized by the
   scientific community nor used by them, and many in the astronomy
   community view these organizations as frauds preying on people ignorant
   of star naming procedure.

Units of measurement

   Most stellar parameters are expressed in SI units by convention, but
   CGS units are also used (e.g., expressing luminosity in ergs per
   second). Mass, luminosity, and radii are usually given in solar units,
   based on the characteristics of the Sun:

          solar mass:       M_\bigodot = 1.9891 \times 10^{30}   kg
          solar luminosity: L_\bigodot = 3.827 \times 10^{26}   watts
          solar radius:     R_\bigodot = 6.960 \times 10^{8} m

   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

   Stars are formed within molecular clouds; large regions of high density
   in the interstellar medium (though still less dense than the inside of
   an earthly vacuum chamber). These clouds consist mostly of hydrogen,
   with about 23–28% helium and a few percent heavier elements. One
   example of such a star-forming nebula is the Orion Nebula. As massive
   stars are formed from these clouds, they powerfully illuminate and
   ionize the clouds from which they formed, creating an H II region.

Protostar formation

   The formation of a star begins with a gravitational instability inside
   a molecular cloud, often triggered by shockwaves from supernovae
   (massive stellar explosions) or the collision of two 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.
   Artist's conception of the birth of a star within a dense molecular
   cloud. NASA image
   Enlarge
   Artist's conception of the birth of a star within a dense molecular
   cloud. NASA image

   As the cloud collapses, individual conglomerations of dense dust and
   gas form that are known as Bok globules. These can contain up to 50
   solar masses of material. 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. 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, producing small patches
   of nebulosity known as Herbig-Haro objects.

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. 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.

   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, 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. Stars that begin with more
   than 50 solar masses can lose over half their total mass while they
   remain on the main sequence.

   The duration that a star spends on the main sequence depends primarily
   on the amount of fuel it has to burn and the rate at which it burns
   that fuel. In other words, its initial mass and its luminosity. For the
   Sun, this is estimated to be about 10^10 years. Large stars burn their
   fuel very rapidly and are short-lived. Small stars (called red dwarfs)
   burn 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,
   fading into black dwarfs. However, since the lifespan of such stars is
   greater than the current age of the universe (13.7 billion years), no
   black dwarfs are expected to exist yet.

   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 and modify the strength of the
   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 these 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 solar masses exhaust their supply of hydrogen
   at their core, their outer layers expand and cool to form a red giant.
   In about 5 billion years, when the Sun is a red giant, it will be so
   large that it will consume Mercury and possibly Venus. Models predict
   that the Sun will expand out to about 99% of the distance to the
   Earth's present orbit (1 astronomical unit, or AU). By that time,
   however, the orbit of the Earth will expand to about 1.7 AUs due to
   mass loss by the Sun and thus the Earth will escape envelopment.
   However, the Earth will be stripped of its oceans and atmosphere as the
   Sun's luminosity increases several thousand-fold.

   In a red giant, hydrogen fusion proceeds in a shell-layer surrounding
   the core. Eventually the core is compressed enough to start helium
   fusion, and the star now gradually shrinks in radius and increases its
   surface temperature.

   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 now follows an
   evolutionary path that parallels the original red giant phase, but at a
   higher surface temperature.

Massive stars

   Betelgeuse is a red supergiant star approaching the end of its life
   cycle
   Enlarge
   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. This process continues, with the successive
   stages being fueled by oxygen, neon, silicon, and sulfur. 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.

   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. In relatively
   old, very massive stars, a large core of inert iron will accumulate in
   the centre 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. 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
   Enlarge
   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.

   Most of the matter in the star is blown away by the supernovae
   explosion (forming nebulae such as the Crab Nebula) 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. 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.

Distribution

   A white dwarf star in orbit around Sirius (artist's impression). NASA
   image
   Enlarge
   A white dwarf star in orbit around Sirius (artist's impression). NASA
   image

   It has been a long-held assumption that the majority of stars occur in
   gravitationally-bound, multiple-star systems, forming binary stars.
   This is particularly true for very massive O and B class stars, where
   80% of the systems are believed to be multiple. However the portion 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.

   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.

   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 (10^11) galaxies in the observable universe. While it is
   often believed that stars only exist within galaxies, intergalactic
   stars have been discovered.

   Astronomers estimate that there are at least 70 sextillion (7×10^22)
   stars in the known universe. That is 230 billion times as many as the
   300 billion in our own Milky Way.

   The nearest star to the Earth, apart from the Sun, is Proxima Centauri,
   which is 39.9 trillion (10^12) kilometres, or 4.2 light-years away.
   Light from Proxima Centauri takes 4.2 years to reach Earth. Travelling
   at the orbital speed of the Space Shuttle (5 miles per second — almost
   30,000 kilometres per hour), it would take about 150,000 years to get
   there. Distances like this are typical inside galactic discs, including
   the vicinity of the solar system. Stars can be much closer to each
   other in the centres of galaxies and in globular clusters, or much
   farther apart in galactic halos.

   Because of their low density, collisions of stars in the galaxy are
   thought to be rare. However in dense regions such as the core of
   globular clusters or the galactic centre, 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 in the cluster with the same luminosity.

Characteristics

   The Sun is the nearest star to Earth
   Enlarge
   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

   Many 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. (See Big Bang theory and stellar evolution.) 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 about one
   million years, while stars of minimum mass (red dwarfs) burn their fuel
   very slowly and last tens to hundreds of billions of years.

Chemical composition

   When stars form they are composed of about 70% hydrogen and 28% 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. 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. The portion of
   heavier elements may also 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.

Diameter

   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 disks of stars are
   much too small in angular size to be observed with current ground-based
   optical telescopes, and so Interferometer telescopes are required in
   order to produce images of these objects. 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.

   Stars range in size from neutron stars no bigger than a city to
   supergiants like Betelgeuse in the Orion constellation, which has a
   diameter about 1,000 times larger than the Sun — about 1.6 billion
   kilometres. However, Betelgeuse 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 proper motion of a star is the traverse velocity across the sky.
   This 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.

   The radial velocity is the movement of the star toward or away from the
   Sun. This is determined by measurements in the doppler shift of
   spectral lines, and is given in units of km/ s.

   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. 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.

Mass

   One of the most massive stars known is Eta Carinae, with 100 – 150
   times as much mass as the Sun; its lifespan is very short — only
   several million years at most. A recent study of the Arches cluster
   suggests that 150 solar masses is the upper limit for stars in the
   current era of the universe. 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.
   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. NASA image
   Enlarge
   The reflection nebula NGC 1999 is brilliantly illuminated by V380
   Orionis (centre), a variable star with about 3.5 times the mass of the
   Sun. NASA image

   The first stars to form after the Big Bang may have been larger, up to
   300 solar masses or more, 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. 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. 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. 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.

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. 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.

   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 at the core and the radius of the star.
   Massive stars can have surface temperatures of 50,000  K. Smaller stars
   such as the Sun have surface temperatures of a few thousand degrees.
   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.

   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).

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 (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 colour 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. 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.) With these parameters, astronomers can also
   estimate the age of the star.

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.

   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, 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 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.

   CAPTION: Number of stars brighter than magnitude

   Apparent
   magnitude  Number
             of Stars
   0         4
   1         15
   2         48
   3         171
   4         513
   5         1,602
   6         4,800
   7         14,000

   Intrinsic or absolute magnitude 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), and it is directly related to a star’s luminosity.

   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 (+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 between two
   stars is calculated by subtracting the magnitude number of the brighter
   star (m[b]) from the magnitude number of the fainter star (m[f]), then
   using the difference as an exponent for the base number 2.512; that is
   to say:

          Δm = m[f] − m[b]
          2.512^Δm = variation in brightness

   Relative to both luminosity and distance from Earth, absolute magnitude
   (M) and apparent magnitude (m) are not exactly equivalent for an
   individual star; 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 further
   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 38,000,000 times more
   luminuous as 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

   CAPTION: Surface Temperature Ranges for
   Different Stellar Classes

   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

   There are different 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-classifications numbered (hottest to coldest)
   from 0 to 9. This system matches closely with temperature, but breaks
   down at the extreme hottest end; class O0 and O1 stars may not exist.

   In addition, stars may be classified by their "luminosity effects",
   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) and VII (white dwarfs). Most stars fall into the
   main sequence, which consists of ordinary hydrogen-burning stars. These
   fall along a narrow band when graphed according to their absolute
   magnitude and spectral type. Our 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.

   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.

Variable stars

   The asymmetrical appearance of Mira, an oscillating variable star. NASA
   HST image
   Enlarge
   The asymmetrical appearance of Mira, an oscillating variable star. NASA
   HST image

   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.

   Pulsating variables are stars that vary in radius over time, expanding
   and contracting as a result of the stellar aging process. 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 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. 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 2.3 to 3.5 over a period of 2.87
   days.

Structure

   The interior of a stable, main sequence star is in a state of
   equilibrium in which the forces in any small volume almost exactly
   counterbalance each other. The balancing forces consist of inward
   directed gravitational force and the opposing pressure from the thermal
   energy of the plasma gas. For these forces to balance out, the
   temperature at the core of a typical star has to be on the order of
   10^7 K or higher. 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 greater than 0.4 solar masses, 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.
   This diagram shows a cross-section of a solar-type star. NASA image
   Enlarge
   This diagram shows a cross-section of a solar-type star. NASA image

   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 near the core or in areas with high opacity as in the outer
   envelope.

   The occurrence of convection in the outer envelope of a main sequence
   star depends on the spectral type. 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 solar masses 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 portion of a main sequence star that is visible to an observer is
   called the photosphere. This is the layer at which the plasma gas 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. 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, 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.

Nuclear fusion reaction pathways

   Overview of the proton-proton chain
   Enlarge
   Overview of the proton-proton chain
   The carbon-nitrogen-oxygen cycle
   Enlarge
   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 converted
   into energy, according to the mass-energy 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 °K for a small M-class star to 40
   million °K for a massive O-class star.

   In the Sun, with a 10^7 °K core, hydrogen fuses to form helium in the
   proton-proton chain reaction:

          4 ^1H → 2 ^2H + 2 e^+ + 2 ν[e] (4.0 M eV + 1.0 MeV)
          2^1H + 2^2H → 2 ^3He + 2γ (5.5 MeV)
          2^3He → ^4He + 2^1H (12.9 MeV)

   These reactions result in the overall reaction:

          4^1H → ^4He + 2e^+ + 2γ + 2ν[e] (26.7 MeV)

   where e^+ is a positron, γ is a gamma ray photon, ν[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.

   In more massive stars, helium is produced in a cycle of reactions
   catalyzed by carbon—the carbon-nitrogen-oxygen cycle.

   In evolved stars with cores at 10^8 °K 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:

          ^4He + ^4He + 92 keV → ^8*Be
          ^4He + ^8*Be + 67 keV → ^12*C
          ^12*C → ^12C + γ + 7.4 MeV

   For an overall reaction of:

          3^4He → ^12C + γ + 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.

   The example below shows the amount of time required for a star of 20
   solar masses 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.
     Fuel
   material   Temperature
            (million Kelvin) Density
                             (kg/cm³)  Burn duration
                                      τ
      H            37         0.0045  8.1 million years
      He          188          0.97   1.2 million years
      C           870          170        976 years
      Ne         1,570        3,100       0.6 years
      O          1,980        5,550      1.25 years
     S/Si        3,340        33,400      11.5 days
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