HOME WEB NEWS IMAGES CLASSIFIEDS YELLOW PAGESPOLLS - SURVEYS WIKI COUNTRIES PHOTOS US UK INDIA
Cities Stocks Weather Movies Coupons Travel Blogs Health Horoscope Maps ASIA EUROPE Classifieds Yellow Pages
|
 2
|
This article is about the astronomical object. For other uses, see Star (disambiguation).
The Pleiades, an open cluster of stars in the constellation of Taurus. NASA photo
A star is a massive, luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun. A star shines because thermonuclear fusion in its core releases energy that traverses the star\'s interior and then radiates into outer space. Almost all elements heavier than hydrogen and helium were created inside the cores of stars.
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 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 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.Bahcall, John N. (June 29, 2000). How the Sun Shines. Nobel Foundation. Retrieved on 2006-08-30. 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, those stars having at least 0.4 times the mass of the SunRichmond, Michael. Late stages of evolution for low-mass stars. Rochester Institute of Technology. Retrieved on 2006-08-04. expand 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.Stellar Evolution & Death. NASA Observatorium. Retrieved on 2006-06-08.
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.Iben, Icko, Jr. (1991). "Single and binary star evolution". Astrophysical Journal Supplement Series 76: 55–114.
Contents |
Historically, stars have been important to civilizations throughout the world. They have been used in religious practices and 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.George Forbes (1909). History of Astronomy (Free e-book from Project Gutenberg), London: Watts & Co.. 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.Tøndering, Claus. Other ancient calendars. WebExhibits. Retrieved on 2006-12-10. 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 the nearest star, the Sun.
The oldest, accurately-dated star chart appeared in Ancient Egypt in 1,534 BCE.von Spaeth, Ove (1999). "Dating the Oldest Egyptian Star Map". Centaurus International Magazine of the History of Mathematics, Science and Technology 42 (3): 159-179. Retrieved on 2007-10-21. Islamic astronomers gave Arabic names to many stars which are still used today, and they invented numerous astronomical instruments which could compute the positions of the stars. In the 11th century, Abū Rayhān al-Bīrūnī described the Milky Way galaxy as multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.Zahoor, A. (1997). Al-Biruni. Hasanuddin University. Retrieved on 2007-10-21.
In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.D. H. Clark, F. R. Stephenson (1981-06-29). "The Historical Supernovae". Supernovae: A survey of current research; Proceedings of the Advanced Study Institute: 355–370, Cambridge, England: Dordrecht, D. Reidel Publishing Co.. Retrieved on 2006-09-24. 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 actually other suns, and may have other planets, possibly even Earth-like, in orbit around them,Drake, Stephen A. (August 17, 2006). A Brief History of High-Energy (X-ray & Gamma-Ray) Astronomy. NASA HEASARC. Retrieved on 2006-08-24. an idea that had been suggested earlier by such ancient Greek philosophers as Democritus and Epicurus.Exoplanets. ESO (July 24, 2006). Retrieved on 2006-10-11. 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.Hoskin, Michael (1998). The Value of Archives in Writing the History of Astronomy. Space Telescope Science Institute. Retrieved on 2006-08-24.
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.Proctor, Richard A. (1870). "Are any of the nebulæ star-systems?". Nature: 331–333. 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.MacDonnell, Joseph. Angelo Secchi, S.J. (1818–1878) the Father of Astrophysics. Fairfield University. Retrieved on 2006-10-02. 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.Aitken, Robert G. (1964). The Binary Stars. New York: Dover Publications Inc..
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.A. A. Michelson, F. G. Pease (1921). "Measurement of the diameter of Alpha Orionis with the interferometer". Astrophysical Journal 53: 249–259.
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.Albrecht Unsöld (1969). The New Cosmos. New York: Springer-Verlag.
With the exception of supernovae, individual stars have primarily been observed in our Local Group of galaxies,Battinelli, Paolo; Demers, Serge; Letarte, Bruno (2003). "Carbon Star Survey in the Local Group. V. The Outer Disk of M31". The Astronomical Journal 125 (3): 1298-1308. Retrieved on 2007-02-04. —by way of example. and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our galaxy"Millennium Star Atlas marks the completion of ESA\'s Hipparcos Mission", ESA, December 8, 1997. Retrieved on 2007-08-05. ). But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.Villard, Ray; Freedman, Wendy L. (October 26, 1994). Hubble Space Telescope Measures Precise Distance to the Most Remote Galaxy Yet. Hubble Site. Retrieved on 2007-08-05. 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"Hubble Completes Eight-Year Effort to Measure Expanding Universe", Hubble Site, May 25, 1999. Retrieved on 2007-08-02. (see Cepheids). However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed; the only exception was faint image of a large star cluster, containing hundreds of thousands of stars, one billion light years away;"UBC Prof., alumnus discover most distant star clusters: a billion light-years away.", UBC Public Affairs, January 8, 2007. Retrieved on 2007-08-02. ten times the distance of the most distant star cluster previously observed.
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.Coleman, Leslie S.. Myths, Legends and Lore. Frosty Drew Observatory. Retrieved on 2006-08-13. 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," known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken. (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers).
Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later 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).The Naming of Stars. National Maritime Museum. Retrieved on 2006-08-13. 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.Adams, Cecil (April 1, 1998). Can you pay $35 to get a star named after you?. The Straight Dope. Retrieved on 2006-08-13.
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:
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).
Stars are formed within molecular clouds; large regions of high density (though still less dense than the inside of an earthly vacuum chamber) in the interstellar medium. 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.P. R. Woodward (1978). "Theoretical models of star formation". Annual review of astronomy and astrophysics 16: 555–584. As massive stars are formed from these clouds, they powerfully illuminate and ionize the clouds from which they formed, creating an H II region.
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
As the cloud collapses, individual conglomerations of dense dust and gas form what 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.Seligman, Courtney. Slow Contraction of Protostellar Cloud. Self-published. Retrieved on 2006-09-05. 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.J. Bally, J. Morse, B. Reipurth (1996). "The Birth of Stars: Herbig-Haro Jets, Accretion and Proto-Planetary Disks". Piero Benvenuti, F.D. Macchetto, and Ethan J. Schreier Science with the Hubble Space Telescope - II. Proceedings of a workshop held in Paris, France, December 4–8, 1995: 491, Space Telescope Science Institute. Retrieved on 2006-07-14.
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.J. G. Mengel, P. Demarque, A. V.Sweigart, P. G. Gross (1979). "Stellar evolution from the zero-age main sequence". Astrophysical Journal Supplement Series 40: 733–791. 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.Sackmann, I.-Juliana; Arnold I. Boothroyd, Kathleen E. Kraemer (11 1993). "Our Sun. III. Present and Future". Astrophysical Journal 418: 457.
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,B. E. Wood, H.-R. Müller, G. P. Zank, J. L. Linsky (2002). "Measured Mass-Loss Rates of Solar-like Stars as a Function of Age and Activity". The Astrophysical Journal 574: 412–425. 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.de Loore,, C.; de Greve, J. P.; Lamers, H. J. G. L. M. (1977). "Evolution of massive stars with mass loss by stellar wind". Astronomy and Astrophysics 61 (2): 251–259. Stars that begin with more than 50 solar masses can lose over half their total mass while they remain on the main sequence.The evolution of stars between 50 and 100 times the mass of the Sun. Royal Greenwich Observatory. Retrieved on 2006-09-07.
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 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 1010 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 fieldsN. Pizzolato, P. Ventura, F. D\'Antona, A. Maggio, G. Micela, S. Sciortino (2001). "Subphotospheric convection and magnetic activity dependence on metallicity and age: Models and tests". Astronomy & Astrophysics 373: 597–607. and modify the strength of the stellar wind.Mass loss and Evolution. UCL Astrophysics Group (June 18, 2004). Retrieved on 2006-08-26. 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.)
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.I.J. Sackmann, A.I. Boothroyd, K.E. Kraemer (1993). "Our Sun. III. Present and Future". Astrophysical Journal 418: 457. However, the Earth will be stripped of its oceans and atmosphere as the Sun\'s luminosity increases several thousandfold.
In a red giant of up to 2.25 solar masses, hydrogen fusion proceeds in a shell-layer surrounding the core.Hinshaw, Gary (August 23, 2006). The Life and Death of Stars. NASA WMAP Mission. Retrieved on 2006-09-01. 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.Iben, Icko, Jr. (1991). "Single and binary star evolution". Astrophysical Journal Supplement Series 76: 55–114. Retrieved on 2007-03-03.
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.
.jpg/58px-Betelgeuse star (Hubble).jpg)
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.What is a star?. Royal Greenwich Observatory. Retrieved on 2006-09-07.
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 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.
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.J. Liebert (1980). "White dwarf stars". Annual review of astronomy and astrophysics 18 (2): 363–398. 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.Introduction to Supernova Remnants. Goddard Space Flight Center (April 6, 2006). Retrieved on 2006-07-16.
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.C. L. Fryer (2003). "Black-hole formation from stellar collapse". Classical and Quantum Gravity 20: S73-S80. 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.
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.Szebehely, Victor G.; Curran, Richard B. (1985). Stability of the Solar System and Its Minor Natural and Artificial Bodies. Springer. ISBN 9027720460. 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 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.Harvard-Smithsonian Center for Astrophysics (January 30, 2006). "Most Milky Way Stars Are Single". Press release. Retrieved on 2006-07-16.
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.What is a galaxy? How many stars in a galaxy / the Universe?. Royal Greenwich Observatory. Retrieved on 2006-07-18. While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered."Hubble Finds Intergalactic Stars", Hubble News Desk, January 14, 1997. Retrieved on 2006-11-06. Astronomers estimate that there are at least 70 sextillion (7×1022) stars in the observable universe."Astronomers count the stars", BBC News, July 22, 2003. Retrieved on 2006-07-18. That is 230 billion times as many as the 300 billion in the Milky Way.
The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion (1012) 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.3.99 × 1013 km / (3 × 104 km/h × 24 × 365.25) = 1.5 × 105 years. Distances like this are typical inside galactic discs, including in the vicinity of the solar system.J. Holmberg, C. Flynn (2000). "The local density of matter mapped by Hipparcos". Monthly Notices of the Royal Astronomical Society 313 (2): 209–216. Retrieved on 2006-07-18. 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."Astronomers: Star collisions are rampant, catastrophic", CNN News, June 2, 2000. Retrieved on 2006-07-21. 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 .J. C. Lombardi, Jr., J. S. Warren, F. A. Rasio, A. Sills, A. R. Warren (2002). "Stellar Collisions and the Interior Structure of Blue Stragglers". The Astrophysical Journal 568: 939–953.
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.
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.Frebel, A.; Norris, J. E.; Christlieb, N.; Thom, C.; Beers, T. C.; Rhee, J.. "Nearby Star Is A Galactic Fossil", Science Daily, May 11, 2007. Retrieved on 2007-05-10.
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.Naftilan, S. A.; Stetson, P. B. (2006-07-13). How do scientists determine the ages of stars? Is the technique really accurate enough to use it to verify the age of the universe?. Scientific American. Retrieved on 2007-05-11.Laughlin, G.; Bodenheimer, P.; Adams, F. C. (1997). "The End of the Main Sequence". The Astrophysical Journal 482: 420–432. Retrieved on 2007-05-11.
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.A "Genetic Study" of the Galaxy. ESO (September 12, 2006). Retrieved on 2006-10-10. The portion of heavier elements may also be an indicator of the likelihood that the star has a planetary system.D. A. Fischer, J. Valenti (2005). "The Planet-Metallicity Correlation". The Astrophysical Journal 622 (2): 1102–1117.
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.Signatures Of The First Stars. ScienceDaily (April 17, 2005). Retrieved on 2006-10-10. 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.Feltzing, S.; Gonzalez, G. (2000). "The nature of super-metal-rich stars: Detailed abundance analysis of 8 super-metal-rich star candidates". Astronomy & Astrophysics 367: 253-265. Retrieved on 2007-11-27. There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.Gray, David F. (1992). The Observation and Analysis of Stellar Photospheres. Cambridge University Press. ISBN 0521408687.
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."The Biggest Star in the Sky", ESO, March 11, 1997. Retrieved on 2006-07-10.
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 in order 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.Ragland, S.; Chandrasekhar, T.; Ashok, N. M. (1995). "Angular Diameter of Carbon Star Tx-Piscium from Lunar Occultation Observations in the Near Infrared". Journal of Astrophysics and Astronomy 16: 332. Retrieved on 2007-07-05.
Stars range in size from neutron stars, which vary anywhere from 20 to 40 km in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter approximately 650 times larger than the Sun—about 0.9 billion kilometres. However, Betelgeuse has a much lower density than the Sun.Davis, Kate (December 1, 2000). Variable Star of the Month—December, 2000: Alpha Orionis. AAVSO. Retrieved on 2006-08-13.
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.Hipparcos: High Proper Motion Stars. ESA (September 10, 1999). Retrieved on 2006-10-10.
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.Johnson, Hugh M. (1957). "The Kinematics and Evolution of Population I Stars". Publications of the Astronomical Society of the Pacific 69 (406): 54. 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. B. Elmegreen, Y. N. Efremov (1999). "The Formation of Star Clusters". American Scientist 86 (3): 264. Retrieved on 2006-08-23.
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.Brainerd, Jerome James (July 6, 2005). X-rays from Stellar Coronas. The Astrophysics Spectator. Retrieved on 2007-06-21.
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.Berdyugina, Svetlana V. (2005). Starspots: A Key to the Stellar Dynamo. Living Reviews. Retrieved on 2007-06-21. During the Maunder minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity.
One of the most massive stars known is Eta Carinae,Nathan, Smith (1998). The Behemoth Eta Carinae: A Repeat Offender. Astronomical Society of the Pacific. Retrieved on 2006-08-13. 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."NASA\'s Hubble Weighs in on the Heaviest Stars in the Galaxy", NASA News, March 3, 2005. Retrieved on 2006-08-04. 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
The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more,"Ferreting Out The First Stars", Harvard-Smithsonian Center for Astrophysics, September 22, 2005. Retrieved on 2006-09-05. 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."Weighing the Smallest Stars", ESO, January 1, 2005. Retrieved on 2006-08-13. 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.Boss, Alan (April 3, 2001). Are They Planets or What?. Carnegie Institution of Washington. Retrieved on 2006-06-08.Shiga, David (August 17, 2006). Mass cut-off between stars and brown dwarfs revealed. New Scientist. Retrieved on 2006-08-23. 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."Hubble glimpses faintest stars", BBC, August 18, 2006. Retrieved on 2006-08-22. 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 c
