Stars
A star is any massive gaseous body in outer space, just like the Sun. Unlike a planet, a star generates energy through nuclear fusion and therefore emits light. All stars except the Sun appear as shining points in the nighttime sky that twinkle because of the effect of the Earth's atmosphere and their distance from us.
The Sun is also a star, but it is close enough to Earth to appear as a disk instead, and to provide daylight.
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 (light from Proxima Centauri takes 4.2 years to reach Earth). Travelling at the orbit speed of the Space Shuttle (5 miles per second -- almost 30,000 kilometers per hour), it would take about 150,000 years to get there.
Distances like this are typical inside galactic discs, where the Sun and Earth are located. Stars can be much closer to each other in the centres of galaxies and globular clusters, or much further apart in galactic halos.
Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old, which is the observed age of the universe. They range in size from the tiny neutron stars (which are actually dead stars) no bigger than a city, to supergiants like the North Star (Polaris) and Betelgeuse, in the Orion constellation, which have a diameter about 1,000 times larger than the Sun about 1.6 terametres. However, these have a much lower density than the Sun.
One of the most massive stars known is Carinae, with 100 -150 times as much mass as the Sun.
Recent work by Donald Figer, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, suggests that 150 solar masses is the upper limit of stars in the current era of the universe. He used the Hubble Space Telescope to observe about a thousand stars in the Arches cluster, a massive young star cluster near the core of the Milky Way, and found no stars over that limit despite a statistical expectation that there should be several.
The reason for this limit is not precisely known, but the Eddington limit is part of the answer. The very 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 star is long extinct, however, and currently only theoretical.
The smallest known star undergoing fusion in its core is AB Doradus C, a companion to AB Doradus A, which has a mass only 93 times that of Jupiter. Smaller bodies are brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants. The minimum mass a star can have is estimated to be in the vicinity of 75 Jupiters.
The smallest known star undergoing fusion in its core is AB Doradus C, a companion to AB Doradus A, which has a mass only 93 times that of Jupiter. Smaller bodies are brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants. The minimum mass a star can have is estimated to be in the vicinity of 75 Jupiters.
Scientifically, stars are defined as self-gravitating spheres of plasma in hydrostatic equilibrium, which generate their own energy through the process of nuclear fusion.
The energy produced by stars radiates into space as electromagnetic radiation, as a stream of neutrinos from the star's core, and as a stream of particles from the star's outer layers (its stellar wind).
The peak frequency of the light depends on the temperature of the outer layers of the star. Besides the emitted visible light, the ultraviolet and infrared components are typically far from negligible. The apparent brightness of a star is measured by its apparent magnitude.
Stellar astronomy is the study of stars and the phenomena exhibited by the various forms/developmental stages of stars.
The majority of stars are gravitationally bound to other stars, forming binary stars.
Larger groups called star clusters also exist.
Stars are not spread uniformly across the universe, but are typically grouped into galaxies. A typical galaxy contains hundreds of billions of stars.
Star Formation and Evolution
As learned by star formation astronomers, stars are born in molecular clouds, large regions of slightly higher density of matter (though still less dense than the inside of an Earthly vacuum chamber), and form by gravitational instability inside those clouds triggered by shockwaves from supernovae.
High mass stars powerfully illuminate the clouds from which they formed. One example of such a nebula is the Orion Nebula.
Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-pressure reactions near the core. Such stars are said to be on the main sequence.
Small stars - called red dwarfs - burn their fuel very slowly and last tens to hundreds of billions of years, far longer than the time elapsed in the universe so far.
According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. They comprise the vast majority of stars and have a diameter and mass of less than one-third that of the Sun (down to 0.08 solar masses, which are brown dwarfs) and a surface temperature of less than 3,500 K. They emit little light, sometimes as little as 1/10,000th that of the sun. Due to the slow rate at which they burn hydrogen, red dwarfs have an enormous estimated lifespan; estimates range from tens of billions up to trillions of years.
Red dwarfs never initiate helium fusion and so cannot become red giants; the stars slowly contract and heat up until all the hydrogen is consumed. In any event, there has not been sufficient time since the Big Bang for red dwarfs to evolve off the main sequence.
The fact that red dwarfs remain on the main sequence while older stars have moved off the main sequence allows one to date star clusters by finding the mass at which the stars turn off the main sequence. In addition, the fact that no red dwarfs have evolved off the main sequence have been observed is evidence that the universe has a finite age.
One mystery which has not been solved as of 2004 is the lack of red dwarf stars with no metals (in astronomy a metal is any element other than hydrogen and helium). The Big Bang model predicts the first generation of stars should have only hydrogen, helium, and lithium. If such stars included red dwarfs, they should still be observable today, but are not. The conventional explanation is that without heavy elements, low mass stars cannot form and the first stars were extremely high mass population III stars which died quickly and produced the metals necessary for low mass stars to form later.
Red dwarf stars are believed to be the most common star type in the universe. Proxima Centauri, the nearest star to the Sun is a red dwarf, (Type M5, magnitude 11.0) as are twenty of the next thirty nearest. However, due to their low luminosity, few are known.
At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs - although none exist yet.
Even at the epoch when black dwarfs exist they will be extremely difficult to detect, emitting thermal radiation at a temperature not much above that of the cosmic microwave background radiation. One of the only ways to detect them may be through their gravitational influence.
Black dwarfs should not be confused with brown dwarfs, which are formed when gas contracts to form a star, but does not possess enough mass to initiate and sustain hydrogen nuclear fusion. (NB: what we now refer to as brown dwarfs were at times called black dwarfs in the 1960s.)
As most stars exhaust their supply of hydrogen, 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 subsume Mercury and Venus.
Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. Larger stars will also fuse heavier elements, all the way to iron, which is the end point of the process.
An average-size star will then shed its outer layers as a planetary nebula. The core that remains will be a tiny ball of degenerate matter not massive enough for further fusion to take place, supported only by degeneracy pressure, called a white dwarf. It will fade into a black dwarf over very long stretches of time.
In larger stars, fusion continues until collapse ends up causing the star to explode in a supernova.
This is the only cosmic process that happens on human timescales; historically, supernovae have been observed as "new stars" where none existed before.
Most of the matter in a star is blown away in the explosion (forming nebulae such as the Crab Nebula) but what remains will collapse into a neutron star (a pulsar or X-ray burster) or, in the case of the largest stars, a black hole.
The blown-off outer layers include heavy elements, which are often converted into new stars and/or planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.
Star Classification
There are different classifications of stars ranging from type O which are very large and bright, to M which is often just large enough to start ignition of the hydrogen.
Some of the more common classifications are O, B, A, F, G, K, M, and can perhaps be more easily remembered using the mnemonic "Oh, Be A Fine Girl, Kiss Me" (variant: change "girl" to "guy"), invented by Annie Jump Cannon (1863-1941).
There are many other mnemonics for star classification; the most frequent addition tacks Right Now Sweetheart for the red dwarf sub-types R, N and S.
Each letter has 9 subclassifications. Our Sun is a G2, which is very near the middle in terms of quantities observed. Most stars fall into the main sequence which is a description of stars based on their absolute magnitude and spectral type.
The Sun is taken as the prototypical star (not because it is special in any way, but because it is the closest and most studied star we have), and most characteristics of other stars are usually given in solar units.
Naming of Stars
Most stars are identified only by catalogue numbers; only a few have names as such. The names are either traditional names (mostly from Arabic), Flamsteed designations, or Bayer designations.
The only body which has been recognized by the scientific community as having competence to name stars or other celestial bodies is the International Astronomical Union (IAU).
A number of private companies (e.g. the "International Star Registry") purport to sell names to stars; however, these names are not 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 how stars are in fact named.
Star Mythology
As well as certain constellations and the Sun itself, stars as a whole have their own mythology. They were thought to be the souls of the dead, or gods/goddesses.
References and additional information
Stellar blast is record-breaker BBC - October 29, 2009
...a gamma-ray burst from a star that died 13.1 billion light-years away.
Most Distant Object Found; Light Pierced "Dark Age" Fog National Geographic - October 28, 2009
The most distant object yet spied in the universe is the
remnant of a star about 13 billion light-years from Earth
that sheds new light on the earliest days of the universe.
Opening Up A Colorful Cosmic Jewel Box Science Daily - October 30, 2009
Simulations Illuminate Universe's First Twin Stars PhysOrg - July 9, 2009
Universe's first stars may have been twins New Scientist - July 9, 2009
Up to half of the universe's first stars may have been born in pairs, a new study suggests.
Rare Magnetar Discovered: Giant Eruption Reveals 'Dead' Star Science Daily - June 16, 2009
Giant Eruption Reveals 'Dead' Star Space Daily - June 18, 2009
An enormous eruption has found its way to Earth after
traveling for many thousands of years across space.
... the dead star belonging to a rare group: the magnetars.
Dead Exploded Star Resurrected in 3-D Space.com - January 9, 2009
Dead Stars Tell Story of Planet Birth PhysOrg - January 5, 2009 < br>
Mystery radio signal could be from universe's first stars New Scientist - January 9, 2009
Team records 'music' from stars BBC - October 23, 2008
Photo: Star Portrait Reveals "Family Tree" National Geographic - August 22, 2008
How stars form amid black hole chaos MSNBC - August 21, 2008
Black hole star mystery 'solved' BBC - August 23, 2008
Astronomers have shed light on how stars can form around a massive black hole, defying conventional wisdom.
'Let there be light' Genesis Moment in a Computer Telegraph.co.uk - August 1, 2008
A primitive star has been born in the heart of a computer, revealing how the basic
building blocks of life were present soon after the Big Bang some 13.7 billion years ago.
Cosmic Rosetta Stone: How The First Stars In The Universe Came Into Existence
Science Daily - August 1, 2008
Big Bang Ripples Formed Universe's First Stars
National Geographic - July 31, 2008
Video Simulation: Universe's First Stars New Scientist - July 31, 2008
Massive Star In Nearby Galaxy Has Mammoth Belt Science Daily - May 28, 2008
Red dwarf emits brightest burst of light ever, according to astronomers MSNBC - May 19, 2008
Newborn Stars: Seeing Dark Filaments Inside A Molecular Cloud Science Daily - March 9, 2008
Stars that Bend Time March 11, 2008
Sun-like Star Flips Its Magnetic Field Like Our Sun Science Daily - February 27, 2008
Planets form twice for two old stars BBC - January 10, 2008
Two old stars may be undergoing a second episode of planet
formation, long after their initial window of opportunity.
"Blue Blobs" in Space Are Odd Stars National Geographic - January 9, 2008
Precursor of Life Molecules Found Around Star Space.com - January 7, 2008
Our Universe: Dark and Messy Space.com - January 7, 2008
White Dwarf Pulses Like A Pulsar Science Daily - January 3, 2008
Cosmic Factories Produce Rubies and Sapphires Live Science - October 9, 2007
Dark matter clues in oldest stars BBC - September 15, 2007
Rare dead star found near Earth BBC - August 20, 2007
Comets Tails and Stars
Astronomers Find New Star 'Family' Live Science - August 8, 2007
Ancient Star Is Nearly as Old as Universe National Geographic - May 12, 2007
Hubble Spies Dazzling Death of a Sunlike Star National Geographic - February 14, 2007
Massive Star Forms by Absorption, Not Collision Scientific American - September 28, 2006
For Stars, Size Determines Destiny, Hubble Confirms Scientific American - August 18, 2006
Stellar explosion revealed in unique detail BBC - July 23, 2006
'Star quakes' shaking up the cosmos News in Science - May 15, 2006
'Dead star' erupts for big show BBC - April 7, 2006
Planets around Dead Stars NASA - April 5, 2006
Merged stars whip up super fields BBC - April 3, 2006
New Kind of Cosmic Object Discovered National Geographic - February 15, 2006
Stars hum themselves to death News in Science - February 10, 2006
NASA's Spitzer Uncovers Hints Of Mega Solar Systems Science Daily - February 8, 2006
Huge Quake Cracks Star Space.com - September 27, 2005
Planet's impact forms dust disk BBC - July 20, 2005
The Biggest Starquake Ever Space.com - July 18, 2005
Hubble spies lord of the stellar rings New Scientist - June 22, 2005
Two burned-out stars are spiralling towards each other New Scientist - June 1, 2005
Bizarre Star Brightens Like Clockwork Scientific American - May 17, 2005
Stars can only grow so big Space.com - March 2005
Light Continues To Echo Three Years After Stellar Outburst Science Daily - February 2005
'Bullet Star' Shines 350 Times Brighter Than The Sun Science Daily - January 2005
Long Lost Star Catalog Found in Plain Sight - Space.com - January 2005
Stunning New Inside Look at Star Birth Space.com - January 2005
Astronomers have identified the three biggest stars known to science BBC - January 2005
Mystery of Magnetic Stars Solved Space.com - November 2004
Deepest Image Of Exploded Star Uncovers Bipolar Jets Science Daily - August 2004
Glowing Results: Rampant Star Birth Left Universal Imprint Space.com - August 2004
Dying Star Goes Out With A Ring Science Daily - August 2004
Comets, Asteroids and Planets around a Nearby Star Space.com - August 2004
Tiny Hot Spot Found on City-Sized Star Space.com - July 2004
Unique Observations Of Newborn Star Provide Information On Solar System's Origin Science Daily - July 2004
Rare Glimpse of Sun-like Star at Birth Space.com - July 2004
Astronomers Measure Mass Of A Single Star - First Since The Sun Science Daily - July 2004
The Heartbeat of a Dying Star Space.com - May 2004
Image of dying star caught by telescope Ananova - April 2004
When did stars first form? Physics Web - April 8, 2004
Hubble Sees Stars as Numerous as Grains of Sand in Nearby Galaxy Hubble - April 8, 2004
The Crazy Cosmos: Stars Near Sun are Wild and Wayward Space.com - April 2004
Chandra X-ray Observatory Images Of N49B, The Remains Of An Exploded Star Science Daily - April 2004
Diamond star thrills astronomers BBC - February 2004
New star emerges from dust cocoon of gas after it was born BBC - February 12, 2004
Mystery of bizarre double star unraveled MSNBC - January 2004
Biggest Brightest Stars Puzzle Astronomers Space.com - January 2004
Found: The best place for alien life BBC - October 2003
Trace gas dates Universe's first stars New Scientist - July 23, 2003
Newfound Stars are Huge, Powerful and Frantic Space.com - July 2003
Astronomers have discovered clusters of stars drifting in what was
thought to be the empty space between the galaxies BBC - July 2003
Astronomers count the stars BBC - July 2003
Strange star puzzles astronomers June 2003 - BBC
Star Search Finds Neighborly Red Dwarf Space.com - May 2003
Hubble glimpses earliest stars May 1, 2003 - BBC
Fifth closest star discovered BBC - February 18, 2003
'Oldest star chart' found January 2003 - BBC
Discovery Of Nearest Known Brown Dwarf Science Daily - January 2003
'Oldest' star found in galaxy BBC - November 2002
Lone neutron star speeds through space
Astronomers investigate pulsar puzzle BBC - August 3, 2000
Scientists 'create' a neutron star BBC - November 15, 1999
White dwarfs are commonly believed to be stellar corpses,
the dense, slowly cooling remnants of low to medium mass stars.
So scientists were surprised to find a high-energy x-ray pulse
coming from the white dwarf known as AE Aquarii. Scientists say the
emission looks like that of a pulsar - a rotating neutron star formed
after a supernova, the life-ending explosion of a massive star.
Starburst Cluster in NGC 3603 NASA - October 5, 2007
Star "Jewel Box" Spotted by Hubble National Geographic - October 3, 2007
Extreme Star Cluster Bursts Into Life Science Daily - October 3, 2007
Glittering star cluster is galactic heavyweight New Scientist - October 3, 2007
From NASA: A mere 20,000 light-years from the Sun lies NGC 3603, a resident of the nearby Carina spiral arm of our Milky Way Galaxy. NGC 3603 is well known to astronomers as one of the Milky Way's largest star-forming regions. The central open star cluster contains thousands of stars more massive than our Sun, stars that likely formed only one or two million years ago in a single burst of star formation. In fact, nearby NGC 3603 is thought to contain a convenient example of the massive star clusters that populate much more distant starburst galaxies. Surrounding the cluster are natal clouds of glowing interstellar gas and obscuring dust, sculpted by energetic stellar radiation and winds. Recorded by the Hubble's Advanced Camera for Surveys, the image spans about 17 light-years.
'Starquake' reveals star's powerful magnetic field New Scientist - September 21, 2007
A computer model of the early Universe indicates the first
stars could have formed in spectacular, long filaments.
Odd Star Sheds Cometlike Tail, Astronomers Say National Geographic - August 16, 2007
A Star with a Comet's Tail NASA - August 16, 2007
Colossal tail trails dying star BBC - August 15, 2007
Ancient star nearly as old as the universe MSNBC - May 12, 2007
The white dwarf star in the Ophiuchus constellation has exhausted
its own nuclear fuel but is now stealing it from a neighboring giant.
NASA's Spitzer Space Telescope has uncovered new
evidence that planets might rise up out of a dead star's ashes.
About 500 milliseconds after the core collapses, the inner core begins to vibrate wildly. And after 600, 700 or 800 milliseconds, this oscillation becomes so vigorous that it sends out sound waves - it hums a note around 'middle C', astronomers say.
... two huge "hypergiant" stars circled by monstrous disks of what might be planet-forming dust.
First evidence of cracks in a neutron star's crust. The star cracked
when it was rocked by the strongest "starquake" ever recorded.
Astronomers hope these oscillations will crack the
mystery of what neutron stars are made of.
so quickly they may ripple the fabric of space-time.
The long lost star catalog of Hipparchus has been under our noses or,
more accurately, slightly above them for more than 1,800 years.
Ancient Astronomer's Work Found on Roman Statue
Lost Ancient Star Catalogue Found - Discovery - January 2005
The similarity between a common bar magnet and a star
There's an intriguing glow across the universe that astronomers
have been trying to pin down for years. Recent observations
appear to have found the source: compact galaxies in the
early universe that were generating stars at a furious pace.
Astronomers have detected a hot spot about the size of a
football field on a city-sized star that is 500 light-years away.
Hubble telescope snares space bug BBC - April 2004
Bug Nebula, a huge mass of gas and dust which hides a hot, dying star.
The Bug Nebula, or NGC 6302, is about 4,000 light-years from Earth in
the southern constellation of Scorpius.
The Chandra image of N49B, the remains of an exploded star, shows a cloud
of multimillion-degree gas that has been expanding for about 10,000 years.
Hubble sends dramatic image of distant star not unlike Van Gogh's "Starry Night" - Reuters - March 2004
- 
The Hubble space telescope captured an image of a distant star that bears resemblance to the famous Vincent van Gogh painting "Starry Night", NASA and the European Space Agency announced. The spectacular image taken February 8 showed the star, V838 Monocerotis (V838 Mon), surrounded by an expanding halo of light "complete with never-before-seen spirals of dust swirling across trillions of kilometers of interstellar space", a statement from the agencies said. "The illumination of interstellar dust comes from the red supergiant star at the middle of the image, which gave off a flashbulb-like pulse of light two years ago," the statement added, describing the image as "nature's own piece of performance art". The outburst event from V838 Mon, located 20,000 light years away from Earth, is probably the source of the dust haze which it illuminates. Van Gogh painted "Starry Night" while at the Saint-Remy-de-Provence asylum in southern France, some 13 months before he committed suicide at the age of 37 at Auvers-sur-Oise, near Paris, in 1890.
Twinkling in the sky is a diamond star of 10 billion trillion trillion carats.
Black hole consumes star but can't digest it all -
This artist's conception shows a super-giant star in front of a black hole in the SS 433 double-star system.
The black hole pulls material from the star, and that material whirls into a disk (shown in white) surrounding
the black hole. All that activity causes the disk to throw off high-speed jets, shown in blue and red.
A team of researchers has found what appears to be the most luminous known
star around, one so massive that it shouldnąt have formed in the first place.
The star, known as LBV 1806-20, tips the scales of stellar masses at about 150
times the heft of the Sun. It shines up to 40 million times brighter than the Sun.
It is the 37th brightest star in the constellation of Gemini to be precise,
42 light years away from Earth and rather like our own Sun.
The star, 37 Gem, is top of a shortlist of the 30 most promising places to
look for life drawn up by Maggie Turnbull of the University of Arizona in Tucson.
Astronomers in Australia say there are 10 times more stars in the visible
universe than all the grains of sand on the world's beaches and deserts.
Achernar is one of the most distorted stars ever observed by astronomers.
It is shaped like a child's spinning top.
Astronomers have stumbled onto a previously unknown star in Earth's stellar
neighborhood, a red dwarf that appears to be the third-closest star system
to our own - appears to be the third closest star to our solar system.
200 million years after the Big Bang
The oldest image of a star pattern, that of the famous constellation of Orion,
has been recognised on an ivory tablet some 32,500 years old.
A brown dwarf is a 'failed star' Bright Southern Star Epsilon Indi Has Cool, Substellar Companion
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