A star is a body that at some time in its life generates itslight and heat by nuclear reactions, specifically by the fusion ofhydrogen into helium underconditions of enormous temperature and density. When hydrogenatoms merge to create the next heavier element, helium, mass islost, the mass (M) converted to energy (E) through Einstein'sfamous equation E = mc squared, where "c" is the speed of light. TheSun is powered by hydrogen fusion, asare many of the other stars you see at night. The fusion doesnot take place throughout the star, but only in its deepinterior, in its core, where it is hot enough. The temperatureat the center of theSun is 15.6 million degrees Kelvin (K =centigrade degrees above absolute zero, -273 C), and the densityis 14 times that of lead. About 40% of the mass of the Sun,occupying about 30% of the radius, is capable of fusing hydrogen. Even under these extreme conditions, the Sun (as well as allother stars) is still a gas throughout. That said, things getmore complicated, as nature creates "substars" called "brown dwarfs" that do not have enoughmass and therefore internal heat to run fullfusion. Even though they do not abide by the formal definitionof a "star," they are still referred to as "stars" even if theirmasses are not much greater than those of planets.
In the second century BC, the Greek astronomer Hipparchus dividedthe stars into six brightness groups calledmagnitudes(nowapparent visual magnitudes (m or V), first magnitudethe brightest, sixth the faintest. The system is still usedtoday, though with a mathematical definition (a star of onemagnitude is 2.512... times brighter than the next fainter) thattakes the very brightest stars and planets through magnitude zeroand into negative numbers. Through the telescope we see muchfainter, to near 30th magnitude (4 billion times fainter than thehuman eye can see alone). Though stars bear some resemblance totheSun, they appear as points in the skybecause they are so far away, the nearest,Alpha Centauri, four light years away. Thelight year is the distance a ray of light will travel in ayear at 300,000 kilometers (186,000 miles) per second, so onelight year is about 10 trillion kilometers (63,000Astronomical Units, where the AU is the average distancebetween the Earth and the Sun). The stars are so far thatdistances were not measured until 1846, by means ofparallax (viewing the star from opposite sides of theEarth's orbit). The most distant stars the unaided eye can seeare over 1000 light years away, which is about the practicallimit of parallax measures. The apparent visual magnitude of astar depends on true visual luminosity (in watts) and distance. To compare true visual luminosities, astronomers calculate theabsolute visual magnitude (M), the apparent magnitude thestar would have were it at a distance of 32.6 light years (10parsecs, where the parsec is the professional unit ofdistance, equal to 3.26 light years). The absolute visualmagnitude of the Sun is +4.83. Absolute visual magnitudes rangefrom around -10 (a million times more luminous than the Sun) tobelow +20 (a million times fainter).
We need to set the stars in context. All those you see at nightare part of our local collection of stars, all part of ourGalaxy. Trillions of othergalaxies flock the Universe, ours one of thelarger ones. The principal part of our Galaxy (our own with acapitol "G") is in the shape of a flat disk about 100,000 lightyears across that contains some 200 billion stars. Our Sun istoward the nominal edge, about 25,000 light years from thecenter, the whole structure at the Sun's distance rotating with aperiod of 200 million years. The "edge" is not sharp, but justgradually fades away to much greater distances. A large portionof the disk's stars are set within pinwheel-like spiral arms thatover millions of years come and go, the stars moving in and outof them as they orbit the Galaxy's center. The arms seem to comeoff of a central bar. Since we are in the disk, we see thecombined light of its billions of stars around our head as thefamed "Milky Way," thecenter of the Galaxy (which contains a supermassiveblack hole of three million solarmasses) located behind the thick star clouds ofSagittarius. Theageof the disk is about 10 billion years. Surrounding the disk is athinly populated rather spherical halo that seems to date toabout 14 billion years.
To create the conditions for such "thermonuclear fusion," starsmust be massive. The Sun has the mass of 333,000 Earths. Starscan range up to about 100 times the mass of the Sun (at whichpoint nature stops making them) down to around 7.5% that of theSun, at which point the internal temperature is not high enoughto run the full range ofnuclear reactions(which requires at least 7 million degrees Kelvin). "Substars"below the 7.5% limit, called "brown dwarfs,"do exist in significant numbers however, and down to around 1/80the solar mass (13 Jupiter-masses) can fuse their naturaldeuterium (heavy hydrogen, withan extra neutron). The lower limit to brown dwarf (substellar)masses is not known. Masses aremeasuredfromdouble stars and can be calculatedfrom luminosity and temperature using the theories of stellarstructure.
Stars are made of the samechemicalelements as found in the Earth, though not in the sameproportions, thechemicalcompositions found from the stars'spectra. Most stars are made almostentirely of hydrogen (about 90% by number of atoms) and helium(about 10%), elements that are relatively rare on our planet. About a tenth of a percent is left over, that tenth containingall the other elements found in nature. Of these, oxygen usuallydominates, followed by carbon, neon, and nitrogen. Of themetals, iron usually dominates. Nevertheless, there is only oneatom of oxygen in the Sun for every 1200 hydrogen atoms and onlyone of iron for every 32 oxygen atoms. However, within thistenth of a percent, the proportions of the numbers of atoms inthe Sun is rather similar to what we find here, in the Earth'scrust. Other stars can deviate considerably, depending on theirstates of aging or upon where they are in theGalaxy. Halo stars, including globularclusters, typically have heavy element contents only a hundredththat found in disk stars, the result of their being older.
Stars are supported, kept from shrinking under their own gravity,by energy generated by internal fusion of lightatoms into heavier ones. Fusion ofhydrogen into helium can take place only under the extremeconditions of temperature and density found in a star's deepcore. The "proton-proton chain" operates inordinary stars (those that have not yet begunthedeath process) with masses more orless like that of the Sun and under (while higher mass stars doit by thecarbon cycle. It begins when twoprotons (bare hydrogen atoms) ram together strongly enough toovercome the mutual repulsion caused by their positive electriccharges and get close enough to stick together under the "strongforce" (which operates only over a very short range). One of theprotons ejects its positive charge in the form of a "positron," apositive electron that hits a normal electron to generate energyin the form ofgamma rays. The conversion creates adeuterium (heavy hydrogen)nucleus as well as a tiny particle called a "neutrino." Detection of neutrinos on Earth allow us to "see" directly intothe solar center. The fusion of the deuterium with anotherproton produces a light form of helium (with two protons and oneneutron), while the fusion of two light helium atoms into anormal helium atom with two protons and two neutrons (with theejection of two protons) completes the process, each reactiongenerating heat and light as a result of a slight loss of mass.
Higher mass stars (with masses greater than about 1.5 times thatof the Sun) fuse hydrogen into helium via the "carbon cycle,"which works only under high-temperature conditions, but is thenmore efficient than theproton-proton chain. It begins when a normal carbonatom (C-12, with 6 protons and 6neutrons) picks up a proton to make radioactivenitrogen-13, one of whoseprotons ejects a positron (positive electron) to make stablecarbon-13 (with the additional ejection of a neutrino). Carbon-13 plus a proton makes normal nitrogen-14, while an additionalproton collision makes oxygen-15, which (like N-13) decays intonitrogen-15. The N-15 collects another proton, and then fallsapart into the original carbon-12 and a helium nucleus. Eachevent produces some energy either itself or through thecollisions of positrons and electrons.
The space between the stars is filled with dusty gas. Thickdustclouds can even be seen with the naked eye within the MilkyWay blocking the light of distant stars and providing much of theMilky Way's structure. Interstellar matter is compressed by theGalaxy's winding spiral arms. The clouds can be furthercompressed through collisions or by blast waves from explodinghigh-mass stars ( supernovae). Lumpsof matter therefore form within the interstellar clouds. Iftheir gravity is great enough, they can condense into one or morestars. Contraction causes more rapid spin, which creates a diskaround the birthing star, from which it can draw matter. Furthercondensation within the disk can createplanets (or even stellarcompanions). The contraction of formingstars raises the internal temperature, finally to the point ofignition ofhydrogen fusion. Gravity wouldlike to make the star as small as possible, but the fusionreactions stabilize it and keep it from contracting any further. The whole life story of a star from here on out is told by thebattle between gravity and nuclear fusion, first one, then theother getting the upper hand. New high mass stars commonly lightup their surroundings to producediffuse nebulae like theOrionNebula.
For decades, astronomers predicted the existence of substars wenow call "brown dwarfs," stars too small and light (ofinsufficient mass, less than 0.073 solar masses) to run the fullnuclear fusion process (from ordinary hydrogento helium). After all, thestar formationprocess should "know" nothing of the conditions under whichnuclear reactions should turn on. Brown dwarfs (which are stillcalled "stars") turned out to be so cool that only new infraredtechnologies could find them. We now know they are very common,so common that newclasses, L and T(cooler than M) had to be made for them. Between 0.073 solarmasses (78 Jupiter-masses) and 13 Jupiter-masses, brown dwarfs dofuse their naturaldeuterium(heavy hydrogen, with an extra neutron) to helium. Below 13Jupiters, fusion stops altogether. As noted above, the lower endof brown dwarf masses is not known. They quite likely overlapthe masses of planets.Planets are bycurrent definition made from the "bottom up," accumulated fromdust in disks surrounding new stars, while stars (including browndwarfs) are made from the "top down," by direct condensation frominterstellar gases. But here even the definitions becomeconfused and might overlap as well.
As a new star condenses from a gaseous lump in interstellarspace, it spins faster, the outer parts of the contracting cloudspinning out into a dustydisk. The dust particles, in orbit about the new star, accumulate,building themselves into planets. Here at home, the planets thatformed close to the Sun (Mercury through Mars) were in anenvironment too hot to incorporate much water or light atoms likehydrogen, so they are made of heavy stuff like iron, silicon, andoxygen. In the outer System, the planets contain huge amounts ofhydrogen and helium and could grow large, their satellites madelargely of water ice.Other stars shouldgrow planets too,planets thatcould be quite different from our own and that are now beingdiscovered.
There are many kinds andclasses of stars. Those that are actively fusing hydrogeninto helium in the middle, that is, in their cores (eitherthrough theproton-proton chain or thecarbon cycle), are called "main sequence" stars. (For historical reasons, main sequence stars are also commonlyreferred to as "dwarfs"). The main sequence is the first stagefollowing birth. In general, main sequence stars have chemicalcompositions similar to that of the Sun. The higher the mass ofthe main sequence star, the greater its diameter and the higherits surface temperature. Dimensions range from about 10% thesize of the Sun (which is 1.5 million kilometers -- 109 Earths --across) to just over ten times solar, and surface temperaturesfrom under 2000 degrees Kelvin to about 49,000 K (the Sun'ssurface is at 5780 K). Around the beginning of the 20th century,astronomers divided the stars (of all kinds includinggiants,supergiants,and others) into seven basic lettered groups that they laterlearned were related to their surface temperatures, which for themain sequence are: O (above 31,500 K), B (10,000 - 31,500 K), A(7500 - 10,000 K), F(6000 -7500 K), G(5300 - 6000 K), K(3800 -5300 K), and M (2100 - 3800 K). A century later, two moreclasses were added to account for faint red stars turned up bynew technologies: class L (1200 - 2100 K) and T (below 1200 K),the whole set now OBAFGKMLT. Class L is a mixture of dwarfs andbrown dwarf substars, while class T consistsentirely of brown dwarfs. The Sun is a G star. The system isdecimalized, making the Sun class G2.Examples of main sequence stars areAcrux,Vega,Sirius,Porrima,Chara,Alpha Centauri A and B, andProxima Centauri. The classes areactually derived from the stars'spectra. The stellar astronomer'sgreatest tool is theHR diagram, a plot ofabsolute visual magnitude against spectral class, in which we cansee nearly all of the stages of stellar life and death. On it,the main sequence is a band that runs from the highest-masshydrogen-fusing stars at the upper left to the lowest masses atthe lower right.
Since the color of a heated body depends on temperature, thedifferentclasses take on different,though subtle, colors, from slightly reddish or orange for classM to orange-yellow for K, through yellow-white to bluish forclasses B and O. Star colors can be noted rather easily evenwith the unaided eye, especially when those close together (a indouble star pairings) contrast againsteach other. Stars of classes L and T, none of which are visibleto the naked eye, range from red through deep red to "infrared"(these optically invisible under any circumstances). Carbonstars likeR Leporis, whose blue spectrahave been removed bylineabsorption, are also deep red. Most of these areadvanced giants. Color can be expressednumerically by the difference inmagnitudes measured at differentwavelengths. The observed colorof a star compared to the color expected from the spectral classallows the calculation of the dimming of the starlight byinterstellar dust.
Main sequence (dwarf) stars have only a certain amount ofinternal fuel available within their hot cores. When thehydrogen fuel has all turned to helium, the stars begin to dieand to produce a number of other different kinds: lower massstars becomegiants, while those of highermass (above roughly 8 or 9 solar masses) intosupergiants. Giants then die aswhite dwarfs, while supergiants explodeassupernovae. The whole process iscommonly known asstellar evolution. Because higher massstars use their hydrogen fuel much more quickly than lower massstars, those of higher mass live shorter lives. The Sun has a 10billion year main sequence lifetime (of which half is gone). Themost massive stars live only a couple million years, the leastmassive for trillions, so long that no star with a mass less thanabout 0.8 solar masses has ever died in the history of theGalaxy. From theory, we calculate that such a 0.8 solar massstar should live for about 12-13 billion years. The Galaxyshould be about as old as its oldest stars, and is thus 12-13billion years old, in accord with the 13.7 billion year age ofthe Universe found from itsexpansionrate.
Begin with stars more or less like the Sun, those with massesfrom about 0.8 times that of the Sun to about 5 times the solarmass. When the fuel in a solar-type star's core runs out, thehelium core contracts under the effect of gravity and heats up. Hydrogen fusion then expands into a shell around the old burnt-out core, and so much energy is produced that the starbrightens and expands by many times over, the expansion coolingthe surface, turning the star into a class Mred giant. When the core temperature hits around 100 million degrees Kelvin,the helium is hot enough to fuse into carbon (through the near-simultaneous collision of three helium atoms) and even a bitfurther, into oxygen. This new power source stops the core'scontraction and the star stabilizes for a time, dimming andheating somewhat at the surface. We commonly see these helium-fusing stars as yellow-orange type K giants. Goodexamples of giant stars areAldebaran andArcturus. Such stars can have diameterstens of times that of the Sun. The giant and subsequent stagesup to the actual death of the star (the end of nuclear fusion)takes roughly 10 or 20 percent of the main sequence lifetime. From about 5 solar masses to 9 or so, helium fusing stars havehigher temperatures and may appear as class F and G giants andevensupergiants.
Giants can also be defined strictly by theirspectra. On theHR diagram, the giants run roughly from themiddle toward the upper right (higher luminosity), where they arefusing helium, are about to do so, or have already done so. Class A and B giants are only somewhat cooler than dwarfs of thesame absolute visual brightness and are not yet fusing helium. Among G and K giants, because of their lower gas densities,temperatures are up to a few hundred degrees cooler than they arefor main sequence dwarfs of the same class.
Between the dwarf and giant stages, stars appear assubgiants. Likegiants,dwarfs, andsupergiants, they can be defined by theirspectra and position on theHR diagram. In the context of stellar evolution, they are stars that havejust given up core hydrogen fusion or are about to do so and,with helium cores, are making the transition to becoming truegiants.
When more massive stars (2 to 8 times that of the Sun) passthrough mid-temperatures either on their way to fusing helium orduring various stages of helium fusion, they can become unstableand pulsate in size, temperature, and luminosity. The first ofthese discovered,Delta Cephei, gavethe name "Cepheid" variable to the group. Cepheids, usuallyclassed as F and G supergiants (though not as massive as truesupergiants), vary by a couple to a fewmagnitudes over periods of one to 100 days. A strict relationbetween absolute magnitude and pulsation period allows us todetermine their distances (period gives absolute brightness, andcomparison to apparent brightness yields distance.) Cepheids arethe major keys to learning distances toothergalaxies. The brightest Cepheid in the sky is Polaris, though the variations are toosmall to be seen by eye. Cepheids occupy the upper range of theHR diagram's "instability strip."
When the helium in the core has turned to carbon and oxygen, thecore shrinks again, and the helium begins to fuse to carbon andoxygen in a shell around the old core, this shell surrounded byanother one fusing hydrogen into helium, the two turning on andoff in sequence. The star now brightens again, expands evenmore, and becomes cooler and even redder than before. As thestar brightens it becomes unstable and begins to pulsate, thepulsations making it vary, or change in brightness. The starbecome so huge, near or greater than the orbit of the Earth, thatthe pulsations can take a year or more. The first of thesefound,Mira inCetus, changes from second or third magnitude totenth, becoming quite invisible to the naked eye. Such stars arenow called "long-period variables" (LPVs) or "Miravariables." Thousands, all cool class M giants, are known. On theHR diagram, such advanced giantsare at the cool end of the "giant branch," theMiras occupying the coolest andbrightest portion. In astronomical jargon, such stars are calledasymptotic giant branch stars (orAGB stars)because of the appearance of their distribution on theHR diagram.
The gases of red giants can circulate upward to the tops of thestars, carrying the by-products of nuclear fusion with them. Oxygen is normally more abundant than carbon. If conditions areright, the surfaces of some stars can change their chemicalcompositions, some becoming very rich in the carbon that was madebelow by helium fusion, resulting in the reversal of the normalratio. Mira variables and other old red giants thus divide intooxygen-rich stars likeMira itself andcarbon stars such as19 Piscium andRLeporis Raised up along with the carbon are elements such aszirconium and many others that have been made in a huge varietyof nuclear reactions that go on at the same time as heliumfusion. Other stars' surfaces are enriched in helium andnitrogen.
Such huge giant stars have low gravities and lose mass throughpowerful winds that blow from their surfaces. Some of the gascondenses into molecules and dust. There may be so much that thestar can be buried in it and become invisible to the eye, theglow of the heated dust seen only by its infrared (heat)radiation. Oxygen-rich giant stars make silicate dust, whilecarbon stars make carbon-dust similar to graphite and soot. Mostof the dust that inhabits interstellar space began this way,though since inception it has been highly modified in the freezerof interstellar space. These stars therefore play a powerfulrole in later star formation. The winds are so strong during thegiant stage of a star's life that it can lose half or more of itsmass back into space, whittling itself down to little more thanthe parts that underwent nuclear fusion.
As a giant star loses almost all of its remaining outer hydrogenenvelope, it comes close to revealing its intensely hot core. Afast wind from the core first compresses the inner edge of theold expanding wind. High-energy radiation from the hot core thenlights up this inner compressed portion, which is now many timesthe size of the whole Solar System. These illuminated clouds,which can be quite beautiful, werediscovered byWilliam Herschel around 1790, who termed them"planetary nebulae" for their disk-likeappearances (they have nothing else to do with planets). Thebest known is theRing Nebula inLyra. Their complex appearances depend toa degree on how matter is lost from the giant stars that makethem. Expanding at rates of tens of kilometers per second, theylast no more than a few tens of thousands of years. From their emission spectra we cananalyze their chemical compositions, and find that many areenriched in the by-products of prior nuclear fusion in the parentadvancedgiant stars.
As theplanetary nebula dissipates into thegases ofinterstellar space, it leaves behindthe spent, old core that now includes the dead nuclear fusingshells. These stars, made of carbon and oxygen and compressedunder their own gravity, have shrunk to about the size of Earth. The first ones found (Sirius-B,Procyon-B, and40Eridani B) were fairly hot and white, so the class acquiredthe name "white dwarf" to discriminate it from the main sequenceof stars (which were originally called "dwarfs" to distinguishthem from the giants). Though small, white dwarfs still containnear the mass of the Sun, giving them astonishing averagedensities of a metric ton per cubic centimeter. The tremendousoutward pressure provided by tightly packed "degenerateelectrons" (which behave like waves that keep them from gettingcloser) prevents gravity from shrinking white dwarfs any further. White dwarfs are therefore also calleddegenerate stars. These small stars, the remains of stars that began their livesbetween 0.8 and 9 or so solar masses, no longer have any sourceof energy generation and are destined only to cool. The coolingtime is so long, however, that all white dwarfs ever created arestill visible, though the oldest are becoming cool, dim, andreddish. (There is no such thing as an invisible, cold "blackdwarf.") The age of the Galaxy calculated (with the aid oftheory) from the oldest white dwarfs roughly agrees with thatderived from the coolest (lowest mass) evolved main sequencestar. On theHR diagram, they fall in aline rather parallel to, but far fainter than, the main sequencedwarfs. Masses of white dwarfs are tied to the original stellarbirth masses and range from about half a solar mass (for a birthmass roughly solar) to a limit of 1.4 times that of the Sun for abirth mass of 8 or 9 solar. Beyond 1.4 solar masses, thedegenerate electrons can no longer provide support, and the coremust collapse, the star exploding as asupernova. Overflow of the limit by massaccreted from a closebinary companion canalso produce collapse and adifferent kind ofsupernova.
As they start to die, higher mass stars (those with masses overabout 9 or 10 times that of the Sun) initially develop the sameway as giants, but then their course of evolution becomes verydifferent. High mass stars are already large and luminous. Astheir dead helium cores contract, heating and firing to fuse thehelium to carbon and oxygen, the stars expand to approach thesizes of the orbits of the outer planets, becoming distended red"supergiants." Excellentexamples arefirst magnitudeBetelgeuse inOrion andAntares inScorpius. Supergiants are so massive, in spite ofgreat mass loss through huge winds, that nuclear fusion canproceed farther than it can in ordinary giants. When the heliumruns out, the carbon and oxygen mixture compresses and heats,causing it to fuse to a mixture of neon, magnesium and oxygen. Hydrogen and helium fusion had already moved outward into nestedshells around the core. When carbon fusion dies out in the core,leaving a mix of neon, magnesium, and oxygen, it too movesoutward into a shell. The neon-magnesium-oxygen mixture now inthe core then heats and fuses into a mix of silicon and sulfur,each fusion stage taking a shorter period of time. During thecourse of their evolution, red supergiants can also contract someand heat to make blue supergiants. The great mass-loss sufferedby supergiants can strip some of them of their outer envelopes tothe point that we see huge surface enrichments of helium,nitrogen, and carbon that have been made by nuclear fusion. Lookfor them scattered across the top of theHRdiagram.
Finally, the silicon and sulfur fuse to iron, an element that isincapable of energy-generating fusion reactions. Gravity nowwins the war that has been going on for the star's lifetime, andsince the iron refuses to support itself, the corecatastrophically collapses. The iron breaks down into itscomponent particles, protons, neutrons, and electrons (theconstituents of atoms), and the whole mass gets compressed into atight ball of neutrons only a few tens of kilometers across. Thecollapse produces a shocking blast wave that rips through thesurrounding nuclear fusing shells and the remaining outerenvelope, and rips the rest of the star apart. On Earth we seethe star explode in a grand "supernova," an event so powerful it is easily visible even inanother galaxy a huge distance away. The part of the star thatis exploded outward is so hot that nuclear reactions produce allthe chemical elements, including a tenth of a solar mass of iron,which then blend with the gasses of interstellar space, out ofwhich new stars are formed. Asupernova canalso be caused by the collapse of awhite dwarf in abinary system.
There are ways of makingsupernovae otherthan through core collapse. Nevertheless, supernovae are stillrare, taking place in ourGalaxy only twoor three times a century. Most are hidden from us by the vastclouds of dust that birth the stars. On Earth we observe aboutfive supernovae per millennium, and have not seen one sinceKepler's Star of1604 (probably created in the collapse of a white dwarf, asdescribed later). The great supernovae of1006,1054 (the "Chinese Guest Star"), and1572 (Tycho's Star)were visible in daylight. Our knowledge of supernovae comesalmost entirely from observing them in othergalaxies, the best of these exploding in1987 (SN1987a) in theLargeMagellanic Cloud, a companion to our Galaxy some 165,000light years away. But keep your eye onBetelgeuse orAntares, which are quite good candidatesfor core collapse. An even better candidate is the southernhemisphere'sEta Carinae, whichunderwent a huge eruption in the 19th century and produced asurroundingnebula, a vastcloud of dusty gas. The star should go off within the nextmillion years or so. At their current distances, the explosionsof such stars would rival the brightness of a crescent Moon. Theblast is so powerful that it if occurred within 30 or so lightyears, it would probably damage the Earth. Fortunately, nocandidate is nearly that close (though such nearby events havealmost certainly happened in the past).
As the debris of a supernova clears, we see a gaseous expandingshell around the old star, the "supernova remnant," whichconsists of the debris of the explosion that is rich in the by-products of myriad nuclear reactions mixed in with localinterstellar matter that is compressed by the mighty blast. Supernova remnants are readily identifiable by their X-rays andradio radiation. We believe all the iron in the Universe hascome from such (and related) explosions. Indeed, betweenordinarygiants,planetarynebulae, and supernovae, all the elements other than hydrogenand helium (and some lithium) were created in or by stars. Themost famous supernova remnant is theCrabNebula inTaurus, the remains ofthe great supernova of 1054, which was well observed by Chineseastronomers. Tens of thousands of years after a supernova event,we may still see the blast waves sweeping through the gases ofinterstellar space, compressing and heating them and perhapsmaking new stars.
At the center of the expanding cloud is a lone neutron starspinning many times per second, with a mass greater than the Sun,a diameter the size of a small town, and an amazing density of100 million tons per cubic centimeter. Aswhite dwarfs are supported by "degenerateelectrons," neutron stars are supported by degenerate neutrons. The magnetic fields of such collapsed stars are magnified alongwith the density to strengths millions of millions of times thatof Earth. The magnetism is so strong that radiation is beamedout the magnetic axis. The axis is tilted relative to therotation axis (like that of the Earth), and wobbles around as thelittle star spins, the beamed energy spraying into space. From adistance, the star looks like a lighthouse: if the Earth is inthe way, we get a blast of radiation, and from here see theneutron star as a "pulsar.Young pulsars emit from low-energy radio waves through high-energy X-rays and gamma rays. As the pulsar ages, it slows, andfinally emits only radio waves, which is the case for most of the600 or so pulsars known. When the rotation period is about 4seconds there is insufficient energy for the pulsar to be seen atall, and it disappears from view. Not fusing anything, theneutron star is held up forever against gravity by pressureexerted its own extreme density.
The collapsing star of a supernova will turn into a neutron staronly if its mass is less than about two or three times that ofthe Sun. If the mass is greater, then even the star's hugedensity cannot hold gravity back, and instead of a neutron starthe supernova creates a "star" that nothing can support againstgravity, and the body contracts forever. At a small enoughradius, the gravitational force becomes so great that light cannot escape, and the star disappears forever into a collapsing "blackhole." What we refer to as the black hole is actually a kindof "surface" at which the velocity required for escape equalslight-speed. What goes on inside is unknown. The center of ourGalaxy, 26,000 light years away, contains asupermassive black hole calledSagittarius A* that carries some three million solarmasses.
Most of stars you see at night have companions, with a great manyobviously double ("binary") even through a modest telescope. Thecomponents of some double stars are nearly equal in mass andbrightness. More commonly, one dominates the other, sometimes tothe point where a little companion is not really visible at all,and detectable only with the most sophisticated techniques. Atthe lowest end, we have stars with low-mass brown dwarfs forcompanions. The stars of some doubles are so far apart that theytake thousands of years to orbit; others are soclose that they revolve aroundeach other in only days or even hours. Gravitational theoryallows us to measure the masses of the stars from the orbits'characters; indeed such measurements are the only way in which wecan find stellar masses. Examples of visually-seen double starsareAlpha Centauri,Acrux,Almach,Albireo, andMizar.
Double stars are vitally important in the measure of stellarmasses, which are derived fromKepler's Laws as generalized by Isaac Newton. Pretend alesser star goes around a stationary more massive one, as seenfor example forAlpha Centauri,Castor, orAlgieba. The first law states that theorbit must be a conic section (circle, ellipse, parabola, orhyperbola), here specifically an ellipse with the more massivemember at one focus, the second law that the orbiting star speedsup in a known way as it gets closer to its mate, slows down as itgets farther away. The crucial third law states that the squareof the orbital period in years equals the cube of the averagedistance between the stars divided by the sum of the masses (insolar masses), which can then be found. In reality, the twostars orbit a common center that lies on a line between thempositioned from each in inverse proportion to the mass ratio. The sum of masses along with the location of the center of mass(and thus the mass ratio) then gives the individual masses, whichcan be used to test theory. The mass of the Sun is found usingthe orbit of the Earth (whose mass is inconsequential).
Stars can also bond into more complicated multiples. There aretwo kinds, stable "hierarchical" systems and unstable "trapezium"systems. In the first, a distant star orbits an inner double(which it senses gravitationally as one) to make a triple (as intheZeta Cancri system), or twodoubles may orbit each other as a quadruple, of whichEpsilon Lyrae (THE famed "Double-Double") is the prime example. In more complex systems, a staror even another double can orbit an inner triple or double-doubleto make a quintuple or sextuple system (likeMizar-Alcor orCastor). The structures of the orbitswill depend on relative masses. In the second kind of multiple,named after theTrapezium (Theta-1Orionis) inOrion'sSword, the member stars are all rather mixedtogether, which allows close encounters to eject stars until somekind of stability is achieved. Trapezium systems must alltherefore be young.
Formation is still contended. The oldest idea involves simplefission. When a new star condenses from the interstellar gases,it spins faster. If the contracting blob is spinning rapidlyenough, it can separate or otherwise develop into a pair or starsrather than a single star. Each of these contracting componentscan further separate into a double, producing a "double-double"star, the most famous of which is fourth magnitudeEpsilon Lyrae. This idea is now widelydiscounted. More likely scenarios involve capture within a densestellar environment, fragmentation of the collapsing birthcloud,and condensation of a companion from a the circumstellar diskthat surrounds a new-born star. Formation ofmultiples is even less understood, especially ofstars with distant fragilely-bound members of the sort we find intheAlpha Centauri system.
If the two stars of a pair are fairly close together, and if theplane of the orbit is close to the line of sight, each star canget in the way of the other every orbital turn, and we see a pairof eclipses, one of which is usually of much greater visibilitythan the other. Eclipsing systems are very important in stellarastronomy, and are used to helpdetermine masses, to find thestars' diameters, temperatures, and even to assess shapes in thecases that the stars' mutual gravities distort each other. Eclipsing doubles are quite common, the most famous secondmagnitudeAlgol inPerseus.
In a double star system in which the two have significantlydifferent masses (by far the most common), the higher mass starwill use its internal hydrogen fuel the fastest and become agiant first. We then see a red giant, or maybe a helium-fusing,orange class K giant coupled with a main sequence star, also verycommon. Eventually, the giant produces its planetary nebula anddies as a white dwarf. Good examples of such systems areSirius andProcyon, each of which are orbited by thetiny dead stars. For each of these systems, and for many others,the white dwarf is by far the LESS massive of the pair, provingthat stars really do lose a great deal of their mass back intointerstellar space.
If the two stars of a double are close together, they caninteract. When the more massive becomes a giant, its surfacesignificantly approaches that of the other star. The lower-massmain sequence star can then raise tides in the giant, distortingit. If the two are close enough, matter can flow from the giantto the main sequence star. Good examples that display suchbehavior areAlgol andSheliak. In more extreme cases, the lostmatter can encompass both stars, creating a "common envelope."Friction will then bring the stars even closer together, makingthe process go yet faster. The stirring of the lost mass cancreate unusually distorted planetary nebulae. At the end, thewhite dwarf created from the giant finds itself very close to theremaining main sequence star. In high mass double stars, thehigher-mass component can explode and produce a nearby neutronstar or even a black hole companion.
Some giant stars have the masses and internal constructions thatallow them to bring by-products of deep nuclear fusion to thestars' surfaces, in the most extreme examples creatingcarbon stars. Mass lost fromone of these enriched giants to a close companion can contaminatethe companion with the giant's newly-formed chemical elements. When the giant becomes a white dwarf we are left with a seeminglysingle star (main sequence or evolved giant) with anodd chemical composition. Onlywith determined observation can we tell that a dim white dwarf ispresent. Among the most prominent examples are "barium stars"(Alphard an example), giants that havevery strong absorptions -- and great overabundances -- of theheavy element barium among several others. All seem to becompanions of what were once mightier stars that had becomecarbon stars and that are now reduced to white dwarfs.
If the white dwarf and main sequence remnant of a close doubleare close enough, the white dwarf can raise tides in the mainsequence star, and mass will flow the other way, from the mainsequence star to the white dwarf. Theory and observation bothshow that the flowing matter first enters a disk around the whitedwarf from which it falls onto the white dwarf's surface. Instabilities in the disk can make such a star "flicker" overperiods of days and weeks, even producing sudden outbursts oflight. The star that became the white dwarf had lost almost allof its hydrogen envelope during its own evolution. When enoughfresh hydrogen from the main sequence star has fallen onto thewhite dwarf, it can, in the nuclear sense, ignite, fusingsuddenly and explosively to helium. The surface of the whitedwarf blasts into space, the star becoming temporarily vastlybrighter. On Earth we see a "new" star or "nova" (meaning "new in Latin) erupt into the nighttime sky,not a new star at all but an old one undergoing eruption.Novae are common, 25 or so going off inthe Galaxy every year, once a generation one close enough toreach first magnitude. Nova Cygni in 1975 rivalledDeneb, giving the celestial Swan twotails.
In a massive double star system, the more massive of the pair maydevelop an iron core and explode as a supernova, becoming eitheraneutron star or ablack hole. Either of these stellarremains in turn may raise tides in the more-normal companion,causing matter to flow into a disk around the collapsed body,from which it falls into an immense gravitational field. Matterin the disk is so hot it can radiate X-rays. From the motion ofthe normal star, we can calculate information on the mass of thecollapsed one. If the mass of the dark orbiting companion isbelow a two-to-three solar mass limit, as inX Persei, it is a neutron star. But if themass is great enough, we can infer the existence of an orbitingblack hole, the best actual proof wehave. Fresh hydrogen falling from the disk onto a neutron starcan produce greatvariability, becomecompressed and fuse to helium, and then explode violently as thehelium fuses to carbon. The result is an X-ray burst similar innature to a nova.
The term "supernova" is derived from "nova" in that the supernovais vastly brighter, no matter that the mechanism of the corecollapse of a supergiant is completely different from themechanism of nova production. White dwarfs, however, can alsoproduce supernovae. No white dwarf can exceed a mass of 1.4times that of the Sun, a limit discovered in the 1930s bySubramanyan Chandrasekhar when he applied relativitytheory to the gases in white dwarfs. If the limit is exceeded,even the white dwarf's enormous pressure cannot hold gravity backand the white dwarf must collapse into a neutron star or a blackhole or perhaps even annihilate itself. There are twoalternative theories for such an event. A massive white dwarfmay accept enough mass from a close main sequence companion andbe pushed over the edge before a nova eruption can take place. The white dwarf then collapses, creating a supernova that isgrander even than one produced by the collapse of a supergiant'siron core. The main sequence star of a double that contains awhite dwarf can also evolve through the giant stage to become awhite dwarf, creating a DOUBLE white dwarf system. If the twohave been drawn close enough together by interaction during acommon envelope phase, they can spiral together by the radiationof gravitational waves predicted by relativity theory. The whitedwarfs then merge, again producing a spectacular supernova. Ineither case, the collapse and resulting explosion makes nuclearreactions that again create all the chemical elements and evenmore iron than in the type ofsupernovaproduced by the collapse of the iron core of a massive star. Kepler's supernova of 1604, the last seen in this Galaxy, wasprobably of this kind.
Stars have a strong tendency to be born in groups, in wholeclusters. If they are bound together tightly enough by their owngravity, they can survive for millions, even billions, of years,even for the lifetime of theGalaxy. Thereare two kinds,open clusters andglobular clusters.Open clusters, the sparser but by far themore numerous of the two, are found in the disk of the Galaxy,and therefore lie largely in the plane of theMilky Way. Many of the closer ones, such asthePleiades andHyades, are easily visible to the naked eye. Someare angularly so large that they makeconstellations of their own, or at leastsignificant parts of them. Thousands of open clusters dot theGalaxy's disk. Though their sizes vary greatly, they typicallycontain a few hundred loosely arranged stars packed within adiameter 10 or so light years across. And though bound togetherby their own gravity, most open clusters gradually break up as aresult of random encounters among stars that speed members to theescape velocity, and because of stretching by tides raised by theGalaxy. Open clusters thus tend to be young, under a billionyears of age (indeed, some are just born), though in the farreaches of the outer Galaxy they can survive for far more than abillion years.
Somewhat related toopen clusters,stellarassociations (commonly calledOBassociations) are large, loosely organized, stellar groupingsthat lie within theMilky Way and aredefined by their young, blueO andB stars. Gravitationally unbound, OB associations areexpanding systems, their stars moving away from individual commoncenters that may have core open clusters. Though they containstars with a full range of masses, associations are bestrecognized through their most massive, luminous, and hottestmembers, which cannot get far away from their birthplaces beforethey expire, rendering ageing associations essentially invisible. OB associations are mostly named for theirconstellations of residence, for exampleOrion OB1,Perseus OB2,UpperScorpius, and so on. Several constellations, inparticularOrion andScorpius, owe their prominence and sparkle tobeing made largely of OB associations. Having massive stars,associations are prime sources ofsupernovae.
While there are thousands ofopenclusters in theGalaxy, there are but150 or so knownglobular clusters. Distinct from open clusters, their home is a huge spheroidalhalo that surrounds the Galaxy's disk. And while openclusters are sparse, loose, and comparatively young, globularsare compact, closely spherical, and can contain over a millionstars packed into a volume only a hundred or so light yearsacross. Their compactness gives them (at least those that havesurvived) long lifetimes. With ages of 11 to 12 billion years,formed when the metal content of the Galaxy was much less than itis today (the increase the result of stellar evolution), they areamong the oldest things known and among the first things to becreated after the Big Bang, the event that formed our Universe.
Clusters of both kinds are profoundly important in establishingthe distance scale in astronomy and in testing and guiding thetheories of stellar evolution -- the ageing process. Clustersare born with an intact array of stars that occupy the entiremain sequence (dwarf sequence) of theHR diagram, in which the higher the mass, thegreater the luminosity. Since high mass stars die first, acluster evolves by losing its dwarf sequence from the top down.Application of evolutionary theory can then tell us the age ofany given cluster by the most luminous and hottest dwarfs stillleft. TheDouble Cluster's hotclass O stars tell that it is young. The most luminous stars ofthe somewhat olderPleiades are ofclass B, while the still olderHyadeshas lost even these. Open clusters range in age from just bornto nearly 10 billion years, which gives the age of the Galaxy'sdisk. By contrast, globular clusters have lost the entire uppermain sequence (dwarf) population down to stars somewhat below asolar mass, which gives them ages 11 to 12 billion years or so,nearly the age of the Galaxy itself. In most dense globularclusters, and even in some open clusters, some stars with masseshigher than the main sequence cutoff linger, refusing to evolve. Since these stars are also bluer in color than the majority ofevolving dwarfs, they are calledblue stragglers.Blue stragglers are believed to becaused by stellar mergers within the dense cluster environments,either by direct collision or by the mergers of closedouble stars, which increases their massesbeyond the cutoff and thus seems to hold back their evolution.
Stars can range in size, depending on mass and age, from only afew kilometers across to the diameter of the orbit of perhapsSaturn. They can range in temperature from near "cold" at only2000 K for an extreme red giant through far over 100,000 K forthe star inside a planetary nebula to over a million K for aneutron star. All the stars you see in the sky will eventuallyexpire, some soon, some not for aeons. Lower mass stars createplanetary nebulae and white dwarfs, while higher mass stars makesupernovae that result in neutron stars or black holes. Doublestars add spice to the product, making novae and a different kindof supernova. All these endings send newly made chemicalelements into the interstellar stew, out of which new stars aremade. As a result, the heavy element content of the Galaxyincreases with time. Ancient main sequence stars, thesubdwarfs, and theirgiant star progeny have a low abundance of metals, whereasyounger stars like the Sun have higher metal contents, allowingus to track the oldest and youngest stars and to determine theage of the Galaxy. New starstherefore contain the by-products of the old, our Earth adistillate of earlier generations. Our Sun will someday make itsown contribution, however modest it may be, to generations ofstars and planets yet unborn.