Stars are large balls of mostly hydrogen. They are so massive that their insides grow really hot, and fusion occurs.

For specific examples, see the UsefulNotes pages on:
* UsefulNotes/TheSun
* UsefulNotes/LocalStars

! Magnitude--Apparent and Absolute

Magnitude is the brightness of a star in the form of a number. Created by Hipparchus or Ptolemy, a star of magnitude 1 is about 2.512 times (the 5th root of 100) brighter than one of magnitude 2. It used to be that 1 was the highest, and 6 was the lowest, but they found that this didn't quite work and had to go into negative numbers, hence the Crab Supernova having -6.

Most famous stars are between 0 and 2. The human eye can see to about 6, with 7.72 being the current world record. Vega is approximately magnitude 0. Sirius, the brightest star in the night sky is at -1.46. -3.9 is the faintest you can see in daylight, Venus ranges from -3.8 to -4.9 (which means it's always brighter than any star apart from the Sun), the Moon is -12.6 when it's full and the Sun is -26.73. This is the apparent magnitude of course.

The farther away the same kind of star is, the dimmer it's going to appear. If a star that's 10 light-years away has an apparent magnitude of 1.5, that same star would have an apparent magnitude of 3.0 if you moved it 20 light-years away. To judge the true, ''intrinsic'' brightness of stars, the magnitude that they would be at if 10 parsecs (32.6 light years) away is determined and called absolute magnitude. For example, Alpha Centauri has an absolute magnitude of 4.38, but an apparent one of -0.27, as it is very close to Sol, relatively speaking (4.3 light years). Far-off (700-900 light years) Rigel has an apparent magnitude of 0.18, but at the 10 parsec distance it shines at absolute magnitude -6.7.

(There is also "planetary absolute magnitude", which is the brightness a planet would show at full illumination from 1 astronomical unit - that is, if the planet was moved to the Earth's orbit and viewed from the center of the Sun.)

! Distance determination

Nearby stars have it determined by something called ''trigonometric parallax.'' It's basically trigonometry that uses the relative movements of objects against a "fixed" background of far-off stars and galaxies- the closer the near star is, the more it will seem to move (you can see this on a train or in a car). Because of the distances involved, the change in angle is very small, less than a second of arc (or 1/3600 of a degree). This is the source of the unit of distance known as a "parsec" it's the distance [3.26 light years] at which an object viewed from two points one AU apart would show a parallax of one second of arc. Most stellar distances are calculated by taking two observations six months apart (with a baseline of 2 AU), but plans exist to send out probes to provide baselines of tens or even hundreds of AU.

But after a certain point current instruments simply can't measure the infinitesimal parallax of truly far away bodies. That's where we turn to an indirect means called the ''standard candle'' -- we look for an object whose intrinsic brightness (i.e. absolute magnitude) is known, then use how dim it appears (i.e. its apparent magnitude) to calculate how far away it must be. Not many objects have predictable brightnesses, but one of the few that do is a type of giant star known as a Cepheid variable. These stars pulse regularly at a rate proportional to their intrinsic luminosity, and some of them are also close enough to have measurable parallax. Therefore, far-off Cepheids (and objects such as the nebulae or galaxies they reside near or within) can have their distance judged simply by noting their pulse rate.

Some stars that aren't Cepheid variables can at least give us a ''ballpark'' hint as to their intrinsic brightness by certain clues in their spectra. When we look at the spectrum of a red star, for example, we can tell whether it's a nearby red dwarf or a distant red giant. While we can't calculate its intrinsic brightness precisely, we can figure it within a couple of orders of magnitude. This trick goes by the rather confusing name of "spectroscopic parallax", even though no actual parallax is involved.

! Heavy element abundance

About 1% of the mass of the sun consists of elements heavier than hydrogen and helium: lithium, beryllium, carbon, nitrogen, oxygen, iron, et cetera. Collectively, astronomers refer to all elements heavier than helium as "metals", even those that aren't metals in the chemical sense. And as it turns out, the abundance of these heavy elements varies from star to star, depending on the chemical composition of the primordial gas-and-dust cloud from which the star originally condensed.

We can tell how much of an element is present in the photosphere of a star by looking at its spectrum. Every element has a characteristic signature, a fingerprint if you will, of absorption/emission lines (sometimes called Fraunhofer lines after their discoverer) at very specific frequencies. Sodium, for example, has two very strong lines right next to each other at 5895.92 and 5889.95 Ångstroms. By looking at how prominent these lines are in the spectrum of a star, relative to their prominence in the spectrum of the sun, we get a pretty good measure of the relative abundance of that element. The overall picture of every heavy element's relative abundance is known as the "metallicity" of the star.

Most of the stars that lie along the galactic plane are rich with heavy elements, like the sun is. These stars are now known as the [[ExactlyWhatItSaysOnTheTin galactic plane population]]; in earlier times, they were referred to as "Population I". These stars are second- or third-generation -- that is, the gas-and-dust cloud out of which they formed was spewed out by the death throes of an earlier star, in which heavy elements had been synthesized. First generation stars, such as those that orbit the galaxy 'way out in the galactic halo, are much poorer in heavy elements, and used to be referred to as "Population II".

The heavy element abundance of a star is important for two reasons:
* More massive stars can take advantage of carbon in their cores, and use it as a nuclear fusion catalyst to vastly speed up the rate at which they burn hydrogen into helium. This nuclear catalyst process is known as the CNO cycle. This makes them much brighter, but also severely shortens their main-sequence lifetimes. A star like the sun (too small for the CNO cycle) will burn for 10 billion years before it evolves off the main sequence, but a big bright star like Sirius won't last longer than about 300 million years total. A Sirius-mass star that lacked heavy elements, though, might last for a few billion years.
* It's not just the star that formed out of that original gas-and-dust cloud. Any UsefulNotes/{{planets}} that form around that star will have also formed out of the same cloud. Thus, if the star is poor in heavy elements, the planets orbiting it will also be poor in heavy elements. There might not be enough carbon, oxygen, silicon, etc. to form rocky planets at all; you might just be left with a few hydrogen-helium gas balls orbiting it.

The amount of metals presented in a star is named by the astronomers metallicity and represented with the letter ''Z'' (in uppercase to avoid confusions with redshift ,that is represented as a lowercase ''z'') or as ''Fe/H''. The Sun, for example, is assumed to have a ''Z'' of around 0.02 meaning roughly 2% of its mass is in the form of heavier elements than hydrogen (whose abundance in a star is represented with an uppercase ''X'') and helium (whose abundance in a star is represented with an uppercase ''Y'').

! Names

Stars have a variety of naming methods:
* Traditional or "proper" names: Most ancient cultures had names for the brighter stars, but generally the ones assigned by [[OlderThanFeudalism the ancient Greeks]] (Arcturus, Sirius, Procyon, etc.) and [[OlderThanPrint medieval Arabs]] (Betelgeuse, Altair, Deneb, Rigel, etc.) are the ones you'll find on star charts today. [[note]]Speaking of the Arab names, due to how UsefulNotes/ArabicLanguage works, Al- (which means "The") is extremely common in star names. Also, very often the Arab names of stars can be translated into "The Something" or "Something (of) The Something"; stars with the long names translatable to the latter version generally have their modern names shortened. And speaking of "Something (of) The Something"-kind of naming, aside from Al-, the words Ras- ("Head") and Deneb- ("Tail") are also very common among different stars, obviously especially for stars in constellations depicting animals. Cardinal directions such as -Schemali/Shamali ("North") and -Genubi ("South") are also common.[[/note]]Others have received proper names more recently - Alpha Pavonis was named "Peacock" by the Royal Air Force because it didn't have a well known one yet, and it was needed on maps as a navigation star for pilots operating in the Southern Hemisphere.
* The Bayer designation: [[AncientGrome A Greek letter followed by the genitive case of the Latin constellation name]], such as "Alpha Centauri" or "Epsilon Eridani". The brightest star in the constellation is named Alpha, the second-brightest Beta, the third-brightest Gamma, et cetera. (There are exceptions, though Beta Orionis (Rigel) is usually brighter than Alpha Orionis (Betelgeuse), for example, and the stars in the Big Dipper are given Greek letters according to their position in the dipper rather than their relative brightness.) Since this naming scheme pre-dates the astronomical use of the telescope, they are only used for stars visible with the naked eye. Some of these objects turned out to be 2 different stars that lay along the same line of sight; their modern names carry an additional superscript to tell the two objects apart, e.g. Zeta[[superscript:1]] Reticuli and Zeta[[superscript:2]] Reticuli.
* The Flamsteed designation: A number followed by the genitive constellation name, such as "51 Pegasi" or "40 Eridani." As with the Bayer designations, this naming scheme pre-dates the widespread use of telescopes in astronomy, and thus only applies to stars visible with the naked eye. These were developed by John Flamsteed, the first Astronomer Royal of England/Great Britain ([[UsefulNotes/TheHouseOfStuart 1675]]-[[UsefulNotes/TheHouseOfHanover 1719]]), albeit with a complicated history that would probably make for an amusing play (involving Edmond Halley and Sir UsefulNotes/IsaacNewton pilfering Flamsteed's catalogue and publishing it without permission, only for Flamsteed's wife to publish the list after his death without numbers, and then having a Frenchman restore the numbers in modern form sixty years later). Flamsteed designations [[strike:seem to be arbitrary]] were originally supposed to have the number increase from west to east, but precession and proper motion have changed the position of some stars relative to celestial longitude since the time the designations were made. Some of them weren't stars at all; for example, what Flamsteed included in chart as "34 Tauri" turned out to be the planet Uranus - as a sidenote, this error left that designation available [[{{Series/Firefly}} for the star at the heart of a certain 'Verse...]]
* Variable stars that don't already have Bayer or Flamsteed designations have their own wonky naming scheme. They're named in order of discovery, starting with "R" -- R Ceti was the first variable star discovered in Cetus, S Ceti was the second variable star discovered in Cetus, etc.. When they get to Z, they go back and use RR through RZ, then SS through SZ, then TT through TZ, etc.. When they get to ZZ, they go back and use AA through AZ, then BB through BZ, etc., all the way[[note]]The letter J is skipped in both positions because it might be confused with the letter I[[/note]] to QQ through QZ. Finally, if still more variable stars are discovered in the constellation, they give up and name the next ones V335, V336, V337, etc..
* Catalog numbers - usually a prefix followed by an ID number or coordinate reference. Some catalogs include the Henry Draper (HD), Smithsonian Astrophysical Observatory (SAO) and "Bonner Durchmusterung" (BD). Early catalogs contained fewer stars, and are only referred to if the star in question first appeared there, e.g. "Wolf 359."
* Single stars can also be named after the individual who discovered them - usually they have some notable property. Examples include Barnard's Star (which has the highest proper motion of any star) and Herschel's Garnet Star (also known as Mu Cephei, it possesses a vivid red hue).

A lot of stars are named in multiple ways, but go primarily by one designation or another. For example, while most people have heard of Rigel or Alpha Centauri, the same folks probably wouldn't recognize Beta Orionis or Toliman, which are alternative names for the same stars (respectively). As another, the star called both Rho[[superscript:1]] Cancri and 55 Cancri is nearly always known by the latter name.

! Types of stars

Most stars are found along something called the Main Sequence, characterized by their balance between inward gravity and outward pressure generated by hydrogen fusion. Other stars exist that are off of it and fusing other elements, or else are dead or dying. These types have varying letters (spectral classifications) applied to them, with numerical sub-groups and a corresponding informal color (you can see the color in a good telescope).

The higher up the scale, the bigger (and brighter) the star, but the faster the rate at which the hydrogen in them is used up and so the shorter their lifespan.

For a number of reasons, very large stars - called giants, supergiants and hypergiants - like to live at the extreme ends of the spectral scales. Giant stars really do not like to be classes F or G, seeming to stay there for a very short time while heating up or cooling down to either extreme. There are a few known, but are decidedly in the minority. In general, most of the visible giants are either class-M or rather cool class-K (Betelgeuse, Arcturus) red giants, or class-B (Eta Carinae, Rigel, Deneb) blue giants.

* Main Sequence Classifications, in order from hottest to coolest:
** O Blue-violet stars. The hottest and most massive main sequence stars, with most of their energy output in the ultraviolet regions of the spectrum. Pretty rare, but also conspicuous. Delta Orionis and Zeta Puppis (Naos) are examples.
** B Blue-white stars, e.g. Rigel, or all the bright stars in the Pleiades.
** A White stars. Sirius A and Vega are examples.
** F Yellow-white stars. Upsilon Andromedae and Procyon are of this type. Canopus is a rare class F giant.
** G Yellow stars. The most famous is a [=G2V=] type known in Latin as Sol, and in English as UsefulNotes/TheSun. Alpha Centauri A, Tau Ceti and Zeta Reticuli are this type as well.
** K Orange stars. Alpha Centauri B, Epsilon Eridani.
** M Red dwarf stars. (No, not [[Series/RedDwarf that one]].) Have very long life-spans i.e. a trillion years. Proxima Centauri and Barnard's Star are of this type. Luminosity class is always V or VI, as more massive types are actually red giants, an entirely different kind of creature.

If you want to memorize the above sequence, use a handy mnemonic like "Oh Be A Fine Girl, Kiss Me" or "Oh Big And Ferocious Gorilla, Kill Mikey."

The term "dwarf star" refers to main sequence stars of spectral classes F, G, K and M. Classes A, B and O usually aren't called dwarfs, because the terms "white dwarf" and "blue dwarf" refer to something else.

Each spectral classification letter is subdivided into 9 numbers, running from 0 (hottest) to 9 (coolest). An F9 star is only ever-so-slightly hotter and whiter than a G0 star, while a G9 star is considerably cooler and oranger than either.

* Non-Main Sequence Classifications:
** W Wolf-Rayet stars, former O-types which have long expended the hydrogen in their core and are using more exotic heavy element fusion, shedding mass rapidly and likely to go supernova sooner or later. They undergo Type-Ib/c core-collapse supernovae, generating spectacular gamma ray bursts. They have an onion like structure with helium on the outside and progressively heavier elements all the way to their core, which ultimately becomes iron just before exploding.
** K or M - Red giant stars - classified as having a similar spectrum to M-type or cooler K-type red dwarfs but with luminosity classes I through IV. These are actually former B- through K-types that have expended their hydrogen and are fusing helium now. Their surfaces are cooler than they were in their hydrogen-fusion days, causing them to shift down the color spectrum to look like red dwarfs even though they may have originally been larger and hotter. They are quite diverse: types of red giant include ordinary, still hydrogen-fusing red giant, red clump giant that starts to fuse helium, assymptotic red giant - a bright giant that already has a carbon core, and red supergiant, which is many-layered and fuses various elements up to silicon. They may either shed their outer layers and become white dwarfs upon expending their helium, or else undergo a Type-II core-collapse supernova. The main difference with Wolf-Rayets, apart from being less massive, is that they still have hydrogen in their outer layers when they explode, and their explosions are less energetic. For eg., Arcturus, Antares, Betelgeuse. Betelgeuse, in particular, is presently on course for a supernova in the near future (read: sometime between tomorrow and a million years) that from Earth would appear brighter than the full Moon.
** D White dwarfs. (No, not [[TabletopGame/{{Warhammer}} that one]].) Small, dead stars that aren't undergoing fusion at all, but are held up against gravity by electron degeneracy pressure, created by quantum effects regarding electrons. Their average densities are on the order of a ton to the cubic centimeter, and the surface gravity is usually around 100,000 G's. They are generally made of a dense core of carbon and oxygen and lack the gravity to fuse these elements further. Sirius B is one of these, there's another orbiting 40 Eridani. This is the final fate of most small to medium stars, including the Sun. Binary white dwarfs can do a bit more though - they may siphon off hydrogen from their companion stars, heating it on their surfaces until it undergoes flash fusion in an event called a nova. If the white dwarf accretes enough material to exceed 1.44 solar masses, electron degeneracy will no longer be able to support the star's weight, and the whole star will collapse in a spectacular explosion called a Type-Ia supernova.
** L or T Brown dwarfs. ([[RunningGag No, not Gary...]] wait, never mind.) Collections of gas that never got big enough to start proper fusion reactions. They usually have some sub-par fusion like deuterium-deuterium and leftover heat of formation to give off a dull magenta glow, but cannot fuse ordinary hydrogen (protium, i.e, raw protons). Sort of intermediate between huge gas giant planets and true stars. The exact amount of mass needed to sustain fusion and not just peter out like this is unclear. A very few actual hydrogen-fusing red dwarfs may be cool enough to fall under the early L-class, as well.
** Neutron stars. The dead remains of heavier stars, known for being extraordinarily dense (a billion times denser than a white dwarf) and occasionally spinning at extraordinary speeds while blasting out radio noise and generating intense magnetic fields (a.k.a., pulsars and magnetars). They are formed from core-collapse supernovae and are composed of pure neutronium, as their name suggests, with whatever charged particles that may be left strewn over their surfaces. Like white dwarfs, they are stabilized by quantum effects, except with neutrons instead of electrons. Also like white dwarfs, neutron stars in binary systems can accrete material from their companion, and eventually ignite it -- except that while a white drawrf that does so can erupt as a nova every few years, a neutron star that does so will erupt as an X-ray burster every few ''minutes''. A neutron star will be the final fate of most of the larger red/blue giant stars, like Spica, Betelgeuse or Rigel.
*** The Magnetar is a common neutron star variation - a star with a magnetic field thousands of times stronger than a vanilla neutron star. However, their magnetic fields decay extremely rapidly in stellar terms, and after about 10,000 years their activity ceases, which leads to estimates of up to 30 million inactive magnetars in the galaxy. Their magnetic field is strong enough to instantaneously rip all of the iron from a person's blood cells from a thousand kilometers away.
*** Pulsars pulse electromagnetc radiation at extremely precise intervals along a thin beam as the star rotates, effectively turn it into an enormous stellar lighthouse. Their period of rotation is precise enough to rival atomic clocks, and they make for useful interstellar navigation points - the [[ Voyager Golden Record]] and the [[ Pioneer plaque]] [[note]]The maps are identical; bottom left on the Record, left on the plaque. Each of the lines represents a pulsar.[[/note]] have a pulsar map of relative locations and frequencies which can be used to locate the Sun, if intelligent aliens were to discover it.
** UsefulNotes/BlackHoles. The ultimate product of core-collapse supernovae, these were once stars whose core gravity was so strong that no force in the known universe could oppose it, so they contracted all the way down into gravitational singularities. This is the final fate of extremely large stars, like Eta Carinae or probably Deneb.
** B VI - Blue subdwarfs. They're spectral class B, and are hence blue-white, but are much smaller and less luminous than the main sequence B-types. They form either when two white dwarfs merge and core gravity needed to restart reactions, or when a companion star strips a relatively small red giant of its outer layers. As a result, they have no hydrogen, and are fusing helium, or carbon/oxygen. They are longer-lived than the usual kind of blue stars, but still no more than some millions of years. More commonly found in globular clusters than in the main galaxy, since star densities there are higher and make the interactions needed to create blue subdwarfs more common.
** Black dwarfs[[note]][[ Now I know what you're thinkin' - that's racist. But actually it's not, it's just cosmic vocab.]][[/note]]. White dwarfs that have finally radiated away all their residual heat. As best we can tell, there aren't any of these yet, since best estimates indicate it would take at least a ''quadrillion'' years for one to come about.
** Blue dwarfs. There aren't any of these yet, either. A blue dwarf is what you get when a small red dwarf is out of fuel. It turns hotter for a brief time, becoming a blue dwarf, then shrinks and becomes inert, turning into a small white dwarf. Such stars never had the mass to even enter the red giant phase, and so instead have this as their intermediate stage before death. Such stars however are also extremely long-lived, and none of them have died yet.
** Blue stragglers. The oddest stars ever discovered, which have discovered the Fountain of Youth. They age ''very'' slowly and avoid the main sequence of stars completely. Long after other stars have gone supernova or entered their red giant stage, they'll remain basically unchanged. Scientists believe these were created through stellar collisions, especially since they're normally found in clusters of fellow stragglers.

There are three other elements to a spectral class designation. The first is a number which roughly indicates where, within a given class, that star falls. Lower numbers are hotter. For example, Alpha Centauri B is a K0 star, meaning that it's very hot for a class K, and is in fact on the borderline of being a class G. Upsilon Andromedae is an F8, meaning that it's quite cool for an F.

The second addition is a Roman numeral representing the star's luminosity class. This has to do with the brightness of the star, but as a rule of thumb, it can be thought of as a size and mass designation. In general, the top three classes started out as unusually massive stars. The numerals are as follows:

* 0. Hypergiants like Eta Carinae or the Pistol Star.
* I. Supergiants like Betelgeuse or Rigel.
* II. Bright giants like Dabih (which is a rare class G giant).
* III. Giants like Arcturus.
* IV. Subgiants like Procyon. These tend to be main sequence stars that have run out of hydrogen to fuse and are expanding toward their red giant stage.
* V. Main sequence stars like the Sun or Sirius. These are sometimes called "dwarf stars", though in fact, those of spectral class G or hotter outshine 80-90% of all stars in the galaxy. (Because most of them are dim red dwarfs).
* VI. Subdwarf stars. There aren't very many of these known - this class is mostly used to designate odd, metal-poor stars. Their low metallicity lets radiation escape easier; consequentially there's less support for the star's outer layers and they tend to shrink and become hotter. This results in a net 1-2 magnitude drop from the brightness a "normal" dwarf star would demonstrate; hence the term "subdwarf".

And finally, some stars' spectral codes have a rider on the end indicating an abundance of a particular element, unusual spectral lines, or other uncommon traits. An example is Alioth (Epsilon Ursae Majoris - the third star in the Big Dipper's handle as you go from the end towards the bowl), which has a spectral class of [=A0pCr=]. The letter "p" stands for peculiar, and "Cr" indicates the peculiarity involves chromium. In this case, the star's magnetic field appears to concentrate the element in bands around its magnetic axis, which is perpendicular to its rotational axis. As Alioth rotates every 5.1 days, from Earth its chromium spectral lines increase and decrease in intensity over that same period as the bands come in and out of view.

! Stellar Evolution

Stars are pretty long-lived compared to us humans, but they do change, age and die. The life-cycle of a star is called stellar evolution and goes through various stages depending on the size of the star in question.

The initial stage all stars go through is the protostar. It is a large, cool gas cloud that slowly contracts under its own weight, building up heat along the way. A typical protostar can be described quite accurately as an "infrared giant": it is large, quite luminous but most of its luminosity is infrared. Protostars can develop protoplanetary discs as they contract. Eventually, they build up enough heat to start a fusion reaction; a bright flash of light and radiation sweeps the protoplanetary disc clean of gas and dust, ending the accretion of planets, and thus a new star system is born.

Some protostars are not massive enough to start a true fusion reaction. They condense into smallish balls the size of Jupiter and briefly go through a low intensity deuterium fusing phase. These not-quite-stars are called ''brown dwarfs'' (example: Teide 1), and their evolution is simple: when the deuterium is all burned up, they start a slow process of cooling and contraction. Eventually they lose all luminosity and become indistinguishable from common gas giant planets, except by mass. This final stage of brown dwarf evolution is called an Y-class brown dwarf (example: WISE 1541-2250).

Other protostars, however, successfully light up their fusion and enter a stage of life called the ''main sequence''. It is called so because it encompasses most of a star's lifespan. The exact lifespan of a star on the main sequence depends on its mass: a low-mass ''red dwarf'' (example: Proxima Centauri) can smolder for trillions of years, because its fusion reactions are so sluggish; more massive stars are brighter yet burn up their hydrogen faster. Most main sequence stars are stable and not variable; only the aforemented red dwarfs can be prone to violent flares. Luminosity of a main sequence star slowly increases as it grows older.

After the star starts running out of hydrogen, its further fate depends on its size and mass. The smallest red dwarfs, with masses 0.08 to 0.3 solar masses (example: Van Biesbroeсk's Star), are homogenous and fully convective, so they do not develop helium cores. Instead, they grow hotter and turn into ''blue dwarfs'', and then become inert ''helium white dwarfs''. Larger dwarfs, with masses 0.3 to 0.5 solar masses (example: Gliese 581), can develop distinct helium cores and evolve in a similar way to heavier stars. They go through subgiant and common red giant phases like mid-size stars.

Mid-size stars, with masses up to 10 solar masses (examples: Sun, Alpha Centauri A and B, Sirius) develop inert cores of helium as they leave the main sequence. The first phase of their evolution is the ''subgiant'' (example: Procyon). Fusion reactions in the core slow down as fuel comes to an end, and move to the neighboring region immediately surrounding the core, called the ''hydrogen-burning shell''. Since the shell has a greater surface area than the core, it produces more radiation pressure. Subgiant stars grow somewhat cooler and larger in size than they were during the main sequence, but not much more luminous.

Once fusion reactions in the core stop completely and move into the shell, the star enters ''red giant'' territory. It starts to swell, growing larger, cooler and more tenuous; although the upper layers of the star cool, the much greater surface makes it much brighter than before. This stage is called the common, or hydrogen-fusing red giant (example: Aldebaran). For small stars with masses 0.3 to 0.5 solar masses, the common red giant phase is the only red giant phase; they just keep swelling and growing more tenuous until they dissipate, leaving an inert helium core which becomes a helium white dwarf.

Heavier stars, however, go through several more transmutations. First, they undergo an event called a helium flash. Though the outer layers of a red giant grow cooler and more tenuous, the helium core does the other way round, it contracts and heats up. Finally, it heats to a degree when helium fusion, called the triple alpha process, becomes possible. The core bursts up, shutting down hydrogen burning in the shell, and the red giant stops growing for a while. These new, helium-fusing red giants are called ''red clump giants'', since they are very uniformly luminous and located in an accurate little clump on the H-R diagram. The swelling stops, stabilizing the giant.

Note that older, metal-poor stars go through a different transformation upon helium flash. They not only stop growing and cooling, they start doing it in reverse! The result is a star called a ''horizontal branch giant'' (example: RR Lyrae), a yellow, white or even bluish-white giant. These giants can be found in the usual haunts of low-metal stars, namely globular clusters and the galactic halo.

After a while, however, the helium core is used up, too. A new, smaller carbon-oxygen core is formed, and helium fusion contunues in a shell around this core (and hydrogen fusion in a larger shell around ''this'' shell). Noticed a pattern yet? Yes, the cooling and swelling process starts anew, and this time it cannot be stopped. This final form of bright red giant is called an ''assymptotic giant'' (example: Mira). It grows to immense size, becomes unstable and starts shedding its outer layers into space. Finally it all vanishes away into smoke, leaving an inert core called an ''carbon-oxygen white dwarf''.

Even heavier stars, massing 10-30 solar masses, go through a different pattern of evolution. Their helium lights up prematurely when there's still hydrogen in their cores, making a normally blue or bluish-white star expand into a ''blue supergiant'' (example: Rigel). After that, it swells and cools similarly to a lighter star, only this time the shell-layering process goes even further with an event called ''carbon detonation'', and carbon starts to burn, too. The end result is the ''red supergiant'' (example: Betelgeuse), a star very similar to a asymptotic red giant, only with more layers, up to iron, and much larger and brighter. It may end like an asymptotic giant, dissipating into an ''oxygen-neon-magnesium white dwarf'', or much more spectacularly.
The catch is that the heavier the element, the harder is it to fuse it, and more of the energy output will be in useless ''neutrinos'' that escape the star, rather than in useful photons (this is called urca-process). This means that each subsequent phase of fusion will be shorter and shorter, until a core of iron forms. Iron is impossible to fuse with any significant energy output, and if the stupid star tries to do so anyway, it starts ''losing'' energy. The core cools and contracts in mere seconds, the shells fall on it causing an enormous starquake, the entire intricate structure is disrupted and blows up in a giant explosion. This is called a ''supernova''. The iron core is compressed even more by this catastrophe and loses its atomic structure, degenerating into neutrons and forming a ''neutron star'', or ''pulsar'' (example: Crab Pulsar).

Stars larger still (30-60 solar masses) go through a more chaotic evolution. Once they go supergiant, they start jumping to and fro from blue supergiant to red supergiant, changing colors several times in their short lifespans. Finally, the supergiant starts to smoke away prematurely, shedding its hydrogen layers. The result is a ''Wolf-Rayet variable star'' (example: Gamma Velorum), which is, much like a large red supergiant, layered in shells of elements, only without the cool, tenuous outer shell of hydrogen. It is very hot, blue and gives a powerful stellar wind. It blows up as a supernova, much like a red supergiant, and the end result may be either a neutron star or a fully collapsed object, a ''black hole'' (example: Cygnus X-1).

Finally, the heaviest stars (60 solar masses and more, the recorded maximum is 265 solar masses) are utterly chaotic in their evolution, resembling very large red supergiants or Wolf-Rayets but irregularly shaped, with lots of bursts, jets, disruptions, protuberances and smoky nebulae. Their powerful fusion furnaces generate radiation so hard that it produces particle-antiparticle pairs of electrons and positrons, severely destabilizing these stars. They go through several supernova-like bursts in their lifetime; one burst isn't enough to blast these superheavy stars to smithereens, you see. In these eruptive bursts, they shed mass measuring in dozens of solar masses at a time, surrounding themselves with misty nebulae (example: Eta Carinae). They spend little or no time on the main sequence, going giant right away upon formation. The final explosion is a hypernova that leaves no remnant and a huge nebula.

Even more massive stars, that are theorized to have existed in the earliest epochs of the Universe, would die not with a bang but with a whimper suffering a process known as photodesintegration that would case them to collapse into a black hole, some mass escaping into relativistic jets. There's, however, a theory according to which these mammoth stars underwent the most fascinating transformation of all: they became quasars, or active galactic cores, and later ordinary galactic cores. In between, they spent some time as ''quasistars'', the transitional critters between star and quasar: enormous black holes completely covered by outer layers of stellar matter held aloft by the radiation of screaming matter pulled into the event horizon. When they ran out of these outer layers, they started to eat stuff around them, and fully transitioned to quasar status.

! Two's Company, Three's Better

Stars are pretty frequently[[note]]While the majority of star systems visible to the naked eye, or visible in any given field when you look through a telescope, are binary or larger systems, this is because the more intrinsically bright a star is the more likely it is to have companions. A comprehensive survey of all stars within a given volume of intra-galactic space would reveal that most of them are in fact solitary red dwarfs.[[/note]] found in pairs, triples or more - Sol is rare for being a single star system. These stars can be very close together (like the twin suns of Tatooine in StarWars), or quite far apart (like Alpha Centauri), but either way it can lead to a very cool AlienSky.

Close binaries are less likely to have useful planets than single stars. But distant binaries/multiples can have a LOT! -- a full system for each of the component stars.

According to one paper published in the late 1970s, a planet in a multiple star system cannot be in a stable orbit unless its orbital radius is either (A) less than one-quarter (1/4) the closest-approach distance between the two stars, or (B) greater than twice the farthest-separation distance between the two stars (so as to orbit both stars as a pair). In Alpha Centauri's case, the A and B stars get within 11.5 A.U. of each other every 80 years, so no planets should exist more than 2.9 A.U. away from either star. Fortunately for writers, this still allows either or both to have a habitable planet.

! Extrasolar Planets

Or exoplanets, for short.

Giant stars are very unlikely to have useful planets: red giants gobble their close-by planets up when they grow, retaining only their outermost worlds, while blue giants, for one thing, blow their protoplanetary disks away and, for another thing, are too short-lived to form planets anyway. Some pulsars have been found to have planets, but these were formed probably after the original star went supernova, from the remaining gas and dust that the explosion left behind. Needless to say, they'd be pretty inhospitable to any known forms of life. Class A stars can host planets (Fomalhaut has one, for example), but almost certainly don't last long enough for life to evolve there naturally. Hotter class F stars (F0 through about F4-5) may have similar problems - they tend to enter their subgiant phases after about five billion years, which is roughly the present age of the Sun. Cooler F, G, and hotter K stars are the most hospitable to Earth-like life to evolve. Red dwarfs (M and cooler K stars) are too dim, that means their habitable zone isn't wide enough to probably have a planet there, with the exception of, perhaps, the possible rare small tide-locked world (see below). Many red dwarfs are also prone to violent solar flares. But there are a hell of a lot of these M-types in the universe, more than any other type, and some percent of them aren't flaring, and some percent of these may have a planet in the life-zone. So quite a number of red dwarfs could be hospitable.

Another wrinkle with red dwarf planets is that a red dwarf's habitable zone is so close to the star that any planets there stand a good chance of being tidally locked; that is, always keeping the same side toward the star, as the Moon does to Earth. Without a very dense atmosphere[[note]] At least one-tenth as thick as Earth's atmosphere[[/note]] (to avert ConvectionSchmonvection), the light side might become searing hot while the dark side becomes cold enough to freeze the air, resulting in a planet with one baked side and the other covered in nitrogen ice. Only if the planet has a dense atmosphere and sufficient water and carbon to become an "eyeball Earth" would it be habitable - the name given because they would be ice-covered except for a liquid ocean in the sub-solar region. Large, bright red dwarfs probably have less of a problem with this since their habitable zones are wider.

By now, a lot of exoplanets have been discovered, but keep in mind that our current methods can generally only discover really big planets (2-3 Earth masses to gas giants) that are very close to their stars, or else planets that happen to be orbiting in a plane where we could see them (partially) eclipse their suns. With our current technology, if we were observing the Sun from Alpha Centauri, we might be able to detect Jupiter and maybe Neptune, but terrestrial planets like Mars, Venus or Earth are right out. That means, if a star's description says "no planets found", don't despair: there may be quite a lot we haven't been able to find yet. And we have no way to detect things like dwarf planets and moons, still - they're hard enough to spot reliably in our own system.

In general, however, don't expect to find ''habitable'' planets around anything other than F, G, K or M main sequence stars or possibly subgiants. Unless your lifeforms are based on ArtificialIntelligence and can live anywhere, have otherwise come from elsewhere and {{Terraform}}ed an inhospitable planet or moon, or have evolved with some exotic form of biology, any exceptions to that rule would be ''extremely'' strange - and without a sufficiently good {{handwave}}, it will look like you don't understand the facts.[[note]]And even these couldn't function in planets that have formed from the gas of a supernova that created a pulsar. Yes, [[ planets have been found around pulsars]], but they get hit by ''extremely'' high radiation.[[/note]]