History UsefulNotes / Stars

** 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 showing the layers where nuclear reactions have altered their composition (some of the most extreme examples are in fact almost bare stellar cores, packing several times the mass of the Sun in a volume similar to the one of our Daystar) 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 and nitrogen or carbon on the outside and progressively heavier elements all the way to their core, which ultimately becomes iron just before exploding. Some of them are unlike most others of their kind rich in hydrogen and are basically O stars in steroids, so luminous that while still fusing hydrogen on their cores mass loss is so potent that they show nitrogen and helium on their surfaces. An example is Gamma Velorum, even if it's actually a double star formed by a Wolf-Rayet star and a luminous O-type companion.
** 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 (known as the Chandrasekhar limit), 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. Near the end of their life, they become super-compressed carbon bodies -- yes, they are basically trillion carat ''diamonds''.

** 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.

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'' (the star is producing so much energy that even the neutrons are breaking apart, and not even a pair-instability event can stop it from contracting) that would cause 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.

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 and RS Puppis). They spend little or no time on the main sequence, going giant right away upon formation. Eventually, they will at random suffer a condition known as ''pair-instability'', where so much energy is wasted creating the aforementioned particle-antiparticle pairs that the core starts loosing energy and contracts in order to compensate -- which is only a temporarily solution as the pair-production will soon catch up to it again and it will have to contract even more. This cycle will continue for a while until the energy density of the core becomes so high that all remaining matter that can suddenly ignites at once, causing a thermonuclear runaway that detonates the core itself (essentially becoming a [[NukeEm huge nuclear bomb]]) into a ''hypernova'', that leaves no remnant and a huge nebula.

** 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 showing the layers where nuclear reactions have altered their composition (some of the most extreme examples are in fact almost bare stellar cores, packing several times the mass of the Sun in a volume similar to the one of our Daystar) 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 and nitrogen or carbon on the outside and progressively heavier elements all the way to their core, which ultimately becomes iron just before exploding. Some of them are unlike most others of their kind rich in hydrogen and are basically O stars in steroids, so luminous that while still fusing hydrogen on their cores mass loss is so potent that they show nitrogen and helium on their surfaces.

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:

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.

The catch is that the heavier the element, the harder is it to fuse it, less energy is released, the ludicrously requeriments of temperature for such fusion reactions translate to faster nuclear reaction rates, 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).

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.

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).

(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.)

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.

* 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...]]

* 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).

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) blue giants.

** 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 and all the bright stars in the Pleiades.
** A – White stars. Sirius A and Vega are examples.
** F – Yellow-white stars. Upsilon Andromedae and Procyon A 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, thanks to both the miserly rate they fuse their hydrogen reserves and being entirely convective, thus having access to all of their nuclear fuel unlike more massive stars. 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.

** 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 showing the layers where nuclear reactions have altered their composition (some of the most extreme examples are in fact almost bare stellar cores, packing several times the mass of the Sun in a volume similar to the one of our Daystar) 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 and nitrogen or carbon on the outside and progressively heavier elements all the way to their core, which ultimately becomes iron just before exploding. Some of them are unlike most others of their kind rich in hydrogen and are basically O stars in steroids, so luminous that while still fusing hydrogen on their cores mass loss is so potent that they show nitrogen and helium on their surfaces.
** 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 (known as the Chandrasekhar limit), 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. Near the end of their life, they become super-compressed carbon bodies -- yes, they are basically trillion carat ''diamonds''.
** 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.

*** 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 [[http://upload.wikimedia.org/wikipedia/commons/5/56/The_Sounds_of_Earth_Record_Cover_-_GPN-2000-001978.jpg Voyager Golden Record]] and the [[http://upload.wikimedia.org/wikipedia/commons/4/4f/Pioneer_plaque_sun.svg 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.

** 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]][[https://www.youtube.com/watch?v=tDkVS-AN4NU&feature=feedlik 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.

* 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.

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 and RS Puppis). They spend little or no time on the main sequence, going giant right away upon formation. Eventually, they will at random suffer a condition known as ''pair-instability'', where so much energy is wasted creating the aforementioned particle-antiparticle pairs that the core starts loosing energy and contracts in order to compensate - which is only a temporarily solution as the pair-production will soon catch up to it again and it will have to contract even more. This cycle will continue for a while until the energy density of the core becomes so high that all remaining matter that can suddenly ignites at once, causing a thermonuclear runaway that detonates the core itself (essentially becoming a [[NukeEm huge nuclear bomb]]) into a ''hypernova'', that leaves no remnant and a huge nebula.

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 [[BinarySuns 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 ''Franchise/StarWars'' and in extreme examples as RealLife [[https://en.wikipedia.org/wiki/Contact_binary contact binaries]] even touching each other), or quite far apart (like Alpha Centauri or especially Alpha and Proxima Centauri), but either way it can lead to a very cool AlienSky. Planets that orbit just one star in such a system are said to have "S-type" orbits, while those that orbit around both stars have "P-type" or "circumbinary" orbits.

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, [[https://en.wikipedia.org/wiki/Pulsar_planet planets have been found around pulsars]], but they get hit by ''extremely'' high radiation.[[/note]]

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.

** 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.

For completeness and from the perspective of an Earth (or another planet)-based observer, binaries can be divided between optical doubles (sometimes more), that appear close in the sky due to projection effects but both stars can be hundreds of light years apart thus unrelated one to each other, and actual star systems where the two or more stars orbit one to each other . A number of these pairs can be resolved with small telescopes often offering some of the most beautiful sights in the sky due to the contrast of colors of the stars (eg, Gamma Andromedae), while others require (much) larger telescopes and finally the tightest examples need either [[https://en.wikipedia.org/wiki/Astronomical_optical_interferometry interferometry]] or [[https://en.wikipedia.org/wiki/Binary_star#Spectroscopic_binaries spectroscopy]] with things being somewhat easier when, as in the case of the famous Algol system, one star eclipses the other.

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 [[BinarySuns 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 ''Franchise/StarWars''), or quite far apart (like Alpha Centauri), but either way it can lead to a very cool AlienSky. Planets that orbit just one star in such a system are said to have "S-type" orbits, while those that orbit around both stars have "P-type" or "circumbinary" orbits.

** 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 showing the layers where nuclear reactions have altered their composition (some of the most extreme examples are in fact almost bare stellar cores, packing several times the mass of the Sun in a volume similar to the one of our Daystar) 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 and nitrogen or carbon on the outside and progressively heavier elements all the way to their core, which ultimately becomes iron just before exploding. Some of them are unlike most others of their kind rich in hydrogen and are basically O stars [[UptoEleven in steroids]], so luminous that while still fusing hydrogen on their cores mass loss is so potent that they show nitrogen and helium on their surfaces.

Luminosities vary from the up to several million times the Sun's one of the most luminous O stars known to the feeble starlight produced by the lowest-mass red dwarfs, shining less than one thousandth times Sol[[note]]And if one factors in just visible light they're still much fainter[[/note]] (a luminosity ratio of roughly 100 billion), while radii vary from the up to two thousand times the one of the Daystar of the largest red hypergiants known[[note]]Such stars have ill-defined edges and suffer extensive mass loss, so sizes are badly determined[[/note]] to the UsefulNotes/{{Jupiter}} or UsefulNotes/{{Saturn}}-sized smallest red dwarfs -if one of the latter was a marble with a size of 1 centimeter, the former would have the size of Louisiana's Superdome-.

** 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 (known as the Chandrasekhar limit), 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.

** 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.

** B – Blue-white stars, e.g. Rigel, or all the bright stars in the Pleiades.

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. Eventually, they will at random suffer a condition known as ''pair-instability'', where so much energy is wasted creating the aforementioned particle-antiparticle pairs that the core starts loosing energy and contracts in order to compensate - which is only a temporarily solution as the pair-production will soon catch up to it again and it will have to contract even more. This cycle will continue for a while until the energy density of the core becomes so high that all remaining matter that can suddenly ignites at once, causing a thermonuclear runaway that detonates the core itself (essentially becoming a [[NukeEm huge nuclear bomb]]) into a ''hypernova'', that leaves no remnant and a huge nebula.

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 [[BinarySuns 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 ''Franchise/StarWars''), or quite far apart (like Alpha Centauri), but either way it can lead to a very cool AlienSky.

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, [[https://en.wikipedia.org/wiki/Pulsar_planet planets have been found around pulsars]], but they get hit by ''extremely'' high radiation. [[ArsonMurderAndJaywalking And you can ]]''[[ArsonMurderAndJaywalking FORGET]]'' [[ArsonMurderAndJaywalking trying to find a planet orbiting a]] ''[[ArsonMurderAndJaywalking black hole]]''[[ArsonMurderAndJaywalking , let alone a HABITABLE one]]![[/note]]