Star

A star is a localised aggregation of matter which, by gravitational compression, gets heated to a temperature at which hydrogen-to-helium fusion takes place.

Stars are suns and the sun is a star. Stars only look small and faint because they are incredibly far away. Unless there is an undiscovered dead star closer, the nearest star to Earth's sun is which is just over 4 light-years distant.

The stellar zoo
Without including their corpses remnants (see further down), that would increase a lot the ratios in luminosity and specially size, stars present very large variations on their properties, especially luminosity, where among those whose properties are better known we've from that really puts the Sun to shame and can be seen even being located in other galaxy to  outshined by our Daystar and so faint that large telescopes are needed to spot it despite its proximity to us — the luminosity ratio between those two extremes is almost 70 billion. Sizes vary between the humongous large enough was it on the Sun's place to reach midway between Jupiter and Saturn and  that is as large as Saturn, and finally masses vary between the almost three hundred solar masses of the already mentioned R136a1 to, with less than 1/10th of the Sun's mass. Finally, (surface!) temperatures show a less extreme variation and range between around the two hundred thousand K of certain and the "just" around two thousand K of those three small ones.

Types of stars
By looking at its and once we know its distance (thus more or less roughly parameters as luminosity, size, and mass), we can classify stars depending both of the former and how luminous and large they are.

The spectral types currently used by astronomers are (note these spectra are the ones of "living" (ie, those that are fusing something) stars, not dead or failed ones such as brown dwarfs, and that every spectral type has a digit attached from 0 ("earlier", and hotter) to 9 ("later", and cooler), even if decimals (2.5, 9.7, etc) are often used to further improve a classification):

W – evolved O-type stars (see below) which have long expended the hydrogen in their cores and are fusing heavier elements (helium onwards) there, shedding mass rapidly in the form of very powerful stellar winds. The hottest ones are blue as an O. They come in three flavors: nitrogen-rich (WN), carbon-rich (WC), and oxygen-rich (WO). The first two types are believed to be fusing helium in their cores into carbon and oxygen, while the latter one (much rarer) are thought to be fusing heavier elements than helium (ie, they're supernova candidates)

– Deep blue stars. The hottest, rarest, and most massive hydrogen-burning stars, even if the most luminous of them may show a Wolf–Rayet-like spectrum, as well as by far the most rare of them. Most of their energy output is in the ultraviolet regions of the spectrum. Note also that the earliest O-type stars known are of spectral type O2.

– Blue stars, less massive and thus less luminous than an O. While still rare, much more abundant than O-type stars.

– Pale blue, not white as generally thought, stars such as the brightest (but not the most luminous) star of the night sky. Most of their light is emitted in the visible part of the electromagnetic spectrum.

– Yellow-white (actually blue-white) stars.

– White (not yellow) stars. Our Sun is one of these.

– (Pale) orange stars.

– Red (actually orange except the coolest ones) stars. These show the largest range in size, from the Jupiter or Saturn-sized low-mass red dwarfs to the huge stars that would go beyond Jupiter's orbit if placed instead of the Sun already mentioned.

– Stars, most of them giants of spectral type F, G, K, and M, abnormally rich in carbon.

– A subtype of evolved M stars, all of them giant, that show evidence of elements produced on its innermost regions carried to the surface by strong convection (what astronomers know as "dredge-up").

– While this classification is used for brown dwarfs, some of the coolest hydrogen-fusing stars have this spectrum as well as exceedingly rare and peculiar objects as They're deep red.

Luminosity classes are as follows. Note how almost all the giant stars, from hypergiants to standard giants, are evolved ones that have stopped fusing hydrogen on their central regions.

Hypergiant stars, often Ia+. Very rare, but including some of the largest and most luminous stars known.

Supergiant stars, further divided into "Ia" (bright supergiants), "Ib" (less luminous (standard) ones), and "Iab" (transitional between both).

Bright giants, intermediate between giant and supergiant stars.

Giants.

Subgiants, former main-sequence stars that are evolving to become a giant or even a supergiant depending on their masses after having exhausted hydrogen on their cores.

Main sequence stars like the Sun, fusing hydrogen into helium on their cores. By far the most abundant type of star.

Subdwarf stars. Most of them are metal-poor (ie, having little of anything heavier than helium) stars, which causes them to be smaller and hotter than their metal-richer brethen.

By combining the spectral type and the luminosity class, you can classify a given star. The Sun is a G2V star, Betelgeuse an M2Iab, Proxima Centauri an M5.5V, Sirius an A1V one, etc. Note that not all combinations are possible, as Wolf-Rayet stars just come in one size (no dwarfs, supergiants, etc), and all S stars known are red giants.

Double and multiple stars
While some stars live alone like the Sun, a significant number of them form pairs or even more with others in what's known as a if there are two or multiple if there's more (triple three, quadruple four, etc) star. While in some cases ("optical doubles"), the alignments are just line of sight chances, in most of the cases, the two or more stars are actually connected by gravity forming such a stellar system ( for more than two of them), and in a significant number of cases, the stars are so close that they cannot be disentangled with a telescope and one must look at spectra to see that there's more than just one star, even if one can suspect that happens if their orbital plane coincides with our line of sight, so they eclipse one to each other and their brightness varies so. Alpha Centauri, the closest star to the Sun, is a triple star system, while Sirius, the brightest star in the sky, a double star system.

Stellar groupings
From above double and multiple stars, one can find where much more stars, typically of the same age and sharing a common place of formation, live together. Two main types of them are distinguished: that are typically young in astronomical terms (from some millions to hundreds of millions of years, with     than that given their low mass means it's easy for them to lose members due to gravitational interactions with other clusters, etc), concentrated in the disk and spiral arms of spiral galaxies such as the Milky Way as well as present in irregular galaxies, and  that are typically much more massive than the former, are in the  of galaxies of all types, and are much older than most open clusters — usually almost as old as the Universe itself, even if much younger  of them are known to exist especially on galaxies with high star formation activity.

Formation
Stars form out of gigantic, rarefied interstellar clouds of gas (by far, mostly hydrogen and helium) and dust named nebulae. Such a cloud will eventually collapse in on itself due to self-gravitation. As the material falls into the center, it heats up, eventually getting so hot that it glows. We now have a protostar, surrounded by a called, that like a cocoon envelopes the forming star.

If the collapsing cloud wasn't very massive — less than about 8% of the mass of the sun — the protostar stage will be the end of the line. The little lump of gas in the middle will be a brown dwarf, which will slowly cool off.

If the collapsing cloud was more massive than this, however, the core at the center of the protostar will get hot enough and pressurized enough that nuclear fusion of light hydrogen will commence. The protostar is now an actual star. It will take some time for all the excess heat generated during the protostar phase to be shed, and for the star to settle down to the brightness it will have for most of its lifetime (called the "main sequence"). This stage between the end of the protostar phase and the beginning of the main sequence is sometimes called the T Tauri stage, named after the 3rd previously-unnamed variable star to be discovered in the constellation of Taurus. Note that at the peak of the protostar stage, a star will be brighter than it will become in the main sequence.

The leftover gas and dust can often be seen surrounding the newly formed star(s). If the stars are hot (thus massive and luminous, ie O- and early B-type stars) enough to ionize the hydrogen gas, we have an (which include some of the most photogenic objects of the sky as the ). Without such kind of stars, we have that ss the name says shine by the reflected gas of the embedded stars.

The chemical makeup of the star is determined entirely by the chemical makeup of the cloud it collapsed out of. Any planets that exist around the star will also have formed out of the same cloud, and will have a similar distribution of elements. Thus, if the star formed out of a cloud that was lying around when the galaxy was new, before the interstellar medium had become enriched with metals, the star and its planets will also be metal-poor.

Note that, contrarily to what one could expect, the more massive a star is, the faster it will fuse its available hydrogen, as larger stellar masses mean higher temperatures in their cores even if densities are lower there, at least while hydrogen is consumed there, which translates to (much) faster rates of nuclear reactions and thus to higher luminosities than smaller ones. The already mentioned R136a1, as well as other less luminous stars but still so much that outshine the Sun by several orders of magnitude, guzzle their fuel reserves so fast that they will be history much before the latter exhausts its hydrogen supply, while the tiny stars of above and others similar to them like the famous precariously fusing their little available nuclear fuel, will still be shining — and without having changed very much since today — long after Sol is gone and will be among the last ones illuminating the Universe, assuming the latter does not croak die before then.

Death
Stars are incredibly hot and shine because nuclear reactions in their cores keep heating them. Stars eventually "die" by either exploding as a supernova or merely cooling to a point where they cannot support nuclear reactions. The ultimate fate of stars can be black holes, neutron stars, or white (or black) dwarf stars.

Beyond main sequence
Before a star actually dies, it goes through a series of death throes. In nearly all cases, this involves some form of a, which may be preceded by a phase: the star expands, usually over the course of up to several hundred million years, to many times its main sequence diameter. In the process, its outer layers cool and thus become redder. The expansion occurs because, when the core runs out of hydrogen, its outward radiative pressure ceases, it begins to contract and get hotter, and a shell of material around the core collapses down onto it. This shell then gets hot and high-pressure enough to start burning hydrogen into helium itself. Due to its greater surface area and how it's eventually "squeezed" by the contracting stellar core as it due to the infalling of helium "ash" produced by the nuclear reactions around the core, the hydrogen-burning shell actually produces more outward radiation pressure than the core did during the star's main sequence lifetime. The red giant phase can last for upwards of several million years, if the star started out small enough.

Eventually, the inert stellar core will become and once it reaches a temperature of around 108K and a density high enough (103kg cm-3),  ensues. For stars with a similar mass to the Sun, this process ("") is explosive, liberating during a few seconds as much energy as an entire galaxy (however, none of that energy arrives to the surface and is instead used to re-expand the stellar core). The star contracts and becomes a smaller, less luminous star to expand again as a red giant once the core helium is exhausted to finally expel its outer layers, forming a, and leaving behind its high-density, inert, core: a white dwarf (see below). Sun-like stars do not fuse elements beyond helium, as they're unable to produce the temperatures and pressures required.

More massive stars have an even more lively old age. Stars of at least 9 solar masses, and even a bit less, are able to burn (fuse) in their cores heavier elements than helium,    less and less energy, in addition to the extreme temperatures and pressures existing in the stellar cores that accelerate nuclear reactions and most of the released energy being released as s causing them to be used up faster and faster (as in, from the millions of years standard core hydrogen fusion lasts to the just years of even days such late nuclear burning processes last) until they arrive to iron, an element whose fusion requires energy. The star then collapses and explodes as a supernova, with the energy of the explosion — similar to that of a galaxy — being able to fuse iron and produce heavier elements. Meanwhile from the outside, things are also quite lively: depending on their mass, they become red supergiants (basically red giants on steroids) and may explode in that stage, or instead extensive mass loss may cause them to loop back to higher surface temperatures and smaller sizes before supernova time.

The most massive stars will not go red supergiant and will instead become hotter-than-red-supergiant hypergiant stars (or will even skip this phase) before contracting due to high mass loss to much smaller radius and much higher surface temperatures, becoming Wolf-Rayet stars, to finally suffer core collapse directly to a black hole and explode as a hypernova (for the most massive of them, direct collapse to a black hole with no hypernova is also predicted as well as the possibility of them simply blowing themselves apart and leaving no remnant). The small, dim red dwarf stars will be almost entirely helium by the time they deplete their hydrogen fuel (many times the current age of the Universe), so there will be no hydrogen to form a hydrogen-burning shell; instead, they will heat up without expanding, becoming a hypothetical blue dwarf, followed by contraction into a white dwarf.

Note that in all the above cases, the presence of a close companion star may royally mess things up (just ask Algol).

Black hole

 * Main article: Black hole

A black hole is an astronomical body so dense that the escape velocity is greater than the speed of light. At the center is the singularity, which is infinitely dense and can't be described by current laws of physics. Its precise nature and effects on the surrounding space is a source of unending scientific debates. Planets and other stars can orbit a black hole much like any other star. Black holes, while black themselves, can have very bright accretion disks. Ironically enough, these include the brightest objects in the universe, quasars.

Neutron star
A neutron star is a compact object that is created in the core of a massive star during a supernova explosion. As their name suggests, neutron stars are composed almost entirely of neutrons. Though they are dead stars, they are still very hot. They are extraordinarily smaller than the original star from which they originated, with a radius of about 12 km. In contrast, the Sun's radius is about 60,000 times that. They typically have a mass between 1.35 and about 2.1 solar masses. As a result of its extreme density, a typical neutron star has a surface gravity of over a hundred billion G's and an escape velocity of about 1/3rd the speed of light. A single teaspoon of its interior would weigh at least two billion tons at the surface of the Earth. Any object falling toward a neutron star would be torn apart by tidal forces before it impacted the surface.

Nuclear pasta and antipasta
Scientists have calculated that neutron clusters in the neutron star's outer crust may be composed of some of the toughest material in the universe, with names like nuclear gnocchi, spaghetti, waffles, lasagna, defects, antispaghetti, and antignocchi, which refer to the shapes of in various lattices that the neutrons stack themselves into. For example, in "spaghetti", neutrons stack into long strings, while in "lasagna", they stack into sheets.

Pulsar
Some neutron stars are known to emit radio waves that pulse on and off. This occurs if a significant proportion of the magnetic moments of the component neutrons are aligned. These neutron stars are called pulsars. The "off" and "on" emission that is characteristic of pulsars is due to the star's rotation. The radio waves only escape from the North and South magnetic poles of the neutron star. If the spin axis is tilted with respect to the magnetic poles, the escaping radio waves sweep around like the light beam from a lighthouse. On Earth, radio astronomers pick up the radio waves only when the beam sweeps across the range of the Earth. The first pulsar was detected in 1967 and for a short time the regular signal was thought to be evidence of extraterrestrial life (the pulsar PSR B1919+21 was originally nicknamed LGM-1 for "little green men").

White dwarf
A white dwarf is a small star composed mostly of electron-degenerate matter. Because a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth, it is very dense, though nowhere near as dense as a neutron star. White dwarfs are faint in comparison to other stars because they're really tiny; although the matter within them no longer undergoes fusion reactions, and all their luminosity comes from the emission of stored heat, they have a lot of stored heat. Their surface temperatures are actually quite high by stellar standards. Over time, they become dimmer as they cool and give off less energy, turning into a theoretical "black dwarf". Because no white dwarfs are older than the universe itself, even the oldest white dwarfs still radiate at temperatures of a few thousand kelvins, and no black dwarfs are thought to exist yet. Interestingly, our own Sun will more than likely become a white dwarf due to the fact that it is too small to become a black hole or neutron star.

If a white dwarf is part of a binary star system, and its companion star expands into a red giant near the end of its lifetime, the white dwarf can accrete material from the other star's outer atmosphere (forming a "mass-exchange binary" system). This drawn-in material will be subjected to the white dwarf's surface gravity, on the order of a hundred thousand G's. When enough material accumulates, the accreted material can get hot enough and pressurized enough to undergo nuclear fusion, resulting in a nova outburst. If the white dwarf accumulates so much material that its mass exceeds 1.44 Solar masses, it will collapse under its own weight and explode, creating a spectacular Type 1a Supernova.

Brown dwarf
Brown dwarfs can be easily confused with other stellar objects. Fortunately, simple tests exist to determine whether it really is a brown dwarf.


 * 1) Lithium is generally present in brown dwarfs, but not in low-mass stars. Stars, which achieve the high temperature necessary for fusing hydrogen, rapidly deplete their lithium when lithium-7 and a proton collide, producing two helium-4 nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is depleted. Therefore, the presence of the lithium spectral line in a candidate brown dwarf's spectrum is a strong indicator that it is indeed substellar. This test can be flawed. Lithium can be seen in very young stars, as they have not had a chance to burn it off. Our sun also contains lithium in its outer atmosphere, as it is not hot enough for the process to take place. In addition, brown dwarfs at the high end of their mass range can be hot enough to deplete their lithium when they are young. Dwarfs of mass greater than 65 $$M_J$$ can burn off their lithium by the time they are half a billion years old.
 * 2) Unlike stars, older brown dwarfs are sometimes cool enough that over very long periods of time their atmospheres can gather observable quantities of methane (an example of a brown dwarf confirmed in this way is Gliese 229B). Main sequence stars cool, but eventually reach a minimum luminosity which they can sustain through steady fusion. This varies from star to star, but is generally at least 0.01% the luminosity of our Sun. Brown dwarfs cool and darken steadily over their lifetimes: sufficiently old brown dwarfs will be too faint to be detectable.
 * 3) Brown dwarfs can be confused with gas giants like Jupiter, but have a much higher density.
 * 4) About all brown dwarfs, no matter what their mass, are about the size of Jupiter.

Society and culture
The following terms need to be distinguished carefully:


 * Astronomy (Greek for "star-naming") is the science that studies outer space, and more specifically, there is stellar astronomy for stars in particular. Astrophysics is a branch of physics studying the same topic, and equivalently we can speak of stellar astrophysics. Professional astronomers and astrophysicists are almost always PhDs, PhD students or scientists of equivalent rank, although the field has a lively amateur community.
 * Astrology, not to be confused with astronomy, is a pseudoscience and a form of divination that claims to be able to predict the future. The main technique used to defraud the customer is cold reading. Professional astrologers may believe in the pseudoscience themselves, but as it is not a science, the field is not taught at the university level.