Star

[[File:Mirfak.jpg|right|thumb|165px|Twinkle, twinkle.]]{{Astronomy}}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{{efn|Unlikely, considering the Universe is too young for white dwarf stars to have cooled the {{wpl|Black dwarf|point of almost undetection}}, and neutron stars and black holes seem to be relatively few and far between}}, the nearest star to Earth's sun is {{wpl|Proxima Centauri||,}} 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 {{wpl|R136a1}} that ''really'' puts the Sun to shame and can be seen even being located in other galaxy to {{wpl|2MASS_J0523-1403||,}} outshined by our [[Sun|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 {{wpl|VY Canis Majoris||,}} large enough was it on the [[Sun]]'s place to reach midway between the orbits of [[Jupiter]] and [[Saturn]] and {{wpl|EBLM_J0555-57|EBLM_J0555-57 Ab||,}} that is as large as the planet Saturn, and finally masses vary between the almost three hundred solar masses of the already mentioned R136a1 to {{wpl|VB 10}}, with less than 1/10th of the Sun's mass and barely massive enough to be able to fuse hydrogen. Finally, (surface!){{efn|Core temperatures range between the 10 million or so K of small hydrogen-burning stars to the ''several billion'' K in the heart of a star just about to go supernova.}} temperatures show a less extreme variation and range between around the two hundred thousand K of certain {{wpl|WR 142|very evolved high-mass stars}} and the "just" around two thousand K of those three small ones.

Do note that these are the ''present'' limits. The very first stars formed after the [[Big Bang]] were born of nebulae with very low or even zero metallicity{{efn|In astronomy, anything that is not either [[hydrogen]] or [[helium]]}}, which would have caused them to be {{wpl|Stellar_population#Population_III_stars|far more massive, thus luminous}} than present-day ones. Conversely in the distant future as such metallicity increases as more stars evolve and die, enriching (or polluting, as you prefer) the remaining gas clouds, [[Ultimate_fate_of_the_Universe#Stelliferous_Era|the higher and lower mass limits will be lowered]].

=== Types of stars ===
[[File:TernaryColorTmap.PNG|right|thumb|165px|Approximate true colors of stars depending of their surface temperatures.]]
[[File:HRDiagram.png|right|thumb|165px|A Hertzsprung-Russell diagram.]]
By looking at its {{wpl|Astronomical spectroscopy#Stars and their properties|spectrum}} 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 – {{wpl|Stellar classification#Class W|Wolf–Rayet star||,}} evolved O-type stars (see below) which have long expended the hydrogen in their cores and are fusing heavier elements (helium onwards) there. The hottest ones and with a deep blue color. They come in three flavors: nitrogen-rich (WN), carbon-rich (WC), and oxygen-rich (WO), the first two types believed to be fusing helium in their cores into carbon and oxygen, while the latter one (much rarer) thought to be fusing heavier elements than helium (ie, they're expected to go supernova or even produce a {{wpl|Gamma-ray burst}} soon). They can basically be considered as bare stellar cores with the outer layers having been stripped away due to strong stellar winds and showing the inner ones where the products of nuclear reactions are present (the ones of hydrogen burning in the WNs, while WCs and WOs show respectively the ones of helium fusion and more advanced nuclear burning stages, {{wpl|WR 102|with the most extreme examples}} cramming a massive star within a radius similar to the Sun's one or even less. Most of them show a lack of hydrogen; however some are hydrogen-rich instead and thought to be very luminous and massive still main-sequence, hydrogen-fusing, stars with very powerful stellar winds.

{{wpl|Stellar classification#Class O|O}} – Deep blue stars. The hottest, by far the rarest, and most massive hydrogen-burning stars, even if the most luminous of them may show a Wolf–Rayet-like spectrum as noted above. 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.

{{wpl|Stellar classification#Class B|B}} – Blue stars, less massive and thus less luminous than an O. While still rare, much more abundant than O-type stars.

{{wpl|Stellar classification#Class A|A}} – Pale blue, not white as generally thought, stars such as {{wpl|Sirius||,}} 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.

{{wpl|Stellar classification#Class F|F}} – Yellow-white (actually blue-white) stars.

{{wpl|Stellar classification#Class G|G}} – White (not yellow) stars. Our [[Sun]] is one of these.

{{wpl|Stellar classification#Class K|K}} – (Pale) orange stars.

{{wpl|Stellar classification#Class M|M}} – 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.

{{wpl|Stellar classification#Class C: carbon stars|C}} – Stars, most of them giants of spectral type F, G, K, and M, abnormally rich in [[carbon]].

{{wpl|S-type star|S}} – 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").

{{wpl|Stellar classification#Class L|L}} – 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 {{wpl|V838 Monocerotis||.}} 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 or at the very least are close to end central hydrogen burning.

{{wpl|Hypergiant|0|.}} Hypergiant stars, often Ia+. Very rare, but including some of the largest and most luminous stars known.

{{wpl|Supergiant_star|I|.}} Supergiant stars, further divided into "Ia" (bright supergiants), "Ib" (less luminous (standard) ones), and "Iab" (transitional between both).

{{wpl|Bright giant|II|.}} Bright giants, intermediate between giant and supergiant stars.

{{wpl|Giant star|III|.}} Giants.

{{wpl|Supergiant star|IV|.}} 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.

{{wpl|Main-sequence star|V|.}} Main sequence stars like the Sun, fusing hydrogen into helium on their cores. By far the most abundant type of star, especially red dwarfs that are considered to be the most abundant stars in the Universe.

{{wpl|Subdwarf star|VI|.}} Subdwarf stars. Most of them are metal-poor 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.

Astronomers use the {{wpl|Hertzsprung–Russell_diagram}} to plot the relationship between the luminosities and spectral classifications (read: temperatures), which is quite handy to study stellar evolution. As can be seen in the one above to the right, most stars lie on the {{wpl|main sequence}} fusing hydrogen in their cores.

=== 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 {{wpl|Double star|double}} 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 from our perspective, in most of the cases, the two or more stars are actually connected by gravity forming such a stellar system ({{wpl|Binary star||,}} or {{wpl|Binary star#Multiple-star examples|Multiple star}} for more than two of them, which offer some of the {{wpl|Albireo|most}} {{wpl|Algieba|photogenic}} {{wpl|Epsilon Lyrae|sights}} of the night sky with a telescope), 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 {{wpl|star cluster||s}} 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: {{wpl|Open cluster||s}} that are typically young in astronomical terms (from some millions to hundreds of millions of years, with {{wpl|NGC 188|very}} {{wpl|Messier 67|few}} {{wpl|NGC 6791|much}} {{wpl|NGC 2158|older}} 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 {{wpl|Globular cluster||s}} that are typically much more massive than the former, are in the {{wpl|Galactic halo||es}} 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 {{wpl|Super star cluster|precursors}} of them are known to exist especially on galaxies with high star formation activity, and finally [[Galaxy|galaxies]] topping this.

== Formation ==

Stars form out of gigantic, rarefied interstellar clouds of gas (by far, mostly [[hydrogen]] and helium) and dust named [[nebula]]e. 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 {{wpl|dark nebula}} called {{wpl|Bok globule}}, 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 {{wpl|emission nebula}} (which include some of the most photogenic objects of the sky as the {{wpl|Great Orion Nebula}}). Without such kind of stars, we have {{wpl|reflection nebulae}} 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,{{efn|In fact, those stars are usually unable to move far away from their birthplaces and their presence is a sign of ongoing star formation.}} while the tiny stars of above and others similar to them like the famous {{wpl|TRAPPIST-1||,}} 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
 [[Ultimate fate of the Universe#Ripping everything apart: Big Rip|die]] [[Ultimate fate of the Universe#Big Slurp: ???|before]] then.{{efn|It is worth of note that such low-mass stars are fully convective, meaning they can burn through their entire hydrogen supply unlike Sun-like and more massive stars, that have access just to the one available on their cores. See [https://www.spaceanswers.com/deep-space/red-dwarfs-the-fascinating-stars-that-live-for-trillions-of-years/ here].}}

==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 hole]]s, neutron stars, or white (and eventually black) dwarf stars.

=== Beyond main sequence ===
Before a star actually dies, it goes through a series of death throes. 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), thanks to such stars being fully convective unlike more massive ones, so there will only be enough hydrogen to form a modest hydrogen-burning shell around a core of helium (see below); instead of going red giant as happens in more massive stars, they will heat up without expanding, becoming a hypothetical blue dwarf{{efn|A bit of a misnomer, [https://ui.adsabs.harvard.edu/abs/2004RMxAC..22...46A/abstract as low-mass red dwarfs are expected to become as hot as the present Sun (ie, they will shine with a white color), at the lightest ones will be cooler]}}, followed by contraction into a white dwarf.

In nearly all other cases (ie, stars similar in mass to the Sun), this involves some form of a {{wpl|red giant||,}} which may be preceded by a {{wpl|Subgiant#Subgiant branch|subgiant}} 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 {{wpl|Schönberg–Chandrasekhar limit|increases its mass}} 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 so the star is much more luminous now than in its main sequence epoch more than compensating such cooling of its outer layers. 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 {{wpl|Degenerate matter|degenerate}} and once it reaches a temperature of around 10<sup>8</sup>K and a density high enough (10<sup>3</sup>kg cm<sup>-3</sup>), {{wpl|Triple alpha process|helium fusion into carbon and oxygen}} ensues. For stars with a similar mass to the Sun, this process ("{{wpl|helium flash||")}} 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 {{wpl|Horizontal branch|smaller, hotter}}, {{wpl|Red clump|and less luminous}} one powered by the combined fusion of helium in the core and hydrogen in a shell around it, as during the red giant phase, to expand again as an {{wpl|Asymptotic giant branch}} star once the core helium is exhausted and the star's inert center, now composed of carbon and oxygen, contracts to become degenerate again while helium and hydrogen burn in shells around the star's core, to finally expel its outer layers, forming a {{wpl|planetary nebula||,}} and leaving behind its high-density, very hot at first, 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 more massive than the Sun will start core helium burning in a non-degenerate core (ie, no helium flash), aborting their transformation into a red giant and becoming {{wpl|Classical_Cepheid_variable|Cepheids}} (very handy to calculate distances up to external galaxies) if they are massive enough. Once core helium is exhausted their evolution is expected to be similar to stars like the Sun, becoming AGB stars, expelling their outer layers as planetary nebulae, and leaving a white dwarf behind.

Those still heavier (at least 9 solar masses and according to some stellar evolution models even a bit less) are able to burn (fuse) in their cores heavier elements than helium, {{wpl|Carbon burning process|each}} {{wpl|Neon burning process|burning}} {{wpl|Oxygen burning process|process}} {{wpl|Silicon burning process|producing}} {{wpl|Nuclear binding energy|less and less energy}} which in addition to the ''extreme'' temperatures, densities, and pressures existing in the stellar cores that accelerate nuclear reactions (and are required for them to take place) and most of the produced energy being released as useless {{wpl|neutrino|neutrinos}}, that unlike useful photons almost do not interact with the star, translate into a high energy production (read: luminosity) in the stellar core causing such nuclear fuels to be used up faster and faster (as in, from the millions of years standard core hydrogen fusion lasts to the just years or even ''days'' or less (''hours'') such late nuclear burning processes last), giving the star's center a layered onion-like structure with heavier elements fusing in deeper layers (ie, during the carbon burning phase the core fuses carbon while helium keeps fusing in a shell around it, and this one is surrounded by another where hydrogen burning is taking place. Once core carbon is exhausted the now inert core contracts until another shell, this one fusing carbon, appears around it and core neon burning ensues once temperatures and densities are high enough in the stellar center and so on), 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 leaving a neutron star or sometimes a black hole behind. Meanwhile from the outside, things are also quite lively: depending on their mass, they become either {{wpl|Super-AGB_star|larger and more luminous versions of AGB stars}} or red supergiants (basically red giants on steroids){{efn|As the already mentioned red ''hyper''giant star {{wpl|VY Canis Majoris||,}} one of the largest known, which is so big that it goes beyond [[Jupiter]]'s orbit.}} and may explode in that stage, or instead extensive mass loss including outbursts caused by vigorous convection in the stellar core and its large luminosity as noted above<ref>[https://arxiv.org/abs/1202.5036 Wave-Driven Mass Loss in the Last Year of Stellar Evolution: Setting the Stage for the Most Luminous Core-Collapse Supernovae]</ref><ref>[https://arxiv.org/abs/1912.10050 The influence of late-stage nuclear burning on red supergiant supernova light curves]</ref><ref>[https://iopscience.iop.org/article/10.1088/0004-637X/780/1/96 SETTING THE STAGE FOR CIRCUMSTELLAR INTERACTION IN CORE-COLLAPSE SUPERNOVAE. II. WAVE-DRIVEN MASS LOSS IN SUPERNOVA PROGENITORS]</ref> and/or the onset of such late nuclear burning processes, up to ejecting the star's outer layers in a pre-supernova explosion because of them sometimes taking place in degenerate matter as per the helium flash<ref>[https://arxiv.org/abs/1505.06712 The Remarkable Deaths of 9 - 11 Solar Mass Stars]</ref>, may cause them to loop back to higher surface temperatures and smaller sizes before supernova time{{efn|In at the very least one case, [https://keckobservatory.org/sn2021yfj/ the very core of the star itself was exposed] just before supernova time.}}).  

The most massive stars will not go red supergiant and will instead become hotter-than-red-supergiant hypergiant stars including to become {{wpl|Luminous blue variable}}s (or will even skip these phases, going Wolf-Rayet directly) before contracting once core helium burning begins due to high mass loss (see, for example, {{wpl|RW Cephei}}, {{wpl|Rho Cassiopeiae}}, and {{wpl|IRC+10420}}) 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, leaving no remnant).

Note that in all the above cases, the presence of a close companion star may royally mess things up (just ask {{wpl|Algol||).}} including as is believed to have happened in precisely Algol mass transfer from one star to the other, and even the possibility of the two stars {{wpl|Blue straggler|merging and forming a more massive and luminous one}}.

===Black hole===
:''Main article: [[Black hole]]''

A '''black hole''' is an [[Astronomy|astronomical]] body so dense that the escape velocity{{efn|Escape velocity <nowiki>=</nowiki> the speed (strictly of course ''velocity'') which anything needs to have in order to ''escape'' the gravitational effect of another object.}} 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, [[quasar]]s.

===Neutron star===
[[Image:Neutronstar.jpg|thumb|right|300px|A speculative diagram of a neutron star.]]
A '''neutron star''' is a compact object that is created in the core of a massive star during a supernova explosion.<ref>[http://imagine.gsfc.nasa.gov/docs/science/know_l1/neutron_stars.html Neutron Stars], ''Imagine the Universe'' from the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center; April 5th, 2009.</ref> 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.{{efn|A ''solar mass'' is defined as the mass of the [[Sun]], about two nonillion kilograms or about 332,950 times the mass of the [[Earth]], or 1,048 times the mass of [[Jupiter]].}}
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.<ref>{{wpa|Nuclear pasta}}</ref><ref>[https://itnews.iu.edu/articles/2017/nuclear-pasta-on-display.php Nuclear pasta on display] (Mar 20, 2017) ''IT News & Events'' (Indiana University)</ref>

====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. <ref>[https://arxiv.org/ftp/arxiv/papers/1002/1002.2522.pdf Possible Constraints on Neutron Electric Dipole Moment from Pulsar Radiation] by C. Sivara (2010) ''arXiv.org''.</ref> 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 {{wpl|Type Ia Supernova}}<ref group=note>Note how some of these supernovae can be produced by the merger of two white dwarfs instead of a white dwarf sucking matter from a companion star.</ref>.

==Brown dwarf==
{{main|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.

#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 <math>M_J</math> can burn off their lithium by the time they are half a billion years old.
#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.
#Brown dwarfs can be confused with [[gas giant]]s like [[Jupiter]], but have a much higher density.
#About all brown dwarfs, no matter what their mass, are about the size of Jupiter.<ref>[http://www.daviddarling.info/encyclopedia/B/browndwarf.html Brown dwarf]</ref>

==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.

==Notes==
{{reflist|group=note}}

==References==
{{reflist}}

[[Category:Astronomy]]

Categories: Astronomy