Stars are large balls of mostly hydrogen. They are so massive that their insides grow really hot, and fusion occurs. For specific examples, see the Useful Notes pages on:
Magnitude- Apparent and AbsoluteMagnitude is the brightness of a star in the form of a number. Created by Hipparchus or Ptolemy, a star of Magnitude 1 is about 2.512 times (the 5th root of 100) brighter than one of Magnitude 2. It used to be that 1 was the highest, and 6 was the lowest, but they found that this didn't quite work and had to go into negative numbers, hence the Crab Supernova having -6. Most famous stars are between 0 and 2. The human eye can see to about 6, with 7.72 being the current world record. Vega is approximately magnitude 0. Sirius, the brightest star in the night sky is at -1.46. -3.9 is the faintest you can see in daylight, the Moon is -12.6 when it's full and the Sun is -26.73. This is the apparent magnitude of course. The farther away the same kind of star is, the dimmer it's going to appear. If a star that's 10 light-years away has an apparent magnitude of 1.5, that same star would have an apparent magnitude of 3.0 if you moved it 20 light-years away. To judge the true, intrinsic brightness of stars, the magnitude that they would be at if 10 parsecs (32.6 light years) away is determined and called absolute magnitude. For example, Alpha Centauri has an absolute magnitude of 4.38, but an apparent one of -0.27, as it is very close to Sol, relatively speaking (4.3 light years). Far-off (700-900 light years) Rigel has an apparent magnitude of 0.18, but at the 10 parsec distance it shines at absolute magnitude -6.7. (There is also "planetary absolute magnitude", which is the brightness a planet would show at full illumination from 1 astronomical unit - that is, if the planet was moved to the Earth's orbit and viewed from the center of the Sun.)
Distance determinationNearby stars have it determined by something called trigonometric parallax. It's basically trigonometry that uses the relative movements of objects against a "fixed" background of far-off stars and galaxies- the closer the near star is, the more it will seem to move (you can see this on a train or in a car). Because of the distances involved, the change in angle is very small, less than a second of arc (or 1/3600 of a degree). This is the source of the unit of distance known as a "parsec" it's the distance [3.26 light years] at which an object viewed from two points one AU apart would show a parallax of one second of arc. Most stellar distances are calculated by taking two observations six months apart (with a baseline of 2 AU), but plans exist to send out probes to provide baselines of tens or even hundreds of AU. But after a certain point current instruments simply can't measure the infinitesimal parallax of truly far away bodies. That's where we turn to an indirect means called the standard candle — we look for an object whose intrinsic brightness (i.e. absolute magnitude) is known, then use how dim it appears (i.e. its apparent magnitude) to calculate how far away it must be. Not many objects have predictable brightnesses, but one of the few that do is a type of giant star known as a Cepheid variable. These stars pulse regularly at a rate proportional to their intrinsic luminosity, and some of them are also close enough to have measurable parallax. Therefore, far-off Cepheids (and objects such as the nebulae or galaxies they reside near or within) can have their distance judged simply by noting their pulse rate. Some stars that aren't Cepheid variables can at least give us a ballpark hint as to their intrinsic brightness by certain clues in their spectra. When we look at the spectrum of a red star, for example, we can tell whether it's a nearby red dwarf or a distant red giant. While we can't calculate its intrinsic brightness precisely, we can figure it within a couple of orders of magnitude. This trick goes by the rather confusing name of "spectroscopic parallax", even though no actual parallax is involved.
Heavy element abundanceAbout 1% of the mass of the sun consists of elements heavier than hydrogen and helium: lithium, beryllium, carbon, nitrogen, oxygen, iron, et cetera. Collectively, astronomers refer to all elements heavier than helium as "metals", even those that aren't metals in the chemical sense. And as it turns out, the abundance of these heavy elements varies from star to star, depending on the chemical composition of the primordial gas-and-dust cloud from which the star originally condensed. We can tell how much of an element is present in the photosphere of a star by looking at its spectrum. Every element has a characteristic signature, a fingerprint if you will, of absorption/emission lines (sometimes called Fraunhofer lines after their discoverer) at very specific frequencies. Sodium, for example, has two very strong lines right next to each other at 5895.92 and 5889.95 Εngstroms. By looking at how prominent these lines are in the spectrum of a star, relative to their prominence in the spectrum of the sun, we get a pretty good measure of the relative abundance of that element. The overall picture of every heavy element's relative abundance is known as the "metallicity" of the star. Most of the stars that lie along the galactic plane are rich with heavy elements, like the sun is. These stars are now known as the galactic plane population; in earlier times, they were referred to as "Population I". These stars are second- or third-generation — that is, the gas-and-dust cloud out of which they formed was spewed out by the death throes of an earlier star, in which heavy elements had been synthesized. First generation stars, such as those that orbit the galaxy 'way out in the galactic halo, are much poorer in heavy elements, and used to be referred to as "Population II". The heavy element abundance of a star is important for two reasons:
NamesStars have a variety of naming methods:
Types of starsMost stars are found along something called the Main Sequence, characterized by their balance between inward gravity and outward pressure generated by hydrogen fusion. Other stars exist that are off of it and fusing other elements, or else are dead or dying. These types have varying letters (spectral classifications) applied to them, with numerical sub-groups and a corresponding informal color (you can see the color in a good telescope). The higher up the scale, the bigger (and brighter) the star, but the faster the rate at which the hydrogen in them is used up and so the shorter their lifespan. For a number of reasons, very large stars - called giants, supergiants and hypergiants - like to live at the extreme ends of the spectral scales. Giant stars really do not like to be classes F or G, seeming to stay there for a very short time while heating up or cooling down to either extreme. There are a few known, but are decidedly in the minority. In general, most of the visible giants are either class-M or rather cool class-K (Betelgeuse, Arcturus) red giants, or class-B (Eta Carinae, Rigel, Deneb) blue giants.
Stellar EvolutionStars are pretty long-lived compared to us humans, but they do change, age and die. The life-cycle of a star is called stellar evolution and goes through various stages depending on the size of the star in question. The initial stage all stars go through is the protostar. It is a large, cool gas cloud that slowly contracts under its own weight, building up heat along the way. A typical protostar can be described quite accurately as an "infrared giant": it is large, quite luminous but most of its luminosity is infrared. Protostars can develop protoplanetary discs as they contract. Eventually, they build up enough heat to start a fusion reaction; a bright flash of light and radiation sweeps the protoplanetary disc clean of gas and dust, ending the accretion of planets, and thus a new star system is born. Some protostars are not massive enough to start a true fusion reaction. They condense into smallish balls the size of Jupiter and briefly go through a low intensity deuterium fusing phase. These not-quite-stars are called brown dwarfs (example: Teide 1), and their evolution is simple: when the deuterium is all burned up, they start a slow process of cooling and contraction. Eventually they lose all luminosity and become indistinguishable from common gas giant planets, except by mass. This final stage of brown dwarf evolution is called an Y-class brown dwarf (example: WISE 1541-2250). Other protostars, however, successfully light up their fusion and enter a stage of life called the main sequence. It is called so because it encompasses most of a star's lifespan. The exact lifespan of a star on the main sequence depends on its mass: a low-mass red dwarf (example: Proxima Centauri) can smolder for trillions of years, because its fusion reactions are so sluggish; more massive stars are brighter yet burn up their hydrogen faster. Most main sequence stars are stable and not variable; only the aforemented red dwarfs can be prone to violent flares. Luminosity of a main sequence star slowly increases as it grows older. After the star starts running out of hydrogen, its further fate depends on its size and mass. The smallest red dwarfs, with masses 0.08 to 0.3 solar masses (example: Van Biesbroeсk's Star), are homogenous and fully convective, so they do not develop helium cores. Instead, they grow hotter and turn into blue dwarfs, and then become inert helium white dwarfs. Larger dwarfs, with masses 0.3 to 0.5 solar masses (example: Gliese 581), can develop distinct helium cores and evolve in a similar way to heavier stars. They go through subgiant and common red giant phases like mid-size stars. Mid-size stars, with masses up to 10 solar masses (examples: Sun, Alpha Centauri A and B, Sirius) develop inert cores of helium as they leave the main sequence. The first phase of their evolution is the subgiant (example: Procyon). Fusion reactions in the core slow down as fuel comes to an end, and move to the neighboring region immediately surrounding the core, called the hydrogen-burning shell. Since the shell has a greater surface area than the core, it produces more radiation pressure. Subgiant stars grow somewhat cooler and larger in size than they were during the main sequence, but not much more luminous. Once fusion reactions in the core stop completely and move into the shell, the star enters red giant territory. It starts to swell, growing larger, cooler and more tenuous; although the upper layers of the star cool, the much greater surface makes it much brighter than before. This stage is called the common, or hydrogen-fusing red giant (example: Aldebaran). For small stars with masses 0.3 to 0.5 solar masses, the common red giant phase is the only red giant phase; they just keep swelling and growing more tenuous until they dissipate, leaving an inert helium core which becomes a helium white dwarf. Heavier stars, however, go through several more transmutations. First, they undergo an event called a helium flash. Though the outer layers of a red giant grow cooler and more tenuous, the helium core does the other way round, it contracts and heats up. Finally, it heats to a degree when helium fusion, called the triple alpha process, becomes possible. The core bursts up, shutting down hydrogen burning in the shell, and the red giant stops growing for a while. These new, helium-fusing red giants are called red clump giants, since they are very uniformly luminous and located in an accurate little clump on the H-R diagram. The swelling stops, stabilizing the giant. Note that older, metal-poor stars go through a different transformation upon helium flash. They not only stop growing and cooling, they start doing it in reverse! The result is a star called a horizontal branch giant (example: RR Lyrae), a yellow, white or even bluish-white giant. These giants can be found in the usual haunts of low-metal stars, namely globular clusters and the galactic halo. After a while, however, the helium core is used up, too. A new, smaller carbon-oxygen core is formed, and helium fusion contunues in a shell around this core (and hydrogen fusion in a larger shell around this shell). Noticed a pattern yet? Yes, the cooling and swelling process starts anew, and this time it cannot be stopped. This final form of bright red giant is called an assymptotic giant (example: Mira). It grows to immense size, becomes unstable and starts shedding its outer layers into space. Finally it all vanishes away into smoke, leaving an inert core called an carbon-oxygen white dwarf. Even heavier stars, massing 10-30 solar masses, go through a different pattern of evolution. Their helium lights up prematurely when there's still hydrogen in their cores, making a normally blue or bluish-white star expand into a blue supergiant (example: Rigel). After that, it swells and cools similarly to a lighter star, only this time the shell-layering process goes even further with an event called carbon detonation, and carbon starts to burn, too. The end result is the red supergiant (example: Betelgeuse), a star very similar to a asymptotic red giant, only with more layers, up to iron, and much larger and brighter. It may end like an asymptotic giant, dissipating into an oxygen-neon-magnesium white dwarf, or much more spectacularly. The catch is that the heavier the element, the harder is it to fuse it, and more of the energy output will be in useless neutrinos that escape the star, rather than in useful photons (this is called urca-process). This means that each subsequent phase of fusion will be shorter and shorter, until a core of iron forms. Iron is impossible to fuse with any significant energy output, and if the stupid star tries to do so anyway, it starts losing energy. The core cools and contracts in mere seconds, the shells fall on it causing an enormous starquake, the entire intricate structure is disrupted and blows up in a giant explosion. This is called a supernova. The iron core is compressed even more by this catastrophe and loses its atomic structure, degenerating into neutrons and forming a neutron star, or pulsar (example: Crab Pulsar). Stars larger still (30-60 solar masses) go through a more chaotic evolution. Once they go supergiant, they start jumping to and fro from blue supergiant to red supergiant, changing colors several times in their short lifespans. Finally, the supergiant starts to smoke away prematurely, shedding its hydrogen layers. The result is a Wolf-Rayet variable star (example: Gamma Velorum), which is, much like a large red supergiant, layered in shells of elements, only without the cool, tenuous outer shell of hydrogen. It is very hot, blue and gives a powerful stellar wind. It blows up as a supernova, much like a red supergiant, and the end result may be either a neutron star or a fully collapsed object, a black hole (example: Cygnus X-1). Finally, the heaviest stars (60 solar masses and more, the recorded maximum is 265 solar masses) are utterly chaotic in their evolution, resembling very large red supergiants or Wolf-Rayets but irregularly shaped, with lots of bursts, jets, disruptions, protuberances and smoky nebulae. Their powerful fusion furnaces generate radiation so hard that it produces particle-antiparticle pairs of electrons and positrons, severely destabilizing these stars. They go through several supernova-like bursts in their lifetime; one burst isn't enough to blast these superheavy stars to smithereens, you see. In these eruptive bursts, they shed mass measuring in dozens of solar masses at a time, surrounding themselves with misty nebulae (example: Eta Carinae). They spend little or no time on the main sequence, going giant right away upon formation. The final explosion is a hypernova that leaves no remnant and a huge nebula. Even more massive stars, that are theorized to have existed in the earliest epochs of the Universe, would die not with a bang but with a whimper suffering a process known as photodesintegration that would case them to collapse into a black hole, some mass escaping into relativistic jets.
Two's Company, Three's BetterStars are pretty frequentlynote found in pairs, triples or more - Sol is rare for being a single star system. These stars can be very close together (like the twin suns of Tatooine in Star Wars), or quite far apart (like Alpha Centauri), but either way it can lead to a very cool Alien Sky. Close binaries are less likely to have useful planets than single stars. But distant binaries/multiples can have a LOT! — a full system for each of the component stars. According to one paper published in the late 1970s, a planet in a multiple star system cannot be in a stable orbit unless its orbital radius is either (A) less than one-quarter (1/4) the closest-approach distance between the two stars, or (B) greater than twice the farthest-separation distance between the two stars (so as to orbit both stars as a pair). In Alpha Centauri's case, the A and B stars get within 11.5 A.U. of each other every 80 years, so no planets should exist more than 2.9 A.U. away from either star. Fortunately for writers, this still allows either or both to have a habitable planet.
Extrasolar PlanetsOr exoplanets, for short. Giant stars are very unlikely to have useful planets: red giants gobble their close-by planets up when they grow, retaining only their outermost worlds, while blue giants, for one thing, blow their protoplanetary disks away and, for another thing, are too short-lived to form planets anyway. Some pulsars have been found to have planets, but these were formed probably after the original star went supernova, from the remaining gas and dust that the explosion left behind. Needless to say, they'd be pretty inhospitable to any known forms of life. Class A stars can host planets (Fomalhaut has one, for example), but almost certainly don't last long enough for life to evolve there naturally. Hotter class F stars (F0 through about F4-5) may have similar problems - they tend to enter their subgiant phases after about five billion years, which is roughly the present age of the Sun. Cooler F, G, and hotter K stars are the most hospitable to Earth-like life to evolve. Red dwarfs (M and cooler K stars) are too dim, that means their habitable zone isn't wide enough to probably have a planet there, with the exception of, perhaps, the possible rare small tide-locked world (see below). Many red dwarfs are also prone to violent solar flares. But there are a hell of a lot of these M-types in the universe, more than any other type, and some percent of them aren't flaring, and some percent of these may have a planet in the life-zone. So quite a number of red dwarfs could be hospitable. Another wrinkle with red dwarf planets is that a red dwarf's habitable zone is so close to the star that any planets there stand a good chance of being tidally locked; that is, always keeping the same side toward the star, as the Moon does to Earth. Without a very dense atmospherenote (to avert Convection Schmonvection), 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 Artificial Intelligence and can live anywhere, have otherwise come from elsewhere and Terraformed 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.