A star is a localised aggregation of matter which, by gravitational compression, is heated to a temperature at which 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 the sun is Proxima Centauri which is just over 4 light years away.
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 reactions. The ultimate fate of stars can be black holes, neutron stars, white (or black) dwarf stars.
 Red giant
Before a star actually dies, it goes through a series of death throes. In nearly all cases, this involves some form of a red giant: The star expands, usually over the course of a few thousand 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 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, 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 a million years, if the star started out small enough.
Two types of main sequence stars are theorized not to pass through a red giant phase: The very large, and the very small. The largest stars, according to some models, will suffer core collapse directly to a black hole and explode as a hypernova. The small, dim red dwarf stars will be almost entirely helium by the time they deplete their hydrogen fuel, so there will be no hydrogen to form a hydrogen-burning shell; instead, they will heat up without expanding, becoming a hypothetical blue dwarf.
 Black hole
- Main article: Black hole
 Neutron star
A neutron star is a compact object that is created in the core of massive star during 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.
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 the matter within them no longer undergoes fusion reactions; their faint luminosity comes from the emission of stored heat. Over time, they become dimmer 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
 Distinguishing brown dwarfs from other stuff
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 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 giants like Jupiter, but have a much higher density.
- About all brown dwarfs, no matter what their mass is, are about the size of Jupiter.
In the Christian sacred scriptures Satan cast down a third of the stars of heaven with his tail as he fell to Earth (Revelation 12:4). This would have resulted in a mass-extinction event at least comparable to the small asteroid that struck the Yucatan peninsula 65 million years ago and wiped out the dinosaurs. In many religions, when the end of the world arrives (presumably in 2012), the stars will fall from heaven.
Throughout the Bible, the authors are apparently ignorant of the relationship between the sun and the moon, and the fact that the sun is a star. The Bible incorrectly refers to the sky as a dome over the earth, with the stars, the sun, and the moon being fixed to the dome. We now know that the stars are an astronomical number of light years away, with the light of many of them being the only thing left for us to witness as they have since exploded, shooting gamma ray bursts that incinerate other unfortunate bodies around them. But these passages are only metaphorical of course.
Islam also gets astronomy quite wrong. Muhammad describes the sky as being made of seven layers, and he arbitrarily placed the stars in the lowest layer, lower than the moon. This is, of course, incorrect, if taken literally. He also assumed that the stars are just lamps to adorn the lower heaven that are also used as missiles to shoot the intruding evil spirits (jinns). Although stellar anti-genie missiles might sound pretty awesome, this hypothesis is provably incorrect: stars follow motions that are predictable and consistent. ("Shooting stars" aren't stars at all, but meteors.)
Zeta Reticuli, where UFOs apparently come from.
- ↑ Escape velocity = the speed (strictly of course velocity) which anything needs to have in order to escape the gravitational effect of another object.
- ↑ 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.
- ↑ 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.
- ↑ arxiv.org/pdf/1002.2522
- ↑ Theory of brown dwarfs at Wikipedia
- ↑ Brown dwarf