Jason Harris Stars What are the stars? Stars are gigantic, self-gravitating spheres of (mostly) Hydrogen gas. Stars are also thermonuclear engines; nuclear fusion takes place deep in the cores of stars, where the density is extreme and the temperature reaches tens of millions of degrees Celsius. Is the Sun a star? Yes, the Sun is a star. It is the dominant centerpiece of our solar system. Compared to other stars, our Sun is rather ordinary; it appears to be so much bigger and brighter to us because it is millions of times closer than any other star. Why do stars shine? The short answer is: star shine because they are very hot. It is really no more complicated than that. Any object heated to thousands of degrees will radiate light, just like stars do. The obvious next question is: why are stars so hot? This is a tougher question. The usual answer is that stars get their heat from the thermonuclear fusion reactions in their cores. However, this cannot be the ultimate cause for the stars' heat, because a star must be hot in the first place for nuclear fusion to be triggered. Fusion can only sustain the hot temperature; it cannot make a star hot. A more correct answer is that stars are hot because they have collapsed. Stars form from diffuse gaseous nebulae; as the nebulous gas condenses to form a star, the gravitational potential energy of the material is released, first as kinetic energy, and ultimately as heat as the density increases. Are stars all the same? Stars have many things in common: they are all collapsed spheres of hot, dense gas (mostly Hydrogen), and nuclear fusion reactions are occurring at or near the centers of every star in the sky. However, stars also show a great diversity in some properties. The brightest stars shine almost 100 million times as brightly as the faintest stars. Stars range in surface temperature from only a few thousand degrees to almost 50,000 degrees Celsius. These differences are largely due to differences in mass: massive stars are both hotter and brighter than lower-mass stars. The temperature and Luminosity also depend on the evolutionary state of the star. What is the Main Sequence? Main sequence The main sequence is the evolutionary state of a star when it is fusing Hydrogen in its core. This is the first (and longest) stage of a star's life (not including protostar phases). What happens to a star after it runs out of core Hydrogen is addressed in the stellar evolution article (coming soon). How long do stars last? The lifetime of a star depends very much on its mass. More massive stars are hotter and shine much more brightly, causing them to consume their nuclear fuel much more rapidly. The largest stars (roughly 100 times as massive as the Sun), will run out of fuel in only a few million years; while the smallest stars (roughly ten percent the mass of the Sun), with their much more frugal consumption rate, will shine on (albeit dimly) for trillions of years. Note that this is much longer than the Universe has yet been in existence.