There are billions of stars in the Milky Way Galaxy, and it is estimated that there are probably millions more in our galaxy alone. A star is an extremely luminous celestial object made up of a highly evolved hydrogen atom surrounded by a cluster of orbiting matter. The Sun is the nearest known star to us, with a mass about five times that of the Sun, and with a temperature similar to that of our planet. Other stars can be much closer to our own and have even larger radii, while all the stars in the Milky Way have approximately the same size as the planet Earth. All of these factors contribute to the rate at which stars burn up as exhaust heat, although the precise nature of this process is not well understood.
Hot stars (also known as main sequence stars) lie in the outer solar system, and are a type of “red” star. They are very hot because of nuclear fusion within their cores, giving off great amounts of radiation in the process. These types of stars tend to dim when a planet crosses or near them; if this happens too often, the star’s surface may turn black, red, or white, and in extreme cases into a red dwarf. While not as bright as main sequence stars, red dwarfs are also very hot, and can reach a red giant stage before shrinking again.
Cold stars (also called white dwarf stars) are very cool, and only about 10% of their total weight is made up of hydrogen (a type of element). They do not give off radiation, and tend to form a very crowded galaxy. They can become very massive, and astronomers use a variety of techniques to determine the composition of these cool objects. Because these dwarf stars are so cool, astronomers can study their atmospheres very closely, obtaining exquisite details of both the gases present, as well as the chemical processes going on. The discoveries made through HST have helped to improve our understanding of space, and the universe.