Astronomy - Stars

As we learned in our last lesson, there are billions of stars in the universe. Because of this, you are probably wondering how we could possibly know anything about individual stars at all. Lucky for us, thousands of people before us have spent their lives mapping the skies and giving names to celestial bodies. Since there are so many stars in the universe, the Muggle International Astronomical Union, the world authority for assigning names to celestial objects, categorizes these stars and charts their locations.
Since there are so many stars in the universe, the IAU uses a different system for newfound stars. Most consist of an abbreviation that stands for either the type of star or a catalog that lists information about the star, followed by a group of symbols. A star develops from a giant, slowly rotating cloud that is made up entirely or almost entirely of hydrogen and helium. Due to its own gravitational pull, the cloud begins to collapse inward, and as it shrinks, it spins more and more quickly, with the outer parts becoming a disk while the innermost parts become a roughly spherical clump. This collapsing material grows hotter and denser, forming a ball-shaped protostar (picture below). When the heat and pressure in the protostar reaches about 1.8 million degrees Fahrenheit (1 million degrees Celsius), atomic nuclei that normally repel each other start fusing together, and the star ignites. Nuclear fusion converts a small amount of the mass of these atoms into extraordinary amounts of energy.
Although our solar system only has one star, most stars like our sun are not solitary, but are binaries where two stars orbit each other, or multiples involving even more stars. In fact, just one-third of stars like our sun are single, while two-thirds are multiples — for instance, the closest neighbor to our solar system, Proxima Centauri, is part of a multiple system that also includes Alpha Centauri A and Alpha Centauri B. Binary stars develop when two protostars form near each other. One member of this pair can influence its companion if they are close enough together, stripping away matter in a process called mass transfer. If one of the members is a giant star that leaves behind a neutron star or a black hole, an X-ray binary can form, where matter pulled from the stellar remnant's companion can get extremely hot — more than 1 million F (555,500 C) and emit X-rays.
Now that we have the basic makeup for stars out of the way, please move on to your homework.