They are usually electrons, protons, or neutrons, but don't have to be. Most are thought to be energized by supernova or other explosive phenomenon, but some are even more energetic than is thought possible. Created by Astronomer Francis Drake, the Drake equation attempts to calculate the number of alien civilizations that exist in the world, using a variety of numbers. Since electrons have the same charge, they repel each other. In an object that is held up by degeneracy pressure, their atoms are so close, only the repulsiveness from their electrons keeps them apart.
Galaxies are large groups of stars nebulae and other objects that orbit the galactic center.
A unit that measures distance in astronomy. 1 light year is the distance light travels in a year, or roughly 5.8 trillion miles. If you want to get more digits than that, calculate it out, light travels 186,282.4 miles per second. A large cloud of dust and gas floating in space. A very small particle produced in large numbers in stellar cores, believed to account for some of the missing mass in the universe.