Galaxies and Stars
There are billions of galaxies in the Universe, each one is a system of billions of stars held together by gravity. Galaxies can be spiral (shaped like pinwheels), elliptical (oval or round), or irregular (no definite shape). Our solar system (including the Sun and the Earth) is located in the Milky Way Galaxy, which is a spiral galaxy. Specifically, we are located on one of the spiral arms of the galaxy, approximately 30,000 light years from the center.
A star is a very large ball of gas, usually hydrogen and helium, that glows from it’s own energy. Stars are different from planets in that they are “self-luminous,” meaning they actually produce light, planets do not do this. The intense heat and pressure within the star actually squeeze the hydrogen atoms together to form helium (other elements may also be formed during this process). This is where all elements of the Universe were formed. This process, which releases tremendous amounts of energy, is called nuclear fusion. Stars are not “burning”, rather they are undergoing continuous nuclear reactions. The Characteristics of Stars chart illustrates the different temperatures and brightnesses of stars. Your can also see a stars life-cycle on this chart. Notice that the Sun is a main sequence star meaning it is average temperature and brightness and that it is about half way through its life. As it dies, it will first swell into a red giant, then shrink into a white dwarf before. Stellar evolution, the way in which a star will change over time, is most affected by the star's mass.