Astronomy

When humans began observing the night sky, they imagined that the twinkling lights they saw formed pictures of people and animals. Today, we call these lights, stars, and the patterns that they form constellations.

Stars are balls of very hot gases that form when gravity pulls a cloud of gas, called a nebula, together with dust. When the mass of gas and dust becomes dense enough, enormous amounts of energy are released, resulting in the light that we see from Earth. Scientists characterize stars based on their color, temperature, size, and chemical composition.

Stars are grouped together in clusters called star systems. Galaxies are large groups of stars and star systems bound together with dust and gas by gravity. Our solar system is located in a galaxy called the Milky Way. Many galaxies, including the Milky Way, are bound by gravity to hundreds or even thousands of other galaxies in regions of space called galaxy clusters.

Most of the stars in our galaxy are main sequence stars, or stars that are in their longest stage of life. Most are also dwarf stars, or small stars with relatively low luminosity. Giant stars, in contrast, are very large and are brighter than the sun.