How are Stars Grouped Together?

Ancient people divided the sky up into groups of stars. This made studying the stars easier. Today we know that the stars—including the Sun—are part of even larger groupings of stars that are all bound to each other by gravity. Galaxies The Sun, the Earth, and the other planets in the solar system are part of the galaxy known as the Milky Way. A galaxy is a huge system of stars, dust, and gas held together by gravity. There are billions of galaxies in the universe.
A few can be seen without a telescope, but they are so far away they look like single points of light. Using powerful telescopes, astronomers have learned that galaxies come in different shapes and sizes. About three-fourths of the galaxies that have been discovered are spiral galaxies. They look like pinwheels. They have bright, bulging middles and wispy arms that fan out from the center. The stars in the arms of the galaxy are circling the center bulge of the galaxy, much as the Earth moves around the Sun. Elliptical galaxies can be almost round or more oval like a football. The largest galaxies we know of are elliptical. There are also elliptical galaxies that are many times smaller than our galaxy. Some galaxies are neither spiral nor elliptical. Galaxies that have no real shape are called irregular galaxies. Irregular galaxies are probably young galaxies in which stars are still forming.