|A typical spiral galaxy|
A galaxy is a massive, gravitationally bound system consisting of stars, interstellar medium of gas and dust, and dark matter. Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion stars, all orbiting a common center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds.
There are probably more than 100 billion galaxies in the observable universe. Most galaxies are 1,000 to 100,000 parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe.
Galaxies can be categorised as elliptical galaxies, spiral galaxies and peculiar galaxies. Elliptical galaxies have an ellipse-shaped profile. Spiral galaxies are disk-shaped with curving, dusty arms. Peculiar galaxies are those with irregular or unusual shapes, these galaxies typically result from disruption by the gravitational pull of neighboring galaxies.
Our Sun is in the milkyway galaxy. The milkyway galaxy is a spiral galaxy. The stellar disk of the Milky Way galaxy is approximately 100,000 light years in diameter, and is believed to be, on average, about 1,000 light years thick. It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars, the exact figure depending on the number of very low-mass stars, which is highly uncertain. The plane of the Milky Way galaxy is visible from Earth as a band of light in the night sky, and it is the appearance of this band of light which has inspired the name for our galaxy. Our milkyway galaxy and our closest neighbour, the andromeda galaxy are on a collision course. In about 3 billion years, the two galaxies will collide with each other.