A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The name is from the Greek root galaxias [γαλαξίας], meaning “milky,” a reference to the Milky Way galaxy. Typical galaxies range from dwarfs with as few as ten millionstars up to giants with hundred trillion stars, all orbiting the galaxy’s center of mass. Galaxies may contain many multiple star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.
A spiral galaxy resembles a pinwheel, with spiral arms coiling out from a central bulge. This galaxy, known as M100, looks much like our home galaxy, the Milky Way. However, the Milky Way has a bar of stars, dust, and gas across its center. Image credit: D. Hunter (Lowell Observatory) and Z. Levay (Space Telescope Science Institute)/NASA
Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). A common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with dusty, curving arms. Galaxies with irregular or unusual shapes are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.
[SinglePic not found]
The most distant galaxies yet observed appear as faint patches of light in this photograph taken by the Hubble Space Telescope. The brighter swirls are galaxies somewhat closer to Earth, and the bright orange object is a star in our own galaxy. The telescope photographed this tiny portion of the sky, called the Hubble Ultra Deep Field, in 2004. Image credit: NASA/ESA/S. Beckwith (STScl) and the HUDF Team
There are probably more than 170 billion (1.7 × 1011) galaxies in the observable universe. Most galaxies are 1,000 to 100,000parsecs 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.
Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.
