Galaxies

Each galaxy contains several hundred billion stars, typically about 100 times as many stars as there are people on our planet. Galaxies (of which the Milky Way is one) are enormous collections of stars, gas, and dust. The different parts of the Milky Way are not static but are in constant motion. The disk rotates about the Galactic center, and at large radii, the rate of rotation does not trail off but remains fairly constant.

Galaxies may be Spiral,elliptical and irregular.

A large amount of evidence suggests that the center of our Galaxy harbors a massive black hole.Galactic centre appears to be the location of a black hole of several million solar masses and is the same source that has been imaged with the new orbiting Chandra X-ray Observatory. The x-ray emissions from the Galactic center show the presence of hot gas and a jet, both of which are often associated with the presence of a black hole.The stars are orbiting a supermassive black hole: a monster in our closet.

Within 1 million parsecs (3 million light-years) of the Milky Way lie about 20 galaxies, the most prominent of which is Andromeda (M31). This galactic grouping, called the Local Group, is bound together by gravitational forces. The generic name for our Local Group is a galaxy cluster. Some clusters contain fewer than the 20 or so galaxies of the Local Group, and some contain many more. The Virgo Cluster, an example of a rich cluster, is about 15 million parsecs from the Milky Way and contains thousands of galaxies, all bound by their mutual gravitational attraction. From the velocities and positions of galaxies in clusters, one thing is very clear. We

cannot directly observe at least 90 (perhaps as much as 99) percent of the mass that must be there. Galaxy clusters, like the outer reaches of spiral galaxies, contain mostly dark matter.

Galaxy clusters themselves are grouped together into what we call superclusters. Many galaxies are grouped in galactic clusters, which, in turn, are grouped into superclusters that are found together on the edges of huge voids.