A galaxy is a huge group of stars, dust, gas, and other celestial bodies bound together by gravitational forces. A cluster of galaxies is a system of galaxies that contains up to thousands of galaxies bound together by gravitational forces.
These huge systems of galaxies are filled with enormous clouds of extremely hot gas (as high as a hundred million degrees) held together by gravitational forces. The mass of this “intracluster medium” is at least equal to (and frequently much greater than) the mass of the stars. The hot clouds of gas emit x-rays but are invisible to the eye.
The largest galaxies in a cluster (generally elliptical galaxies) are pulled to the center of the cluster by gravitational forces. Large clusters capture nearby galaxies with their enormous gravitational forces, continuing to grow over time.
A supercluster is a huge group of up to a thousand galaxies. These tend to be shaped as flattened disks, sheets, or filaments. These superclusters then form surfaces like the surfaces of bubbles, with virtual voids in between (Margaret J. Geller, John P. Huchra and Valerie De Lapparen, 1986). In 1989, Geller and Huchra discovered this “Great Wall,” a sheet of galaxies that extends for at least 500 million light-years, perhaps even more. This may be the largest structure in the universe.
in the Local Group
|Large Magellanic Cloud (LMC)||30,000||163,000|
|Small Magellanic Cloud (SMC)||20,000||190,000|
The Local Group is the cluster of galaxies to which we belong. It is a group of about 30 galaxies that is about 5 million light-years across. The largest of the galaxies are Andromeda galaxy, Triangulum, and our Milky Way. The Local Group also includes Fornax, the Large and Small Magellanic Clouds, M32, M33, M101, and 9 dwarf spheroidal galaxies.