Wednesday, February 24, 2010

Intergalactic Space

The Local Group contains dozens of galaxies contained within a volume about 10 million light years across and its two largest galaxies are the Andromeda galaxy and the Milky Way galaxy. Both galaxies account for over 80 percent of the visible light of the Local Group. The third most prominent galaxy in the Local Group is the Triangulum galaxy and it has approximately one-fifth the luminosity of the Milky Way Galaxy. The rest of the galaxies in the Local Group are irregular galaxies, dwarf irregular galaxies, dwarf elliptical galaxies and dwarf spherical galaxies. Many of these smaller galaxies are either in orbit around the Andromeda galaxy or the Milky Way galaxy. Within the Local Group, many more tiny galaxies are still probably waiting to be discovered.

The mutual gravitational attraction from matter within the Local Group dominates over the expansion of the universe. The galaxies in the Local Group range in size from the Andromeda and Milky Way galaxies which contain hundreds of billions of stars each to tiny galaxies which hold fewer than a million stars. The Andromeda galaxy and the Milky Way galaxy are separated by a distance of 2.5 million light years and they are approaching each other at a rate which will most probably result in a merger over the next few billion years. Such a merger will lead to the formation of a giant elliptical galaxy. It is very unlikely that stars will collide when both galaxies merge as stars in galaxies are typically spaced extremely far apart from each other.

Located 700 million light years away, in the vicinity of the constellation Boötes as seen from the Earth is one of the largest known voids in the Universe and it is called the Great Void or Boötes Void. This region of empty space is estimated to be 400 million light years across. For comparison, the luminous disk of the Milky Way galaxy is 100000 light years across and you will need 4000 Milky Way galaxies placed end to end to cover the span of the Boötes Void. Within this incredibly vast void, several galaxies have been detected to extend in a rough tube-shape through the middle of the void. Astronomer Greg Aldering once mentioned: “If the Milky Way had been in the center of the Boötes void, we wouldn’t have known there were other galaxies until the 1960s.”