The Astronomy of the Local Group

The Local Space Neighborhood Contains Over 30 Galaxies

Aug 12, 2009

The Milky Way Galaxy is one of 30-35 galaxies that are in the space neighborhood known as the Local Group. Here are some interesting facts about this "neighborhood."

As big as the Milky Way galaxy is, it is nevertheless one of over 30 in the Local Group of Galaxies. This is the space neighborhood that covers most of the sky.

Local Group

The size of the Local Group is spread out over the distance of 3 – 6 million light years. Some of the neighbors include the Andromeda Galaxy, located about 2 million light-years from the Milky Way. Both galaxies are at opposite sides of the Local Group. Other galaxies in the Local Group include the large and small Magellanic clouds. The Large Magellanic Cloud is about 163,000 light-years from the Milky Way with a diameter of 30,000 light-years. The Small Magellanic Cloud is further away, located about 190,000 light years from the Milky Way. It has a diameter of 20,000 light-years. These are some of the largest.

The Sagittarius Dwarf is the closest galaxy to the Milky Way. However, it is only seen as a dim patch of light. It was discovered only recently in 1994. The main reason the Sagittarius Dwarf was not identified sooner is that it is very dim, and spread out over the sky, and there are many stars in front of it. The distance to the Sagittarius Dwarf is about 55,000 light years.

Not only are there many galaxies, there are different types of Galaxies They are elliptical, spiral, or irregular. With over half being elliptical, the remaining are either spiral or have an irregular shape.

Because of the distance most of these galaxies were detected only by their infrared radiation; this is due to the galactic dust which hides the visible light. Hence there are some galaxies that may still be discovered. This may be possible with the new Large Binocular Telescope in Arizona.

About 90 percent of the galaxies, are considered low surface bright (LSB) galaxies. These galaxies, in other words, are very dim, with only a magnitude between 24 and 25. In comparison, without a telescope, your eyes can just barely see magnitude 6 stars. The sun has a magnitude of -26, and a full moon -13. Stars in the magnitude range of 16 to 22 are extremely faint; they require powerful telescopes for observation. The faintest objects detected by the Hubble Space Telescope have a magnitude around 29. The range of stellar magnitudes from 16 to 22 is used as a guide for measuring the background sky brightness.

Because of gravitational attraction between the two largest galaxies, the Milky Way and the Andromeda are moving towards each other instead of expanding along with the rest of the galaxies in the universe. The attraction rate is about 119 km/s. In fact, this gravitational attraction is so strong that the entire Local Group is collapsing and in about 3 billion years, the Milky Way and Andromeda will collide.

Supercluster

Even with the size described, the Local Group is part of The SuperCluster.This cluster contains some 50 separate clusters, each with its own set of thousands of galaxies. Its center is located about 50 million light years away. The Supercluster appears to be made of two major structures: a flattened disk, which has a thickness of about 5 million light-years and contains about 60% of the bright galaxies, and a roughly spherical halo, which contains the remaining 40% of the bright galaxies in a small number of clouds.

The Local Group is only one of many galactic groups. It is possible that there undiscovered galaxies, and new technologies will push back on the darkness and discover other galaxies, even undiscovered Superclusters.

Sources

The Local Group

Brightness of the Local Group

The copyright of the article The Astronomy of the Local Group in Deep Space Astronomy is owned by George Garza. Permission to republish The Astronomy of the Local Group in print or online must be granted by the author in writing.