The Local Group of Galaxies was first recognized by Edwin P. Hubble and described in his The Realm of the Nebulae.

The Milky Way and the Andromeda Galaxy (also known as M31) are by far the most massive members of the Local Group and are considered the dominant members of the Group. The rest of the Group consists primarily of these two galaxies satellites, such as The Magellanic Clouds and many dwarf galaxies, though there are a number of members that are independent of the two dominant members.

The Local Group is actually spread in an approximate volume of ten million light years (sci.astro FAQ, Hartmut Frommert and Christine Kronberg) in diameter. Membership appears to be dynamic and has undergone significant change over time. There are currently approximately 30 members of the Local Group.

Many scientists are studying future scenarios for the Group, to give some indication of the fate of our home galaxy. The most common theories state that our galaxy and M31 will collide, creating a massive elliptical galaxy, and that the Local Group will eventually be "swallowed" by the almost incomprehensibly huge Virgo Supercluster of galaxies.

In addition, it appears that the Local Group is in gravitational interaction with at least four neighboring galactic groups.

The Local Group of Galaxies is a loose collection of galaxies close to the Milky Way that is considered to be gravitationally bound. Even as the billions or hundreds of billions of galaxies that make up the observable universe, the galaxies in the Local Group will orbit around each other, or in many cases, will eventually merge into a single super galaxy.

The members of our local group consist of:

  • The Milky Way: our home galaxy, the smaller of the two major partners.
  • The Andromeda Galaxy: Larger than the Milky Way, but on the same order of magnitude and with a similar structure. The Andromeda Galaxy and the Milky Way are heading towards each other at pace where they will probably collide and merge in a billion or so years.
  • The Large Magellanic Cloud and the Small Magellanic Cloud, satellite galaxies of the Milky Way that still have some galactic structure.
  • The Triangulum Galaxy, another small spiral galaxy, close to Andromeda and probably in orbit around it.
  • A host of small, irregular dwarf galaxies, either in orbit around the predominant partners, or scattered between them.
The exact mass of all of these members is unknown, because it hinges on a series of subjects that are not fully known or understood by science: how dark matter and dark energy work. These same questions are also relevant to whether the Local Group will eventually coalesce, move apart, or both. This is also why the question of whether the Local Group will eventually join the rest of the Virgo Supercluster is not known. Billions of years from now, the remnants of our local group might just be drawn into the Great Attractor, forming into a megagalaxy, or the local group might merge together and be alone, with no other galaxies in the sky.

Astronomical distances and time scales are hard to understand, and past a certain point, the structure of the universe is hard to comprehend. The scale of the local group, however, is not that complicated, since it involves only a few members. Our Milky Way galaxy is 100,000 light years across, and the Andromeda Galaxy is on a similar scale, although probably twice as big. They are separated by around 2 million light years, or 20 times their size. The subsidiary galaxies are closer and smaller. All of this takes place in three dimensions, but if it is imposed on a two dimensional map, it looks a lot like a map of cities. The cities would be about 10 or 20 miles across, and would be about 150 miles apart from each other, while their suburbs would be a few miles across and a few miles outside of the main cities. And then, somewhere between them, would be the small towns. In fact, a pretty good comparison (for me), is to think of the Milky Way as Portland, Oregon the Magellanic Clouds as its suburbs; Seattle, Washington as the Andromeda Galaxy, Tacoma as the Triangulum Galaxy...and the scattered dwarf galaxies, as, perhaps, Aberdeen or Chehalis. While not entirely accurate, this is probably the best analogy for understanding the approximate scale of our Local Group.

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