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.