Welcome to the Galactic Neighborhood: the Local Group of Galaxies

The visible galaxies in the Local Group simulation, shown in the lower right, only trace a tiny fraction of the vast number of dark matter halos, revealed in the upper left. John Helly, Till Sawall, James Trayford, Durham University. Used by permission.

We live inside an immense spiral galaxy called the Milky Way. You can see it as it appears from the inside on a dark night. It looks like a faint band of light running through the sky. From our vantage point, it's tough to tell that we're actually inside a galaxy, and that conundrum had astronomers puzzled until the early years of the 20th Century. In the 1920s, strange "spiral nebulae" were discussed and debated, with some scientists arguing that they are simply part of our own galaxy. Others maintained that they are individual galaxies outside the Milky Way. When Edwin P. Hubble observed a variable star in a distant "spiral nebula" and measured its distance, he discovered its galaxy was not part of our own. It was a momentous finding and led to the discovery of other galaxies in our nearby neighborhood.

The Milky Way is one of about fifty galaxies called "The Local Group". It's not the largest spiral in the group. There are larger ones along with some oddly shaped galaxies such as the Large Magellanic Cloud and its sibling the Small Magellanic Cloud, along with some dwarfs in elliptical shapes. The Local Group members are bound together by their mutual gravitational attraction and they stick together quite well. Most galaxies in the universe are accelerating away from us, driven by the action of dark energy, but the Milky Way and the rest of the Local Group "family" are close enough together that they stick together through the force of gravity.

Local Group Stats

Each galaxy in the Local Group has its own size, shape, and defining characteristics. The galaxies in the Local group take up a region of space about 10 million light-years across. And, the group is actually part of an even larger group of galaxies known as the Local Supercluster. It contains many other groups of galaxies, including the Virgo Cluster, which lies about 65 million light-years away.

The Major Players of the Local Group

There are two galaxies that dominate the local group: our host galaxy, the Milky Way, and the Andromeda galaxy. It lies some two and a half million light-years away from us. Both are barred spiral galaxies and almost all of the other galaxies in the local group are bound gravitationally to one or the other, with a few exceptions.

Milky Way Satellites

The galaxies that are bound to the Milky Way galaxy include a number of dwarf galaxies, which are smaller stellar cities that have spherical or irregular shapes. They include:

  • Sagittarius Dwarf Galaxy
  • Large and Small Magellanic Clouds
  • Canis Major Dwarf
  • Ursa Minor Dwarf
  • Draco Dwarf
  • Carina Dwarf
  • Sextans Dwarf
  • Sculptor Dwarf
  • Fornax Dwarf
  • Leo I
  • Leo II
  • Ursa Major I Dwarf
  • Ursa Major II Dwarf

Andromeda Satellites

The galaxies that are bound to the Andromeda galaxy are:

  • M32
  • M110
  • NGC 147
  • NGC 185
  • Andromeda I
  • Andromeda II
  • Andromeda III
  • Andromeda IV
  • Andromeda V
  • Andromeda VI
  • Andromeda VII
  • Andromeda VIII
  • Andromeda IX
  • Andromeda X
  • Andromeda XI
  • Andromeda XII
  • Andromeda XIII
  • Andromeda XIV
  • Andromeda XV
  • Andromeda XVI
  • Andromeda XVII
  • Andromeda XVIII
  • Andromeda XIX
  • Andromeda XX
  • Triangulum Galaxy (third largest galaxy in the local group)
  • Pisces Dwarf (unclear if it is a satellite of the Andromeda Galaxy or the Triangulum Galaxy)

Other Galaxies in the Local Group

There some "oddball" galaxies in the Local Group that may not be gravitationally "bound" to either the Andromeda or the Milky Way galaxies. Astronomers generally lump them together as part of the neighborhood, although they are not "official" members of the Local Group. 

The galaxies NGC 3109, Sextans A and the Antlia Dwarf all appear to be gravitationally interacting but are otherwise unbound to any other galaxies.

There are other nearby galaxies that seem to not be interacting with any of the above groups of galaxies, including some nearby dwarfs and irregulars. A few are being cannibalized by the Milky Way in an ongoing cycle of growth that all galaxies experience. 

Galactic Mergers

Galaxies in close proximity to each other can interact in colossal mergers if conditions are right. Their gravitational pull on each other leads to a close interaction or an actual merger. Some galaxies mentioned here have and will continue to change over time precisely because they are locked in gravitational dances with each other. As they interact they can rip each other apart. This action — the dance of the galaxies —  significantly alters their shapes. In some cases, the collisions end up with one galaxy absorbing another. In fact, the Milky Way is in the process of cannibalizing a number of dwarf galaxies. 

The Milky Way and Andromeda galaxies will continue to "eat up" other galaxies. There's some evidence the Magellanic Clouds might merge with the Milky Way. And, in the distant future Andromeda and the Milky Way will collide to create a large elliptical galaxy that astronomers have nicknamed "Milkdromeda". This collision will commence in a few billion years and radically alter the shapes of both galaxies as the gravitational dance commences. The new galaxy they will eventually create has been nicknamed "Milkdromeda".