Welcome to the Galactic Neighborhood: the Local Group of Galaxies

Local20Group20Dark20Matter20and20stars.jpg
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.

Our planet orbits a star inhabiting an immense spiral galaxy called the Milky Way. We can see the Milky Way as part of our night sky. 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, astronomers discussed strange "spiral nebulae" they were seeing in photographic plates. They'd been known to exist since at least the mid 1800s, when Lord Rosse (William Parsons) began finding these objects through his telescope. By the early 20th century, some scientists held the view that these spirals 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, including the members of the Local Group.

Milky Way Galaxy
An artist's concept of what our galaxy looks like from outside. Note the bar across the center and the two main arms, plus smaller ones. NASA/JPL-Caltech/ESO/R. Hurt

The Milky Way is one of about fifty galaxies in the group. It's not the largest spiral; that would be the Andromeda Galaxy. There are also many smaller ones, including oddly shaped 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.

A map of the Local Group of galaxies.
A graphical representation of the Local Group of galaxies, including our own. It contains at least 54 individual members. Antonio Ciccolella, CC BY-SA 4.0

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.

Andromeda and the Milky Way colliding, as seen from the surface of a planet inside our galaxy.
Andromeda and the Milky Way are the two largest members of the local group. In the distant future, they will be colliding. This artist's concept shows that collision from the point of view of a planet in the Milky Way. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

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.

Galaxy NGC 3109
This member of the Local Group is called NGC 3109, as seen by the Galaxy Explorer spacecraft. It may be interacting with another nearby galaxy. NASA/GALEX 

There are other nearby galaxies that do not seem to be interacting with any of the above groups of galaxies. They include some nearby dwarfs and irregulars. Others 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. 

hubble rose galaxies
A group of interacting galaxies as seen by Hubble Space Telescope. NASA/ESA/STScI

The Milky Way and Andromeda galaxies will continue to "eat up" other galaxies as time goes by. This appears to be what has happened to create most (if not all) of the galaxies we see today. In the distant past, smaller ones merged to become larger ones. Large spirals then merge and create ellipticals. It's a sequence that has been observed throughout the evolution of the universe.

Will Mergers in the Local Group Affect Earth?

Certainly the ongoing mergers will continue to reshape the Local Group galaxies, changing their shapes and sizes. The ongoing evolution of galaixes will almost certainly affect the Milky Way, even as it goes about gobbling up smaller galaxies. For example, 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.

Fast Facts: The Local Group

  • The Milky Way is part of the Local Group of galaxies.
  • The Local Group has at least 54 members.
  • The largest member of the Local Group is the Andromeda Galaxy.

Sources

  • Frommert, Hartmut, and Christine Kronberg. “The Local Group of Galaxies.” Messier's Telescopes, www.messier.seds.org/more/local.html.
  • NASA, NASA, imagine.gsfc.nasa.gov/features/cosmic/local_group_info.html.
  • “The Universe within 5 Million Light YearsThe Local Group of Galaxies.” The Hertzsprung Russell Diagram, www.atlasoftheuniverse.com/localgr.html.