The Local Group

The Definition of the Local Group

  • The Local Group is the cluster of galaxies which includes the Milky Way galaxy.
  • It is a galactic group or a small cluster of galaxies that have gravitational influence over each other.
  • Consisting of more than 54 galaxies, including dwarf galaxies and larger ones like the Andromeda and Milky Way.
  • The Local Group spreads over an estimated 10 million light-years in diameter.
  • The group lies within the Virgo Supercluster, which contains many other groups and clusters of galaxies.

The Major Members of the Local Group

  • The two heaviest galaxies – the Andromeda galaxy (M31) and our own galaxy, the Milky Way – are the dominant members.
  • These two spiral galaxies are similar in size and shape and are on a collision course, predicted to merge in about 4 billion years.
  • The Triangulum galaxy (M33) is another notable member and the third largest of the Local Group.
  • Many other galaxies in the Local Group are so-called dwarf galaxies, including the Large and Small Magellanic Clouds, which are satellites of the Milky Way.

Characteristics of the Local Group

  • The Local Group has a barycentre (centre of mass) located between the Milky Way and the Andromeda galaxy.
  • It is made up of galaxies that are gravitationally bound to each other, which means they are moving relative to one another instead of just expanding with the universe.
  • There are empty regions or voids within the group, where no galaxies exist. Such voids are typical in the large-scale structure of the universe.
  • The velocities of galaxies within the Local Group indicate its future: the Milky Way and Andromeda will eventually merge into a single large galaxy.

Observations of the Local Group

  • The main galaxies of the Local Group are visible to the naked eye, but many of the smaller ones require a telescope to observe.
  • The Andromeda galaxy is the farthest object that can be seen with the naked eye from Earth.
  • Studies of the Local Group have provided critical insights into galaxy formation and evolution, the nature of dark matter, the expansion rate of the Universe, and the properties of stars and gas within galaxies.
  • The Local Group acts as a key source for astronomers to understand the structure and evolution of galaxies.