Astronomy and the Local Group

The Local Group is a collection of 85 gravitationally bound galaxies near our Milky Way. It includes our home galaxy, the Andromeda Galaxy, and many smaller dwarf galaxies. It’s a small, but important part of the larger universe that astronomers study.

The biggest members of our local group are the two largest spiral galaxies, the Milky Way and Andromeda. The local group also contains ellipticals, irregulars, and dwarf galaxies. It has a diameter of about 10 million light-years, and it is located between the Milky Way and Andromeda. It is also connected to other groups of galaxies in a bigger structure called the Virgo Supercluster.

Aside from being interesting astronomical objects, the Local Group galaxies help astronomers learn more about the universe at large. For example, observing the Local Group’s galaxies has revealed that 26 percent of the universe is made of a substance called dark matter. Dark matter is not visible, but it can affect the movement of the normal, visible material in the universe. Astronomers can search for this dark matter using a technique called gravitational lensing, which allows them to use distant galaxies as a microscope. The Local Group’s galaxies have also helped astronomers understand how and where black holes form.

Observing the Local Group’s galaxies gives astronomers a microcosm that represents the universe at large. It’s a laboratory close at hand that lets them experiment with various physical phenomena, from examining the stars in nearby galaxies to studying what’s in their black holes.

Aside from the big galaxies, the Local Group contains a handful of mid-size members. Two of them are satellites of Andromeda, the elliptical M32 and the irregular M110. Andromeda itself has an entourage of orbiting galaxies, including the irregulars The Magellanic Clouds and Segue 2, plus a number of dwarf ellipticals. These low-metallicity galaxies are like time capsules that allow astronomers to study the universe as it was shortly after the Big Bang, before stars “polluted” the space around them with heavy elements such as iron and nickel.