Key Takeaways:
- Edwin Hubble initially identified approximately 11-12 galaxies within the Local Group, a term he coined, while current estimations place the number of members at 125 or more.
- The Local Group's total mass, approximately 2 trillion solar masses, is primarily concentrated in its two largest galaxies, the Milky Way and Andromeda, each possessing roughly 1 trillion solar masses.
- The Local Group's diameter is estimated to be around 10 million light-years, with member galaxies clustered around the Milky Way and Andromeda.
- Several galaxies within the Local Group, including NGC 147, NGC 185, NGC 205, M32, M31, and others, are observable by amateur astronomers under optimal conditions, using medium-sized telescopes.
American astronomer Edwin Hubble (after whom the Hubble Space Telescope is named) coined the term “Local Group” for the galaxies he had identified as moving through space with the Milky Way. The term appeared in his 1936 book, The Realm of the Nebulae. He identified 11 definite members with a possible 12th. Today, astronomers have shown that the Local Group contains 125 members with several dozen others still up for possible inclusion.
One of the weirdest things about the Local Group is that all the galaxies in it contain a combined mass of 2 trillion Suns. The two largest members, however — the Milky Way and the Andromeda Galaxy — each have a mass of about 1 trillion Suns. That means that the 123 (or more) smaller members contribute little to the overall mass of the group. That makes sense because most galaxies in the Local Group are dwarfs, some of which are only slightly larger than star clusters.
Although most of the members congregate around either the Milky Way or the Andromeda Galaxy, astronomers believe the Local Group has a diameter of about 10 million light-years.
Go observe some of them
Most of the galaxies in the Local Group are far out of reach of amateur telescopes. But several dozen can be spotted by observers. Here’s a list of the ones brighter than magnitude 10 in order of their right ascension. Any medium-size telescope should reveal some details to you on a dark, clear night. In the list, the name is first // then the magnitude // then the size.
NGC 147 // 9.5 // 13′ by 8′
NGC 185 // 9.2 // 12′ by 10′
NGC 205 // 8.1 // 22′ by 11′
M32 // 8.1 // 8′.7 by 6′.5
The Andromeda Galaxy (M31) // 3.4 // 190′ by 60′
The Small Magellanic Cloud // 2.3 // 320′ by 185′
The Sculptor Galaxy (NGC 253) // 9.0 // 40′ by 31′
IC 1613 // 9.2 // 16′ by 15′
ThePinwheel Galaxy (M33) // 5.7 // 71′ by 42′
The Fornax Dwarf Spheroidal // 9.0 // 17′ by 13′
The Large Magellanic Cloud // 0.6 // 645′ by 550′
NGC 6822 // 9.0 // 16′ by 14′
Good luck!
