Our galactic neighbourhood is a dumbbell whose two ends are falling together
The Milky Way belongs to a family of galaxies shaped like a dumbbell. We and our satellites form one lobe; the Andromeda Galaxy and its companions form the other, roughly three million light-years away. The two lobes are closing at about 123 kilometres per second, and eventually the whole group should fuse into one giant galaxy.
Edwin Hubble coined the name in his 1936 book The Realm of the Nebulae, describing a typical small group isolated in the general field and listing eleven members, from Andromeda down to faint dwarfs. The census has grown since: 134 galaxies are now known within a megaparsec of the centre, most of them dwarfs, and more surely hide behind the dust and stars of our own galaxy. The whole group weighs something like two trillion Suns and spans about 17 million light-years.
Two giants dominate. The Milky Way and Andromeda are both spirals of roughly a trillion solar masses each, and the group's centre of gravity lies slightly nearer Andromeda, which may be the heavier. Each keeps a retinue of satellites, including the Magellanic Clouds on our side and compact M32 on Andromeda's. The Triangulum Galaxy, the third spiral, weighs far less; whether it orbits Andromeda is unclear, but a close pass between them two to four billion years ago set off star formation across Andromeda's disc.
The edges are fuzzy. Some dwarfs float alone, bound to neither giant. A distant cluster around NGC 3109, the Antlia-Sextans group, probably lies beyond the boundary where the Local Group's gravity can hold anything, making it a separate group. As the Milky Way and Andromeda merge and their combined mass concentrates, that boundary may widen enough to pull it in. Tidal debris records past encounters too, such as a stream of gas being stripped from the Magellanic Clouds.
Over tens of billions of years the members are expected to coalesce, with the Milky Way and Andromeda collision as the main event. Most expect a single elliptical galaxy at the end, but that is debated: some think the merged system could keep a spiral structure for a while, form a lasting superspiral, or drift toward a lenticular shape.
Source: Local Group
