The Local Group is a collection of over 50 known galaxies bound together by gravity, spread across roughly 10 million light-years of space. Three large spiral galaxies anchor the group: the Milky Way, the Andromeda Galaxy, and the Triangulum Galaxy. The rest are small dwarf galaxies, most of them orbiting one of the two largest members.
The Three Major Galaxies
The Milky Way and the Andromeda Galaxy (also called M31) dominate the Local Group. They sit about 2.5 million light-years apart, and the gravitational center of the entire group falls somewhere in the space between them. Andromeda was long thought to be significantly more massive than the Milky Way, but a revised estimate from the Observatoire de Paris put its total mass at about 450 billion times the mass of our Sun, roughly half to a quarter of previous calculations. That makes it comparable to the Milky Way rather than dramatically larger.
The Triangulum Galaxy (M33) is the third-largest member, about half the size of the Milky Way. It sits around 2.7 million light-years from Earth, relatively close to Andromeda, and some evidence suggests it may be gravitationally bound to Andromeda rather than independently orbiting the group’s center.
Satellite Galaxies of the Milky Way
The Milky Way has its own entourage of smaller galaxies. The two most prominent are the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), visible to the naked eye from the Southern Hemisphere. The LMC sits about 160,000 light-years away, while the SMC is roughly 200,000 light-years out. Both are irregular galaxies with active star formation, and they may be on their first pass around the Milky Way rather than long-term satellites.
Beyond the Magellanic Clouds, the Milky Way’s “classical” satellites include a handful of well-studied dwarf galaxies: Sagittarius (the closest, at about 85,000 light-years, currently being torn apart by the Milky Way’s gravity), Fornax, Sculptor, Carina, Draco, Ursa Minor, Leo I, Leo II, and Sextans. These are mostly dwarf spheroidal galaxies, meaning they’re roughly spherical, contain very little gas, and have largely stopped forming new stars.
Modern sky surveys have pushed the count much higher. Dozens of ultra-faint dwarf galaxies have been detected in the past two decades, many of them so dim they contain only a few thousand stars. The total number of Milky Way satellites likely exceeds what we’ve found so far, because the faintest ones can only be spotted when they’re relatively close, and large portions of the sky remain incompletely surveyed at the necessary depth.
Satellite Galaxies of Andromeda
Andromeda has its own swarm of companions. The two best known are M32 and M110, both visible through backyard telescopes near Andromeda’s bright disk. M32 is a compact elliptical galaxy that may be the stripped core of a much larger galaxy that collided with Andromeda a few billion years ago. M110 is a dwarf elliptical galaxy, one of the few small galaxies that still shows faint signs of recent star formation despite being classified as elliptical.
Beyond those two, Andromeda hosts a growing list of dwarf satellites designated Andromeda I through Andromeda XXI and beyond. Discoveries continue: Pegasus V, identified in recent years, is an ultra-faint dwarf satellite of Andromeda so faint and old that it may have stopped forming stars during the very early universe. Another candidate, Pisces VII, could be a satellite of the Triangulum Galaxy or an isolated ultra-faint dwarf, pending better distance measurements.
Types of Dwarf Galaxies in the Group
The dozens of smaller Local Group members fall into a few broad categories. Dwarf spheroidals are the most common type. They’re gas-poor, no longer forming stars, and tend to orbit close to one of the large spirals. Dwarf irregulars, by contrast, are gas-rich and still actively producing new stars. They tend to sit farther from the major galaxies, where gravitational interactions haven’t stripped away their raw material. A handful of “transition” types fall somewhere in between, with properties of both categories.
This pattern isn’t random. Galaxies that orbit close to the Milky Way or Andromeda get their gas pulled away over time, shutting down star formation. Isolated dwarfs farther from the group’s center keep their gas longer and continue making stars. Simulations show that below a certain mass threshold, roughly 10 billion times the mass of the Sun, the conditions for forming stars in a small galaxy become increasingly difficult, which helps explain why ultra-faint dwarfs are so extraordinarily dim.
Where the Local Group Ends
The Local Group isn’t just an arbitrary label. It has a physical boundary defined by gravity. Astronomers identify this edge using what’s called the zero-velocity surface: the distance at which a galaxy’s outward motion from the expanding universe exactly balances the gravitational pull of the group. For the Local Group, that boundary sits at a radius of about 3.8 million light-years (1.18 megaparsecs) from the group’s center.
Galaxies beyond that radius are being carried away by cosmic expansion. A small chain of galaxies including NGC 3109, Antlia, Sextans A, and Sextans B sits just outside this boundary at about 5.5 million light-years from the group’s center, receding from us. They’re neighbors, but not members.
The nearest galaxy groups beyond the Local Group include the M81 Group, the Sculptor Group (sometimes called the Sculptor filament), the Centaurus A/M83 Group, and the IC 342/Maffei Group. These neighboring groups have similar gravitational boundaries, with zero-velocity radii in the range of 2.9 to 4.2 million light-years. Together with the Local Group, they form part of the larger Virgo Supercluster.
The Milky Way-Andromeda Merger
The Local Group’s two largest galaxies are heading toward each other. Data from the European Space Agency’s Gaia spacecraft refined the timeline: Andromeda and the Milky Way will collide in about 4.5 billion years, roughly 600 million years later than earlier estimates predicted. The updated measurements also suggest the collision will be more of a glancing blow than a direct head-on impact, which changes how disruptive the initial encounter will be. Over the following billions of years, the two galaxies will eventually merge into a single large elliptical galaxy, reshaping the heart of the Local Group entirely.
The Triangulum Galaxy’s fate in this scenario is less certain. Depending on its exact orbit, it could be absorbed into the merged galaxy, flung outward, or settle into a distant orbit as a satellite of the new, larger system.