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The Milky Way has a lot of neighbors, but what is the closest galaxy to the Milky Way? Most people would answer Andromeda, or perhaps the Magellanic Clouds. But, in fact, the closest galaxy to the Milky Way lies within the Milky Way itself! The Canis Major Dwarf Galaxy is about 42,000 light years from the galactic center, and a mere 25,000 light years from us (which puts it closer to us than the center of our own galaxy, which is 30,000 light years away from the Solar System).
The Canis Major Dwarf Galaxy was discovered in 2003 when astronomers analyzed infrared images of the Milky Way. Infrared images allow astronomers to see through the gas an dust that are part of the galactic disk. Canis Major has a lot of M-Dwarf stars – cool, red stars that shine brightly in the infrared – that stick out in infrared images. The Milky Way became the size it is now by eating up other galaxies like Canis Major, and it continues to do so today. Stars from the Canis Major Dwarf Galaxy are already part of the Milky Way, making the nearest galaxy to the Milky Way part of the Milky Way itself.
It’s on the small size for a galaxy, and contains only about a billion stars. For comparison, the Milky Way has 200 to 400 billion stars.
Much like in the Guinness Book of World Records, there is a history of galaxies we thought held the record for being the closest to our own, but other contenders rose up to break these records. Discovered in 1994, the Sagittarius Dwarf Elliptical Galaxy held the previous record for closest galaxy, at 75,000 light years away.
The Andromeda Galaxy (M31) is the closest spiral galaxy to us, and though it’s gravitationally bound to the Milky Way, it’s not the closest galaxy by far, being 2 million light years away. The Large and Small Magellanic Clouds are two irregular dwarf galaxies about 180,000 and 210,000 light-years away, respectively. They were thought to be orbiting the Milky Way, but that may not be the case..
All of these galaxies make up part of what is called the Local Group, which is a group of more than 30 galaxies that lie within 4 million light years of the Milky Way. Here’s a great article from the Spitzer Space Telescope’s website on the Milky Way’s family of close galaxies and a video by the same author on the subject.