科学美国人60秒:Galaxy of a Thousand Stars

Our Milky Way galaxy is on the heavy side. We have hundreds of billions of stars, and an even more massive amount of dark matter.

Now astronomers have found the ying to the Milky Way’s yang. With just 1,000 stars, the dwarf galaxy Segue 2 becomes the smallest one known. Researchers published measurements of the galaxy’s size in the Astrophysical Journal. [Evan Kirby et al., Segue 2: The Least Massive Galaxy]

Segue 2 is one of dozens of “companion” galaxies that orbit the Milky Way. Scientists who model how galaxies form expect there to be thousands of small galaxies orbiting nearby, even if most are too faint to see with existing technology. The discovery of Segue 2 offers astronomers a hint that the neighborhood around the Milky Way really is clumpy with tiny companions.

Researchers think that when Segue 2 first formed over 100 million years ago, it was much larger and brighter. But repeated orbits around the Milky Way stripped away most of its membership. The last thousand stars and a smattering of dark matter thus claim the title of smallest galaxy known. For now.

—Michael Moyer