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Giant galaxies such as our own Milky Way formed long ago, from collisions between many smaller galaxies. Dozens of dwarf galaxies, which are are similar to those early building blocks, orbit the Milky Way today. And astronomers recently discovered a likely dwarf galaxy that’s crashing into the Milky Way even now, showing us the process of galaxy growth in action.
The dwarf galaxy resides near the head of Hydra, the water snake, the longest constellation in the sky. It zigzags up the southeastern sky this evening, although you need a starchart to pick it out.
Hydra I is just 40,000 light-years from Earth — a fraction of the distance to most of the Milky Way’s other companion galaxies.
Hydra I is anything but bright. While the Milky Way contains hundreds of billions of stars, Hydra I consists of only thousands of stars. But those few stars have a fairly high level of iron — a trait of larger galaxies. That suggests that Hydra I used to be bigger, but the Milky Way’s gravity tore it apart and grabbed most of the smaller galaxy’s stars for itself. In fact, a recent study estimates that Hydra I has lost more than 99.99 percent of its stars to the Milky Way.
Hydra I is so modest that it might be a star cluster instead of a galaxy. Either way — a disintegrating galaxy or a disintegrating star cluster — Hydra I demonstrates how the Milky Way grew into a cosmic colossus that rules an empire of dozens of lesser galaxies.
Script by Ken Croswell, Copyright 2015