Latest News: First discovered in the 1960s, the comet-shaped “Smith Cloud” -- depicted in this artist's illustration -- is 11,000 light-years long and 2,500 light-years across. The cloud, which is invisible at optical wavelengths, is plummeting toward our galaxy at nearly 700,000 miles per hour.
Astronomers long thought that the Smith Cloud might be a failed, starless galaxy, or gas falling into the Milky Way from intergalactic space. If either of these scenarios proved true, the cloud would contain mainly hydrogen and helium, not the heavier elements made by stars.
Hubble was used to measure the chemical composition and found heavier elements that could only come from stars. The cloud appears to have been ejected from within the Milky Way and is now boomeranging back. It will plow back into the Milky Way's disk in about 30 million years. When it does, astronomers believe it will ignite a spectacular burst of star formation, perhaps providing enough gas to make 2 million suns.
Though this settles the mystery of the Smith Cloud's origin, it raises new questions: How did the cloud get to where it is now? What calamitous event could have catapulted it from the Milky Way's disk, and how did it remain intact? The answers may be found in future research.http://hubblesite.org/newscenter/archive/releases/2016/04