Astronomers say that in approximately 20 to 40 million years, a giant cloud of hydrogen gas, known as Smith's Cloud, will slam into the Milky Way galaxy, resulting in the formation of countless stars, which could make for the biggest fireworks display in the Local Group. The front edge of the cloud has already begun to hit the galaxy.
"The leading edge of this cloud is already interacting with gas from our Galaxy. This is most likely a gas cloud left over from the formation of the Milky Way or gas stripped from a neighbor galaxy. When it hits, it could set off a tremendous burst of star formation. Many of those stars will be very massive, rushing through their lives quickly and exploding as supernovae. Over a few million years, it'll look like a celestial New Year's celebration, with huge firecrackers going off in that region of the Galaxy," said Felix J. Lockman of the National Radio Astronomy Observatory (NRAO), one of the astronomers who headed a study to research the cloud. The University of Wisconsin-Whitewater also participated in the study.
Astronomers used the Robert C. Byrd Green Bank Telescope (GBT) to study the cloud, which was first discovered by an astronomer in 1963. The study found that as many as a million new stars, much like our own Sun, could form from the gas when it does collide with our galaxy. 
Currently it is 8,000 light years away from full impact of our galaxy, and is traveling at over 150 miles a second. It is 2,500 light-years wide and over 11,000 light-years long. Astronomers believe it will strike the Milky Way's disk at a 45-degree angle.
It was earlier believed that the cloud was traveling away from the Milky Way or might have been part of the galaxy itself but, due to the technology at the time, images were not clear enough to see what was really happening. Using the highly sensitive GBT, astronomers have now been able to capture over 40,000 different images of the cloud. The researchers now believe that the cloud is, in fact, moving toward the Milky Way and pushing the gases of the galaxy up onto itself as it progresses.
"If you could see this cloud with your eyes, it would be a very impressive sight in the night sky. From tip to tail it would cover almost as much sky as the Orion constellation. But as far as we know it is made entirely of gas -- no one has found a single star in it," added Lockman.

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Hydrogen gas cloud to smash into Milky Way galaxy