The prevailing theories are that the enormous cloud is left over from when the Milky Way was forming or is made from gas stripped off a neighboring galaxy.
To help resolve these mysteries, scientists made more than 40,000 observations of the cloud using the Robert C. Byrd Green Bank Telescope in West Virginia.
They found the cloud is 11,000 light-years long and 2,500 light-years wide, and extends across 15 degrees of the sky—about the size of 30 full moons lined up edge to edge.
If the cloud were visible with the naked eye, it would be a "very impressive sight," Lockman said.
"From tip to tail it would cover almost as much sky as the Orion constellation," he said. "But as far as we know it is made entirely of gas—no one has found a single star in it."
Cosmic Collision
According to Lockman, it's not clear what allows the gas in the cloud to hang together—clouds are fairly diffuse when they're floating in space.
But Lockman and his team have already seen that Smith's Cloud is substantive enough to push some of the Milky Way's gas ahead as it nears the edge of the galaxy.
"It is also feeling a tidal force from the Milky Way and may be in the process of being torn apart," Lockman said.
Robert Benjamin, a team member and an astronomer at the University of Wisconsin in Whitewater, said that the cloud's gas is compressing as it gets nearer to the Milky Way.
Even before it collides with the galaxy full bore in 20 to 40 million years, parts of the cloud could grow dense enough to form a few stars, he added.
This is probably not the first time such a collision has happened, the scientists add.
Similar hydrogen clouds have been spotted buzzing around many galaxies, including Andromeda and the M81 galaxy, 11.8 million light-years from Earth.
Some theories even propose that a ring of stars near the sun called Gould's Belt was created by a similar collision as the one predicted to occur between Smith's Cloud and the Milky Way.
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