Image: A false-color radio telescope image of Smith's Cloud
Bill Saxton  /  NRAO / AUI /NSF
A false-color radio telescope image of Smith's Cloud, which is headed toward a collision with the Milky Way, taken by the Robert C. Byrd Green Bank Telescope, the world's largest steerable radio telescope.
updated 1/11/2008 1:28:52 PM ET 2008-01-11T18:28:52

A colossal cloud of gas is racing toward a collision with our galaxy, and when it hits, the crash could trigger an intense burst of star formation.

The collision and stellar light show will occur in 20 million to 40 million years, an astronomer announced here today at a meeting of the American Astronomical Society.

The cloud, dubbed Smith's Cloud after the astronomer who discovered it in 1963, is just 8,000 light-years from our galaxy's disk. Jam-packed with enough hydrogen to make a million stars like the sun, it is 11,000 light-years long and 2,500 light-years wide.

"My guess is that this [gas cloud] is a remnant of the original formation of the Milky Way in the way that comets and meteors are remnants of the formation of the solar system," said Jay Lockman, of the National Radio Astronomy Observatory (NRAO) in Green Bank, W.Va.

If you could see the cloud, it would span 30 times the width of the moon.

"From tip to tail it would cover almost as much sky as the Orion constellation," Lockman said. "But as far as we know it is made entirely of gas — no one has found a single star in it."

For decades after the cloud's discovery, scientists were puzzled over its allegiance because the available images lacked any detail. They didn't know whether it belonged to the Milky Way, or if the cloud was moving — either getting blown out or falling into our galaxy.

Lockman and his colleagues made their recent observations of the cloud with the National Science Foundation's Robert C. Byrd Green Bank Telescope, the largest steerable radio telescope. Since the cloud is made of cold gas, it emits only in the radio wavelengths, Lockman said. It does not generate any visible light.

Image: Impact near the sun
Bill Saxton  /  NRAO / AUI / NSF
A graphic shows Smith's Cloud, at bottom, heading toward an impact site on one of the Milky Way's spiral arms. Our solar system is on a different spiral arm, as shown.

Results showed Smith's Cloud is plunging into the Milky Way, not heading out. And it's falling in at more than 540,000 mph (869,000 kilometers per hour).

"We are able to see it rubbing up against the outer atmosphere of the Milky Way," Lockman told "It's not only coming in, it's starting to push up gas in front of it."

He added, "It is also feeling a tidal force from the gravity of the Milky Way and may be in the process of being torn apart."

Tidal forces of gravity, like the moon tugging on Earth, pull the front parts of an object greater than the regions on the far side.

He said the cloud would likely strike a region somewhat farther from the galactic center than our solar system. The addition of new gas into our galaxy along with the shock of the collision may trigger a burst of rapid star formation.

"When it hits, it could set off a tremendous burst of star formation," Lockman said. "Many of those stars will be very massive, rushing through their lives quickly and exploding as supernovae."

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