A giant, invisible gas cloud is hurtling toward our galaxy at over one million kilometres per hour and when it collides it will unleash an explosion of star formation. Luckily, that collision won't happen for about 30 million years. 

That's just part of the news released this month by a team of astronomers working with the Hubble Space Telescope to learn more about the so-called "Smith Cloud" (it's named after Gail Smith, the doctoral student who found it in the 1960s), one of the hundreds of similar gas clouds swirling around our galaxy at mind-boggling speed.

What makes the Smith Cloud different from the rest is that scientists know quite a bit about its size and trajectory in space. At about 11,000 light years in length and 2,500 light years across, the most enduring mystery of the Smith Cloud was its origin.

Until now, it was thought that the giant cloud may have been a galaxy that never fully formed, devoid of the kinds of atomic ingredients necessary for star formation. Alternatively, it may have been a stream of hydrogen gas flowing into the Milky Way from intergalactic space.

It's telling us that the Milky Way is a bubbling, very active place where gas can be thrown out of one part of the disk and then return back down into another.
- Andrew Fox, astronomer

But by measuring its chemical composition for the first time, the research team has concluded that the cloud was ejected from the Milky Way like a great cosmic belch millions of years ago and is steadily boomeranging back towards us.

They used an instrument onboard Hubble called the Cosmic Origins Spectrograph, which breaks the light emitted from an object into colours and measures the intensity of each colour. Scientists can use the colours to divine all sorts of different information about the object, including what sorts of atoms make it up. 

In this case, the research team focused the COS on the bright cores of three galaxies located billions of years beyond the Smith Cloud, and looked to see how much of that light, in particular ultraviolet light, was absorbed by the cloud.

Smith cloud

That's key because the presence of sulphur, which absorbs lots of ultraviolet light, is an indicator that other heavy elements — the kinds necessary for star formation — are also in the cloud. 

As it turns out, they found lots of sulphur — as much as is found in the outer portion of the Milky Way, which is not a coincidence. 

If the Smith Cloud was just a failed galaxy or a stream of hydrogen, two theories previously floated to explain its existence, then scientists would have only found evidence of hydrogen and some helium. 

"The cloud is an example of how the galaxy is changing with time," said the team's leader Andrew Fox, of the Space Telescope Science Institute in Baltimore, Maryland, in a statement. 

"It's telling us that the Milky Way is a bubbling, very active place where gas can be thrown out of one part of the disk and then return back down into another."

Now astronomers will look to figure out what led to the Smith Cloud's violent ejection from the Milky Way and why it remained intact.