Huge gas cloud hurtling towards our galaxy could trigger the creation of 200 million new stars

The Smith Cloud, which was spat out from the edges of the Milky Way 70 million years ago, is on its way back

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The Independent Online

A giant gas cloud hurtling towards the Milky Way could form two million new stars when it finally collides with our galaxy.

The discovery was made after astronomers managed to figure out the chemical composition of the Smith Cloud, a huge cloud of gas which is approaching the edges of the Milky Way at a speed of around 193 miles per second.

The Smith Cloud has been on science's radar since it was discovered in 1963, but its origins and chemical makeup were a mystery for a long time. Some thought that the cloud could be a starless galaxy or just a giant body of gas, falling in to the Milky Way from intergalactic space.

However, by observing the cloud with the Hubble Space Telescope, a team of scientists from the University of Notre Dame in Indiana, led by Dr Nicolas Lehner, have determined the that cloud is made up of elements also found in the Sun - revealing that it originated in the outer limits of the Milky Way, but was somehow ejected into space around 70 million years ago.

The astronomers were able find out the composition of the cloud by observing the light from distant galaxies that passed through it. By observing which wavelengths of ultraviolet light made it through the cloud, they discovered it is as rich in sulfur as the outer disk of the milky way, meaning it originated in our galaxy - if it came from somewhere else in outer space, it would not be tainted with so much sulfur.

smithcloudtrajectory.jpg
This diagram shows the 100-million-year-long trajectory of the Smith Cloud as it arcs out of the Milky Way and boomerangs back in (NASA/ESA/A. Feild (STScI))

No one yet knows how the Smith Cloud was spat out into space, but it's on its way back. In around 30 million years, the cloud will collide with the galaxy - a long time in human years, but a relatively short period in cosmic terms.

When the cloud does make its way back into the Milky Way, astronomers believe it will trigger a huge star formation event. The Smith Cloud is around 9,800 light years long (or around 58 quadrillion miles), so it contains enough matter to generate over two million new stars in the galaxy.

Lehner says the discovery shows that gas recycling is an important mechanism in the evolution of galaxies.

The team's study, On the Metallicity and Origin of the Smith High-Velocity Cloud, authored by Dr Lehner, Dr Andrew J. Fox and a number of others, was published this month in the Astrophysical Journal Letters.

Such a huge star formation event rarely happens so close to the Earth, so the Smith Cloud could provide fertile research ground for astronomers well into the future.

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