Picture this: a massive invisible cloud 2,500 light years across and 11,000 light years long, as massive as 2 million suns, hurtling towards you at Mach 800 (800 times the speed of sound). To bring things into perspective, the fastest man-made object – the Helios 2 German-American solar probe, reached speeds close to Mach 200 and a standard .50 caliber bullet reaches about Mach 2.6
The Smith cloud, discovered by astronomer Gail Smith in 1963, is a part of a class of objects called high velocity clouds, which are basically large collections of gas or dust moving at speeds in excess of 70 km per second. These clouds are mostly found in the outer reaches of a galaxy or the ‘halo’ region. Recent imaging by the Hubble Space telescope, using the Hubble Cosmic Origins Spectrograph, gave us a chemical analysis of the cloud’s contents. The cloud had hydrogen and helium as its main constituents as expected.
The surprising revelation was that the Smith cloud has about half the sulphur composition of our sun. This indicates that the cloud is not of primordial origin or that it was not formed from the leftover gas at the edge of the galaxy. Astronomers now agree that the Smith cloud must have been ejected from our own galaxy, from a part with similar proportion of elements as our sun, some tens of millions of years ago by some colossal cosmic event. This cloud is now heading back towards us due to the immense gravitational pull of the Milky Way.
Though this settles the problem of origin, there are many unanswered questions such as what kind of violent explosion catapulted the gas cloud so deep into space, and how did the cloud remain intact as it barreled through the galaxy. In a sense, HVCs are the Robin Hoods of our galaxy – they start out at regions at the outer rim with high gas content and redistribute the star building resources to the other parts of the galaxy.
This gives us an insight into the redistribution mechanism of star forming materials. The proximal nature, and sheer size of the Smith cloud give us an amazing opportunity to study this phenomenon. Astronomers estimate that when Smith cloud rejoins the Milky Way, it will pave the way for the formation of nearly 200 million suns.
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