There are many types of objects in space that just can’t be seen with visible light, and many more that have very different features when observed across the electromagnetic spectrum. A prime example of the former is a molecular cloud. Cold, incredibly huge, and full of low density Hydrogen, these clouds are the raw material for star forming galaxies. If stars begin to form within them, they can be seen as gorgeous nebulae, but when alone in the darkness of space we need to look for the dim signature of radio waves they emit.
The Smith cloud, named after it’s discoverer Gail Smith, who found it in the 1960s, is one of many molecular clouds that are flying around the outer arms of the Milky Way galaxy. However, this cloud is unique in the way it is plummeting into the core of our Galaxy at over a million kilometers per hour. This is an amazingly fast speed for an object that is over 11,000 light years long and 2,500 light years across! If it were visible in the sky, it would be 15 degrees across. That’s the size of your outstretched hand at arm’s length, about 30 times the diameter of the Moon.
The Smith cloud is passing silently by, but it is a ticking time bomb because eventually, some 30 Million years from now, it will collide with the center of our galaxy, where it will heat up and condense to form 2 Million suns worth of stellar material.
It was originally thought to be a remnant from a distant galaxy, but measurements of the Sulfur in the cloud have given astronomers a way to test where it came from. If the Sulfur content is similar to that of molecular clouds in the outer regions of the Milky Way, then it came from our own galaxy. If it is significantly different, than it must have come from somewhere else. The measurements revealed that the Smith cloud did indeed come from our own galaxy, and that some interaction must have started it on it’s collision course toward the galactic center.
But what event caused it’s current trajectory? Even though we know where it came from, there is always a greater mystery to solve.