Astronomy Jargon 101: Giant Molecular Clouds

In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll be a source of inspiration from today’s topic: giant molecular clouds!

They’re called “clouds” because like many things in the universe, they’re just diffuse clumps. The word “molecular” appears because they’re cold enough for their elements to bind up as molecules. And lastly, they’re “giant” because they’re…big. The smallest are only a dozen or so lightyears across, but the largest span over 650 lightyears in diameter.

Most giant molecular clouds (or GMCs for short) in the Milky Way are located in a band between 11,000 and 24,000 lightyears from the center of the galaxy, and almost all are contained within the main disk. These clouds are major sites of star formation. Each GMC contains enough material to form anywhere from 10 thousand to 10 million sun-sized stars.

An individual cloud can persist for millions of years if left undisturbed. But it it comes too close to another cloud or is struck by a supernova blast wave, it can destabilize. When that happens, the cloud collapses in on itself. Pieces of the cloud break off and continue collapsing, eventually forming the embryo that will develop into a stellar system. A single giant cloud can fragment into thousands, and even millions, of individual stars. The largest ones will illuminate portions of the cloud before the intense radiation evaporates it entirely.

Visually, the GMCs aren’t much to look at. They are much denser than average – even though they occupy less than 1% of the volume of the inner regions of a galaxy, they take up over half of the density. At those densities, visible light can’t punch through, so they are best seen as black blotches that obscure background stars. Common clouds include the Taurus Molecular Cloud, the Carina Nebula, and the Orion Molecular Cloud, which are all visible with a decent backyard telescope.