Molecular Clouds in Everyone's Favorite Constellation

I'll admit right off the bat that I'm being unoriginal with this one. I originally found this image by way of Phil Plait at Bad Astronomy, who called it the best astronomy photograph of 2010 (actually it's a mosaic of 32 photos, all taken by an amateur astronomer, no less!). It is, of course, Orion--perhaps the most recognizable constellation in the sky. But a judicious use of long exposures and filters shows us that there's far more to this constellation than meets the eye.

Click through to the source for various (much) larger versions.

Aside from the stars themselves, the brightest object is M42, the Great Nebula in Orion. Located in Orion's "sword," this nebula (which is visible to the naked eye, even in light-polluted Cambridge) is a 2000-solar-mass molecular cloud which has collapsed into a region of explosive star formation--the closest such region to Earth, and one from which astronomers have gleaned enormous amounts of information about how stars form from collapsing molecular clouds.

Not far away, a pinkish smear lies across the leftmost star in Orion's belt, Alnitak (literally, "the belt" in Arabic--some constellations look like the same thing no matter what culture you're from). Look closely enough at it and you might be able to pick out the silhouette of a horse's head, pointed sideways. Appropriately enough, this is the Horsehead Nebula, another region of star formation connected to M42. Because the process of cloud collapse and fragmentation has progressed so far for M42 and the Horsehead Nebula, each of them feature alternating regions of total transparency and opacity. The opaque regions are dense clumps of gas and dust, soon to form into new stars.

The activity in this photo extends well beyond these two nebulae, though. Perhaps the most dramatic feature of it is the sweeping red band stretching from between Orion's shoulders to his ankles. This is Barnard's Loop, and it's actually a spherical bubble of gas 300 light years across. Barnard's Loop is a chilling companion to the stellar nursery in the heart of Orion--along with that star formation came the dramatic deaths of massive, short-lived stars. At the ends of their lives, they went out with quite a bang. leaving behind this enormous--and still expanding!--sphere of gas.

Continuing up to the top of the image we can see another brilliant cloud of bright red gas, the Lambda Orionis nebula. Despite its brightness, most of the gas in it has already formed into new stars, most of them only five million years old. The open cluster formed from this cloud is easily visible as a dense collection of stars at its center, dominated by a single 28-$M_\odot$ behemoth--Meissa, or Lambda Orionis.

If you look closely at the image, you can see countless other wisps and trails of gas--many of these were once parts of long-dead stars, and likely will be again. But one more easily distinguishable cloud stands out, down in the bottom right. Known as the Witch Head Nebula (the reason for this should be obvious, at least of you turn your screen upside down), it is just barely starting to experience star formation--nearly all of the light that makes it visible comes from mighty Rigel, which--at $18$ solar masses and an incredible $10^{6}$ solar luminosities!--is the seventh brightest star in Earth's sky, and the brightest in Orion.

All of the nebulae visible here are a part of the enormous Orion Molecular Cloud Complex, which encompasses not only these but another half-dozen or so nebulae and molecular clouds which are either out of frame or too faint to be visible here. Ultimately, the existence of these clouds--and the stars that emerge from them--is due to density waves propagating through our galaxy, which give it (along with other spiral galaxies) its distinctive arms. Our own arm of the Milky Way is known as the Orion Spur--seeing just how much is going on in the constellation Orion, it's not hard to see why.