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Every now and again, I am reminded that the beauty of astronomy can underlie truly terrifying objects.
Take, for example, the simply gorgeous nebula called Sharpless 2-308 (the 308th object in Stewart Sharpless’s catalog of extended bright gas clouds). It lies about 5000 light-years away, in the constellation of Canis Major, the big dog (not too far in the sky from Sirius, the brightest apparent star in the night sky):
The image of Sh2-308 above was taken by Thierry Demange, Richard Galli, and Thomas Petit at their Atacama Photographic Observatory using a Takahashi 150 mm telescope (a seriously nice piece of equipment that, frankly, has me droolingly jealous). It’s a combination of multiple images taken in two filters: one (called H-alpha) that emphasizes red light from warm hydrogen in the nebula, and another (called [OIII]) that emphasizes green light from low density oxygen atoms. The total exposure time is about 47 hours, so this is a very deep image!
This is, to my knowledge, the deepest image of the nebula I’ve ever seen, and fantastic detail is visible. It’s clearly a spherical bubble in space; it’s translucent and gets brighter near the edges, which is precisely what you expect from a thin shell of gas in space.
What catches my eye in this image is how much greener it is than red. Most gas clouds in space are rich in hydrogen, and so glow fiercely red. You have to be careful here, because, for one thing, filters can sometimes unbalance the color ratios, and for another, oxygen in a gas cloud tends to emit more light, kilo for kilo, than hydrogen. So, a little bit of oxygen can shine as brightly as a lot more hydrogen. Also, in this image the red has been enhanced to bring it out; in most other images of Sh2-308, there’s hardly any red emission at all.
The gas in the nebula is being energized by a single star, called EZ Canis Majoris (or sometimes WR 6), which you can see just to the right of the nebula’s center. Everything about it makes my neck hair stand on end. It started off life as a very massive star, probably 25 times or more times the mass of the Sun. Stars like that are hot and extremely bright (and blue; in the photo it looks red, again because that color has been enhanced somewhat). As they near the ends of their lives they can become somewhat unstable, blowing off fierce winds of gas that escape into space. We call these kinds of stars Wolf-Rayet stars.
That gas is mostly hydrogen and, in the case of EZ Canis Majoris, that wind blasted out a long way, far enough it’s spread very thinly. The total amount of matter blown out must have been huge; there’s very little hydrogen left in the star, itself! That means it long ago blew off almost all of its outer layers, which are primarily hydrogen, leaving behind the hydrogen-deficient lower layers and core of the star. The nebula as we see it now is likely from the later stages in the star's life, and is rich in heavier elements. Studies of this nebula show it to have lots of nitrogen, which is what’s expected from such a star. That element is produced at high rates in the star’s interior and dredged up by convection (rising gas), where it can then be blown out into space.
Interestingly, Sh 2-308 is actually deficient in oxygen (most of it is transformed into nitrogen inside the star) compared to other nebulae. I find that funny, since it’s the brightest thing we see in the photo. But spectra show this to be the case, and it goes to show you that it can be hard to make conclusions about astronomical objects based solely on images.
Another interesting bit: The star is off center in the nebula, likely because it’s moving rapidly with respect to other stars (we see this often in such nebulae). The expanding gas plows into this material and slows rapidly, while the star barrels on. It’s moving more or less left-to-right in this image, and you can see the gas is brighter on the right: As the gas piles up it intensifies the light.
The scale of this structure is nearly unimaginable. The nebula is something like 60 light years across, which is huge. Most stars that blow a wind as they near the ends of their lives can manage to get something a couple of light years across. Even big stars might only create a nebula 10 light years in size. EZ Canis Majoris blows them all away, a testament to its ferocity.
Speaking of which, the star is probably ready to blow, too. It’s likely very near the end of its life, and will eventually explode, becoming a supernova. It’s roughly 5000 light years away, far too distant to hurt us here on Earth, but when it goes, it’ll easily become the brightest star in the sky for a few months, shining 50 times brighter than Venus, and making Sirius look like a pale dot.
When will that happen? It’s hard to say. It could happen tonight, or it may not happen for a hundred thousand years. I doubt it will take too much longer than that, though; massive stars live short, violent lives, and by the time they show the characteristics we see in EZ Canis Majoris, the clock is ticking. When it does go, the nebula will light up like fireworks from the intense flash, and eventually the debris from the supernova will plow through it, sweep it up, and send it out into the galaxy. It’ll eventually collide with other gas clouds which may collapse and form new planets, too. It’s a cosmic recycling program, at the cost of a titanic star undergoing a colossal explosion.
Like I said, sometimes beauty and terror go hand in hand. That’s very often the case when it comes to objects like these.