When a star struggles to be free of its chrysalis

Bad Astronomy
The entire universe in blog form
Feb. 12 2010 8:59 AM

When a star struggles to be free of its chrysalis

I have a fondness for bipolar nebulae: double-barreled gaseous clouds formed when stars are born, and sometimes as they age and die. I've seen a lot of them, and studied a lot of them, so I was surprised to see this image from the Gemini North telescope of a BPN I'm not that familiar with, called Sharpless 2-106:

gemini_sharpless2-106

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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Oooo, pretty! Sharpless 2-106 is about 2000 light years away, located in a region of the galaxy known for birthing stars. The nebula is only about two light years across -- small for a star-forming region, but still over 2,000 times bigger than our entire solar system!

Deep in the middle of the cloud is a star struggling to be born. It may have about 15 times the mass of the Sun, big enough to put it squarely into the "massive star" category. It's flooding the nebula with ultraviolet radiation, causing the gas to glow. Different atoms glow at characteristic colors, allowing us to identify what elements are present, at what quantities, and even at what temperatures. In this case, special filters were used to pick out the elements helium (purple), hydrogen (red), oxygen (green), and sulfur (blue). The result is not really a true-color image -- it's not what your eye would see if you were out there floating around -- but it's close. Amazingly, to me, each filter was exposed for only 15 minutes, resulting in a one-hour total exposure time for this image!

[Note: the purple glow surrounding that bright star is just an internal reflection, light scattering around inside the telescope. That's most likely a bright foreground star blasting out more light in the purple filter than the others; it doesn't mean that star has a giant shell of helium around it!]

The nebula is double-lobed because the star is probably surrounded by a thick disk of material: gas, dust, silicates and other junk swirling around that forms the star itself (and perhaps planets, though we can't tell in this case because there's simply too much stuff there obscuring our view). A typical disk is on the order of the size of our solar system, so is invisibly tiny in this image.

But the star is blowing out material too in a stellar wind. It gets stopped by the equatorial disk, so it can only blow up and down, above and below the disk, forming these two great lobes that stretch for trillions of kilometers.

If we compare this image to one taken in the infrared by Subaru, we learn even more:

gemini_spitzer_sharpless

Like the Orion Nebula picture the other day, the IR image shows that a cavity is being carved out the surrounding gas, most likely from the winds from that massive star. Streamers of gas can be seen on the left, probably formed as the outflowing gas slams into dense knots of surrounding material, a bit like a sandbar that forms when water flows around a patch of sand. You can also see lots more stars than in the optical image, including many bright ones you don't see at all in the optical. The thick dust surrounding Sharpless 2-106 blocks the optical light from stars, but IR can pierce that veil and reach our telescopes, showing us the hidden treasures.

We see bipolar nebulae all over the place... I have another one I'll be telling you about soon, one of my very favorite objects in the whole sky. If you've been reading my blog for more than a couple of weeks you've already seen it, probably without even knowing it. But that's the only hint I'll give for now. Stay tuned and I'll tell you all about it. Promise!

Until then, soak in the beauty of this nascent star, which, in a few million more years, will blow away the tattered remnants of its cocoon, and emerge as another bright blue-white star to light up our galaxy.

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