Hubble sees a gaseous necklace 13 trillion km across

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Aug 12, 2011
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I've been accused of having a big head (which is literally true; finding hats that fit properly can be difficult), but even I wouldn't have any trouble squeezing the 13 trillion kilometer (8 trillion mile) wide Necklace Nebula around my noggin:

[Click to enlarynxate.]

This Hubble image shows the so-called planetary nebula, which is the product of a dying star. Deep in the center of the ring are actually two stars circling each other. As one started to die, it puffed up, literally engulfing the other star. This spun up the larger star, and the centripetal force flung off material in a huge disk well over a light years across. As the star lost its outer layers, the much hotter inner core was exposed, flooding the gas with ultraviolet light, causing it to glow like a neon sign.

Or, more accurately, a hydrogen/oxygen/nitrogen sign, the gases highlighted in this image (shown as green, blue, and red, respectively). See the knots of pink emission in the ring? As the gas was expelled, the speed of the wind increased with time while the density decreased. This faster wind caught up with and slammed into the slower wind, creating clumps and other features. You can see how the gas appears to be streaking away from the center of the ring; that's real, as the fast wind carves away the slower one.

You can also see faint red blobs at the upper right and lower left, well away from the ring itself; those are probably the caps of a very faint (in this image, invisible) hourglass shaped nebula. The disk prevents the wind from expanding along the equator of the system, so it blows up and down, out, creating two lobes of material. Those caps are all you can see, where the gas gets mildly compressed at it expands into the gas surrounding the star system.

If this whole thing looks a bit familiar, well, it should: it's very similar to Supernova 1987A, which I've written about a bazillion times (seeing as how I studied it for six years for my PhD). In this case, the central star(s) is lower mass, so not as hot as the explosion that flash-ionized the gas around the supernova. That's why it's fainter, even though at 15,000 light years away it's actually ten times closer than 87a!

I love planetary nebulae. They're weird, and pretty, and tell us a lot about how stars similar to the Sun die. In our daily lives death is rarely beautiful, but in astronomy it almost always is.

Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

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