One Ring Nebula to Rule Them All

Today I have something besides another globular cluster picture for your perusal. It's a picture of a nebula fairly famous in astronomical circles that I've seen prob-ably hundreds of times in the telescope (it's a popular target during the summer) but have never actually imaged before.

Perhaps it's appropriate that I have this picture less than a week after The Hobbit came out, as this object, Messier 57, is popularly known as the Ring Nebula. It's a small planetary nebula (small on the sky, not physically) found in the constellation Lyra, the Lyre, best seen during the summer and autumn. When I say small, it's only about 1.5 by 1 arc-minutes in diameter; compare that with Messier 55 from my last post, at 19 arc-minutes across. I've therefore cropped out the central region for easier viewing.

Messier 57, the Ring Nebula, in Lyra, at 100% resolution from the camera.

The Ring Nebula is about 2,300 light-years from Earth, and is currently about two and a half light-years across. Measurements of its expansion rate suggest that it has been expanding for about \(1,610\pm240\) years.

The processes forming the Ring Nebula have to do with the life cycles of stars. When stars about the mass of the Sun exhaust the hydrogen in their cores, they go through a complex process of fusing the helium produced by hydrogen fusion into heavier elements, then those into heavier elements, up the periodic table till they get stuck at carbon, having insufficient mass to fuse it to anything higher. During this time, due to other concurrent processes, their atmospheres swell up to become hundreds of times larger than before. As the star runs out of fusible material in its interior it gradually loses its grip on its outer atmosphere which puffs off into space, and which would have been observed starting sometime between A.D. 250 and A.D. 670.

This escaped atmosphere is what we're actually seeing when we look at the Ring Nebula. The core of the progenitor star has contracted down to a small white dwarf of mostly carbon about the size of Earth, but containing about the mass of the Sun. It currently has a temperature of about 125,000 K (~225,000 \(^\circ\)F) and lights up the surrounding atmosphere like a beacon as it blows away. The white dwarf at the center of the Ring Nebula is too faint to be seen in this picture, but is estimated to weigh about 20% more than the Sun currently does.

One of the reasons that I haven't had a picture of this famous (and not un-photo-genic) nebula up before, is because I'd already taken a picture of it...sorta. Sometime during the summer of 2010, I think, I tried imaging it using the narrow-band filters on the imager. Unfortunately, the night I chose had some very thin, high clouds, and I quickly learned that just because a star is bright enough over the entire visible light spectrum to serve as a guide star, does not mean it will be bright enough when you are only looking at the minuscule fraction of its light that comes through a narrow-band filter. Basically, it lost tracking during the exposure, the resulting picture was ruined, and I just never got around to imaging it again, there being plenty of other objects in the summer and autumn sky to keep me busy. This September I finally got around to imaging it and I'm glad I did, for completeness' sake if nothing else.

Globular Cluster Photo Series (Part 10): M56

Last Friday while I was up at the Vis the weather was nice enough for me to be able to take the following picture for my globular cluster collection:

Messier 56 in Lyra.

This is Messier 56, a fairly average globular cluster located 32,900 light years away from us in the direction of the constellation Lyra. It is about 84 light years across, comparable to many of the other clusters I've photographed, and because of its distance it covers a fairly small 8.8 arcminutes on the sky (a little less than a third the size of the full moon).

There isn't that much interesting about this cluster, according to the sources I looked at. There are about a dozen variable stars known in it, a decent number but nowhere near the 112 found in Messier 15. The most interesting piece of information I found about it is that it is approaching us at the high velocity of about 145 km/sec. Other than that, not much to say about it, yet another celestial jewel in the collection.