I don’t get a chance very often to combine two previous posts, but I was thinking recently about planetary nebulae — winds of gas blown off by dying stars — and remembered my very favorite one in the whole sky, Abell 39:
Isn’t that awesome? It’s like it’s right out of Star Trek. I’ve written about the giant halos surrounding some planetary nebulae before, and also about why some objects look like smoke rings. In a (nut)shell, as a star like the Sun begins its long, slow path to dying, it expands into a red giant and blows off a thick wind of matter. This material expands spherically in most cases, streaming off in all directions into space and forming what’s called a giant outer halo.
In most planetaries (like the famous Cat’s Eye nebula, and the less famous but also cool NGC 6826) the outer halo slams into material floating in interstellar space, causing it to get all clumpy or form a bright rim as the surrounding matter gets plowed up. But the rim around Abell 39 isn’t like that; while it’s bright, it’s actually an illusion! Near the edge, we are seeing through more material than we are through the center, and that would be true no matter from what direction we see the nebula. That makes the outer edge look brighter than the inner parts, giving the nebula the appearance of a vast, eerie smoke ring.
It’s pretty rare to get such a near-perfect circle of gas from a planetary nebula, and to be honest Abell 39 is one of only a very few I’m aware of. One reason it’s so perfect is that it’s located well above the plane of the galaxy. Down here, in that plane, there’s copious gas and dust. But Abell 39 is well away from all that, so its expanding red giant wind can retain its almost exactly spherical shape.
It’s also huge: 5 – 6 light years across, which is twice as large or more than most other planetaries, implying it’s old. Given the expansion velocity and size, it must be 20,000 years old or so. It’s also 7000 light years way, so it’s fairly faint, making it a rare target for amateur astronomers… though not unknown.
Readers with a keen eye might have noticed the star in the middle is a bit off-center. No one knows why. I’d love to see a telescope like Hubble or Gemini North get a deep view of this smoke ring. Why is the star misplaced? Why is the limb on the lower left brighter than on the right? Plus, the hint of detail in this image would be greatly enhanced in a deeper exposure, and that would really be incredible to see.
Hard on the heels of my post on Abell 39 last week comes another nebula that forms perhaps an even more perfect circle: PN G75.5+1.7, aka the Soap Bubble Nebula:
That’s really cool. As I pointed out in the earlier post, these are called planetary nebulae, and are the results of the dying stars blowing off winds of gas. They are very rarely circular, instead coming in all kinds of fantastic shapes. It’s thought that you might not get a PN unless the star is binary or swells up to eat its planets as it dies; when that happens the star can get spun up and eject the gas more easily.
It’s not really a circle, of course: it’s a sphere, or more properly a spherical shell. It really is like a soap bubble! The bright edge is due to an effect called limb brightening, which I explained in that earlier post.
This isn’t really well understood, but to get one this symmetric the star must be a loner, and spherical ones are pretty rare. The Soap Bubble is extremely round, maybe even more than Abell 39, so that in itself is interesting. It’s also located near to a vast complex of gas and dust, which is weird: I’d expect the surrounding material to mess up the nice, neat, spherical structure of this nebula. Most likely the Soap Bubble is actually between us and the complex, in relatively empty space.
Also nifty is that this object was discovered by an "amateur", Dave Jurasevich. I know Dave; he works at Mt. Wilson in California and helped us run the 100" Hooker telescope when we were filming Bad Universe. He’s a good guy, and discovered the Soap Bubble in 2008 while photographing the sky in the constellation Cygnus. It’s a good story, which he recounts on his site.
The picture above is from the 4 meter Mayall telescope at Kitt Peak, taken by Travis Rector (his name may be familiar; I’ve posted stuff by him before). He used two filters: one picks out light from warm hydrogen (colored orange in the picture) and another that selects oxygen (colored blue). Planetary nebulae strongly emit at that latter wavelength — it’s not that they have more oxygen, it’s that oxygen is a very vigorous producer of light in thin gas — so it looks blue in this photo.
In the comments of my Abell 39 post are links to even more pictures of nearly circular planetaries. If I had known about all these a few years back when I was working on Hubble, I would’ve applied for time to observe them. A spectrum of these guys might reveal a lot more about them, and give us a clue as to why they are so nearly perfect. It would tell us if they are expanding evenly in all directions, for example, and maybe the density of the material around them.
I wonder how many of these exist? I’ve never seen a catalog of them. It would be interesting to know more about them, like position in the sky, age, and so on. Someday, our own Sun will expand into a red giant. If it swallows Mercury and Venus, as we expect it will, it might be a planetary nebula too. I wonder what it will look like, 6+ billion years from now…?
Ah... bystander, now that you've cleaned up your perfect circle act with the Soap Bubble Nebula, i can give you a rating of-->
I would suggest that you keep your rating inside, as it seems to be rather warm in Oklahoma right now.Think
<<Dave Jurasevich is a degreed engineer whose area of expertise in the chemical and petro-chemical industry has afforded him travel to many interesting parts of the world. Retiring from that field after a long and rewarding career, Dave has been fortunate to marry his passion for astronomy with continued engineering interests as Superintendent of the historic Mount Wilson Observatory in Southern California. Being involved in the operation of a facility like Mount Wilson is a unique opportunity and an honor for this former Chicagoan who, as a budding amateur astronomer in the 1950’s, grew up admiring the deep-sky photographs taken by the big reflectors on that mountain. To walk in the footsteps of the great staff astronomers of Mount Wilson, such as George Ellery Hale, Edwin Hubble, Harlow Shapley, and Walter Baade, is a “dream job” that Dave considers a very special privilege.
Dave’s enthusiasm for astronomy was kindled early in life by his father’s interest in science and evolved over a 50 year span with such memorable experiences as his first view of Saturn in 1958 through a large Newtonian reflector, igniting a profound passion that remains strong to this day. His progression as an amateur astronomer included grinding and figuring his first telescope mirror at Chicago’s Adler Planetarium, basic visual observing with a 2.4 inch Unitron refractor in the backyard on chilly Midwestern winter nights, and more recently on to sophisticated amateur equipment outfitted with a variety of CCD cameras for capturing the magnificence of the night sky. Dave’s main emphasis nowadays in digital astro-imaging encompasses monochrome narrowband work to showcase the timeless beauty of emission nebula in the Milky Way’s rich galactic plane. In 2008, while doing a wide-field mosaic composition of the IC 1318 area in Cygnus, Dave discovered a new planetary nebula hidden deep within the HII region surrounding the oft-imaged Crescent Nebula (NGC 6888). Dubbed the Soap Bubble Nebula because of its nearly perfect symmetry, the discovery was formally recognized and announced by the International Astronomical Union on 16 July 2009 in their CBET “Telegram” 1876, being designated by the much less flamboyant but technically correct moniker of PN G75.5+1.7.
Climbing exploits of which Dave is particularly proud include summiting a Himalayan peak in the Everest region, climbing some 1000 peaks world-wide, authoring a guide book to climbing in the Southwestern US, and a first ascent route up a major peak in Yosemite National Park. He lives in the Pasadena area with his wife, En Lee, and two teenage children, Rachel and Aaron, where in their free time they enjoy traveling, hiking and camping.>>
Re: BA: A nearly perfect circle in space
Posted: Sat Aug 06, 2011 2:37 pm
by Voyager3
The nature of the universe being what it is, I'm surprised there aren't more perfect spheres/circles out there.
Very nice pictures.
APOD: The Planetary Nebula Show (2003 Jun 14) (2001 Oct 03)
