There is something aesthetically pleasing about symmetry, whether it’s on Earth or as it is in the heavens. That may be one of the reasons I love planetary nebulae, the eerie and beautiful structures created when dying stars cast off their outer layers. They come in many strange shapes, and oddly it’s quite rare to find one that appears perfectly circular.
But they do exist, and one of the best examples I have seen yet is Abell 33, the glowing winds from a star located something like 1500 light years away:
How gorgeous is that? [Click to embiggen.] This image was made by BABlog regular Adam Block, using the 0.8 meter (32") Schulman Telescope at the Mount Lemmon SkyCenter. He combined images through four filters for a total observation of 630 minutes, over ten hours, to make this dream-like shot.
A few things jumped out at me. First, that bright star on the edge of it giving it a diamond-ring look is almost certainly just a chance alignment. It’s probably much closer or farther away than the nebula, and coincidentally lined up. But it’s very pretty!
Second, it looks like the central star may be a binary, two stars orbiting each other. I measured their separation on the image, and assuming the distance to the nebula really is 1500 light years (though that’s fairly uncertain; I was surprised that there aren’t many papers in the literature about this object), those two stars are no closer than about 0.03 light years from each other — about 2000 times the distance of the Earth from the Sun! That’s a long way, though not unheard of in binaries. Still, I can’t rule out a coincidental alignment.
Also, it’s obvious the central star(s) is off-center! That does happen, usually if the star is moving rapidly through space. The wind it expels gets "blown back" by gas in between the stars, like steam coming from an old-time train’s spout as it rolls down the track. If that were the case, though, I’d expect the nebula itself to be compressed along one side, which it isn’t. That’s a bit of a mystery.
Third, of course, is how circular it is. The outer rim is nearly a perfect circle, which is really uncommon! You might think that in space spheres are common (which would then look like circles as seen from the outside; see here for why), but in fact that’s rarely the case for planetary nebulae. Most are oval, or barrel-shaped, or something even weirder. It has to do with the way the dying star blows off winds, streams of subatomic particles from its surface. If a star is just sitting there, or rotating very slowly, then sure, the wind might expand as a sphere.
Apparently, though, for most stars something happens to increase their spin before they become planetary nebulae. One thing that could do it would be planets: big planets orbiting the star close in would get swallowed up as the star expands into a red giant (the first step in becoming a planetary nebula). The planet orbits the star faster than the star itself spins, so the planet acts like an egg beater or a whisk, stirring up the star’s insides, and spinning it up. We know planets are common — it’s possible most stars have them — so this explanation makes a lot of sense.
But if you look more closely at the picture of Abell 33, things get weirder. The rim is circular, sure, but the fog on the inside is not. It has two oval holes in it, slightly off center. Moreover, the two ovals are parallel, slanted a bit at the same angle. I’m pretty sure this is a sign that the nebula itself may be more barrel shaped, and the two dark ovals are the open ends of the barrel structure. It makes me think it’s a lot like NGC 1514 (inset here; click to embiggen), which has a similar structure (and also a binary central star, hmmm). In the infrared, NGC 1514 looks like a space station! I’d dearly love to see a high-res infrared shot of Abell 33. Would it have those weird rings too?
All in all, this is a pretty interesting nebula, and I’m fairly surprised it hasn’t been studied more. There are some unusual things going on here, which would make for a pretty good project for someone, I’d bet. Until then, though, I guess we’ll have to be satisfied with just soaking in the beauty of this enigmatic object.
Image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona
I love a good coincidence. On Monday morning, I posted a gorgeous picture of the planetary nebula Abell 31, an object formed when a dying star blows off its outer layers in a series of winds which collide with each other. I mentioned that these nebulae are usually symmetric — Abell 31 happens not to be because it’s moving rapidly through space, and the gas through which it moves is compressing one side of it. But events like that are not the norm; most planetaries show stunning symetric features… like Henize 3-1333, as you can see in this nice Hubble image of it:
[Click to ennebulenate.]
It looks like a flower, doesn’t it? The petals you see are actually sculpted lobes of gas. I’m guessing it undergoes periodic episodes where it blows out gas in focused beams, which then move outward and form those features as they plow into gas previously blown out by the star. It’s a guess, but it fits what’s known about the inner regions of the cloud near the central star. There’s a thick disk of material surrounding the central star, something like 30 billion km (20 billion miles) across, far larger than our solar system. Every six years or so, the central star appears to dim, which may be due to the inner part of the disk itself becoming unstable and puffing up, blocking the light. This disk may also be responsible for shaping the outflow of the gas from the star, forming those petals.
If you’re wondering just how much material the star is blowing out, it turns out to be about the mass of the Earth every year! That’s a tiny fraction of the star’s mass, but it blows out this much every year for thousands of years. Eventually that wind will turn off, and all that will be left is the very hot (30,000° C or about 55,000° F) core of the star, which will then cool over the next few billion years. Long before then the expanding gas around it will dissipate, and all that’ll be left is a very diffuse cloud of material that will mix with the ethereally thin matter between the stars.
Well that, plus images like this one. And, of course, the knowledge we’ve gained studying how stars die.
Credit: ESA/Hubble & NASA
I’ve been doing this astronomy thing for a while, OK? I’ve seen galaxies, clusters, stars, planets… so many I’ve lost count. So it’s hard to find something I’ve never seen before, or even heard of before. So when astrophotographer Adam Block sent me a note about the nebula Abell 31, my first reaction was, "Say what now?", and then I clicked on the picture, and my second reaction was "What the what?" Then my third reaction was to soak in the beauty of this gorgeous object, and my fourth reaction was to nod my head slowly, thinking, "Ahh, I see what’s going on here."
Let me share:
See what I mean? What a beauty! [Click to ennebulenate.]
This image was taken with the 0.8 meter Schulman Telescope at Mt. Lemmon in Arizona, and is the result of an astonishing 21 hours of exposure time in various filters! Right away, that was a clue as to why I had never heard of this object: it’s incredibly faint. A quick perusal of amateur astronomers’ sites proved that to be correct; not too many have observed this jewel because it’s barely detectable.
Abell 31 is a planetary nebula, a cloud of gas formed when a dying star expels winds of matter. At first the wind is slow, but then as the star continues to expire the velocity increases. The faster wind catches up with and slams into the slower wind, forming fantastic shapes, usually displaying amazing symmetry.
Abell 31 isn’t symmetric though! One side is sharply defined, and the other diffuse, fuzzy. I was pretty sure right away what I was seeing: this nebula is in motion, moving through space, and interacting with the thin material between the stars. A literature search confirmed this suspicion: it’s moving through space at relatively high speed. In this picture, the direction of motion is toward the bottom of the frame, and the gas leaving the star in that direction is compressed. Gas heading the other way is moving downwind, and relatively untouched. Think of it like blowing on a dandelion; the side toward your mouth gets compressed, while the other side stays fuzzy.
The red gas is hydrogen, and the blue is oxygen. Interestingly, oxygen is probably located throughout the entire nebula, but only in the center is it close enough to the central star to get lit up and glow. The star doing all the work here is a white dwarf, basically the core of a normal star like the Sun, but exposed after all the outer layers of the star have been shed to form the nebula.
Abell 31′s white dwarf central star is tiny, only about 4 times bigger than Earth (or about 0.04 times the size of the Sun), but it’s incredibly hot, blazing away at about 85,000° Centigrade (150,000°F)! It has about half the mass of the Sun, meaning it probably started out life as a star with twice the mass of our star or so, and lost the rest as it aged and blew those winds. Judging from how fast those winds are blowing outward, the star probably started dying in earnest about 130,000 years ago, after a billion or more years of normal life. What used to be a star a couple of million kilometers across is now this lovely object nearly ten light years wide — that’s 100 trillion kilometers from end to end!
At its distance from the Earth of 2000 light years that makes it pretty big in the sky, roughly half the size of the full Moon. With its light spread out so much, no wonder it’s faint. Too bad: it’s a fascinating object, worthy of study. And it’s also fantastically beautiful, worthy of wider recognition.
And I’d love to see a set of Hubble observations of this! I studied planetaries for several years, and I still sometimes get the desire to poke and prod them again. I’ll just have to wait, I suppose, and be satisfied with images as lovely as this one.
Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona
160,000 light years away sits the Large Magellanic Cloud, an irregular dwarf galaxy that orbits our own Milky Way galaxy. It’s a fascinating object, actually, filled with stars, gas, dust, and all the usual trinkets a galaxy has.
It also has an assortment of globular clusters — roughly spherical collections of a few hundred thousand stars bound by their own gravity orbiting the cluster center like bees buzzing around a hive. NGC 1846 is one such globular cluster, and it looks like most of the others, if a bit sparse and loosely distributed. But it has something that does make it rather special. You can see it if you peruse this lovely Hubble Space Telescope picture that was just released:
[Click to embiggen, or get a much larger version.]
Isn’t that pretty? As much as I like it, the most interesting thing in it, though is actually rather difficult to see here. Look at the center of the cluster, then let your eye go straight down, nearly to the bottom of the frame. See the green spark there? It’s the only green thing in the entire picture. It’s not a star — there are no green stars — but it used to be…
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)
[Click to ennebulenate.]
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. Read More
Look, I’ve been around the block a few times. I’ve spent my whole life as an astronomer, so I’ve seen pretty much every big, bright object there is in the sky.
However, "pretty much" != "all". It’s still possible to surprise me, and folks, let me tell you: the Gemini telescope’s observation of the nebula Kronberger 61 did just that!
Wow! [Click to ennebulenate.] It looks like a buckeyball or a soccerball; my wife pointed out it looks like the shape you get when you use a bubble maker to make a bunch of bubbles all stuck together. Kn 61, as it’s called, is actually a planetary nebula, the gas flung off by a star like the Sun as it dies. You can get the details (along with many pretty pix) of how this works in a recent post of mine. In a nutshell, when a star runs out of useable hydrogen fuel in its core, it expands into a red giant and expels a huge wind of gas. This strips away the outer layers of the star, revealing the hot, dense core. Ultraviolet light from that star then lights up the surrounding gas, making these gorgeous nebulae.
The exact mechanisms for this process, however, are still not clear. Read More
When I was younger, it was pretty common on clear nights to see me at the end of my driveway with my telescope. And one of my favorite targets to observe was (and still are) globular clusters: hundreds of thousands, and sometimes millions of stars all bound together in a tight ball due to their gravity. And one of the best of those is the fabulous M15… and when it’s seen by Hubble, well, it’s simply spectacular:
Holy wow! Click to englobulenate – I had to shrink the image a lot to get it to fit here, so as gorgeous as this is it’s a shadow of the higher-res version… or the ginormous full-res one!
M15 is relatively nearby as globulars go, about 35,000 light years. Over 150 of these objects orbit our galaxy, and so some are quite far away. Not only is it close, but M15 is also fairly densely-populated, its stars orbiting each other like bees around a beehive, making it a pretty easy target for amateur astronomers. It was one of the first things I’d go after once it got dark in the autumn, and it would appear as a fuzzy ball in my 25 cm ‘scope. Of course, when you aim the 2.4 meter mirror of Hubble at it, well. You can see for yourself.
This false-color image is a combination of two pictures; one taken in visible light (colored blue; in reality the filter used let through yellow and red light), and the other in near-infrared (colored red). That selects out redder stars; the brightest ones are red giants, stars nearing the ends of their lives, and the fainter ones are lower-mass stars that are still busily fusing hydrogen into helium in their cores.
If you look to the left and a bit below the cluster’s center, though, a blue glow sticks out among all the red. If you do grab the seriously super high-res version of the image, you get a much better look at it. I’ve zoomed in on it here. It’s clearly not a star; the blue halo is much larger than any star image, and you can see the rim on the left hand side is bright. What gives?
One of my favorite types of objects in space are the thin, ethereal shells of gas stars create when they die. So I was thrilled* to see this new image of one taken in exquisite detail by the Hubble Space Telescope:
[Click to supernovenate.]
I studied weird soap bubbles like this for quite some time for both my post-graduate degrees, and they still fascinate me. This one, called SNR 0509, is actually a very interesting example. There are lots of ring-shaped objects in the sky — the Helix nebula (seen below) may be the most canonical — but usually the ring itself is thick, the width of the band being a large fraction of the object diameter itself. Why does SNR 0509 have such a thin ring?