May Astrobite, or “How Is a Pixel Like a Bucket?”

Just a quick post tonight to point out my most recent Astrobites article which came out on the 22nd. This one was very interesting to write. It's about a paper which I read when it first came out back in February on the arXiv. (It's pronounced “archive,” and it's a website where most papers in physics and astronomy and several over sciences are hosted freely available; it's undoubtedly revolutionized the areas it serves by making it easier to communicate results, and I can't imagine trying to do research without it.) I'd been stockpiling recent papers that looked interesting for a week or two before sitting down to write, but none of them really seemed to call to me, till I finally remembered this interesting paper I'd read about CCD systematics.

CCD stands for charge-coupled device, which is the technology behind most digital cameras nowadays. Astronomers adopted them very rapidly back in the 1970s soon after they were invented, and they're responsible for a very wide variety of astronomical research since then. Despite coming up on fifty years old, the authors of the paper I wrote about managed to find a new, never-before-seen form of subtle systematic errors in sixteen out of twenty-two instruments they investigated. The thing that really blew my mind while browsing the abstract and got me to read the paper? They noticed an effect that was proportional to the number of 1's in the binary representation of the value of various pixels in the image.

If you just said “What‽” out loud like I did upon reading that, check out the paper! It's really well written and does a good job of explaining their findings with some really good, high-quality graphs. If you don't know what that means or why it sounds so weird, maybe check out my astrobite—I spent several hours wrestling with an analogy involving grids of buckets and sprinklers in an attempt to render the technical details more approachable, so hopefully I've explained it there in a way that makes sense.

Basically, the top part should be a flat line at zero, not…this.

The results of this paper, while not necessarily highly problematic, are likely to be very far reaching and will affect a lot of people and their science, so now that it's been officially published as of May 11th I expect we'll start seeing some more papers popping up on arXiv related to the issue it reveals. (arXiv allows people to upload “preprints” of papers that have been submitted to journals and are in the process of peer review, which is how I was able to read it back in February.) That's it from me for now though, a hui hou!

Snagging a Supernova (SN 2014j)

Last week Tuesday, January 21, an astronomer at the University of London Observatory was teaching some undergraduate students how to run the CCD imager on a 14-inch telescope during a short break in the clouds. At about 7:20 PM local time, they noticed a new star in the nearby galaxy Messier 82. This star did not show up in archival images of the galaxy, and after using a second set of equipment to make sure it wasn't an artifact they realized they were dealing with something very real. It turned out to be the closest type-Ia supernova in 42 years, and was duly designated SN 2014j.

Messier 82 is a funny-looking galaxy located at a distance of about 11.5 million light-years away, close to another galaxy called Messier 81. They're practically next door in galactic terms – only 6 times further away than the Andromeda Galaxy – and are easily visible in moderately sized telescopes.

I know this personally both because I've seen them before at the Vis, and because I went up there Sunday night in order to try to get an image of the supernova. When I got there I discovered some bad news: I couldn't get the telescope mount to connect to the control software on the computer (probably a cable went bad). The telescope mount still had its built-in control software, so on a whim I polar-aligned it as best I could then told it to go to Capella, the brightest star close to M82. Amazingly, when I took a short exposure with the CCD Capella was visible on the first try. I then tried sending it to M82, and while that wasn't visible in the resulting image, its neighbor M81 was. Finally, after some touch-and-go work manually directing the telescope with its built-in directional controller I was able to get both M81 and M82 (supernova easily visible) in the same frame.

Having miraculously overcome the first major hurdle of finding the target, I was left with the sad reality that without being able to connect the mount, I couldn't get auto-guiding going, which meant that exposures longer than about 12 seconds started to show unmistakable signs of star trails. This kept me limited to short exposures. The resulting picture is hideous, not helped at all by Blogger's re-scaling of the limits. I'm a bit embarrassed to be showing such a poor piece of work, but it's a supernova, I simply can't refrain from talking about and showing it. Here's the picture I got, with M81 the blur at the top, M82 the blur at the bottom, and SN 2014j marked by the tiny straight lines.

I looked back through my blog and saw that I've never imaged this pair of galaxies before, so for the record M81 is actually a lovely face-on spiral galaxy, while M82 is...odd. It's apparently a spiral galaxy too, but it's almost edge-on to use, and it has these really weird filaments of dust in front of it in optical light. (You can kind of see the thickest of them in the image near the middle of the galaxy, as a dark bar that bisects it.) These filaments often make it look as though the galaxy is exploding in long exposures, and it has the amusing nickname of the Cigar Galaxy due to its shape in small telescopes.

I hope to get a better picture of these beautiful galaxies and perhaps the supernova that graces one of them, but my near-future plans have been put on hold by the arrival of a large storm system bringing snow and cloud cover to Mauna Kea. The forecast is bad through the weekend at the moment, but, as always, we shall see...

Adventures at the James Clerk Maxwell Telescope

Two weeks ago I was privileged to be invited up to the summit of Mauna Kea to the James Clerk Maxwell Telescope as part of my job. Well, I say that, but really the only reason I was there was as a "warm body" – safety regulations require a minimum of two people together up at the summit at all times, in case of medical emergency, so my specific job had nothing to do with it.

Which I don't mind at all! It meant I got to go up with no real responsibilities besides crisis management in the event that something came up, which it didn't. I was therefore free to get a tour of the telescope that I officially work for, and wander around to get some pictures.

For instance, this black machine here is the water vapor meter, whose output I work with a lot. It measures the amount of water vapor in the air, which directly (and dramatically) affects the opacity of the atmosphere in the sub-millimeter wavelengths we observe in. It's been having some issues, so it was actually replaced with a different (silver) one just last week.

This big blue machine down here is SCUBA-2, the Sub-millimeter Common User Bolometer Array (2). This is another instrument whose output I work with on a regular basis. It's basically the best sub-millimeter camera in the world today.

It also happens to be the coldest place in the known universe. You think outer space is cold? Out far away from hot stars and galaxies, in the inter-galactic voids of space, the temperature can drop to about 2.7 kelvins, the temperature of the cosmic microwave background radiation, which is about 2.7 degrees Celsius above absolute zero (which is -273.15 °C or -459.67 °F).

That's cold alright, but it's still quite a bit hotter than the temperature of the SCUBA-2 imaging arrays, which are kept a mere one-tenth of a kelvin above absolute zero.

(The reason for such cold temperatures is that, in order to get a reasonably high signal-to-noise ratio from the electronic imaging arrays used in astronomy and digital cameras, the imaging array itself has to be cold enough that it's not emitting too much electromagnetic radiation in the portion of the spectrum it's trying to take a picture of. Consumer digital cameras work because they aren't glowing at visible wavelengths. Another way to think about it is that trying to take images in the sub-millimeter portion of the spectrum [which comes from objects that are themselves quite cold already] is like trying to take pictures with a digital camera that is on fire. The amount of light given off by the camera at that point is so much that you won't catch much light coming from what you want to photograph by comparison.)

Anyway, this is a panorama I took from behind the telescope looking up at the back of the dish. It's slightly misleading; it looks like it's looking at the sky, but that's actually the world's largest piece of Goretex (no joke). It also looks like it has a square opening, but that's a result of the image stretch due to the panorama. It's actually more like a rectangular slice in the side of a cylinder. (Unfortunately I forgot to take a picture of it from the outside.)

Finally, here's a high dynamic range image of the back of the dish. I had to take this one lying on the floor to get it all in, hence the slightly off-horizontal angle.

For those who don't know what the JCMT looks like from the outside, here's a picture of it from back in 2010 while I was up on a summit tour. It's the white cylinder on the middle-left. The opening is on the left side of the cylinder in this picture (the whole building rotates to point the telescope), but it wasn't actually open at the time.

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 28): M55

Today I have another globular cluster picture for you, and this one just happens to be the next in the Messier catalog: Messier 55, in Sagittarius. This globular cluster is much closer than M54, at a moderately distant 17,600 light-years. It appears almost twice as large on the sky at 19.0 arc-minutes, but is a mere third its actual size at 96 light-years in diameter. It's also a lot less compact than M54 (class XI out of XII), and really looks quite nice.

Messier 55 in Sagittarius.

Not every object in Charles Messier's catalog was discovered by him (and he gave credit where it was due), and M54 is one such object. It was discovered by an astronomer named Nicholas Louis de Lacaille from an observatory in South Africa in 1752. Messier, having heard of this discovery, tried several times to locate the cluster starting in 1764, but was stymied by its low apparent height from his location in Paris (it is located 30 degrees south of the celestial equator, which makes it rather difficult to see from mid-northerly latitudes). In fact, it wasn't until 1778 – 14 years later – that Messier was actually able to find it, after which he included it in his famous catalog of objects.

All in all, M55 is a rather nice looking cluster, if I say so myself.

Globular Cluster Photo Series (Part 27): M54

It's been a while since I had any astronomical images to show, hasn't it? I haven't been able to use the imager for a while now, due to a combination of poor weather and being busy, but I do have a few images from September lying around that I never got around to reducing. Today I have the first of those, a picture of the globular cluster Messier 54 in Sagittarius.

Messier 54 is an interesting globular in several ways. For starters, it doesn't actually belong to our galaxy – or at least is a relatively recent acquisition. It appears to originate from the Sagittarius Dwarf Elliptical Galaxy (or SagDEG), a small nearby satellite galaxy of the Milky Way currently residing opposite the galactic core from us. SagDEG has four known globular clusters of its own, of which Messier 54 is the largest and main one.

Because it's on the other side of the core, M54 is the most distant cluster I've yet photographed, at a whopping 87,400 light-years away, easily surpassing the next most distance cluster I've shown here (M53, 58,000 light-years). For comparison, the Milky Way Galaxy itself is only about 100,000 light-years across. Despite its great distance, M54 still appears a relatively large 12.0 arc-minutes across on the sky, fully one-third the diameter of the full Moon. At its distance, that translates into the incredible diameter of about 306 light-years, making M54 larger than nearly every other globular cluster in the Milky Way (and certainly all the ones I've shown so far). It is also very luminous, shining with the light of 850,000 Suns, being outshone only by the brilliant cluster Omega Centauri (which is also a lot closer).

M54 is also one of the denser globular cluster, being a class III on the density scale (with class I being the densest and XII the least dense). It's also possible, according to a 2009 paper, that there may be a black hole with a mass 10,000 times that of the Sun at the center of the cluster, which is unusual for a globular cluster. All in all, it's a fascinating cluster.

Nebula, in Three Parts

Today I've got something a little different from the usual globular cluster pictures I've had a lot of recently. This is a picture of Messier 20, the Trifid Nebula, a fascinating object in Sagittarius, the Archer.

The Trifid Nebula, Messier 20, in Sagittarius.

This nebula gets its name from the way it appears divided into three parts by the dark nebula stretching across it. It got this name long before anyone knew what it was exactly, but the number three is also important to this object for another reason: it nicely illustrates all three types of nebulae.

The first type of nebulae, emission nebulae, are represented by the reddish region at the top. This red light comes primarily from hydrogen atoms in the gas being excited by copious amounts of ultraviolet light from the hot, young stars inside and around the nebula. The particular wavelength responsible is at 656.28 nanometers and is so important and wide-spread that it has its own name: hydrogen-alpha, or H-alpha for short.

The second type of nebulae is the blue reflection nebula seen below the emission nebula. These nebulae come about from starlight being reflected off of tiny dust grains in the gas cloud. The reason it appears blue is because the dust grains preferentially reflect blue light, the same way that the molecules in the Earth's atmosphere preferentially scatter blue light. Further from the young stars than the emission nebula is, the gas in the reflection nebula isn't being excited to emit in visible wavelengths very strongly.

Finally, the third type of nebulae, dark nebulae, also come about as result of dust and are represented by the dark clouds and bands of dust in front of the emission and reflection nebulae. Dark nebulae are full of tiny dust particles containing organic molecules that are extremely effective at absorbing visible light. Soot is actually fairly close in composition to these dust particles, so you have some idea of just how dark they are in visible light. Fortunately, they are much more transparent at other wavelengths, allowing us to probe their structure in infrared and radio wavelengths.

Also, to be comprehensive, there is a fourth type of nebula that typically gets its own name: planetary nebulae. These are really a subset of reflection nebulae, as they are the puffed-off atmospheres of old Sun-like stars that are being illuminated by the white dwarf core of the star, but they are different enough from typical emission nebulae to warrant their own designation.

Globular Cluster Photo Series (Part 26): NGC 5466

Today I have a picture of the surprisingly sparse globular cluster NGC 5466 for your perusal. NGC 5466 is located very far away in the constellation Boötes at 51,800 light-years from Earth, making it the third-farthest I've shown so far after M53 and M72. At this great distance its larger-than-average size of 166 light-years in diameter gives it a visual angle of 11.0 arc-minutes, about a third the width of the full Moon. Interestingly, NGC 5466 makes nearly an isosceles triangle with Earth and the galactic center, being about 52,800 light-years away from the core.

NGC 5466 in Boötes.

As you can see from the picture NGC 5466 lacks any sort of concentration in its core, in sharp contrast to most globular clusters. It's almost difficult to tell that it's a globular cluster at all. In fact, under the globular cluster classification scheme devised by Harlow Shapley in 1927, NGC 5466 is a class XII, the most loosely concentrated class there is (class I being the most highly concentrated towards the center). You may have noticed that there is general lack of stars both in the globular and in the foreground of this image; this is because Boötes is located away from the galactic plane, and thus there are relatively fewer stars between us and the cluster.

Finally, in an interesting historical aside, it turns out that NGC 5466 was discovered by William Herschel exactly two hundred and five years to the day before I was born, back in 1784.

Globular Cluster Series (Part 25): NGC 6293

Wow, number twenty-five already. It seems like just yesterday that I decided to make a photographic catalog of the Milky Way's globular clusters, but it's already been a year. Today's picture is the moderately-sized cluster NGC 6293 in the constellation Ophiuchus, the Serpent Bearer. NGC 6293 is pretty similar to the cluster I showed off last time, NGC 6541. NGC 6293 is a bit further away at 31,000 light-years (compared to 22,800), and also a bit physically smaller at 71 light-years in diameter (compared to 100), which combine to give it much smaller size on the sky, only 7.9 arc-minutes compared to 15.0.

Globular cluster NGC 6293 in Ophiuchus.

Both NGC 6293 and 6541 are at about the same distance from the galactic center: 6,200 light-years for NGC 6293, and 6,800 for NGC 6541. Interestingly, despite the fact that NGC 6293 is both fainter and smaller than 6541, it was discovered first, in 1785 by William Herschel (discoverer of the planet Uranus), while NGC 6541 wasn't discovered until 1826. This may have something to do with the fact that NGC 6541 is much more southerly than 6293, which would make it appear much fainter and harder to see for the northern European astronomers who discovered both of them.

Globular Cluster Series (Part 24): NGC 6541

Boy, it seems like I've been showcasing globular clusters for a long time now (and in a sense I have, stretching back to June of last year), but I'm still only up to twenty-four so far. Even accounting for the fact that from my location I can see "only" about 80% of the sky, and that there are some globular clusters that are realistically too small and faint for me to capture, there are probably still at least 50–75 globular clusters I can feasibly hope to capture. So, I'm barely a third done at this point, at most.

Anyway, the globular cluster I want to showcase today is called NGC 6541. It's the only globular cluster in the small southern constellation Corona Australis (the Southern Crown), and it turns out to be a lovely little gem of a cluster about 22,800 light-years away from us. Despite this great distance it appears quite large on the sky at 15.0 arc-minutes in diameter, about half the size of the full Moon. This puts its physical size at just about 100 light-years, making it in the upper 50% of globular clusters.

NGC 6541 in Corona Australis.

Although far from the Sun, NGC 6541 is pretty close to the galactic center, only about 6,800 light-years away. For such a large and pretty cluster I wasn't able to find too much information about it other than that it was discovered in 1826, which strikes me as fascinating. I mean, we've known about the existence of this cluster for less than 200 years, less time than the United States has been a country.

In many ways, astronomy, despite being the oldest science, is still completely fresh and new. We've only been able to build telescopes for the past 400 years, and it wasn't until the last century that we've been able to create telescopes capable of exploring more than the minuscule sliver of the electromagnetic spectrum that is able to penetrate our atmosphere. We still know so little about things in our universe, or even our own galaxy, because it's only been within the last 50 years or so that we've developed the ability to even detect them. So much to learn and discover...enough to last many lifetimes.

And that's why I love astronomy.

Globular Cluster Photo Series (Part 23): NGC 6356

Well. This is a first. Today's globular cluster is the first one I've showcased here on my blog that doesn't have an article about it on Wikipedia. I don't use Wikipedia for all my information but it does make a nice central repository of info, so I had to do a little more digging around tonight. This sort of thing will probably start happening more and more as I exhaust the Messier objects and the brighter or more famous NGC ones. (I already talked a bit about the New General Catalogue in my post about NGC 3201, the first NGC globular cluster I showcased back in March.)

Anyway, NGC 6356. It's located in the constellation Ophiucus at the rather large distance of 49,200 light-years from us. At this distance, I calculate its diameter to be about 115 light-years (since I couldn't find a number anywhere) given its size of about 8.0 arc-minutes on the sky (full Moon is about 30). This would actually make it among the larger clusters of the Milky Way, but its great distance makes it appear small (though it's still among the top 50% of clusters by area on the sky).

NGC 6356 in Ophiucus.

NGC 6356 is also located pretty far from the galactic core outside the galactic plane, at about 24,400 light-years (that's about a quarter of the way across the entire Milky Way galaxy). Most globular clusters are closer to the core (and disk) of the galaxy.

Another interesting thing about NGC 6356 is that it is located about 80 arc-minutes from another globular cluster, Messier 9 (which I wrote about on August 5th). That distance is small enough that I could actually catch both clusters in one picture with a well-placed shot, though I didn't know about it until tonight. Once I found out, however, I was able to match up star patterns seen in both pictures and put them together to make the picture below. Messier 9 is the cluster on the left, while NGC 6356 is the one on the right.

Messier 9 (left) and NGC 6356 (right) in Ophiucus.

While looking at this picture, keep in mind that M9 is about 90 light-years across while NGC 6356 is about 115; it's only NGC 6356's greater distance that makes it look about the same size. Also, since this is two pictures reduced and composited separately, the brightness scale between them is not uniform.

So, all in all NGC 6356 turns out to be a rather interesting cluster for an object not interesting enough to have a Wikipedia page. A hui hou!

Globular Cluster Photo Series (Part 22): M68

Today I have a picture of the globular cluster Messier 68, located in the constellation Hydra.

Since globular clusters orbit the center of the galaxy like everything else in it, the vast majority of them are found in the hemisphere containing the galactic core. Messier 68 is an oddity, a large globular cluster found in the hemisphere opposite the galactic center. Physically, it is about 106 light-years in diameter, and appears about 11.0 arc-minutes across on the sky (about a third of the width of the full Moon). It is located at the not-too-shabby distance of 33,300 light-years from us, and given that it is further from the core than we are, it is perhaps not too surprising that it is approaching us at 112 kilometers/second. M68 contains a fairly average number of 42 variable stars (discovered so far).

Messier 68 in Hydra.

Hydra is pretty far south for a Messier object (declination –26° 44′ 38.6″), and as a result observers in the mid- to far-northern hemisphere tend to see it through a lot of atmosphere low on the horizon, which often led past observers to estimate its brightness as fainter than it really was. Thankfully, being in Hawai‘i means that I can see it pretty well. And other than that, there really isn't too much to say about it. A hui hou!

Globular Cluster Photo Series (Part 21): M19

Today I've got a picture of the globular cluster Messier 19. Much like M9 that I showcased last time, M19 is one of the globular clusters closest to the galactic core. It is about 28,000 light-years from Earth, and about 5,200 light-years from the core, narrowly beating M9 (at 5,500 light-years) for closest globular cluster to the core that I've showcased so far. M19 is the most elliptical globular cluster known (though it is only slightly noticeable in this picture), and is about 140 light-years across the long way. This is a good 50% bigger than M9, so since they are about the same distance away M19 looks a lot bigger on the sky at 17.0 arcminutes to M9's 12.0. (The full Moon for comparison is about 30 arcminutes across.)

Messier 19 in Ophiuchus.

Other than its great ellipticity (which may actually be a visual effect due to intervening dust extincting the light on one side), M19 is a fairly standard globular cluster with little to say about it despite its great size. It contains a rather small number of variable stars and was one of the earlier objects that Charles Messier observed. And other than that I really can't find much more interesting information about it. A hui hou!

Globular Cluster Photo Series (Part 20): M14

Today I have a picture of the globular cluster Messier 14 in Ophiuchus. M14 is a rather large cluster around 100 light-years across, and at a respectable distance of about 30,300 light-years. The cluster's large physical size gives it a size of 11.0 arcminutes on the sky, about a third the width of the full Moon. The entire cluster shines about 400,000 times as bright as the Sun, but is just below naked-eye visibility at its distance.

Messier 14 in Ophiuchus.

M14 is known to contain 70 variable stars, quite a decent number for a globular cluster. In 1938 a nova went off in the cluster, but this fact was not discovered for 28 years until photos of the cluster taken in 1938 were examined in 1964. It's estimated from these photos that the nova reached a peak brightness over 5 times that of the brightest non-nova stars in the cluster. This is only the second nova known to have appeared in a globular cluster (and the first was in 1860, before it could be photographed).

Globular Cluster Photo Series (Part 19): M9

The last globular cluster I showed a picture of, Messier 64, was quite far away from the galactic center. Today I'm going to go in the opposite direction with a picture of Messier 9. This cluster is moderately far from Earth at a distance of about 25,800 light-years, but that's because it happens to be one of the closest globular clusters to the galactic core. The distance between M9 and the core is only about 5,500 light-years, which is pretty small when you remember that the Galaxy is about 100,000 light-years across. (For comparison, our Solar System is about 23,000 light-years from the core.) M9 is a average-sized cluster about 90 light-years across, which at its distance translates to a size of about 12.0 arcminutes (about a third as large as the full Moon).

Messier 9 in Ophiuchus.

Partly as a result of being so close to the center of the Galaxy Messier 9 is retreating from us quite quickly, at a rate of 224 kilometers per second (just a hair over half a million miles per hour). It is also located close to the dark nebula Barnard 64, which you can see as the region to the upper-left of the cluster that appears to be devoid of stars. This dark molecular cloud (made up of interstellar gas and dust) is probably something like what the Orion Nebula and the Lagoon Nebula would look like from the other side.

Closest Quasar

Today I have a picture of a very unique object to show you: the first quasar ever discovered. I had meant to take this picture before the transit of Venus, but the combination of poor weather and a heavy workload made it impossible. Luckily for me, Virgo was still high in the sky when I took this picture on the 11th of July.

A short note about quasars: quasars (short for quasi-stellar objects) are believed to be comparatively small accretion disks around supermassive black holes in distant galaxies. They are known as quasi-stellar objects because they appear star-like to all but the most powerful telescope due to their extremely great distances. They give off tremendous amounts of light all across the electromagnetic spectrum, from X-rays to infrared. Some also give off copious amounts of gamma rays and radio waves.

The quasar below is thought to be the closest quasar to us at 2.44 billion light-years away. Yes, that's billion with a "b". Our Milky Way galaxy is about 100,00 light-years across, so nearly 25,000 galaxies like the Milky Way could fit between this quasar and our galaxy. And that's the closest quasar to us. Most quasars are much further away, and due to their extreme luminosity are some of the farthest objects visible in the universe. This quasar, though, is likely the farthest object you could reasonably expect to see through an amateur-sized telescope.

Anyway, enough explanation. Here it is, entry 273 in the Third Cambridge Catalog of Radio Sources, 3C 273 itself!

3C 274, the closest quasar to us. Located 2.44 billion light years away in the constellation Virgo.

Kindly hold all applause until the end of the blog post. I know it's not much to look at, but it's remarkable because of what it represents. With a 4-inch telescope and a CCD camera you and I can see the light from the mind-bogglingly intense region of warped space-time around a black hole with more than 800 million times the Sun's mass from over 2,440,000,000 light-years away. It's staggering to me that such a thing is even possible. Hopefully you can see why this is such an amazing picture, even if it isn't as showy as some of the ones I put up here.

3C 273 is an interesting object because it was the first quasar to have its spectrum taken (due to the fact that it is the brightest quasar in the visible light range), which helped show that it wasn't a star and was in fact very much further away than previously thought. Although we know more about quasars now than ever before, they continue to remain mysterious objects and there are many questions about them yet to be answered.

Celestial Lagoons

Today I have a picture of the Lagoon Nebula, a lovely star-forming region in Sagittarius. It is similar in nature to the famous Orion Nebula and is similarly visible, very faintly, to the unaided eye. It is over five times larger than the Orion Nebula (110 light-years across vs. 24), but appears slightly smaller on the sky due to its greater distance (3,000-4,000 light-years away, compared to ~1,300 for Orion).

Messier 8, the Lagoon Nebula in Sagittarius.

Like the Orion Nebula, the reddish color comes from hydrogen ionized by hot, massive young stars embedded in the nebula. The blue color comes from light scattering off dust in the cloud, similar to the way air molecules scattering light causes the sky to look blue.

The Lagoon Nebula is also similar to the Orion Nebula in that they both offer looks into the cavernous interiors of gigantic clouds of cool gas and dust. From the outside these clouds appear dark and boring, and you can see that slightly around the edges of the nebula. But when young stars inside them blow away the gas around them and offer a view inside, the sight is spectacular. Not unlike geodes, now that I think about it. (Geodes, for those who don't know, are rocks that look like any other rock on the outside to the untrained eye, but which contain beautiful crystal formations on the inside if broken apart.)

Image Stretch and the Effects Thereof

Today I just want to briefly discuss a very important decision that went into making the transit of Venus video I posted last time. That decision was how to stretch the images from the CCD camera. You see, the CCD camera has a wide range of sensitivity, with 65,536 different light levels it can record. Most monitors cannot display anywhere near this amount of contrast, so the image has to have that range compressed down into what the monitor can display. In theory, this means that a lot of detail is going to be lost, and that's where stretching comes into play.

Stretching an image's histogram basically means reassigning how the compression takes place. When you're looking at an astrophoto, you may have 65,536 different levels of brightness, but a large number of those are probably going to be so close to black as no matter. You can then adjust the stretch so that they simply display as black, leaving more of the dynamic range of the monitor available for seeing detail in the brighter regions. Essentially, the image stretch lets you decide which regions of the image you want to see detail in based on their brightness.

To illustrate how important the stretch is, let me give you some examples. Immediately upon applying the reduction process to one of the images of the transit and importing it into GIMP, we get this picture:

To the left you can see the Sun with Venus in front of it and a few sunspots visible near the middle of the disk. On the right you can see the default linear stretch currently applied to the image. Basically, it maps a zero value in the image to black, a value of 65,535 to white, and linearly compresses everything in between. The horizontal scale is the brightness levels of the original image, the vertical scale is the brightness levels on the monitor, and the gray lines are a histogram representing how many pixels there are at each brightness level. What it tells us is that this image has a lot of very dark pixels (on the left, all the background), a lot of very bright pixels (on the right, the center of the Sun's disk) and a moderate number in between (around the edge of the Sun, and things like sunspots).

Speaking of sunspots, I quickly discovered that there were more than could be seen with just the default linear stretch. By cutting off the left end of the histogram pretty dramatically with another linear stretch, I could bring out details as yet unseen:

In some ways, this worked quite well – sunspots had much better definition, and you could see more of them. The main problem to me was that it made the outside edge of the Sun look a bit grainy. I ultimately decided that it was not quite good enough, and continued looking for a suitable stretch. My next attempt, which I'm calling an “exponential” stretch, looked like this:

This stretch is similar to the previous linear stretch, but leaves more detail in the dark areas. Again, the sunspots looked good, but it looked even worse around the edges than before. After playing around with variations on both of these (plus several others), I finally came up with the curve that I would end up using:

This curve is, ultimately, a compromise. It doesn't show the sunspots quite as well as either of the previous curves, but it looks much better along the edges (at least it does at 100% magnification, they all look about the same in these pictures). It took me quite a while to decide on this curve, because I'd keep looking at it and tweaking it and trying to get the absolute best curve possible, especially since I was going to be applying it to the next one hundred and eighty-seven images and didn't want to have to go back and redo it. It wasn't a decision lightly made, I'll say that much.

I also see that, having sat down to write a “brief” blog post, I managed to write a 650-word essay. I don't know why that always seems to happen. I hope you at least found it interesting. I've come to appreciate the image stretch more and more as I (hopefully) get better as an astrophotographer, and it should show in some of my upcoming pictures. A hui hou!

Globular Cluster Photo Series (Part 18): M53

Today's picture (I'm slowly getting through the ones I have lying around) is the globular cluster Messier 53. M53 is a bit large as far as physical size goes, measuring about 220 light-years across, which gives it a visual diameter of 13.0 arcminutes at its estimated distance of a whopping 58,000 light-years from us. M53 is genuinely far out there, as it is also about 60,000 light-years from the galactic center, almost twice as far away from it as we are.

Messier 53 in Coma Berenices.

Like Messier 64 that I showcased yesterday, M53 is located in the constellation Coma Berenices, or Berenice's Hair, a small, faint constellation near Leo. According to legend, Queen Berenice II of Egypt had long, golden hair that she was quite proud of, and which she promised to sacrifice to Aphrodite if her husband King Ptolemy III Euergetes returned safely from his military expedition against the Seleucids. When he did so, she cut her hair and donated it to the temple, only for it to turn up missing the next morning. Thinking quickly, the court astronomer, a guy by the name of Conon of Samos, told the furious royal pair that the gods had apotheosized Berenice's hair into a constellation, indicating a misty patch of stars that have ever after borne that name. Interestingly, Ptolemy (the 2nd-century astronomer, not the king) did not include it in his definitive list of 48 recognized constellations, considering it part of Leo, though he did refer to is as “the lock [of hair]”. (He considered it to be the tuft of hair at the end of Leo's tail, which does make sense given its place in the sky.)

M53 contains a total of 47 RR Lyrae variable stars, a not inconsiderable number. Its stars also happen to be even lower in elements heavier than hydrogen and helium than most other globular clusters, which are already much lower than stars like the Sun. And other than that, I'm afraid there really isn't much more of interest to write about it tonight. A hui hou!

More Nebulae! Flames and Horseheads.

Today I have Yet Another Nebula Picture (YANP), this time of two nebulae close enough on the sky to share a field of view. One of the them, the Horsehead Nebula, is probably one of the more distinctive and well-known nebulae out there, while the lesser-known Flame Nebula is a visual treat.

Also, these nebulae are nice because they lie near a prominent feature in the night sky, one that many (if not most) people are familiar with. Take a look at the picture below:

The Flame and Horsehead Nebulae in Orion. Click for a larger version.

See that bright star there? That is Alnitak, and you probably know it better as the left (or eastern) star in Orion's belt. The large bright region to its left is the Flame Nebula. Its bright red glow comes from the hydrogen atoms in the cloud being ionized by Alnitak's intense ultraviolet radiation. Alnitak itself is an O-class star, the hottest and most luminous class of stars, and puts out nearly a million times more light across the electromagnetic spectrum than the Sun does.

Just below Alnitak you can see the prominent outline of the Horsehead Nebula silhouetted against a backdrop of glowing hydrogen. In fact, there's a very nice, nearly straight line where a large dusty cloud blocks the light of the nebula behind it that extends for over half a degree below Alnitak.

What's cool is that this cloud of dust is also responsible for the Flame Nebula's dramatic shape, because the same dust cloud that causes the Horsehead Nebula is also in view in front of the Flame Nebula's backdrop of glowing hydrogen. So in a way, these two nebulae are intimately linked.

Another way to illustrate just how dark and dusty the molecular cloud that's producing these nebulae is, is to note that there is no inherent difference in the density of stars across the picture. The left side of the picture has just as many stars as the right side (you're looking through the plane of the Galaxy, after all); it's only the presence of the dust cloud that's blocking out the light from the ones behind it. (The ones you see on the left are either stars in front of the cloud, or ones seen where it is thinner.) All in all, a very beautiful section of the sky, one with a lot of very young, hot, and luminous stars.