Thanksgiving Stars

Well, Thanksgiving came and went this week without me doing much to celebrate (mostly because it's been a pretty busy week, and next week I'll be attending a mini-conference at Monash University here in Melbourne). I do, however, have some things to be thankful for! I've recently finished two pieces of art, have another nearing completion, and, most exciting of all, some of my art is now in the stairwell of the building I work in at Swinburne!

In order, then: I finished a Christmas present for Christian, another student from Germany who started this year. He's working on another aspect of the larger project my PhD is a part of. My project involves measuring the value of the fine-structure constant, using solar twins, or stars similar to our Sun. His project involves finding more solar twins, at greater distances in the Milky Way, to be used as targets for my work. Anyway, because of that, I decided to paint him a little solar twin of his own. And for fun, I named it “Sonne Doppelgänger” (solar twin in German).

“Sonne Doppelgänger”, 20×20 cm, acrylic with glass beads on canvas.

I'm quite happy with this one, although I managed to get my shadow in the only good picture I have of the finished work. I think my practice with painting stars is paying off—I actually like this sun-like star even better than the one I did for my Main Sequence series of paintings. I used the glass beads on it again, and managed to capture the effect slightly in this photo, but it looks so much better in person. One of the nice things about being a painter is being able to create things that bring me pure joy when I see them. And another nice thing is being able to give them away as personalized presents!

Speaking of presents, I finished another piece intended as a gift for the new director of our department, who'll be starting officially in March but will be here in Melbourne next week, and attending the Christmas party the week after that. As she works on galaxy formation and globular clusters, I figured I'd do a nice painting of a galaxy, but didn't really have a good idea of exactly what until a few weeks ago. Then, I had an amazing idea: paint a picture of a galaxy, then paint several globular clusters on little wooden disks which could be positioned on the canvas with magnets. Repositionable globular clusters! I loved the idea so much I went out and bought a canvas and started painting it the same day. I've been working on the various pieces involved, ordering magnets, and so on, and last night it finally all came together. Behold:

“Galaxy in Motion,” 40×30 cm(?) acrylic on canvas, with acrylic and glass beads on wooden disks and magnets.

I glued magnets to the back of each of the wooden disks (I actually used clear acrylic paint as the glue), and made a matching red wooden cube for each one with an attached magnet to stick on the back to hold the globular clusters in place.

Two globular clusters and associated magnetic handles.

I'm really, really pleased with how this idea came out, and I'd love to more things involving magnets and painting in the future—just need to think of some cool ideas now!

Finally, perhaps the coolest thing to happen this week is that we got permission to hang some of our artwork up in the stairwell in the building I work it at uni. (Funnily enough, it was originally the art building for the university, as shown in the letter designation it still keeps, ‘AR.’) Last week we blocked out where about half the works we had in mind would go, and Tuesday I came in to find someone in the process of hanging them up. This initial batch of 17 pieces only includes my star series from me, but I've got another piece that's nearly finished and Tenuous Transport ready to go up when we get everything together for the second wave.

Anyway, you've seen my star series before, but I just couldn't help taking another picture of them, finally hung the way I'd always envisioned, in a manner reminiscent of the main sequence on the Hertzprung-Russell diagram they were inspired by:

So those are some of the things I'm especially thankful for this Thanksgiving, among others. Hauʻoli Lā Hoʻomaikaʻi, (happy Thanksgiving) everyone! A hui hou!

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.

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.

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!

Globular Cluster Photo Series (Part 17): M3

Today I have a picture of one of the biggest and brightest globular clusters in the Northern Hemisphere, narrowly beaten out by the likes of M13. This globular cluster, Messier 3, is found in the northerly constellation of Canes Venatici, the Hunting Dogs, just below the handle of the Big Dipper asterism.

M3 is, as its name indicates, the 3rd object in Messier's popular list of bright non-comets, and as such it may have the distinction of being the object that prompted him to begin a systematic search for these objects instead of simply cataloging ones that he came across by chance.

M3 is a bright cluster that currently stands as the record-holder for largest number of variable stars – 274 – found in a globular cluster to date. It has an angular size on the sky of 18.0 arcminutes, a bit less than two-thirds of the width of the full Moon and ever-so-slightly smaller than M13's 20.0. It is actually a bit larger than M13 physically at 180 light-years across (10 more than M13),  but is also a bit further away from us at 33,900 light-years compared to M13's 25,000. Even this far away from us, further away than the center of our own galaxy, it still shines just bright enough to possibly be seen with the naked eye under pristine conditions at apparent magnitude 6.2. With any sort of magnification, of course, it looks quite nice. (For comparison, the distance to the supermassive black hole Sagittarius A* at the center of our galaxy is 25,900 \(\pm\) 1,400 light-years.)

Messier 3 in Canes Venatici. Click for larger picture.

M3 also beats out M13 in the number-of-stars department, having perhaps 500,000 compared to M13's 300,000 which puts it at about half the number of Omega Centauri, and in the top few percent for Milky Way globulars overall. This unusually large population is probably one reason so many variable stars have been discovered in it.

Currently M3 is cruising at about 40,000 light-years away from the galactic core, which is quite respectable when you consider that the Milky Way is only about 100,000 light-years across, and which puts M13 even further from the core than we are from M13. (You know you're an astronomer when you find yourself saying things like “only one-hundred-thousand light-years” – that's only about six-hundred-thousand trillion miles, for you curious.) Despite this distance, it probably has a better view of the core than we do as it is about 33,000 light years above the plane of the disk, while we have to look through all the dust and gas in the plane of the galaxy between us and the core.

Globular Cluster Photo Series (Part 16): NGC 3201

The globular cluster I have for you today is the first one I've imaged not found on Charles Messier's famous list. Instead, it bears the New General Catalogue (NGC) number 3201. The New General Catalogue was [and remains] one of the most comprehensive lists of deep-sky objects visible with typical amateur equipment ever, even though it was originally compiled in the 1880's. It contains a core group of 7,840 objects, and was later revised to include an additional 5,386.

Anyway, NGC 3201 is a lovely cluster far enough south in the constellation Vela the Sail that Messier wouldn't have been able to see it from his location in Paris. It's about 15,000 light-years away and perhaps 80 light-years across, which combines to give it the impressive visual size of 18.6 arcminutes, nearly two-thirds the width of the full Moon and twice as wide as M97 from yesterday.

In the northern hemisphere, most of the brightest objects are in the Messier catalog; thus, almost by definition, anything without a Messier number is not going to be as bright or as impressive as something with one (there are some exceptions both ways, but it's a general rule of thumb). In the southern hemisphere, this is not the case simply because Messier couldn't see down there.

This was well demonstrated the night I got the data for M79 which I showed in the previous post, and NGC 3201, which turned out to be the more impressive one. Here's the picture for comparison:

NGC 3201 in Vela.

M79 turned out to be another fairly small nondescript cluster (although its possible extra-galactic origins make it pretty cool in other ways). NGC 3201 was rather impressive by comparison. I was even able to make it out faintly by eye in one of our 14-inch telescopes, which was neat. Come to think of it, this is probably the southernmost globular cluster I've imaged to date.

Globular Cluster Photo Series (Part 15): M79

Today I have another Messier globular cluster to show you, Messier 79. This one is located just below Orion's feet in the constellation Lepus, the Hare. M79 is fairly small on the sky, just 8.7 arcminutes across (less than a third the width of the full Moon). This is mainly due to its large distance of about 42,000 light years from us, as the cluster itself is roughly average in size at about 118 light-years across.

Messier 79 in Lepus.

There are two very interesting things about Messier 79, and they may be related to each other. First, M79 is in an unusual position for a globular cluster: it's almost directly opposite from the galactic core in the sky. By far the majority of globular clusters are located somewhere in the same hemisphere as the core (and hence are visible during the summer); M79 is one of the very few that's not.

Secondly, M79 may not be native to the Milky Way. It is possible that it (along with three other smaller, fainter globular clusters) were originally satellites of another dwarf galaxy that is in the process of being absorbed into the Milky Way. A candidate galaxy was discovered back in 2003 and is known as the Canis Major Dwarf Galaxy based on the constellation in which it was first discovered in. I say "candidate" because it is still not entirely certain if this galaxy actually exists as an independent galaxy or is simply an over-dense part of the Milky Way. This is because it lies behind the galactic disk from our vantage point, making it impossible to see in visible light and difficult to analyze.

If the Canis Major Dwarf Galaxy does exist it would be the closest external galaxy to us, being only about 25,000 light-years away, and actually closer to us than to the Milky Way's core. It seems to correspond with a structure known as the Monoceros ring, an incredibly long stream of tidally disrupted stars that stretches over 200,000 light-years long and wraps around the Milky Way three times. If M79 was originally a satellite of the CMDG it would help to explain its strange position (although it's not impossible for a true Milky Way globular to have such a position; it's just rather unlikely).

Anyway, enough about this unusual globualr cluster; tune in tomorrow to see our very first non-Messier one!

Globular Cluster Photo Series (Part 14): M71

Today's picture is of the globular cluster Messier 71 in the tiny constellation Sagitta, the Arrow (not to be confused with the much larger and more familiar constellation Sagittarius, the Archer). M71 is an unusual globular cluster between 12-13,000 light-years away with a diameter of about 27 light years, fairly small for a globular cluster. It has a small apparent diameter of only 7.2 arcminutes (less than a third the width of the full Moon).

Messier 71 in Sagitta.

Sagitta is located in the plane of the Milky Way from our point of view, which explains the high stellar density in the background of this image. M71 was for a long time (up until the 1970's, in fact) thought to be a dense open cluster rather than what it actually is, a loose globular cluster. One reason was that the stars in M71 are younger than is typical for globular clusters, although that fact simply turned out to mean that M71 is a relatively young globular cluster. M71 also lacks a particular kind of variable star called RR Lyrae stars (after the prototype RR Lyrae) that are common in globular clusters, which turned out to be related to its age: its stars are too young to have become RR Lyrae-type variables yet. In fact, M71 contains only 8 known variable stars, though one of them is an interesting irregular variable.

This lack of RR Lyrae stars is one reason the distance to M71 is known to no better than a thousand light-years. RR Lyrae stars make good standard candles within our Galaxy, as the relation between their periods and their luminosities is well-known. They are also much more common than the other type of variable star commonly used as standard candles, Cepheid variables. RR Lyrae stars can be found at all angles in the sky (in contrast to Cepheids which is are strongly associated with the galactic plane), and consequently make up 90% of the variable stars found in globular clusters.

Anyway, that's it for tonight, I need to get some sleep. A hui hou!

Globular Cluster Photo Series (Part 13): M92

Today I have an image of the globular cluster Messier 92 for your viewing pleasure. M92 is one of the more spectacular globulars in the sky, but is unfortunately outshone by the slightly more spectacular M13 with which it shares the constellation Hercules. M92 is smaller than M13 at 109 light-years across compared to its 170, but is at roughly the same distance, about 26,700 light years away (M13 is about 25,000). These two factors combine to give it an apparent diameter on the sky of 14.0 arcminutes, a bit smaller than M13's 20.

Messier 92 in Hercules.

M92 is a nice looking globular cluster, still large enough on the sky to look interesting, and fairly concentrated. M92 boasts one of the few eclipsing binary systems known in globular clusters. It has another, more interesting claim to fame, however. The Earth's spin axis slowly precesses over time, taking about 26,000 years to describe a large circle on the sky. (Think of a top slowly wobbling in a circle as it spins. It's the same physical principle.) Precession is the reason that Thuban (a star in Draco) was the North Star for the ancient Egyptians rather than Polaris like it is today. Anyway, in about 14,000 years precession would point the Earth's axis less than a degree away from M92, leading to M92 being a sort of North Cluster. For comparison, M92 is about a fourth of a degree across, so you can see just how close that would be. Polaris itself is about a degree from the North Celestial Pole, which is small enough that it doesn't matter for everyday navigation aiding.

Pretty interesting, no? A hui hou!

Messier Globular Cluster Collage

How's that for a noun cluster? This picture is a collage of all the pictures of globular clusters I've taken so far, all nicely labeled.

My housemate Jonathan told me the title immediately made him think of ‘assorted candies.’

These are all at the original size as they appeared in the images I took, they haven't been scaled relative to each other (well, they're original size when you click on the image to see the full version). When you look at them like this it's easy to see why Omega Centauri is one of the best ones to see visually. For comparison, Omega Centauri is about the size of the full moon on the sky. They're arranged in no particular order. They're also not at the same quality level, and I might end up re-doing some of them such as M107 if I get the chance. I've submitted this picture for the slideshow at the annual Volunteer Appreciation Banquet for Mauna Kea volunteers tomorrow, which I'm really looking forward to!

Globular Cluster Photo Series (Part 12): M80

Today for your perusal I have an image of the globular cluster Messier 80. This is a very populous cluster with several hundred thousand stars and also one of the more densely populated ones, as all those stars are contained in a sphere only about 95 light-years across. Given M80's appreciable distance of about 32,600 light-years from us that translates to a somewhat smaller angular diameter of 10.0 arcminutes.

Messier 80 in Scorpius.

One curious incident in M80's history started around the time of the outbreak of the Civil War on May 21, 1860, when a nova (dubbed T Scorpii) was observed that briefly and spectacularly outshone the rest of the cluster (although it was still invisible to all but the most sensitive eyes with ideal dark sky conditions). Like all novae, this one was most likely the result of a white dwarf accreting mass from a larger binary companion star which eventually builds up to the point that it flash fuses extremely rapidly -- think an H-bomb with the mass of a small planet distributed across the surface of the star -- and blows away any gas that didn't fuse, leading to a massive brightening of the star. Novae in globular clusters are rather rare as a rule, with only a few others known. Interestingly, observations of M80 with the Hubble Space Telescope have only revealed two candidate binary systems for novae, a lot fewer than were expected based on models given the density of M80's core.

Globular Cluster Photo Series (Part 11): M5

Today for your consideration I have a picture of the globular cluster Messier 5 in Serpens Caput, the Head of the Snake. With a diameter of about 165 light-years this is one of the larger clusters of the Milky Way, and its gravitational pull dominates a humongous volume of over 34 and a half million cubic light-years!

Messier 5 in Serpens Caput.

M5's apparent diameter of 23.0 arcminutes is also on the larger end of the scale for Milky Way globulars. This is despite its great distance of about 24,500 light years from Earth. The number of stars it contains is unknown exactly; it is estimated to be above 100,000, and possibly as high as 500,000 which would put it above the more famous M13 in Hercules. 

Being as bright and large as it is, it is perhaps not too surprising that M5 was discovered by the astronomer Gottfried Kirch 62 years before Charles Messier would rediscover it 1764 and make it famous. It contains 105 variable stars, a very large number for a globular cluster. These are what help pin its distance down so exactly.

One interesting thing I noticed about this cluster in comparison to the last cluster I showcased, M56, is the relative dearth of foreground stars in this frame. That's because seeing M56 requires looking through the disk of the Milky Way, while M5 is found by looking outside of it. It really does make quite a big difference!