Tag: globular cluster


By Phil Plait | July 27, 2012 10:00 am

Globular clusters are some of the most stunning objects in the sky. Composed of hundreds of thousands of stars, over 150 of these compact beehives orbit our Milky Way galaxy alone. Some are close enough that even through a small telescope they reveal a breathtaking beauty, individual stars sparsely distributed in their outskirts becoming more cramped and crowded until they blur into a generalized smear in the middle.

When you use a bigger telescope to look at them, you get wondrous beauty:

[Click to massively apiaryenate, and you really, really want to.]

This picture is from Adam Block, using the 0.8 meter Schulman Telescope on Mt. Lemmon in Arizona, and shows M5, one of my all-time favorite globulars (I posted a Hubble image of this cluster a while back, too). It’s located in the constellation of Serpens, visible over the entire populated region of Earth at this time of year. I spent many nights finding this cluster both when I was younger and also when I was in graduate school and teaching a lab class in observational astronomy. It’s a piece of cake to find since it’s bright and big. In fact from very dark skies it’s just visible to the naked eye, one of the few that are.

This picture from Adam reveals it in its glory. It’s roughly 150 – 200 light years across and 25,000 light years away, and you really get a sense of its hundred thousand stars (or more). Globular clusters like this are very old – 12 billion years or so. Any star in M5 with a mass of more than the Sun would have died long ago, turning into a red giant, blowing away its outer layers, and becoming a faint, hot, white dwarf. Even some lower mass stars have become red giants – you can pick them out pretty easily in the picture – and only stars with significantly lower mass are still chugging along, merrily fusing hydrogen into helium in their cores like the Sun does.

The cluster also contains over a hundred "blue straggler" stars – stars which are surprisingly blue given their age. These were a mystery for decades, but it’s now understood that they’re the result of binary stars, where one star is feeding off the material from another, and also from actual stellar collisions, where two stars physically collide and merge! Collisions like that are so rare they’re non-existent in the volume of space our Sun occupies, but globular clusters have a lot of stars tightly packed into a small region. Collisions are more common, and can result in a star that looks much younger than it really is.

There are so many reasons to love a good globular. I enjoy their relative simplicity – they’re only made of stars! – and structural symmetry, and how they can be observed with everything from your eyeballs to the most sophisticated telescopes on or above the Earth. And if you read about them, their physical history, composition, and orbits, you’ll find there is a vast amount to know about them, and a vast amount yet to learn. To my scientifically curious mind, that makes them just about the perfect object for study.

And, of course, they are surpassingly beautiful. That never hurts.

Image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

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CATEGORIZED UNDER: Astronomy, Pretty pictures

Ho hum, another boring gorgeous sparkling beehive of stars

By Phil Plait | May 10, 2012 6:45 am

Orbiting our galaxy in the lonely depths of intergalactic space, 160+ globular clusters are among the oldest structures we know. They’re composed of thousands, sometimes hundreds of thousands, of stars, all held together by their mutual gravity. I always think of them as beehives, with the stars buzzing around on orbits all tilted willy-nilly.

The European Southern Observatory just released this picture of the globular cluster M55 using the VISTA telescope., and it’s very pretty:

[Click to englobulenate, or grab the 6Mb 3k x 3k pixel version!]

Honestly, there’s not a lot of science I can add to this that I haven’t written about a bazillion times before (see Related Posts, below). M55 is 17,000 light years away toward the galactic center, which is relatively close as these things go. It’s big, 100 light years across, so from Earth it looks to be roughly 2/3 the size of the Moon. In this unusual picture by VISTA, we’re seeing it in infrared — at 1 micron (colored blue in the picture) and 1.5 microns (colored red), so stars that look red are really much cooler than the Sun.

But other than that, it’s just another run-of-the-mill globular. Which is remarkable enough! And you know what: despite their clunky name, there’s no such thing as an ugly globular cluster. That’s reason enough to share this lovely picture.

Image credit: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit

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CATEGORIZED UNDER: Astronomy, Pretty pictures

Desktop Project Part 12: The galactic deep diver M9

By Phil Plait | April 6, 2012 9:01 am

[The Desktop Project is my attempt to clear off all the amazing images I’ve been collecting on my computer’s desktop over the past few weeks – I’ll post one great picture every day until they’re gone. That way, I clear off my computer and you get to drink in a beautiful image and some science. Everyone wins!]

I love globular clusters. They’re amazing objects: hundreds of thousands of stars all orbiting each other like bees around a beehive, incredibly old (most are well over 10 billion years and counting), and are a laboratory of how stars age and die. And I used to love looking at them through my own telescope when I was a kid, avidly going from one to another.

M9, though, isn’t one that I remember observing. I’m not sure why. It’s easily bright enough to see, and big enough to get a sense of its globularness. But even if I had seen it, I don’t think it would’ve looked quite as amazing as this shot from Hubble:

[Click to hugely apiaryenate.]

Yegads. Funny though: all in all, as globulars go, M9 is rather unremarkable. It’s average in size, in distance (about 25,000 light years from Earth), and in most other ways. It does have the distinction of being the globular cluster nearest the center of the Milky Way, lying less than 6000 light years from the core. But that’s a bit of a cosmic coincidence: globulars orbit the Milky Way, so whichever one is closest is just a matter of orbital characteristics and time.

Still, hmmm. I imagine M9’s orbit must be pretty elliptical to bring it that close in, though. Some globulars orbit very far out on paths that never take them too close to the center. So perhaps in this way it is interesting. Why is its orbit shaped like that? Did it interact with some other object billions of years ago, changing a circular orbit into one that’s elongated? Its velocity through space is relatively high (PDF) as you might expect from an object at the bottom of its orbit– a position called perigalacticon, if you want to impress your friends at parties.

And yet, for all its unremarkableness, look at it! It’s gorgeous! Even the mundane in astronomy possess surpassing beauty. It’s another reason I love this science so much.

Image credit: NASA & ESA

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Desktop Project Part 3: The massive massiveness of M54

By Phil Plait | March 28, 2012 7:00 am

[Over the past few weeks, I’ve collected a metric ton of cool pictures to post, but somehow have never gotten around to actually posting them. Sometimes I was too busy, sometimes too lazy, sometimes they just fell by the wayside… but I decided my computer’s desktop was getting cluttered, and I’ll never clean it up without some sort of incentive. I’ve therefore made a pact with myself to post one of the pictures with an abbreviated description every day until they’re gone, thus cleaning up my desktop, showing you neat and/or beautiful pictures, and making me feel better about my work habits. Enjoy.]

I post a lot of pictures of globular clusters, because they’re just so darn pretty. They’re roughly spherical collections of thousands of stars, all born at the same time, and bound together by their own gravity. But M54 is special, for more than one reason… but mostly because in this Hubble image it’s jaw-droppingly gorgeous:

See? [Click to embiggen, or grab the huge 15Mb version.]

It’s the second most massive globular known (the most massive is Omega Centauri), with well over a million times the mass of the Sun. That’s a lot. It’s very far away, too, at a distance of about 90,000 light years, making it one of the more distant globulars known. It’s big — 300 light years across, and also incredibly luminous, shining with over 800,000 times the Sun’s light. Only its vast distance dims that glory to us.

Also among its oddities is that it harbors what’s called an intermediate mass black hole, one that’s far larger than the kind you get when a star explodes (which have usually up to a few dozen times the Sun’s mass), but still far less than the huge ones found in the centers of galaxies like ours (which are millions or billions of times the Sun’s mass). M54’s black hole has something like 9400 times the Sun’s mass, which is still substantial! These kinds of black holes were only discovered a few years ago for the first time, and how they form is still something of a mystery.

But I haven’t even mentioned the weirdest thing: M54 may be part of another galaxy! It actually sits right at the center of a dwarf galaxy called the Sagittarius Dwarf Elliptical Galaxy, a small galaxy currently being torn apart and assimilated by the Milky Way. That’s right: we’re eating it. It’s likely that it formed in that galaxy and has been co-opted by the Milky Way. Since M 54 was discovered by Charles Messier in 1778, this makes it the first extragalactic globular cluster ever seen!

Image credit: ESA/Hubble & NASA

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Top 24 Deep Space Pictures of 2011

By Phil Plait | December 14, 2011 5:30 am

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

The green ghost of a distant dead star

By Phil Plait | November 22, 2011 11:00 am

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…
Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures

Gorgeous globular hides hundreds of rejuvenated stars

By Phil Plait | October 3, 2011 11:30 am

I will never, ever get tired of insanely gorgeous images of globular clusters.

Holy. Haleakala. [Click to embiggen, or get the ridiculously huge 3900 x 4000 pixel version.]

That is Hubble’s view of M 53, a cluster of several hundred thousand stars crammed into ball about 60,000 light years away — well outside the Milky Way itself, but bound to it, orbiting our galaxy. It’s probably 12 billion years old, but it looks like some of the stars in it have opted for a little cosmetic surgery…

In our galaxy, stars are so far apart that collisions between two of them almost never happen. But in globular clusters stars are so closely packed that many of them have apparently literally collided with each other, merging into objects called blue stragglers. Globulars are old, so having blue, massive stars is weird; they have short lifespans, and should’ve all blown up as supernovae or at least turned into red giants billions of years ago.

When these objects were first discovered in globulars they were really surprising, and while we still don’t understand everything about them, it’s a fair bet they result from two stars having a very, very close encounter. If two older, low mass red stars pass close to each other at low speed, their gravity can cause them to become bound to each other (it helps if a third star is involved; it can steal away energy from the other two, making it easier for them to become stuck together). Over time, they can spiral together and merge, forming a single, more massive, hotter object: a blue straggler. They’re seen in many globular clusters, and tend to be more common where stars are thickest, as you’d expect.

Over 200 of them have been found in M 53 alone, and at first glance, if you didn’t know better, you’d think they were far younger than the ancient stars around them. In a way, I suppose, they are.

But don’t judge. If you were a 12 billion year old star, you might want a facelift, too.

Image credit: ESA/Hubble & NASA

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A cluster's masquerade

By Phil Plait | September 8, 2011 6:40 am

When I first saw this picture of NGC 2100, I thought it was a globular cluster. But I was wrong. That happens sometimes. Still, it’s worth it to see such a magnificent photo:

Yegads! What a shot! [Click to enstellarnate.]

Globular clusters are tightly packed collections of thousands of stars in a roughly spherical shape (hence their name), and are generally very old. But upon second glance, the stars of NGC 2100 in this image didn’t look quite right to me. There didn’t appear to be enough, for one thing, and though they’re highly concentrated in the center, the distribution around the core seemed off somehow.

Turns out that’s correct. Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures

The new VLT Survey Telescope delivers spectacular images

By Phil Plait | June 8, 2011 7:32 am

[Update: I originally had called this the Very Large Survey Telescope, but have learned it’s actually the VLT (for Very Large Telescope) Survey Telescope. I’ve corrected this in the title and below. I like my less-redundant name for it better, but it’s best to be accurate.]

The European Southern Observatory is an agency that governs some of the best telescopes on the planet, and they just added a new eye on the sky: the VLTe Survey Telescope (VST), a 2.6 meter ‘scope in Chile. There are lots of telescopes of similar size dotting our planet, but what makes this one special is its huge field of view — a solid one degree across, twice the diameter of the Moon on the sky – and the resolution of the camera: a terrifying 268 megapixels!

When you put that together, you get some dazzling pictures, like this one of the globular cluster Omega Centauri:

[Click to englobulenate to a 4000 x 4000 pixel 13 MB image, or grab yourself the internet-choking 14,540 x 14,540 pixel 280 MB version.]

Omega Cen is one of the largest globular clusters of the 150 or so orbiting the Milky Way galaxy, a collection of hundreds of thousands or even millions of stars all orbiting the cluster center willy-nilly like bees swarming around a hive. Telescopes like the VST will allow astronomers to survey these clusters quickly and deeply, which is important because it’s sometimes difficult to know what stars are in the cluster and which happen to be in the background or foreground. You have to get a good census of cluster membership before moving on to studying how old the stars are, what they’re made of, and how they behave. Since globulars are among the oldest objects in the Universe and are tied with galaxy formation, understanding them leads to understanding a great deal more.

VST also took this spectacular picture of the star-forming region M17, also known as the Omega nebula:

Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures

Spectacular and sparkling, but what is it?

By Phil Plait | May 23, 2011 7:19 am

Globular clusters are among the most spectacular of objects in the entire night sky. Compact balls of hundreds of thousands of stars, well over a hundred orbit our galaxy at various distances. When viewed by Hubble, the result is nothing less than jaw-dropping:

[Click to embiggen, and please do; I had to crop the image to get it to fit and the full-size version is even more spectacular!]

This view of Terzan 5, as it’s called, is gorgeous! The thing is… Terzan 5 may not really be a globular cluster. Sure, it’s a cluster, and it’s globular, but it may not be what we usually think of as a globular cluster.

When I read the press release for the picture, the name Terzan 5 looked familiar. So I searched my blog, and found I’ve written about this object before. That post was about a ground-based Very Large Telescope picture of the cluster, seen here. The picture looks odd because Terzan 5 lies in a very crowded region of the Milky Way, lousy with dust. That interstellar junk tends to scatter away blue light, making objects look redder. The dust blankets across Terzan 5, but is thicker in one half than the other, making that side redder than the other.

Terzan 5 itself is also unusually dense, with stars packed in it more tightly than is usual for a globular cluster. Not only that, but studies have shown the stars in the cluster appear to fall into two different age groups; one significantly older than the other. That’s weird. Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures

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