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
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
I have a tale of death, near death, and undeath to weave for you, but first, gaze upon the jewel-like beauty of the glittering denizens of M30:
This image was taken by the Advanced Camera for Surveys on board the Hubble Space Telescope. I had to cut it and compress it drastically to get it to fit on the blog, so you very much want to click on it to embiggen it massively and see it in its fully resolved glory.
The image is of the insanely beautiful globular cluster M30, an ancient city of a few hundred thousand stars located 28,000 light years away in the constellation of Capricornus. The cluster is ancient, about 13 billion years old, making it as old or even older than the Milky Way itself. The core of the cluster is unusually dense as such things go, which is why it was studied. Where better to find vampires and thrillseekers?
Like people, stars are born, age, and die. Stars born with more mass tend to die off more quickly, consuming their fuel at far higher rates than their lower-mass brethren. These stars tend to be blue, so in an old cluster like M30 you’d expect to see no blue stars at all; they should all be long gone. And yet, there are quite a few — astronomers call them blue stragglers. Where did they come from?
One theory, which has been borne out by observations, is that blue stragglers are in tight binary systems, with a dead star in such close proximity to a normal star that it can siphon off the normal star’s gas, using it to rejuvenate itself. This would make them vampires, of course, sucking the life force of other stars in an attempt to stay young.
But another idea was that dead stars might also physically collide with other stars and merge, forming a single star that would burn blue and bright. In an environment like that near our own Sun this kind of collision can almost literally never happen; even considering the entire Milky Way Galaxy over its entire lifetime a head on collision has probably never happened out here in the stellar suburbs.
But that cluster M30 is pretty densely populated with stars, and collisions are far more likely. What observations like this one of the cluster (and also of an ancient cluster called NGC 188) have shown is that the blue stragglers appear to have two different sub-groups; one that appears to have come from the vampire stars, and another from stars that have collided: thrill-seekers, stars that have physically slammed into each other and merged, their combined mass separating them from the other blue straggler group.
Blue stragglers have been known since the 1950s, and the idea that they were pulling gas off nearby stars was proposed to explain them, too, but it’s only with our modern instrumentation that we can not only show that this is true, but that a second, far-fetched-sounding scenario of collisions also contributes.
I find it wonderful and extremely uplifting that an image as spectacular and gorgeous as the one above — it became my desktop wallpaper as soon as I saw it! — not only satisfies our desire for beauty and art, but can also be tapped to deliver incredible science that boosts our awe of what Nature can do. I love that we can understand such things, but you know what I love even more? The idea that we have only begun to understand the Universe.