Globular clusters are, well, globe-shaped collections of stars. Millions of ‘em, usually, packed into a ball just a few light years across. They’re much smaller than galaxies, and in fact galaxies like the Milky Way have hundreds of them orbiting it, like bees swarming around a hive.
The stars in globulars are usually very old, indicating that studying these beasties can tell us a lot about how galaxies form. So scientists pointed Hubble at a cluster of galaxies — a gravitationally-bound collections of many, sometimes thousands, of galaxies — to see what globulars they could see. What they found was pretty cool.
The cluster they chose was the Virgo Cluster, the nearest such swarm of galaxies. It’s only 55 or so million light years away, which is practically in our face. That makes the globulars easier to see, and in their surveys the astronomers say they were able to distinguish something like 90% of the total numbers of globulars (from the ground, they’d look like stars) — they bagged over 11,000 globulars in the Virgo cluster, an incredible number (there are about 2000 galaxies in the cluster).
Of particular interest are the dwarf galaxies (like IC 3506, pictured above from the Hubble survey; nearly all the "stars" in that picture are actually globular clusters), dinky galaxies that have fewer stars than massive ones like the Milky Way. Since they’re smaller, they tend to have fewer globulars orbiting them, and are more sensitive to forces around them.
Astronomers found two interesting effects: dwarf galaxies within about 3 million light years of the center of the Virgo cluster had more globular clusters orbiting them than dwarf galaxies farther out, but dwarf galaxies closer in, less than 130,000 light years from Virgo’s center, tended to have few or no globulars at all!
This is being interpreted as meaning that globular clusters tend to form more easily in slightly denser environments such as those near (but not at) the center of galaxy clusters. However, if they venture too close to the core, they have their globulars stripped away.
M87 is a massive elliptical galaxy smack in the heart of Virgo. It probably has something like a trillion stars, five times the number the Milky Way has. Over billions of years it’s built up its mass, eating the stars from other galaxies as they fly by (it also eats entire galaxies). Apparently it’s done that with globulars from dwarf galaxies too, stripping off their globular clusters and keeping them for its own. That explains why no dwarf galaxies too close to M87 at the center of Virgo have globulars (and a chemical analysis of the globulars orbiting M87 indicate they are low in heavy elements like iron, suggesting they come from farther out in the cluster).
The image there is of M87. Take a close look; just about every single point of light you see there is a globular cluster! The beam coming from the center is a blast of particles and radiation being emitted from M87’s central supermassive black hole (which is a whole ‘nuther story unto itself).
Although the Milky Way is not really a part of any cluster (it is moving toward the Virgo Cluster but is not really a part of it) our environment is different, and the way we formed is probably different as well from galaxies in Virgo. But just like comparative anthropology, comparative galaxology (OK, I made that up, but you know what I mean) can tell us more about how we got here, and what’s happened since the Milky Way formed 10 billion years ago. It’s a big puzzle, but we have big pictures to help us figure it out.
August 5th, 2008 at 11:23 am
Globular clusters are, IMHO, the only deep sky abjects that look better through a telescope than imaged. Photographic images tend to burn out the centre of the image, but the eyeball (with a good telescope) can resolve stars right to the middle.
August 5th, 2008 at 11:29 am
I agree with Andy. IMHO using a 8″ backyard telescope, globular clusters are the best sights outside the solar system.
August 5th, 2008 at 11:30 am
My head sometimes hurts when I try to put the scale of things in the universe into perspective. “It’s only 55 or so million light years away, which is practically in our face”. Which is true, but, dang! A “trillion” stars in a galaxy?! Simply amazing stuff.
August 5th, 2008 at 11:32 am
Confused about the numbers here. The NASA release says they found 11k globulars in the cluster, but if you click through to the caption of the M87 picture, the ESA page says M87 alone has 13k globulars. How does that work?
August 5th, 2008 at 11:35 am
Xerxes, that’s an interesting point. M87 has at least 4000 GCs, but it’s not clear how many there are in total, but it might be as high as 13,000. I don’t think the exact number is known, which would explain the discrepancy.
August 5th, 2008 at 11:42 am
um, wow.
“comparative galaxology (OK, I made that up, but you know what I mean) ”
I thought the new official word was “evolution”.
August 5th, 2008 at 11:53 am
Hmm, I did some more research and I think this release is probably related to this impressive series of fifteen(!) papers: http://arxiv.org/find/all/1/ti:+EXACT+The_ACS_Virgo_Cluster_Survey/0/1/0/all/0/1
Paper number 4 confirms the number 13k GCs for M87, so I think that must be right. Probably the 11k number is just the number of GCs considered in this particular survey of GCs in the Virgo cluster. From looking at it, I think they took detailed images of just 100 representative galaxies. So if those are typical galaxies (and not the largest), then the actual number of GCs should be about 22 times higher or around 242k. That sounds a bit high, so probably the selected galaxies are larger than average.
August 5th, 2008 at 11:53 am
Does this cencus cast any light on the distribution of Dark Matter?
August 5th, 2008 at 11:59 am
How do stars not collide with one another in such a packed fashon? Amazing!
August 5th, 2008 at 12:02 pm
Very cool post Phil. I’m with Andy Beaton and NoAstronomer. Looking at globulars through my 8″ Dob is the most fun you can have outside the solar system (with your clothes on)
August 5th, 2008 at 12:18 pm
> How do stars not collide with one another in such a packed fashon? Amazing!
Sometimes they do. Try checking Wikipedia or Google for “blue stragglers”.
August 5th, 2008 at 12:31 pm
Wow!
August 5th, 2008 at 12:36 pm
WOW! It’s amazing to be sharing a universe with something so amazing! Thanks for showing me this.
August 5th, 2008 at 1:07 pm
Wait, wait, wait. I just set that M87 image as my wallpaper. You’re telling me that almost every speck of light in that image is a globular???
That’s mind-boggling.
August 5th, 2008 at 1:24 pm
Looking at the massive (and active) galaxy M-87, I am very curious if there is life there (including intelligent life) since the galaxy is so enormous. Of course the galaxy is so distant, we will most likely never know. Since it is an active galaxy, does that preclude the formation of life at all?
August 5th, 2008 at 1:43 pm
Re: BMcP’s comment – There is a notion that higher metallic content as found in population 1 stars would likely be necessary for planets (and life as we know it) to evolve. The stars within globular clusters are population 2 stars that have low metal content, perhaps making life less likely there. But this is a speculation and hardly a theory. If there is life deep within a globular cluster, they have a spectacular night sky!
August 5th, 2008 at 1:54 pm
Strangely the Milky Way just has about 150 GC’s, very few for a – well, considered – big galaxy. The Andromeda Galaxy, which is a little bit bigger than the MW, has many more GC’s (I don’t know the exact number, but I think it has about 1k or more) and it’s environment should be almost the same as ours. Maybe we just haven’t found them, but that would seem strange to me, because we see them on far distances and not in our backyard? (Or maybe that’s just the point. In Germany we say “Man sieht den Wald vor lauter Bäumen nicht”. I’m sorry not having a good translation – maybe someone can help me
).
August 5th, 2008 at 2:12 pm
Dr. Flimmer,
Babblefish says “Man sieht den Wald vor lauter Bäumen nicht” means “One does not see the wood before loud trees”. Not speaking German myself, I am going to assume that is indeed exactly what you meant to say.
August 5th, 2008 at 2:25 pm
Re: Chip
I was referring to the galaxy M-87 itself
Although I have also read what you stated, that globular clusters are made up of mostly older Population II stars, and thus “metal-poor” and unlikely to have rocky or icy worlds. Of course, you are right, there may be life in some of the globular clusters that may defy the odds, as we know them.
I wonder, has direct evidence of Population III stars ever been found?
August 5th, 2008 at 2:44 pm
I think “Man sieht den Wald vor lauter Bäumen nicht” is usually rendered in English as “You can’t see the forest for the trees.” In English, it’s usually meant to imply that if you focus too much on little details (trees), you can’t understand the greater structure (forest).
From my reading today, I’ve found that GCs are really very mysterious. I liked this article on the subject: http://arxiv.org/abs/astro-ph/0006167 Unfortunately, it’s now a bit dated at 8 years old.
This difference between the Milky Way’s GCS and Andromeda’s really emphasizes how strongly their properties are affected by the galaxy’s history. It seems that GCs can be formed during extreme events in a galaxy’s life: its original collapse, starbursts, mergers, etc. I wonder if Andromeda’s wealth of GCs might be related to the unusual double-core structure it has.
August 5th, 2008 at 3:07 pm
PsyberDave,
thank you! Indeed that’s what I meant.
August 5th, 2008 at 3:10 pm
How do we know the Milky Way is 10 billion years old? Is it radioisotopic analysis? Looking at the most metal-poor stars and determining their age? Probably a whole mess of other indicators as well.
August 5th, 2008 at 3:17 pm
Phil, edit-function is really NEEDED
Reading it again I have a feeling that Babblefish did a one-to-one translation – and did it wrong, maybe.
).
Assuming that “One does not see the wood before loud trees” does not make much sense (so it seems to me, which doesn’t need to count
I checked it myself with a “german translation site” (Leo) and it gives the english translation
“not to see the wood for the trees”, which seems to be better english to me and maybe makes more sense, doesn’t it?
And probably a translation is not given at all – we will see
August 5th, 2008 at 3:35 pm
“He can’t see the forest for the trees.” Well known expression in English.
August 5th, 2008 at 3:44 pm
BTW, “lauter” in this context doesn’t mean “louder”, but something more like “the sheer number of”. A more literal translation is “One does not see the forest for the sheer number of trees.” My earlier rendering is just the idiomatic English version.
August 5th, 2008 at 3:50 pm
Globular clusters are simply cool. With my 8″ telescope (I bought a second-hand Schmidt-Cassegrain about 20 years ago, still going strong) I’ve managed to bag about 2/3 of the Milky Way’s globulars. Being in the southern hemisphere helps.
August 5th, 2008 at 5:00 pm
I’m confused. I thought Virgo, like most constellations was only an apparent structure or grouping due to the location of Earth. Your post seems to suggest that Virgo is some real collection of Galaxies and such.
August 5th, 2008 at 5:18 pm
On the linguistic note, in Spanish we say: ‘Cuidado, porque puede ser que los árboles no te dejen ver el bosque’. Literally: ‘Watch out, the trees might not let you see the forest.’
Besides, it is interesting to point out that galactology would be ‘the science of milk’!
August 5th, 2008 at 9:17 pm
Your post seems to suggest that Virgo is some real collection of Galaxies and such.
Within the constellation of Virgo (an arbitrary chunk of sky) exists a supercluster of galaxies. Because the supercluster is mostly in the part of the sky we call Virgo, it gets called the Virgo Supercluster.
August 5th, 2008 at 9:28 pm
Humbling really. Little ol’ us out in the boonies somewhere with trillions of stars out there. Wish ET would give us a wave.
August 5th, 2008 at 9:30 pm
Because the supercluster is mostly in the part of the sky we call Virgo, it gets called the Virgo Supercluster. Cool. I don’t have to accuse the bad astronomer of astrology then.
‘Cuidado, porque puede ser que los árboles no te dejen ver el bosque’
Cuidado, porque puede ser que los árboles te maten al caer encima
August 5th, 2008 at 11:16 pm
A good article from Phil after a long time…!! Pls give more of these….
August 6th, 2008 at 12:03 am
Somehow I doubt the “134,000 light years” figure, as well. M87 has got to be that big all by itself. And 11K GCs is just five per galaxy, where you say our punkish Milky Way has hundreds all to itself. Xerxes did a good job investigating what you *might* have meant, but he shouldn’t have to.
Phil, I know you’re not getting paid for this, but couldn’t we have better numbers? Otherwise you might as well just post pictures and say “ooh, pretty” and “James Randi makes my heart thump ironically!!!”.
August 6th, 2008 at 1:41 am
Although the Milky Way is not really a part of any cluster (it is moving toward the Virgo Cluster but is not really a part of it)
I always thought we (the Milky Way) were part of the Local cluster, or the Local group, wich in turn was part of the local supercluster, or the Virgo supercluster.
How do you mean, “not really part of”?
Bear with me people, I’m less than a layman when it comes to astronomy, tho it’s been an interest of mine since I was a kid.
/P
August 6th, 2008 at 3:11 am
Not to nit pick, but in your post, you say: “they bagged over 11,000 globulars in the Virgo cluster, an incredible number”, but in the text under the M87 image you linked to, it says, “The monstrous elliptical galaxy M87 is the home of several trillion stars, a supermassive black hole, and family of 13,000 globular star clusters.
M87 is the dominant galaxy at the centre of the neighbouring Virgo Cluster of galaxies, which contains some 2,000 galaxies.”
So, I’m seeing a discrepancy between the two descriptions. In yours, it’s 11,000 GCs in a cluster of 2,000 galaxies, and in the space telescope page, it says 13,000 GCs in M87 alone, which, if true, is mind-blowing. Could you clarify?
Incidentally, I saw my first cluster when I was researching for a play that I worked on which had an astronomer as its main character. Unfortunately, the playwright was not an astronomer, so the science was a bit off. But the cluster was amazing beyond words.
August 6th, 2008 at 6:12 am
@ Phil
Thank you! More stunning photos and text.
The Virgo Cluster is too big for me to comprehend. And, the universe is bigger.
Um, with all that energy spewing out of its center, wouldn’t that preclude life as we know it, on any planets with the stars of M87? That flare has to be awesome, to look that big from this far away. Really mind boggling.
August 6th, 2008 at 7:08 am
Don Snow,
.
a planet being right in the way of the jet will not be able to host life, that’s clear. Every form of life as we know it would be vaporised due to the high-energy particles and radiation.
But in the suburbs of the galaxy I think it’s not impossible to find a planet with some forms of life.
But you’re right. Sitting on the night-side of a planet in M87 and watching this incredibly hugh stream of light (I guess it’s much brighter than the stream of the Milky Way, which I cannot see at all in the glow of the cities) must really be a sight to remember
August 6th, 2008 at 7:14 am
The stream coming out of M87’s black hole. How does it escape the
tremendous gravity of a black hole?
August 6th, 2008 at 7:25 am
Phil – perhaps you might know anything related to this off-hand, but I was wondering if there was any work that related the glob pop to a galaxy’s halo or bulge pop (and therefore, with the established correlation between bulge size and masses of the galaxy’s black holes). It seems to me there might be good reason to extend the correlation of black hole mass and bulge size and brightness beyond those bulge parameters to glob population and the halo as a whole. In other words, linking a galaxy’s glob population to it’s supermassive black hole mass. Some very preliminary and rough calculations seem to suggest that there is. What do you think?
August 6th, 2008 at 7:30 am
Edward Says: “The stream coming out of M87’s black hole. How does it escape the tremendous gravity of a black hole?”
Simple. That material was never inside the the “tremendous gravity” represented by the black hole’s event horizon: that ionized gas in the jet comes from the extremely energized accretion disk immediately surrounding the black hole.
August 6th, 2008 at 7:42 am
“Man sieht den Wald vor lauter Bäumen nicht”.
Basically, word for word, in order, it translates, “Man sees the forest through lots of trees not.”
August 6th, 2008 at 8:37 am
@Edward:
I don’t think it’s matter that’s escaping the black hole, but energy, some type of radiation. Read something about “massless particles” can and do escape it, which in turn causes the black hole to evaporate over time. Mass turning into energy that escapes the black hole, decreasing the mass etc.
Hawking Radiation was it? someone correct me if I’m wrong.
/P
August 6th, 2008 at 8:51 am
Edward,
the stream is called a “jet”. It’s a thin tube of a magnetic filed. Through this tube rush charged particles (electrons, positrons, protons, etc) at nearly the speed of light. How this particles are accelerated is still a mystery. It’s likely that magnetic field lines of the accreation disk (the disk of particels surrounding the black hole before they fall in) are connected in some way with the jet-field lines and stripe away some particels. But they get that fast remains unknwon.
August 6th, 2008 at 11:04 am
So globular clusters of stars are like little tiny galaxies, which are part of other galaxies, which are part of galaxy groups, which are part of galaxy clusters.
My head hurts.
August 6th, 2008 at 11:52 am
Pouria,
Hawking radiation is when a pair of “virtual particles” are created very close to the event horizon. What are “virtual particles”? The vacuum of space is not “empty” or a place of “no” energy. There is a vacuum-energy. And due to “E=mc^2″ a particle and it’s anti-particle can be created “out of nothing”. They annihilate each other again, so nothing has really happened. That’s why they are called “virtual”.
If they are created very close to the event horizon of a black hole one of the particles can fall in before they annihilate and the other is left behind and becomes real. But, the two particles have “stolen” their energy from the vacuum-energy of space which would be against the law of energy-conservation. So the energy is taken from the black hole which ate one particle but it must give back the “mass-energy” of TWO particles, so it looses ernergy and so mass.
But: The particles in the jet are not massless. You can have electrons, protons, positrons, etc in such a jet with incredibly high speeds – as I said before, they are very closed to the speed of light. The particles do NOT come OUT of the black hole but from it’s accreation disk. But how they are accelerated to such speeds is still a mystery, as I also said before.
August 6th, 2008 at 5:28 pm
Wait. Im sorry…black holes evaporate? does anyone have proof to substantiate this? (and just because Steven Hawking says doesnt count) talk about woo.
August 6th, 2008 at 8:24 pm
Me too.
So, to the experts – what is wrong with that picture?
Um, we had that question a few threads back. It comes out of GR research in the 70’s.
I’m not conversant with GR. But AFAIU there are two ways that black holes may loose energy on, and so evaporate.
The first way is to extract mass-energy from excited black holes. (Black holes are dynamic objects with mass, momentum, angular momentum and charge.) Let me cite the old GR bible (”Gravitation”, Misner, Thorne, Wheeler; also called “MTW” for short):
“A Kerr-Newman black hole – which is rotating or charged or both – is not dead. The rotational and electromagnetic contributions to the mass-energy can be extracted.”
So first you have the gravitational radiative dynamics of a freshly collapsing black hole. Then it starts to stabilize it can continue to leak off energy as a Kerr-Newman black hole by being perturbed by the environment, say an accretion disc, until it is a “dead” Schwarzschild (non-rotating, non-charged) black hole.
The second way is through Hawking radiation, which Hawking established as a possibility by physics, not by “say so” obviously. I assume string (brane) models can give the same results. And finally I assume that the final evaporation of black holes are not well understood. (That, I believe, is one of the uncertainties of LHC detection of the same.)
I believe none of these ways are experimentally proven by direct observation. But there are ongoing attempts to use optical systems to simulate the event horizon of black holes to possibly observe analogue processes.
August 27th, 2008 at 3:51 am
Phil – nice summary of the science! I know this is a bit late, but I just found this thread. Regarding the question of why the survey found 11,000 globular clusters in the entire galaxy cluster whereas M87 alone has 13,000, the answer has to do with the number of galaxies in the survey and the field of view of Hubble.
We actually only looked at 100 of the over 2,000 galaxies in Virgo. Also, the field of view of the camera we were using (the Advanced Camera for Surveys) is sufficiently small that for large galaxies like M87, we only measure a fraction of the total population of globulars. We know from other observations and that M87 has ~13,000, but we only directly observed around 1,000 with HST.
Keep up the good work!
December 16th, 2008 at 7:31 pm
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