Globular clusters

[Outbackmanyep]

Hey guys,
I have read many times that globular clusters are collections of very old stars, but what keeps them together and how did they form? Or is that question still an open one??

Cheers!

[Paddy]

I read one article recently which suggested that they may be old dwarf galaxies captured by the Milky Way (or other large galaxy as appropriate) and stripped of their outlying stars to leave only a core of stars closely bound by their own gravity. I imagine that their are quite a few other theories.

[xelasnave]

Their orbits suggest they have been gobbled up.:eyepop:..

I think they must be the cores of galaxies left after the host galaxy has stripped the outer stars... they may have been spirals but have now lost their arms..

I dont know if this is right but I think spiral galaxies travel thru space oriented like a pie being thrown in ones face.:).. so their direction of travel sees them meeting other galaxies the same way..the smaller galaxy passes thru and starts to orbit loosing its outer stars..

the orbit continues but only the core is left ..

and maybe as time passes the globular moves more to the orbit of the galactic plan where the core of the smaller galaxy is finally consumed..or rather its old structure disappears as its stars get scattered in the galactic plan of the host galaxy...why do they stay in a bunch.. gravity is acting upon them... how ....I dare not say.

So if old galaxies could it be that each hide a black hole??? is this where the galaxy gets its extra mass.. who knows what does a globular weight?

alex:):):)

[Outbackmanyep]

You would think that if it was the core of a consumed galaxy then the stars would have scattered long ago?...OR I wonder if the individual stars in a globular orbit a common centre, like a galaxy, or wether they are stationarily bound like a crystals lattice?.....very interesting!!

Thanks Paddy and Alex!

[ngcles]

Hi All,

As _I understand_ it, may be a few of the GCs that are indeed remnant cores from small galaxies gobbled up by the Milky Way (Omega Centauri NGC 5139 might well fall into this category), but the vast majority are not.

The GCs are all uniformly ancient stars -- among the very first stars born within the primordial Milky Way. Way, way back in the mists of time, when it was collapsing from its roughly spherical gas cloud, but before most of the material had collapsed to the disc, the first stars were born into vast, vast clusters -- in some cases more than a million stars. All the high-mass stars in these clusters evolved quickly and exploded as supernovae leaving behind only the low-mass stars we see today -- about 10gyr on. The stars of the GCs are different from the general population of stars we see today; they are in effect fossils. Because they came out of the material of the big-bang they contain virtually no metals (ie material heavier than 2He). Because they were not born of the disc, the globulars do not inhabit the disc but are arranged in a more-or-less spherical halo around the core.

Gravity (the same force that sticks us to the ground) (Alex excepted ;)) is what holds 'em together. With those clusters that pass through or close to our core, they do undergo stripping and loose stars that usually end-up populating the halo. Two examples of clusters that have either undergone or are undergoing stripping are Pal 5 and E3 -- the latter is likely our galaxy's least massive and least luminous GC. No doubt over time, some of the Milky Way GCs have been completely evaporated in this way.

The Milky Way's original retinue of clusters have likely been added to significantly by the Milky Way gobbling up little galaxies and stealing their clusters. For example, the Sagittarius Dwarf Elliptical that is currently undergoing dismemberment is likely in the process to be donating several more GCs -- from memory, these include M54 (which _might_ be the core of that galaxy) Rup 106, Pal 12, Arp GC 2 and maybe a couple of others. These "donated" GCs can be inferred from their distances, motions and (slightly different) metalicity.

Similarly, the recently discovered (still disputed though) Canis Major Dwarf _may_ have donated M79, NGC 1851 and NGC 2298 that are all, suspiciously enough, located at the roughly the same distance from the core of the Milky Way a long, long way out from the core (where you probably wouldn't expect to find 1, let alone 3) and quite near (relatively) each other. There is no other evidence at this stage to support that hypothesis. Richard Lane who works at Sydney Observatory is currently doing a lot of research on the alleged Canis Major Dwarf in order to prove/disprove its existence.

The reason why Omega seems different from the "other" GCs, lies in the fact that its stars have low metalliciity (like a GC) but, there are three slightly but distinctly different bands of metallicity within its population inferring that some of its stars were born at different times -- which would be highly unusual for a true GC.

Best,

Les D
Conntributing Editor
AS&T

[Paddy]

Hmmmmm - very interesting!

[Zuts]

Great post Les,

Thanks for that

Paul

[OneOfOne]

I must admit, I have asked myself the exact same question on a number of ocassions. I have never really read much in books or magazines either, just that they are low metalicity and very old (population II, never worked out why they call the older ones population II and the "galactic" one population I).

Thanks for the thoughts Les, makes a lot of sense.

[bojan]

It is just a convention, well they have to call them somehow :)
There is also population III, theory says some of them should still be around but none of those are found yet.. They are stars that formed first after BB, from H and traces of He, no metals at all...

[Paddy]

"Because they were not born of the disc, the globulars do not inhabit the disc but are arranged in a more-or-less spherical halo around the core."

I have one question arising from this. If the Milky Way's GCs are so old and were formed with the galaxy itself, what has prevented the gravity of the disc pulling them into the disc?

[bojan]

Probably the fact that they were further away :shrug:, They are also in orbits, so they can not be pulled towards the centre.
Also,the fact that disk stars are younger means that disk collapsed more recently, for the stars to start their own formation.

[xelasnave]

Does anyone have a link to an artist impression showing their orbit in relation to the disk in general terms ?
alex

[coldspace]

Hi Alex and friends,

Found this link explaning some new theories on some Gobular clusters.
http://video.search.yahoo.com/video/play?vid=725204&vw=g&b=0&pos=1&p=globular+clusters&fr=yfp-t-501

Regards Matt.

[xelasnave]

Thanks Matt I am going out now but I am sure looking forward to this when I get back.
alex

[erick]

There's even more to like about NGC 2808 now! :)

[xelasnave]

I cant get the dam thing ..I rushed home from Makas forgoing a second cupof coffee...
But this is different eh...so I wonder what we can read into this???
There were stars with metal but as they are so old so alien race has removed all the metal no doubt...just kidding for the folk who wonder what I really think about stuff....
alex

[Outbackmanyep]

So if GC's are metal poor, and they were formed around the same time the milky way formed, Whats the life expectancy of metal poor stars? What is the typical spectral class of the individual stars?

So the more massive stars end up going supernova, billions of years on there are no traces of the supernovae, so how fast do the clusters move about??
Fascinating!

Les, i think you should make a note to put a story in to AS&T, i'd like to know more!!!

Cheers!

[xelasnave]

I think the view of how galaxies form etc could do with alternatives... it makes little sense that these things have been there since year dot...
metal poor or not we still have the prospect that after all these years they are still in orbit...orbits are temporary .. why should they orbit from the start and still be going strong .. does not add up to me... I feel they must be recent acquisitions of out galaxy ... also I dont like the big bang approach to the explanation of galaxy formation really... why should everything collapse into the disk and these things continue their merry path.. for no other reason that to fit the premise of a cloud of gas collapse which is dictated by the time frame allowed by the big bang really...

We need an active debate here .. we can review the knowledge as good as anyone and maybe contribute to human understanding... so have a go folks... come up with an explanation for the observations... you observe so what do you take from your observations... can you provide an input that will take humans further in their quest to understand all there is to understand

alex

[ngcles]

Hi Outback & All,

I'll make a note of that mate, but I'm no astrophysist and don't pretend that I am one -- I'm just an amateur with a slightly-better-than-most knowledge of such things -- mainly because I work at Sydney Observatory and need to know it to answer questions from the public and also many pesky HSC/Yr 12 physics students.

As for life expectancy of metal-poor stars, I think it would make hardly (if any) difference, except perhaps for the high-mass stars.

Low-mass stars make their energy via the proton-proton reaction -- a quite slow reaction chain. A succinct explanation is set out here:

http://en.wikipedia.org/wiki/Proton-proton_chain

Metals play no part as either fuel or catalyst.

In high-mass stars the dominant reaction is via the CNO cycle -- a much faster and more vigorous reaction chain. A succinct explanation is here:

http://en.wikipedia.org/wiki/CNO_cycle

Where the "metals" play a significant role as a catalyst but are conserved.

The fabled Population III stars that are said to have existed in the early Universe were _extremely_ massive stars that were _extremely_ metal poor because all the baryonic matter in the Universe at that time was basically 1H, 2He and _minute traces_ of 3Li and 4Be.

So how did they "burn" without metals? It is thought via proton-proton but because of this they could be much, much larger than today's most massive stars without tearing themselves apart. Maybe up to 250 solar-masses they were. It was this initial generation of super-stars that seeded the very first metals into the interstellar medium when they exploded as supernovae. From this (so the theory goes), came the Population II stars that are (still) very metal poor and are found in the Hub and Halos of spirals and in the GCs.

These so called Population III stars (I still reckon they should have called them "Population 0" stars) have not been observed directly because they only existed for a brief time in the very earliest epoch of the Universe. If we are going to find them (or at least their spectra), in all probability we have to look at light from the very first generation of stars that is so distant it will probably only be found in the gravitational lenses.

As for the "typical" spectrum of the stars in a GC, it is quite late -- late F or G generally. The Colour-Magnitude (or H-R) diagram for a G.C looks distinctly different to an open or galactic cluster. This is not due to metallicity but because of age. They have very short main-sequences from M upward to a turn-off point to red-giant-hood quite low at about the G mark. There will also be pronounced Red-giant branch where there are a large number of individual stars and most importantly a well-populated AGB (horizontal branch) where the RR Lyrae stars live.

By contrast, even for a large/massive open cluster, you would expect a long main sequence right on up to B or even O type stars, and then perhaps the odd red-giant and nothing at all on the horizontal branch.

Apart from metallicity, this C.M diagram is how a true globular can be easily distinguished from an open cluster -- all true G.Cs are 10-12 Gyr old and have CM diagrams of this sort with _well populated_ red-giant _and_ horizontal branches.

As a matter of interest, there are quite a few "GCs" that are commonly catalogued or mapped as part of the LMC and SMC, but many of them are not "true" globulars. They may be globular in size (ie >10^5 stars, but they are not globular in "population" -- ie extremely old, metal-poor stars. Indeed the only "true" GC in the SMC is NGC 121 -- all the others marked as such in many star maps are either very populous or very old open clusters (mostly both). The LMC has about 7 "true" GCs (from memory).

Paddy wrote:

"I have one question arising from this. If the Milky Way's GCs are so old and were formed with the galaxy itself, what has prevented the gravity of the disc pulling them into the disc?"

The simple answer is "the conservation of angular momentum". Plus, gas is "sticky", stars (or balls of them) aren't.

There is a Wikipedia page on GCs that by and large also looks pretty good (accurate and brief) and sets out a lot of this stuff (and other interesting facts).

It is here:

http://en.wikipedia.org/wiki/Globular_cluster

Interesting ain't they!


Best,

Les D

[xelasnave]

I feel we are very privelged Les to have your input ...just think how many folk are now better informed... great stuff..we now know what you know
alex

[Outbackmanyep]

I hope you had time to take a breath in all that Les! I'll have a read of the links!
Cheers!

[Paddy]

From Les' post,

'Paddy wrote:

"I have one question arising from this. If the Milky Way's GCs are so old and were formed with the galaxy itself, what has prevented the gravity of the disc pulling them into the disc?"

The simple answer is "the conservation of angular momentum". Plus, gas is "sticky", stars (or balls of them) aren't.'

Thanks Les for your detailed and informative entries - fantastic! Please be patient if I pick your brains a tad more. My understanding is quite limited, but I would have thought that if conservation of angular momentum was to prevent GCs moving into the disc, their orbits would have to be roughly orthogonal (now I haven't used that word in some decades to the plane of the disc). If the orbit was in a plane parallel to the disc, I imagine the plane of orbit of the GC would gradually merge with that of the disc.So do they orbit in a plane "perpendicular" to the plane of the disc or have I misunderstood? Could you also say a little more about gases being sticky whilst GCs are not and how this influences their positions/orbits?

I looked at the entries in wikipedia and they are very,very helpful, but you are adding a lot more information with your entries here and I really appreciate your contribution.

[Brian W]

Yes we are privileged but for myself I now have read what he wrote but I sure don't know what he knows!:lol:

[ngcles]

Hi Paddy & all,

Perhaps I have expressed myself in a slightly clumsy fashion.


_So far as I understand_ (and again, I'm no astrophysist or dynamycst) the orbits of the GCs are random about the galactic centre at all angles to the plane of the disc. Some may be at right-angles, most probably not. Some have shorter orbits than others. Some, the outer halo and extreme outer halo clusters (like NGC 2419) have such huge orbits, they orbit the Milky Way but never move through the disc at all.

Once a GC (as a whole) is in an orbit around the centre of mass of the Milky Way, it will remain in that orbit unless an outside force acts to alter its orbit, and its orbital angular momentum as a whole will remain constant. The majority of the galaxy is fairly empty -- a GC can pass through the disc and stars won't crash into one another unless they are very, very, very unlucky. The disc therefore will not do much to brake the cluster's motion and change its orbit.

However, the repeated passages through the disc and core will have an effect on individual cluster stars and tidal forces will eventually rob the cluster of individual members and therefore mass making it easier in turn for the next member to escape, and the next, and the next and so on.

Clusters that make repeated (frequent) dives through the disc or more importantly through the core of the Milky Way loose members that will straggle both behind and in front of the cluster in its orbit but generally remaining in the galactic halo. This is what is happening to Pal 5 at present (and several other clusters) and eventualy they will completely evaporate, loosing all their stars into the halo. The tidal tails of Pal 5 are in total about 13,000 ly long. The cluster will eventually become a "stream" of stars within the halo. I seem to remember reading somewhere that if the Milky Way does not add to its globulars, then in 10gyr time less than 1/2 what are there now will remain then. It is easier for small GCs to evaporate than
big ones.

Gas behaves diferrently to stars (that are point-like in the scheme of things). It is easy for a two clusters of stars to pass right through each other and exit the other side completely unscathed. Gas has volume and is viscous and when gas-cluds ram into each other they can easily loose momentum (angular or otherwise) and become stuck together (the likely result is star formation). Think of the stars as "needle-like" bullets and the gas like a block of wood. Which one will pass through a human body most easily at high-speed? We see this all the time in merging and interacting galaxies the stars won't hit each other -- the gas clouds do and violently. Interacting/merging galaxies are alive with star-forming activity for just this reason.

Paddy wrote:

"If the orbit was in a plane parallel to the disc, I imagine the plane of orbit of the GC would gradually merge with that of the disc."

But they can't orbit parallel to the disc, they have to orbit the centre of mass of the galaxy, but their orbits are generally all well tilted with respect to the plane of the disc. Some outer halo clusters never sully themselves with the disc. The inner halo clusters spend half their time above, half below. Think of them as being a bit like the comets that are not necessarily confined to the plane of the solar-system, but come and go at all angles -- but all still orbit the Sun!

That's how I understand it works anyway. Please, if there is a physicist out there and I've got it wrong -- let us all know!

Best,

Les D

[OneOfOne]

Another factor that reduces the impact suffered by any GC going through the plane of the galaxy is that it is so thin. So for the millions of years it may take a cluster to go around the galaxy once, only a very tiny amount of this time will be spent under the potential influence of stars in the parent galaxy. Sort of like the rings of Saturn, they are very thin and consist mostly of nothing. Any effects caused by stars interacting with the galaxy would be fleeting at best, gravitationally or otherwise.

It would be interesting to know if any GCs have been found actually inside the galaxy and just passing by? Statistically I would expect this would be very unlikely for the reasons above.

[Paddy]

Many thanks Les, that's a very helpful discourse! I now feel a lot clearer. You keep saying you're no astrophysicist, but the amount of fascinating material that you can impart is truly impressive. So much to learn, so little time!