What is the center of a globular clusters made of?

[vrmuth]

what's the center of a globular cluster , is it a star or can it be a black hole ? how are they formed and bound to gravitational pull of the milky way

 

[DEvens]

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

Looks like it's only stars.

 

[mfb]

Mainly empty space.
There can be a black hole, but that is not necessary. Individual stars can stay close to the center for a while, but they are not massive enough to avoid perturbations from other stars which move them around over time.

 

[Garth]

One question of course is that of the internal orbital mechanics; once formed how do globular clusters remain so tightly bound for nearly the age of the universe? Open clusters have lifetimes < 100Myrs. or so.
Globular Cluster

Globular clusters have a very high star density, and therefore close interactions and near-collisions of stars occur relatively often. Due to these chance encounters, some exotic classes of stars, such as blue stragglers, millisecond pulsars and low-mass X-ray binaries, are much more common in globular clusters. A blue straggler is formed from the merger of two stars, possibly as a result of an encounter with a binary system. The resulting star has a higher temperature than comparable stars in the cluster with the same luminosity, and thus differs from the main sequence stars formed at the beginning of the cluster.Globular cluster M15 may have an intermediate-mass black hole at its core.

So what keeps the globular cluster so tightly bound, for example, are there BH's at the centre?

Astronomers have searched for black holes within globular clusters since the 1970s. The resolution requirements for this task, however, are exacting, and it is only with the Hubble space telescope that the first confirmed discoveries have been made. In independent programs, a 4,000 M☉ intermediate-mass black hole has been suggested to exist based on HST observations in the globular cluster M15 and a 20,000 M☉ black hole in the Mayall II cluster in the Andromeda Galaxy.[44] Both x-ray and radio emissions from Mayall II appear to be consistent with an intermediate-mass black hole.

These are of particular interest because they are the first black holes discovered that were intermediate in mass between the conventional stellar-mass black hole and the supermassive black holes discovered at the cores of galaxies. The mass of these intermediate mass black holes is proportional to the mass of the clusters, following a pattern previously discovered between supermassive black holes and their surrounding galaxies.

Garth

 

[Safak Mert]

what's the center of a globular cluster , is it a star or can it be a black hole ? how are they formed and bound to gravitational pull of the milky way

Globular clusters are beautiful and weird. There are many mind boggling questions associated with them. This was the best text answering many of those questions -including yours- I have read so far. "A Thousand Blazing Suns: The Inner Life of Globular Clusters http://www.astrosociety.org/pubs/mercury/9904/murphy.html "

 

[davenn]

So what keeps the globular cluster so tightly bound, for example, are there BH's at the centre?

The first obvious choice would be mutual gravitational attraction between the members of the globular cluster
Since there are vastly many more stars in a CG than in an open cluster. The gravitational attraction would be more substantial
As mfb said, there may be a BH at the core of some of them, but it wouldn't be necessary to keep the GC compact.

Open clusters, on the other hand, are generally likely to be less than a couple of 100 stars, with many much less than that, a few dozen or so.
They are also spread out over a larger area meaning mutual gravitational attraction is going to be lower and this would allow for
members of the OC to drift apart over time.

Dave

 

[davenn]

Globular clusters are beautiful and weird. There are many mind boggling questions associated with them. This was the best text answering many of those questions -including yours- I have read so far. "A Thousand Blazing Suns: The Inner Life of Globular Clusters http://www.astrosociety.org/pubs/mercury/9904/murphy.html"

interesting looking article ... will get a chance to read it soon

thanks
Dave

 

[Garth]

The first obvious choice would be mutual gravitational attraction between the members of the globular cluster
Since there are vastly many more stars in a CG than in an open cluster. The gravitational attraction would be more substantial
As mfb said, there may be a BH at the core of some of them, but it wouldn't be necessary to keep the GC compact.

Open clusters, on the other hand, are generally likely to be less than a couple of 100 stars, with many much less than that, a few dozen or so.
They are also spread out over a larger area meaning mutual gravitational attraction is going to be lower and this would allow for
members of the OC to drift apart over time.

Dave

Hi davenn!

Yes, of course, however the question arises of the stability of the member stars' orbits over ~ 10Gyrs. With perturbations of many nearby neighbours mergers between members and ejections from the globular system must be common, far more so than in the galaxy at large.

Obviously they have survived and are stable over such long periods but mathematically the stability question is an interesting one.

Garth

 

[marcus]

One question of course is that of the internal orbital mechanics; once formed how do globular clusters remain so tightly bound for nearly the age of the universe? Open clusters have lifetimes < 100Myrs. or so.
Globular Cluster
So what keeps the globular cluster so tightly bound, for example, are there BH's at the centre?
Garth

Some nice stuff in that wikipedium! It pointed to something about intermediate-mass BH,
http://hubblesite.org/newscenter/archive/releases/2002/18/text/
which in turn pointed to:
http://arxiv.org/abs/astro-ph/0209314
http://arxiv.org/abs/astro-ph/0209315

http://arxiv.org/abs/astro-ph/0508251
http://arxiv.org/abs/astro-ph/0209313

 

[TEFLing]

Is it possible, that GCs are the tidally stripped down cores, of previously larger and more distended objects?

Can individual GCs be resolved, at cosmological distances? If so, then are they larger?

Gravity causes less massive objects to be scattered outwards, whilst more massive members sink towards the center... Less massive members farther from GC center might be easier to strip off

 

[davenn]

Can individual GCs be resolved at cosmological distances?

yes, tho I haven't found any images or info on if the individual stars in the GC's can be resolved

There are many images showing GC's around other galaxies

 

[TEFLing]

T

yes, tho I haven't found any images or info on if the individual stars in the GC's can be resolved

There are many images showing GC's around other galaxies

Those would be very important observations... If so then GCs are largely intact ancient relics

 

[vrmuth]

Some nice stuff in that wikipedium! It pointed to something about intermediate-mass BH,
http://hubblesite.org/newscenter/archive/releases/2002/18/text/

that 's a nice article , thanks but it didn't answer how they become the satellite of a galaxy

 

[liometopum]

Click here for a link from Galaxy Zoo that has some very nice images of globular clusters.

Here are some sample images.

This is Messier 53:

and here is NGC 5466

Thanks to ElisabethB, of Galaxy Zoo, for help with this.

 

[marcus]

that 's a nice article , thanks but it didn't answer how they become the satellite of a galaxy

May I conjecture? Imagine that a swarm of globular clusters forms first, before there IS a galaxy
a swarm not only of globular clusters as we know them, but of smaller collections of various sorts: proto-galaxies, dwarf galaxies, blobs of stars and gas, etc.
then there is a kind of "bottom-up" coalescence rather than "top down".

These globular clusters, if I remember correctly, do not orbit AROUND our galaxy, they typically orbit through, they "yo-yo" through the galactic plane
When dwarf galaxies do that, they lose stars as they pass through. They get slightly disrupted. So they gradually contribute stars to the main galaxy. Outliers get shaved off.

So I am imagining that the globulars do not have to "come into" orbit or " become satellites" of a galaxy. It is the main galaxy which "becomes" in their midst. Out of a the swarm of smaller clots and curdles. With the help of the dark matter halo. they are prior.

Perhaps the globulars are the SURVIVORS of this process and look the way they do because, like pebbles, they have been worn smooth by many passes through the plane of
collective rotation.

BTW there is a Wippy Kidium on "Galaxy formation" . I don't think structure formation and galaxy formation in particular is a completely solved problem. It probably doesn't hurt to imagine various alternatives.

 

[Quds Akbar]

A globular cluster is a group of stars bound together by gravitational effects which only increases as you approach the center and the number of stars per a given area increases too.

 

[davenn]

A globular cluster is a group of stars bound together by gravitational effects which only increases as you approach the center and the number of stars per a given area increases too.

yes, that's correct as can be seen by the images posted up the page

Would be an interesting place to be on a habitable planet around one of those stars

Dave

 

[mfb]

The star densities there are not good for the long-term stability of planetary orbits.

But it could give an amazing view of the milky way.

 

[davenn]

The star densities there are not good for the long-term stability of planetary orbits.

Yup I would suspect that

But it could give an amazing view of the milky way.

Uh huh, specially if in one well above the galactic plane

That made me think of the view of the MW from one of the Magellanic Clouds, would also be an awesome sight

Dave

 

[wabbit]

But it could give an amazing view of the milky way.

You'd need to be in its remote suburbs though, otherwise I suspect the sky would be so bright from all the nearby stars surrounding you that you wouldn't see the milky way at all. Which would be quite a sight in its own right...

 

[Garth]

You'd need to be in its remote suburbs though, otherwise I suspect the sky would be so bright from all the nearby stars surrounding you that you wouldn't see the milky way at all. Which would be quite a sight in its own right...

Something like this?...

Inside view of a globular cluster from The Harvard-Smithsonian Center for Astrophysics.

Garth

 

[wabbit]

Nice : ) I'd have thought the colors would look less saturated, like those of stars seen from earth, but maybe that's incorrect ?

 

[Garth]

Nice : ) I'd have thought the colors would look less saturated, like those of stars seen from earth, but maybe that's incorrect ?

Well it is only an artist's impression...

Our night vision (detected by the rod cells of the retina) is basically insensitive to colour, so colour perception improves with brightness. The nearer stars within a globular cluster will be brighter and therefore the colours will be more noticeable to human vision.

Assuming, of course, that there are humans there to see the view...

Garth

 

[wabbit]

Well it is only an artist's impression...

Our night vision (detected by the rod cells of the retina) is basically insensitive to colour, so colour perception improves with brightness. The nearer stars within a globular cluster will be brighter and therefore the colours will be more noticeable to human vision.

Assuming, of course, that there are humans there to see the view...

Garth

Ah yes good point... Now I wonder how Antares would look like if it had the apparent size of the Sun..
Edit : blinding of course: ) Somewhat farther so we can look at it with full color vision instead
Edit2 : quite white it seems. From Wikipedia article on thermal radiation anything above ~1700K is whitish, and Antares is 3400K

 

[Garth]

Edit2 : quite white it seems. From Wikipedia article on thermal radiation anything above ~1700K is whitish, and Antares is 3400K

Antares, Betelgeuse, Aldebaran, Arcturus and others are red giants and can be seen with a distinctly reddish hue to the naked eye. (Surface temps ~ 3,500 K)

Rigel is a blue white supergiant with a bluish hue, (Surface temp ~12,000 K+), whereas if you want a pure white star you can do no better than good old Sirius. (Surface temp ~10,000 K)

Our own Sun of course is yellowish with a surface temp of ~5,800K.

Each star radiates more or less as a Black Body and the colour is determined by the photosphere temperature.

Garth

 

[wabbit]

Hmm our sun is white by definition of white no ? I thought the yellowish tint was due to the atmosphere (the counterpart of that being the sky is blue)

 

[davenn]

Hmm our sun is white by definition of white no ?

only from our optical point of view ... we are so close to it
If you could get a decent distance from it, it wouldn't be the white/ blue-white of the high temperature stars

As Garth say, its a mid temp star compared to the full range of stellar colours/temperatures

 

[wabbit]

only from our optical point of view ... we are so close to it
If you could get a decent distance from it, it wouldn't be the white/ blue-white of the high temperature stars

As Garth say, its a mid temp star compared to the full range of stellar colours/temperatures

Right but white is in the middle of the range (red white blue), and I don't see how the spectrum depends on distance.
What I meant was "white" is a human perception of a neutral balance of colors, and I assume sunlight is pretty much what we mean by that.
But maybe I'm missing something ?

Edit : if you look at the sun in a telescope thru a neutral filter say mylar it is very white indeed. Warning: Never view the Sun without proper safety precautions, you could seriously damage your eyesight

 

[davenn]

Right but white is in the middle of the range (red white blue),

not sure where you get that from ?

and I don't see how the spectrum depends on distance.

it doesn't ... but our perception does
as I said, with the sun so close, its a blinding white light, put at a few lightyears distance and you will see it for the real colour it is

 

[wabbit]

it doesn't ... but our perception does
as I said, with the sun so close, its a blinding white light, put at a few lightyears distance and you will see it for the real colour it is

I don't think so. Look at the sun through a solar filter.

 

[davenn]

Look at the sun through a solar filter.

solar filters come in all variations of colour renditions ... that's a very poor comparison

 

[davenn]

see where the sun is on the standard H-R diagram ...

 

[wabbit]

solar filters come in all variations of colour renditions ... that's a very poor comparison

Yes but good ones have a flat transmission curve. See for instance http://www.alpineastro.com/Solar_Observation/Images/DERF_Curve_Lg.jpg

 

[wabbit]

see where the sun is on the standard H-R diagram ...

View attachment 79343

Yes G2 star. The colors on his chart are illustrative,not accurate.

 

[davenn]

accurate enough to give a good indication compared to the other stars in the sequence and you will find that is the description in most places

 

[wabbit]

not sure where you get that from ?

That the range goes red white blue ? You can call it red yellow white bluish if you prefer : )

 

[davenn]

Yes but good ones have a flat transmission curve. See for instance http://www.alpineastro.com/Solar_Observation/Images/DERF_Curve_Lg.jpg

I haven't seen one yet that doesn't change the colour of what is being viewed

 

[davenn]

believe what you like
I have had enough of your argumentative attitude !
I have put you on ignore

 

[wabbit]

I haven't seen one yet that doesn't change the colour of what is being viewed

Try Baader they ain't bad

 

[wabbit]

Dr. William Harris, and his Ph.D. student, Jeremy Webb, of McMaster University, wrote an article for Astronomy magazine, July 2014, titled "Life Inside A Globular Cluster".

Very nice! especially the view from apogalacticon

 

[wabbit]

About the color of the sun, a good summary is found at http://solar-center.stanford.edu/SID/activities/GreenSun.html

 

[liometopum]

You can see the view from different locations within a globular cluster at Jeremy Webb's page,
http://www.physics.mcmaster.ca/~webbjj/liagc.html
Three images:
1. View from the center
2. View from near the half-light radius of the cluster
3. View from apogalacticon

 

[liometopum]

Here is a link to a globular cluster lecture by Dr. William E. Harris of McMaster University.
http://physwww.mcmaster.ca/~harris/mcgill09aug.pdf

He covers a number of interesting things, among them being:
The Milky Way has 150 GCs
The Sombrero galaxy has about 1900 GCs
Discusses the size distribution of GCs
Formation via Young Massive Clusters in starburst dwarf galaxies
Some have black holes, and some do not
Globular clusters evaporate (the stars leave the GC). They all evaporate, except the big ones take a long time to do so.

 

[davenn]

You can see the view from different locations within a globular cluster at Jeremy Webb's page,
http://www.physics.mcmaster.ca/~webbjj/liagc.html
Three images:
1. View from the center
2. View from near the half-light radius of the cluster
3. View from apogalacticon

cool link and nice artistic impressions thankyou

I think it would be amazing to be able to look down on the Milky Way from above the galactic plane from such a "close" distance,
rather than from edge on as we currently do

The last image gives a hint of that view

Dave

 

[davenn]

Globular clusters evaporate (the stars leave the GC). They all evaporate, except the big ones take a long time to do so.

Yeah, I would suggest a very long time. Spectral analysis of GC stars indicate that many of them are very old stars

 

[vrmuth]

You'd need to be in its remote suburbs though, otherwise I suspect the sky would be so bright from all the nearby stars surrounding you that you wouldn't see the milky way at all. Which would be quite a sight in its own right...

even if you are in remote suburbs you will be able to see the MW only for half of the year . can you see how ?

 

[wabbit]

even if you are in remote suburbs you will be able to see the MW only for half of the year . can you see how ?

Oh it's even worse than that : you also need your star to be on the right part of its orbit around the center of the cluster lest the cluster blocks the view. Lots of geometric constraints to combine...

 

[liometopum]

The Senior Editor of Astronomy Magazine, Michael E. Bakich, has given approval for us to post the article, "Life Inside a Globular Cluster", written by William E. Harris and Jeremy Webb, from the July 2014 issue, on Physics Forums. See the attached pdf.

I wish to thank Michael E. Bakich, and Astronomy Magazine, for this gracious gift to the members of Physics Forums!

Please thank Astronomy Magazine by liking this post, or better yet, giving a gift subscription to a friend, or even yourself. (I am fully responsible for that plug)

 

[liometopum]

I recently spotted this arXiv article, and it fits this post well.

Two stellar-mass black holes in the globular cluster M22
The paper made it into http://www.nature.com/nature/journal/v490/n7418/full/nature11490.html.

I'd not seen it before, but the Milky Way globular cluster, M22, apparently has at least TWO black holes. From the abstract:
"...we report the presence of two flat-spectrum radio sources in the Milky Way globular cluster M22, and we argue that these objects are black holes of stellar mass (each ∼ 10–20 times more massive than the Sun) that are accreting matter." and M22 may contain a total population of ∼ 5–100 black holes."

Here are a few interesting extracts from the arXiv paper:
1. ...the objects cannot be black-hole/black-hole binaries, and instead are probably in binary systems with Roche lobe-overflowing companions
[ for an explanation of the Roche Lobe, see ]

2. Stellar-mass black holes will mass-segregate to the core of the cluster.

3. The presence of black holes in a globular cluster can lead to an expansion of the core radius through interactions between black holes and stars. This could explain why M22 has the fifth largest core radius among luminous ( >∼ 2 × 105L ) Milky Way globular clusters

 

[Chronos]

IMO, globular clusters probably preceded ordinary galaxies in the universe. I suspect most of them harbor black holes of significant mass. There is observational evidence favoring this idea, which also explains why they tend to be populated by ancient stars.