why don't globs collapse? ... or do they?

janoskiss (Steve H) · #1 29-09-2005, 10:23 PM

Some questions about globular clusters:

Are globular clusters stable structures? Or are they just a bunch of stars collapsing under their mutual gravitational pull?

But if they are stable (or metastable even), what is the dynamics stopping their collapse?

Has there been any research that looks at redshifts from individual stars in a globular cluster, to determine the velocity of each star, and to eventually get a detailed picture of glob dynamics?

I haven't a clue of the answers; just contemplating what might be, after seeing Tony's fab pic of Tucanae 47.

mickoking · #2 29-09-2005, 10:34 PM

Mate, that is a good question and I too would like to know the answer.

gaa_ian (Ian) · #3 29-09-2005, 10:41 PM

Not sure of the exact dynamics ... but I do know that some Globs undergo "Core collapse"
bringing a lot of stars close together <1 LY.
But Globs are very old for the most part as old or older than our Galaxy, so they must be pretty stable.
This is a good source of Info on all the NGC objects, including globs.
http://www.seds.org/~spider/ngc/ngc.html

ballaratdragons (Ken) · #4 29-09-2005, 10:41 PM

Collapsing or stable it would be daytime all the time on any Planet orbiting any stars within a Glob! The amount of light coming in from all directions would be blinding.

Good question Steve.

janoskiss (Steve H) · #5 29-09-2005, 10:47 PM

Imagine if they were collapsing, what would happen when eventually a million or so stars collided. A Small Bang perhaps? Kaboom!

ballaratdragons (Ken) · #6 29-09-2005, 10:52 PM

If enough stars in the core collided it could set off a chain reaction moving outwards and engulf the whole glob. Maybe

It may have already happened but not reached us yet!

gaa_ian (Ian) · #7 29-09-2005, 11:02 PM

Here is some more good info on the dynamics within the core of a globular (47Tuc in fact)
http://www.space.com/scienceastronom...es_010518.html

mickoking · #8 29-09-2005, 11:02 PM

Maybe Globs have black holes at their core?

gaa_ian (Ian) · #9 29-09-2005, 11:04 PM

Apparently Not! , see above link.

janoskiss (Steve H) · #10 29-09-2005, 11:05 PM

I suspect that although from here globs look really compact, there is vast space between the stars. From any star the nearest star would be lightmonths or years away, while the diameter of the star would only be some lightseconds. So the stars would be tiny compared with the distances between them.

Then one could think of the interaction between the stars as interaction between very small particles. And if this picture is correct (= good enough) the stars would very rarely collide, and would orbit each other and their common centre of gravity. But I would like to know what is really going on, and what the best astronomical observations tell us.

janoskiss (Steve H) · #11 29-09-2005, 11:14 PM

Great link Ian! Millisecond binaries??? That is beyond mindboggling. A cosmic centrifuge too scary to contemplate.

mickoking · #12 29-09-2005, 11:18 PM

I checked the link. looks like there are many near misses (forming binaries) and collisions (increasing the mass of the star and eventually becoming neutron stars). there must also be many stellar mass black holes as well.

Correct me if I am wrong but gravity will eventually win and in the faaaaaaar future the globular will eventually collapse?

janoskiss (Steve H) · #13 29-09-2005, 11:35 PM

Quote:

Originally Posted by mickoking

Correct me if I am wrong but gravity will eventually win and in the faaaaaaar future the globular will eventually collapse?

Collapse and/or disperse. The smaller stars have the best chances of escaping by being slingshotted away.

Argonavis (William) · #14 05-10-2005, 09:49 PM

My understanding is that, as a result of gravitational interactions, most globulars would be dominated at their centre by a massive binary pair. This massive binary pair would take up residence and gravitationally throw off any intruders.

Globular clusters do undergo what is termed a "gravitational collapse", but this is where the core becomes very heavily populated by massive stars, whilst the outer layers of stars are dispersed due to the gravitational interaction between them. This is easily observed in the eyepeice, comparing the highly concentrated core of 47 Tucana with the more evenly dispersed appearance of Omega Cent. All globulars display different degrees of central concentration.

Even the most concentrated of globulars would still maintain about 1 light year between stars, so that the average star is surrounded by a sphere of space 1 light year in diameter before you find another star. Whilst they are all playing celestial billiards in there, actual collisions and merger events would still be rare.

It would certainly be true that any planet associated with a star in a globular would experience night skies ablaze with 1st magnitude stars, I am not sure that it would be totally overwhelming. Stars in globulars are K dwarfs, whose luminosities are less than the Sun's. Bright, but not blazing so. I suspect that there would be dark gaps. How many we can only but imagine.

The study of the dynamics of globulars is a field in its own right, and is quite facinating. Gravitational interactions between the players means that in any encounter between 2 stars, the more massive will lose energy and its orbit will contract, so bringing it closer to the centre, whilst the less massive star will gain energy and move to a higher orbit and possibly exit the cluster. Hence over time the core of a globular becomes dominated by massive stars, and the less massive stars tend to be lost to interstellar space. This is core collapse, and the concentration of a globular clusters core is an excellent indication of how old it is, as least in theory. In practice there are other influences at work.

gaa_ian (Ian) · #15 05-10-2005, 10:45 PM

Excellent summary Argonavis ....
They are certainly interesting places, Globs, both visually and from a scientific perspective !

ving (David) · #16 06-10-2005, 09:43 AM

globs make such an interestig topic

I have always wondered how they form in the first place. I heard somwhere that it had something to do with colliding galaxies

astroron (Ron) · #17 06-10-2005, 09:58 AM

It used to be thought that stars do not merge or collide,but it was found that there where a lot of what are called blue straglers found in certain Globular Clusters,such as 47 Tuc and Omega Cent, and it is now thought that this is caused by the merging of Solar mass stars and reigniting the star formation proccess.astroron

Rodstar (Rod) · #18 06-10-2005, 09:54 PM

It is incredible that this question should be discussed at the moment, I had a friend over the other night who was looking in the scope at 47 Tuc for the first time, and asked the same question: why doesn't gravity cause its collapse? I was stuck for an answer, but now have a few things I can pass on. Thanks guys!

Argonavis (William) · #19 07-10-2005, 12:23 PM

I am not sure why some people think that globulars should collapse. No-one seems to think that the solar system will collapse, and the forces acting on both are the same. Whilst the masses of the planets are smaller than stars, the distances are smaller too, and gravity's effect drops off with the square of the distance. All of these bodies maintain a forward momentum that keeps them in free fall orbit indefinitely until another body acts on them. Then it gets interesting.

janoskiss (Steve H) · #20 07-10-2005, 12:49 PM

Motion of solar system bodies is common knowledge. But when most people look at stars and star clusters, they probably imagine a static picture of stars suspended in space.