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 | | From: | David Brodeur | | Subject: | Difference between globular & open cluster | | Date: | Sun, 23 Jan 2005 01:12:32 GMT |
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 | How clear is the dividing line between globular clusters and open
clusters?
When I look for a definition of a globular cluster, I come up with
that they are:
* roughly spherical
* large (10^4 - 10^6 stars)
* very old (~10^10 years).
It seems to me that age is not necessarily intrinsic to the
definition, but merely indicative of the history of globular formation
in our particular galaxy. And yet I read of M71 being included, and
M11 excluded from the globular club due to their age, as deduced from
their color-magnitude diagrams.
OTOH, we have an extragalactic cluster like NGC 1850 in the LMC, which
is sometimes classed as a globular despite being only 40 million years
old.
So what does it take for a cluster to be considered a globular? Number
of stars, density profile, luminosity function, dynamics?
I'd appreciate any information you can give, or point me to.
David
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| | From: | Aidan Karley | | Subject: | Re: Difference between globular & open cluster | | Date: | Sun, 23 Jan 2005 19:00:08 GMT |
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| In article , David Brodeur
wrote:
> How clear is the dividing line between globular clusters and open
> clusters?
>
> When I look for a definition of a globular cluster, I come up with
> that they are:
> * roughly spherical
> * large (10^4 - 10^6 stars)
> * very old (~10^10 years).
>
Your definition missed a couple of important points: A GC is
also gravitationally bound (held together by the mutual gravitation of
it's stars) and has a strong central condensation of stars, but no
significant loose gas. The central condensation, rough sphericity and
gravitational binding all go together as a suite of features.
Open clusters typically don't have the central condensation, and
often have some remnant gas from their formation.
--
Aidan Karley,
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233
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| | From: | David Brodeur | | Subject: | Re: Difference between globular & open cluster | | Date: | Mon, 24 Jan 2005 07:22:08 GMT |
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| Aidan Karley wrote:
>In article , David Brodeur
>wrote:
>> How clear is the dividing line between globular clusters and open
>> clusters?
>>
>> When I look for a definition of a globular cluster, I come up with
>> that they are:
>> * roughly spherical
>> * large (10^4 - 10^6 stars)
>> * very old (~10^10 years).
>>
> Your definition missed a couple of important points: A GC is
>also gravitationally bound (held together by the mutual gravitation of
>it's stars) and has a strong central condensation of stars, but no
>significant loose gas. The central condensation, rough sphericity and
>gravitational binding all go together as a suite of features.
> Open clusters typically don't have the central condensation, and
>often have some remnant gas from their formation.
Are open clusters not gravitationally bound? I would think that the
very old ones at least must survive due to gravitational cohesion. The
remnant gas seems like it would be an age-related feature.
Central condensation seems like a more essential characteristic. But
would a globular cluster have this property initially, or would it
take time for the condensation to develop?
I have found a lengthy article on globular cluster dynamic by G.
Meylan and D.C. Heggie at
http://victorian.fortunecity.com/durer/481/GCReview.ZIP which looks
like it may answer some of my questions.
Thanks,
David
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