# The Naked Scientists Forum

### Author Topic: astronomical QOTW  (Read 3784 times)

#### ukmicky

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##### astronomical QOTW
« on: 09/05/2006 02:10:51 »
1 How do they tell the difference between a dwarf galaxy and a star cluster?

Michael

#### Soul Surfer

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• keep banging the rocks together
##### Re: astronomical QOTW
« Reply #1 on: 09/05/2006 09:45:12 »
Someone has been watching the sky at night.

They said there that galaxies show evidence of Dark matter  star clusters in our own galaxy dont.

The answer is all in the velocities of the stars.  It is quite easy to measure the velocities of stars in the line of sight using doppler effect on the spectrum lines and if you do that for a lot of stars in the star cluster or galaxy you will get an indication of the way the stars are moving about.  It may be just random or there may be rotation.  It is also possible to estimate the mass of the cluster/galaxy by counting the stars and noting how big they are (the spectrum also tells you that) The nature of the stars in the cluster can also give an indication of the age.  Now if the galaxy/cluster is reasonably old it is reasonable to assume that the structure is gravitationally bound and stable and therefore is subject to the Virial theorem  (look this up in Google) this describes how the orbital velocites in any bound structure must be shared out and allows the mass of the cluster to be estimated in a different way.  If the cluster masses defined by the stars and their relative velocites are very similar it is a star cluster.  If the mass of the cluster defined by the velocities is much bigger there must be dark matter in the cluster to hold it together and it is a galaxy.

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#### ukmicky

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##### Re: astronomical QOTW
« Reply #2 on: 09/05/2006 22:22:05 »
Damn someone else has been watching the sky at night, i thought i was the only one. but i must say that was a superb reply.

Michael

#### neilep

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##### Re: astronomical QOTW
« Reply #3 on: 09/05/2006 23:40:48 »
I love The Sky At Night....Do you think they'll continue with it when Patrick joins the constellation ExitEarthus ?..I hope so.

I reckon they're grooming that Chris bloke (another Clever Chris !!) to take over...he virtually runs the show now anyway I think !

Men are the same as women, just inside out !

#### ukmicky

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##### Re: astronomical QOTW
« Reply #4 on: 10/05/2006 02:32:20 »
Patricks having trouble prononcing his words so i cant see him lasting much longer and its not the same with chris unfortuanatly

Michael

#### neilep

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##### Re: astronomical QOTW
« Reply #5 on: 10/05/2006 03:07:30 »
quote:
Originally posted by ukmicky

Patricks having trouble prononcing his words so i cant see him lasting much longer and its not the same with chris unfortuanatly

Michael

yep, you're right there !..His voice is slurred isn't it ?...he's still a fast talker, perhaps if he slowed down a bit !........

..he's too set in his ways to change now I suppose though. I think he's older than the Universe !

Men are the same as women, just inside out !

#### JimBob

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##### Re: astronomical QOTW
« Reply #6 on: 11/05/2006 03:27:58 »
http://www.thenakedscientists.com/forum/topic.asp?TOPIC_ID=3633&whichpage=2

This article I posted says they are graded by their luminosity. What is the reson for the different definitions?

The mind is like a parachute. It works best when open.  -- A. Einstein

#### Roy P

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##### Re: astronomical QOTW
« Reply #7 on: 12/05/2006 22:34:15 »
quote:
Originally posted by ukmicky
Patricks having trouble prononcing his words

Ha-ha-ha -- poor Patrick. I have to record it so that I can replay those words and sentences I can't understand. Oh yes, the Chris Bloke is destined to take over.

Roy P

#### ukmicky

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##### Re: astronomical QOTW
« Reply #8 on: 12/05/2006 23:19:06 »
Your article is what made me originally think about the question which soul surfer anwsered But basically a star cluster is mainly bound together by the gravity of its own stars whereas a galaxy is mainly held together by dark matter

Michael
« Last Edit: 12/05/2006 23:22:08 by ukmicky »

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##### Re: astronomical QOTW
« Reply #8 on: 12/05/2006 23:19:06 »