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mark81

Star Cluster Magnitudes

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Hi All,

Can someone tell me how the magnitude of a star cluster is measured?  Is it by the brightness of stars within the cluster or the cluster as a whole.  I as wondering, if you had two clusters near each other which both had the same magnitude, surely if you could see one, you should be able to see the other..? I think that's what I'm asking... thanks

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The apparent magnitude of a cluster is a measure of the total light received from all the stars in it.

So your eye sees the same amount of light from a Mag 6 cluster as from a Mag 6 star.

But since the cluster is spread over many square arc-seconds of sky it looks much dimmer.

Surface brightness ( Light received per unit area of sky) is a better indication of how easy it is to see an extended object.

Stellarium etc usually quotes both measures.

 

 

Edited by lenscap
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On 21/09/2018 at 15:56, mark81 said:

Hi All,

Can someone tell me how the magnitude of a star cluster is measured?  Is it by the brightness of stars within the cluster or the cluster as a whole.  I as wondering, if you had two clusters near each other which both had the same magnitude, surely if you could see one, you should be able to see the other..? I think that's what I'm asking... thanks

Lenscap's explanation of cluster magnitudes is spot on.

It sounds like you're having an issue with a particular cluster. If so, which is it? It may help to provide a thorough explanation if we know.

Most good catalogues will provide the magnitude of the brightest individual star in the cluster, which is also a good indication of whether the object will be visible or not.

You *can* see open clusters where the brightest star is below the limit of your scope, but all you'll see is a faint glow. 

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5 hours ago, DeepSkyBagger said:

Lenscap's explanation of cluster magnitudes is spot on.

It sounds like you're having an issue with a particular cluster. If so, which is it? It may help to provide a thorough explanation if we know.

Most good catalogues will provide the magnitude of the brightest individual star in the cluster, which is also a good indication of whether the object will be visible or not.

You *can* see open clusters where the brightest star is below the limit of your scope, but all you'll see is a faint glow. 

thanks,

It wasn't a cluster in particular, I was looking around Auriga and enjoying m36, 37 and 38.  I just noticed a number of other clusters on my Sky Atlas when I came in.  After looking them up, their magnitudes were around 8-11 and I was just wondering whether my ST80 could pick any up - I'm guessing not.... but I'll have a go..

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22 hours ago, mark81 said:

thanks,

It wasn't a cluster in particular, I was looking around Auriga and enjoying m36, 37 and 38.  I just noticed a number of other clusters on my Sky Atlas when I came in.  After looking them up, their magnitudes were around 8-11 and I was just wondering whether my ST80 could pick any up - I'm guessing not.... but I'll have a go..

Good shout. Having a go, is always the best policy when it comes to deep sky. :thumbright:

 

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