Messier 5

[Graeme]

This is Globular Cluster M5 in Serpens. It's a collection of hundreds of thousands of stars 24,500 light years from us. There are 150 known Globular Clusters orbiting the Milky Way.

The stars to the right of the image appear to not be collimated but the ones on the left are ok. Not sure what would cause this! I might have to get the telescope looked at.

M5-02-EL.jpg

[ChrisGreaves]

There are 150 known Globular Clusters orbiting the Milky Way.

This week's question: What is the difference between a cluster of stars that is a Globular Cluster, and a cluster of stars that is a Galaxy, such as our Milky Way?
Is it a matter of number of stars, or shape of cluster, or diameter of cluster, or what? Who decides that one bright smudgy light is a galaxy while another smudgy light is a cluster?
Thanks, Chris
P.S. I have left it to Hans to tell you that this is a brilliant photo. C.

[HansV]

P.S. I have left it to Hans to tell you that this is a brilliant photo. C.

Brilliant photo!

Why are globular clusters not considered galaxies?

[BobH]

I was gobsmacked, first by the 24,500 LIGHT YEARS distance then by the brilliance of the photo.

Thank you, Graeme!

[Graeme]

This week's question: What is the difference between a cluster of stars that is a Globular Cluster, and a cluster of stars that is a Galaxy, such as our Milky Way?

As per Hans' link, the total mass of the stars in a globular cluster is far less than the total mass of a galaxy. Historically though there may have been a link between merging globular clusters and early galaxies, no one really knows!

P.S. I have left it to Hans to tell you that this is a brilliant photo. C.

Brilliant photo!

Why are globular clusters not considered galaxies?

Cheers, brilliant link!

If you zoom in on the stars on the right side of the image you can see that it is in fact not a brilliant photo. When I get to the bottom of my collimation problem I'll find another globular cluster and capture its photons with better clarity!

I was gobsmacked, first by the 24,500 LIGHT YEARS distance then by the brilliance of the photo.

They are very photogenic! The first time I saw one visually (M13 I think) was a bit of a wow moment!

[ChrisGreaves]

As per Hans' link, the total mass of the stars in a globular cluster is far less than the total mass of a galaxy. Historically though there may have been a link between merging globular clusters and early galaxies, no one really knows!

Hi Graeme, and thanks for the brilliant ripostes.

I have to believe that astronomers have some sort of measure of what makes a globular cluster and what makes a galaxy.

I can understand that, if globular clusters, one by one, gravitate together and ultimately form a galaxy, that there must be a time when that region of space stops being a cluster and becomes a galaxy. That event might be signaled (Boolean) by the formation of a black hole (whether we on Earth can detect it or not).
Twin stars orbiting a common centre of gravity might be excluded from the definition of "cluster" whereas three stars with a common centre of gravity could be defined by earthlings as a cluster.

Right now I am groping with a poor analogy - that of bunches of grapes on a grapevine, where each of the grapes lives within a grape-cluster ("bunch of grapes") which is defined by the common stalk. If we follow a path from a grape towards the source of its food, we come to nodes or junctions where we are joined by the stem of another grape, but there comes a time when we reach a junction that does not support a grape - it supports a leaf or a twig, and at that time we can make the Boolean distinction that we have reached the boundary of a grape bunch/cluster.
The grape vine itself, if we continue traveling upwards until we reach the point (downwards!) at which the stem dives into the soil, would then be a galaxy.
And all the grapevines stretching from Bordeaux to The Barossa Valley would be our Universe.

Cheers, Chris