Exam question: how to determine cluster type based on spectrum information?

[jezura]

Hi all, I'm learning for my final exam and looking for help with one question:

Cluster 1: contains Main Sequence A-class stars.

Cluster 2: hottest Main Sequence stars from this cluster are F-class stars.

Question: What types are cluster 1 and 2 and what is their relative age?

Correct answer: Both are open clusters and 1 is younger than 2.

My question: How should I find out that cluster 2 is an open cluster and not a globular cluster?

Thx!

[JulianO]

Hi all, I'm learning for my final exam and looking for help with one question:

Cluster 1: contains Main Sequence A-class stars.

Cluster 2: hottest Main Sequence stars from this cluster are F-class stars.

Question: What types are cluster 1 and 2 and what is their relative age?

Correct answer: Both are open clusters and 1 is younger than 2.

My question: How should I find out that cluster 2 is an open cluster and not a globular cluster?

Thx!

Well basically the age of the stars. OBAF are all very large stars with relatively short lifetimes. Therefore these are recent structures.

Globular clusters are very old typically, usually around the same age as the galaxy itself, so they are reddish in colour and composed of low mass stars that can burn for several billion years, So typically in globular clusters its those further down the spectral class, as in such as GK and M.

(i.e, K class stars have a lifetime of 10¹¹ years, F around 10¹⁰ and B around 10⁷ years)

[jezura]

Thanks!

Just to explain my thoughts: I got confused by this diagram:

http://en.wikipedia.org/wiki/File:M3_color_magnitude_diagram.jpg

(it says that globular clusters - M3 in this case - contain F-class stars)

After some more google-searching I found out that the diagram is not really correct - all the stars should be moved a bit to the right.

[Merlin66]

The definative "Stars and Clusters" by Cecilia Payne-Gaposchkin, p158 gives the spectral details of the brightest 22 globular clusters.

17 are F class and the balance G class. See also the B-V graphs on page 164.

I think the #2 question is a generalisation that there are no O-B type stars in older open clusters.........