I urge anyone who wants to know the science behind any NGC object to plug it into Google. The Wikipedia articles tend to sort right to the top of the search list and these are a great way to get the quick overview on any object in the NGC. You can drill down to more and more detail or just surf around in the general area for a tour of the science of Astronomy. It’s a great way to spend an evening during a blizzard. Briefly if you wonder how scientist draw the line between open and globular clusters it’s all in the HR diagram. A deep sky photo of the entire cluster is taken with a standard B (for blue) and another with a standard V filter. Then each star is measured in each photo and the results are graphed. The V value is plotted vertically and the B-V is plotted horizontally. Most of the stars will fall on the main sequence whose left end is called the cutoff. The B-V of the stars at the cutoff gives us the color and the age of the brightest stars still on the main sequence and the age of the cluster itself. The V gives the apparent magnitude of these stars and distance to these stars and the cluster itself. NGC2477 is 700 million years old and has about 300 members. Globulars are much older and larger. This one looks like a globular because it’s so close at 4200 lightyears. A similar example farther north is NGC7789 also called the “Magnificent Cluster”. Either one is worth a long leisurely look with the weather allows. DT

From the photo I can see why it could be called either a globular or an open cluster. To me it resembles M11, an open cluster.

Daniel, I really appreciate your explanation. Thanks, Joe

________________________________ From: daniel turner <outwest112@yahoo.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Wed, February 24, 2010 7:15:36 PM Subject: Re: [Utah-astronomy] NGC 2477 Open or Globular Cluster

I urge anyone who wants to know the science behind any NGC object to plug it into Google. The Wikipedia articles tend to sort right to the top of the search list and these are a great way to get the quick overview on any object in the NGC. You can drill down to more and more detail or just surf around in the general area for a tour of the science of Astronomy. It’s a great way to spend an evening during a blizzard.

Briefly if you wonder how scientist draw the line between open and globular clusters it’s all in the HR diagram. A deep sky photo of the entire cluster is taken with a standard B (for blue) and another with a standard V filter. Then each star is measured in each photo and the results are graphed. The V value is plotted vertically and the B-V is plotted horizontally. Most of the stars will fall on the main sequence whose left end is called the cutoff. The B-V of the stars at the cutoff gives us the color and the age of the brightest stars still on the main sequence and the age of the cluster itself. The V gives the apparent magnitude of these stars and distance to these stars and the cluster itself.

NGC2477 is 700 million years old and has about 300 members. Globulars are much older and larger. This one looks like a globular because it’s so close at 4200 lightyears.

A similar example farther north is NGC7789 also called the “Magnificent Cluster”. Either one is worth a long leisurely look with the weather allows.

DT

_______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php Visit the Wiki: http://www.utahastronomy.com

_______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php Visit the Wiki: http://www.utahastronomy.com