APOD: Double Cluster in Perseus (2014 Jan 23)

[Anthony Barreiro]

I love the blue brilliance of the Pleiades, of course, but the cluster's lack of red giants is a mystery to me.

Thanks Ann, Judy, and the Nitpicker for your help.

As an interested amateur with no formal education in astrophysics, I love the ongoing dynamic tension between my growing understanding of stars and their utter complexity and bottomless mystery. All I have learned to do is to ask (hopefully) more intelligent questions.

[rstevenson]

That's a quite good description of science, Anthony. That's what scientists do, they learn to ask more intelligent questions, over and over again. You're a (budding) scientist!

Rob

[Ann]

Scientifically speaking, if you know something, despite all evidence suggesting something else, it is likely that you don't really know that something.

Statistically speaking, there are enough possibilities in the universe for me to be not puzzled by the lack of red giants in some clusters (and the existence of red giants in others ).

Well and nitpickingly spoken, Nitpicker.

My ramblings were motivated by my own confusion at the similarity yet difference of the two members of the Double Clusters. Why does one of them have red giants but not the other?

On the other hand, one of the papers that Geckzilla provided links to claimed that the halo population of the Double Cluster probably belongs to both clusters, in a way that is slightly similar to a planet orbiting the center of mass of a binary star. (Or rather, imagine a whole retinue of planets orbiting a binary star!) And five out of six red supergiants in the Double Cluster appear (by visual inspection) to belong to the halo population, where they might be orbiting both of the clusters. So when it comes to why one of the clusters in the Double Cluster has red giants but the other has none, it could be that the question is moot.

I can't resist showing you this infrared picture of the Double Cluster. The large yellow-green stars are the red supergiants, whereas the small blue-white stars are the non-red stars. The core of NGC 869 is obvious, but the core of NGC 884 is almost lost in the glare of one of the red supergiants.

Ann

[Anthony Barreiro]

My ramblings were motivated by my own confusion at the similarity yet difference of the two members of the Double Clusters. Why does one of them have red giants but not the other?

On the other hand, one of the papers that Geckzilla provided links to claimed that the halo population of the Double Cluster probably belongs to both clusters, in a way that is slightly similar to a planet orbiting the center of mass of a binary star. (Or rather, imagine a whole retinue of planets orbiting a binary star!) And five out of six red supergiants in the Double Cluster appear (by visual inspection) to belong to the halo population, where they might be orbiting both of the clusters. So when it comes to why one of the clusters in the Double Cluster has red giants but the other has none, it could be that the question is moot.

I can't resist showing you this infrared picture of the Double Cluster. The large yellow-green stars are the red supergiants, whereas the small blue-white stars are the non-red stars. The core of NGC 869 is obvious, but the core of NGC 884 is almost lost in the glare of one of the red supergiants.

Ann

Thanks Ann. I read that paper, or as much of it as I could understand, but didn't make the connection to the possibility that if the red supergiants are orbiting the common center of mass of both clusters, it's just a coincidence that most of the red supergiants we see from our perspective are lined up in front of NGC 884, rather than NGC 869. The answer to my puzzlement seems to be: the universe is big, and I am not at the center of the universe!

[geckzilla]

It's also difficult to say with certainty which stars aren't actually part of any given cluster. A couple of chance alignments could easily throw you off. Must be why at least one of those papers I noticed they focused on studying the very centers of the clusters rather than the extended regions around them.

[Anthony Barreiro]

It's also difficult to say with certainty which stars aren't actually part of any given cluster. A couple of chance alignments could easily throw you off. Must be why at least one of those papers I noticed they focused on studying the very centers of the clusters rather than the extended regions around them.

Yes, I did understand that much. This whole discussion has been very helpful to me. I'm not at a level where I can digest much from astrophysics journal articles, but if somebody explains it slowly and doesn't use too many big words, I can understand a bit more over time ... .

[owlice]

Anthony, you might be interested in the Astrobites site, which is written by grad students for undergrads; see here: http://astrobites.com/