Revealing A Hybrid Star Cluster


Almost a century ago, astronomers Shapley and Melotte began classifying star clusters. This rough, initial go-around took in the apparent number of stars and the compactness of the field – along with color. By 1927, these “classes” were again divided to include both open and globular clusters. But there are some that simply defy definition.

According to Johns Hopkins astronomer Imants Platais, there is one case which has puzzled astronomers for decades: a well-known, seemingly open star cluster in the constellation of Lyra, named NGC 6791.

“This cluster is about twice the age of the sun and is unusually metal rich (at least twice the Sun’s metallicity),” said Platais, of the Henry A. Rowland Department of Physics and Astronomy’s Center for Astrophysical Sciences. “A couple of decades ago, it was also found that NGC 6791 contains a handful of very hot but somewhat dim stars, called hot subdwarfs. The presence of such stars in an open cluster is rare, though not unique.”

Why are these hot subdwarfs an anomaly? The facts about star clusters as we know them are that globular clusters are notoriously metal poor, while open clusters are metal rich. “The massive stars that create much of the metals live for only a short time, and when they die, they spit out or eject the metals they have created.” says the team. “The expelled metals become part of the raw material out of which the next stars are formed. Thus, there is a relationship between the age of a star and how much metal it contains: old stars have a lower metallicity than do younger ones. Less massive stars live longer than higher mass stars, so low mass stars from early generations still survive today and are studied extensively.”

A team led by Platais and Kyle Cudworth from The University of Chicago’s Yerkes Observatory set out to solve the mystery of NGC 6791 by taking a census of its stars. Their findings revealed several luminous stars in the horizontal branch of the HR diagram… Stars that would normally be found in globular clusters. The hot subdwarfs were confirmed to be genuine cluster members, but they now “appear to be simply the bluest horizontal branch stars”. What’s wrong with this picture? NGC 6791 contains simultaneously both red and very blue horizontal branch stars – making it both old and metal rich. Quite simply put, studying star clusters is key to understanding stellar evolution – unless the cluster starts breaking the rules.

“Star clusters are the building blocks of galaxies and we believe that all stars, including our own sun, are born in clusters. NGC 6791 is a real oddball among about 2,000 known open and globular star clusters in the Milky Way and as such provides a new challenge and a new opportunity, to our understanding of how stars form and evolve,” said Platais, who presented this work last week at the 218th meeting of the American Astronomical Society in Boston.

So… what about star clusters in other galaxies? Three hybrids have been discovered (2005) in the Andromeda Galaxy – M31WFS C1, M31WFS C2, and M31WFS C3. They have the same basic population and metallicity of a globular cluster, but they’re expanded hundreds of light years across and are equally less dense. Are they extended? Or perhaps a dwarf spheroidal galaxy? They don’t exist (as far as we know) in the Milky Way, but there’s always a possibility these hybrid clusters may call other galaxies home.

Until then, we’ll just keep learning.

Original Story Source: John Hopkins University.

8 Replies to “Revealing A Hybrid Star Cluster”

  1. Astronomer Imants Platais: “Star clusters are the building blocks of galaxies and we believe…”
    I would like to put on a suit of armour, walk up to Imants Platais, place my left hand on his right shoulder, and then…

    1. giggle! i caught that in platais comment and reckoned you would as well. now, since the word has been called to my attention, i can’t “un-see” it.

      no one escapes the spanish inquisition!

  2. It’s almost as everything has its own place and purpose and all the galaxies and clusers together made the unique life on Earth possible. I would not be surprised if the science actually proved the non-existence of any of humanoid life in the whole galaxy for the sole purpose of the life on Earth being possible…

  3. I know how you guys would like to promote your site, and I think you deserve it but I think google’s implementation of *publicly* plastering your “likes” all over the internet and in their search engine is completely obnoxious and one-sided, and not only that they want to associate it with your real name so you have a trail all over the web for eternity of your activities. No thanks.

  4. “A Hybrid Star Cluster” Quite disturbing nonsense.
    Differences between open star clusters and globular clusters is based on dynamics; NOT population nor chemical compositions. I.e metal poorness, as stated in this article.

    For example, do these intermediate clusters have hard core or soft core binaries or do they also suffer core collapse like the globular clusters?

    l’d admit, NGC 6791, whose main sequence has all but disappeared. I.e. This is unlike the turn off point of a typical globular star cluster

    As for “By 1927, these “classes” were again divided to include both open and globular clusters” is so wrong I’m dumfounded. In fact, initial classification was the physical divisions between the open clusters and globulars — open clusters ‘a’ to ‘g’ and globular clusters as I to XII.

    Please learn something more about open and globular clusters if you want to look knowledgeable on the topic. Open clusters and globulars are very different in many respects, and are mostly dynamically chalk and cheese.

    1. RE:

      Please learn something more about open and globular clusters if you want to look knowledgeable on the topic.

      To be fair to the author Tammy, she was merely reporting the news release from John Hopkins University (the source link is at the bottom of the article); therefore, if you’ve got an axe to grind on the terminology used, then I suggest that you send them a note.

      1. Nah. None of the papers related to this cluster or press release says this is some hybrid cluster. They just don’t exist, and are unlikely to be found in nature. It is an attempt by this author to ramp up attention to this article using specious ideas unrelated to our current astrophysical knowledge.
        Not mentioning dynamics at all, shows the shallowness of the story.

      2. “None of the papers related to this cluster or press release says this is some hybrid cluster.”

        The JHU press release titled:

        ““Oddball” Star Cluster is a *Hybrid*, JHU Astronomer Finds” mentions:

        “This adds to the peculiarities previously known of being both metal rich and old. Thus, NGC 6791 is the first known star cluster juxtaposing the properties of open and globular clusters and, as such, represents a new class of star clusters, which likely originated in the central Bulge region of the Milky Way.

        Essentially, this new work – which appeared in the May 20 issue of Astrophysical Journal Letters – has revealed the *hybrid* nature of this star cluster in our Galaxy.”

        Seems the JHU press release mentions the word *hybrid* twice, as related previously by IVAN. Your point concerning lack of any mention of the stellar dynamics in the JHU press release on NGC 6791 is noted.

        FWIW, the paper by Platais et al can been found here:

        [….and the word *hybrid* does not, as you note correctly, appear in this paper]

        *asterisks added by me

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