BDanielMayfield wrote:Today’s apod is eye opening as to the relativity of the open-ness of “open” clusters. I had thought that open clusters were called that to distinguish them from globular clusters in which the stars are permanently bound together gravitationally, whereas in open clusters the stars all eventually disperse. I didn't know that an “open” cluster could be both old and compact as NGC 2158 clearly is. This cluster looks like a wimpy version of a globular cluster. My question is, will NGC 2158's stars disperse given enough time, or is it gravitationally bound?
There is no sharp dividing line between globular clusters and open clusters. Check out this very interesting page which contains information and pictures of M53 and NGC 5053
. (Check out this page
, too.) M53 and NGC 5053 are both globular clusters, and they are at similar distances from us, about 60,000 light-years. They are probably of similar ages, likely 10-12 billion years old. Both are typically metal-poor and contain blue horizontal stars, the kind of stars that are only found in metal-poor populations. Both are made of gas that has probably been recycled through only one or two previous generations of massive stars. Yet they are very different, because M53 is a typically rich globular cluster, whereas NGC 5053 is loose and star-poor.
Why do they look so different? There can be only one answer. M53 was the product of a truly amazing blaze of star formation, much, much bigger and fiercer than, say, R136
in the Large Magellanic Cloud. NGC 5053, by contrast, probably started out no bigger or heavier than R136. Over billions of years, NGC 5053 has mostly but not completely evaporated, in the same way that R136 will have shrunk to an unimpressive faint loose cluster ten billion years from now. M53 has also lost a lot of stars, but because of the much higher gravity it started out with, it has held on to its constituent stars much more efficiently than NGC 5053.
How can we say, then, that rich cluster NGC 2158 is an open cluster, when faint loose cluster NGC 5053 is a globular? It has to do with the age and metallicity of these clusters. NGC 2158 is only about one billion years old
, according to Wikipedia. At this age, it is sure to be much, much more metal-rich than NGC 5053; that is, the gas that the cluster was born from had been enriched by heavy elements produced by many generations of massive stars and supernovae. Also, the color-magnitude diagram is sure to look different for NGC 2158 than for NGC 5053. NGC 2158 is sure to have hydrogen-fusing stars that are more massive than the Sun, while NGC 5053 is sure to lack them. And while NGC 5053 is sure to have blue horizontal branch stars, NGC 2158 is sure to lack them.
Here is the color-magnitude diagram for NGC 5053
. Note at upper left a small but definite population of relatively bright blue horizontal stars. Here is a color-magnitude diagram for NGC 2158
, although it must be pointed out that it is not a B-V diagram but a J-H diagram, which I am not so good at reading. Two B-V diagrams would have made the comparison easier.
There is one anomalously blue star in NGC 2158, which is seen at 7 o'clock in NGC 2158 in today's APOD. Personally I can't help wondering if this star might not be a foreground star, perhaps even a star that belongs to cluster M35.
P.S. Happy 232 (233?), starsurfer!