[madbadgalaxyman (Robert)]

The first section of this post is copied from an email I wrote about some of the unusual types of massive (at least 10,000 solar masses) and compact and centrally-concentrated Star Clusters that are not the familiar Globular Star Clusters with their exclusively old (9-13 billion year old) stellar populations. There are several different types of these Extremely Rich and Very Centrally Concentrated star clusters that look a lot like globulars, but which are not exactly globular star clusters.

Then I include an MPIA press release, which tries to answer the following question:
"will the young star cluster NGC 3603, which is already looking a lot like a globular, eventually turn into a more familiar 'old' globular star cluster?"

Then I attach a review paper about Nuclear Star Clusters, which are another variety of rich, massive and centrally-concentrated Star Cluster that greatly resembles a globular.
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From madbadgalaxyman's email:

At about 10,000 solar masses or more (as being the value of the total cluster mass), young and intermediate-aged star clusters in our own Galaxy and other galaxies, adopt a steep and centrally-concentrated Radial Profile of surface density..... which is similar to the radial profile that is observed in the canonical "old" Globular Star Clusters.
[[ “surface density” (in professional astronomical jargon) refers to the apparent two-dimensional density of stars or of surface brightness, rather than the actual three-dimensional density of the stars in a cluster ]]

NGC 3603 is an example of this type of cluster. Other clusters of this type include:
- the Very Compact cluster R136 at the core of the Tarantula Nebula
- Westerlund 1 (which is significantly obscured, and in our own Galaxy)
- the Arches and Quintuplet clusters that are near to the center of our own Galaxy.

Another object which is arguably a low-mass Globular Star Cluster that is not old......is Messier 11 (this object is normally regarded as a high-mass open star cluster.)

These objects are often called "Super Star Clusters" or "Massive Compact Young Clusters” in the literature, but essentially they can be thought of as looking a lot like young globular star clusters.(normal globulars are some 9-13 billion years old, which is nearly as old as the oldest stars in the universe).

It would seem that these possible"young globular clusters" can be formed just about anywhere that the pressure & density of the cold molecular ISM reaches a very high value, for instance:
- where the ISM of a galaxy gets a mighty shock that is caused by a galaxy collision or a galaxy merger (e.g. in NGC 4038/9 and in NGC 3256)
- in the largest molecular gas complexes, which can most commonly be found in the "starburst" regions at the centres of barred spiral galaxies and in the spiral arms of non-dwarf Sc galaxies.

(( A good example of a galaxy with some intermediate aged (<7 billion years old) globular clusters is NGC 1316, which has undergone one or more mergers with other galaxies. ))
(( Some tentative evidence has been produced for the existence of massive & compact & centrally-concentrated young "globular-like" Star Clusters of up to 100,000 solar masses within the galaxy M83, but I don’t know if there is any recent work that has 100% confirmed this idea.))

A related type of star cluster, which again looks very similar to an ordinary globular star cluster, is the "nuclear star cluster" which is often found at the very center of many galaxies (including our own!). Nuclear Star Clusters can be a lot more massive and luminous than even the most massive “old” globular star clusters, and nuclear star clusters can contain stars of various ages. NSCs are easiest to see in type Sd galaxies (e.g. NGC 300 and NGC 7793) , due to the very light screen of dust that exists within this type of galaxy.

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MPIA Press release about NGC 3603
(this press release is not 100% true to the actual science results, but it is better than nothing!!)

WILL NGC 3603 BECOME A GLOBULAR CLUSTER ?

Stars in Motion: High precision follow-up study of star movement shows surprising unrest in massive star cluster

Using the NASA/ESA Hubble Space Telescope, astronomers from the Max Planck Institute for Astronomy in Heidelberg and the University of Cologne have completed a long-term study of one of the most massive young star clusters in the Milky Way, comparing two observations that were made ten years apart. The comparison, which relies on extremely precise measurements, reveals the motions of several hundreds of stars, which prove to be at odds with current models of how such clusters evolve, stellar motion not having “settled down” as expected. The results have been published in the Letters section of the Astrophysical Journal.

Ordinary star clusters ("open star clusters") disperse over time, as the different stars go their own separate ways. Very massive and compact clusters are thought to be different. In the long term, this can lead to the development of massive aggregations of stars known as "globular clusters", whose tightly-packed stars remain gravitationally bound to each other for billions of years.

With a mass of more than 10,000 suns packed into a volume with a diameter of a mere 3 light-years, the massive young star cluster NGC3603 is one of the most compact stellar clusters in the Milky Way. (For comparison: in our own immediate stellar neighborhood, the same volume contains no more than a single star, namely the Sun.) Could this be a globular cluster in the making?

To find out, a team of astronomers led by Wolfgang Brandner (Max Planck Institute for Astronomy, Heidelberg, MPIA) tracked the movement of the cluster's many stars. Such a study can reveal whether the stars were in the process of drifting apart, or about to settle down. It also serves to distinguish members of the star cluster from unrelated stars that, as viewed from Earth, just happen to fall along the same light of sight.

By using two observations, made ten years apart with the same camera aboard the Hubble Space Telescope, and by performing an intricate analysis to account for all possible disturbances, Brandner and his colleagues were able to reach the required accuracy.

All in all, the team observed more than 800 stars. About 50 of these were identified as foreground stars, which are unrelated to the cluster. From the remaining sample of more than 700, the astronomers were able to obtain sufficiently precise speed measurements for 234 cluster stars of different masses and surface temperatures. Boyke Rochau (MPIA), the paper's lead author, who performed the data analysis as part of his PhD work, explains: "Once our analysis was completed, we reached a precision of 27 millionths of an arc second per year. Imagine you are in Bremen, observing an object that is located in Vienna. Now the object moves sideways by the breadth of a human hair. That's a change in apparent position of about 27 millionths of an arc second."

The results for the motion of these cluster stars were surprising: According to widely accepted models, which reproduce what is actually observed in older globular clusters, the average stellar speed in a cluster like the one in NGC3603 should depend on mass: Stars with lower mass should move faster, and those with higher mass should move more slowly. The stars for which precision measurements were possible represent a range of masses between 2 and 9 times that of the Sun. Yet all of them move at about the same average speed of 4.5km/s (corresponding to a change in apparent position of a mere 140 micro-arc seconds per year). Average speed does not appear to vary with mass at all.

Apparently – and surprisingly – this very massive star cluster has not yet settled down. Instead, the stars' velocities still reflect conditions from the time the cluster was formed, approximately one million years ago. Team member Andrea Stolte from the University of Cologne explains: "For the first time, we have been able to measure precise stellar motions in such a compact young star cluster. This is key information for astronomers trying to understand how such clusters are formed, and how they evolve."

Vexingly, the question of whether or not the massive young cluster in NGC3603 will become a globular cluster remains open. Given the new results, it all depends on the speeds of the low-mass stars, which were too faint to allow for precise speed measurements with the Hubble Space Telescope.

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Here is a review paper about Nuclear Star Clusters, from IAU Symposium S266 in the year 2009:


NSCs_best version.pdf

You will note, from reading this paper, that there are many different types of small spheroidal stellar objects, ranging up to the size of dwarf spheroidal galaxies!!

See also this conference:

http://moca.monash.edu/conferences/compact _____________________________
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