Dating a Cluster – A New Trick


Finding the ages of things in astronomy is hard. While it is undoubted that the properties of objects change as they age, the difficulty lies in that the initial parameters are often so varied that, for most cases, finding reliable ages is challenging. There’s some tricks to do it though. One of the best ones, taught conceptually in introductory astronomy courses, is to use the “main sequence turn-off” of a cluster. Of course, applying any of these methods is easier said than done, but a new method may help alleviate some of the challenges and allow for smaller errors.

The largest difficulty in the main sequence turn-off method lies in the inherent scatter caused by numerous sources that must be accounted for. Stars that lie along the same line of sight as the cluster being observed can add extraneous data points. Any interstellar reddening caused by gas may make stars appear more red than they should be. Close binary stars that cannot be spatially resolved appear brighter than they should be as an individual star. The amount of heavy elements in the star will also effect the fitting of the model. All of these factors and more contribute to an uncertainty in any calculation that requires an accurate Hertzsprung-Russell Diagram. Tricks to correct for some of these factors exist. Others cannot (yet) be accounted for.

Thanks to all these problems, fitting the data can often be challenging. Finding the point where the cluster “peels away” from the main sequence is difficult, so one of the tricks is to look for other points that should have significant numbers of stars to provide extra reference points for fitting. Examples of this include the horizontal branch and the red clump.

The new technique, developed by a large team of international astronomers, uses “a well defined knee located along the lower main sequence” which they refer to as the Main Sequence Knee (MSK). This “knee” appears in H-R diagrams of the clusters taken in the near-infrared and is largely independent of the age of the cluster. As such, it provides a stable reference point to improve corrections for the general main sequence turn-off method. Additionally, since this system uses infrared wavelengths, it is less prone to contamination between gas and dust.

To test this new method, the group selected a globular cluster (NGC 3201) as a test case. When their method was applied, they found that their derived age for the cluster was consistent with ages derived by other methods.

However, the new method is not without difficulties of its own. Since the knee is at the faint end of the main sequence, this requires that exposure times for target clusters be sufficiently long to bring out such faint stars. Fortunately, with new telescopes like the the James Webb Space Telescope, these faint stars should be in reach.

11 Replies to “Dating a Cluster – A New Trick”

  1. All this is true, however, the problem, especially for the young clusters is the stars are formed over period of time and not all at once. Therefore the line of the zero-aged main sequence (ZAMS) is not known to and certainty. This is probably the largest error in determining the cluster’s ultimate ‘average’ age.
    Stellar physics largest difficultly with the H-R Diagram is the appearance of double (or even multiple) main sequences – varying in either XYZ composition parameter or in having separate periods of star formations.

    As time moves on, theses evolutionary differences are not as pronounced, but there are other changes as well. I.e. Loss of stellar members by the cluster’s dynamics.

    The truth is that many areas of the H-R Diagrams are used in constraints on cluster ages. It is a bit naive to suggest that the MSK is somehow uniquely new or for “Others cannot (yet) be accounted for.”

    A “new method” isn’t exactly how I’d put it. About instead saying “enhanced method” is closer to the truth.Better still. You don’t necessarily need the James Webb Telescope to improve these theories. Such a connection is tenuous at best.

    Note; Love these stellar evolution and the thorn they leave in the intelligent designers!!

  2. At least this is better than EU Anaconda, who still believes star evolve up along the main sequence instead of AWAY from it!

    Truly hilarious!

  3. You’re quite right that waiting for the ZAMS to even out is a problem. I could have listed it with all the other difficulties, but I figured I had enough.

    And I think it’s fair to say that some of the things I listed cannot be properly accounted for with some techniques. With a simple photometric survey unresolved binaries can’t be picked out. If you go through spectroscopy, you can, but even then, only for ones that have some of their motion along our line of sight.

    I also think “new method” is reasonable here. The overall idea (finding a distinct point to use as a corrective factor) isn’t new. In fact, I exploited it during my undergraduate internship (

    What is new is what feature it uses. So there is a novel element that I think justifies me calling it “new”.

    And if you like stellar evolution posts, I’d suggest reading through some of the posts I’ve done on my blog regarding the topic. Generally I take special care to point out how they contradict the Creationist point: (

  4. It is my understanding that main sequence stars evolve by peeling off the HR diagram and moving along a path more or less heading to the upper right. This puts a lot of ordinary stars in the red giant region. Of course a former F through K class star that end up there is different from a big Antares-like star that can be 20 or more times the mass of the sun.


  5. The assumption of isochronal behaviour is always a problem is both open and globular clusters.

    Jon, an interesting article that highlights many of the problems is that of the southern intermediate age NGC 4815 and its quite unusual CMD a. This appears in the paper ; Carraro, G. and Ortolani, S. >A HREF=””>”Deep CCD BV photometry of the poorly studied NGC 4815″ , A&A.Sup.Ser., 106, 573 (1994)

    This ideally highlights the ‘killer’ issues of the “age range”, Z parameter variations and with issues like the appearance of dual main sequences. This article also highlights in an extreme case of problems with field stars to related to the cluster and these unresolved binaries. (it is a good basic article to grasp the larger issues in this news item and this paper in question)

    This same cluster is discussed in more detail by Prisinzano, L. “Luminosity and mass function of galactic open clusters
    I. NGC 4815
    A&A. 369, 851 (2001)

    [Unlinked – we are seemingly only allowed one link per post. You can find it in the ADS.]

    Here they conclude;

    “NGC 4815, which has an age of 0.5 Gyr (Carraro & Ortolani 1994), has a MF [mass function] slope fully consistent with this value. The MFs of the clusters older than 0.5 Gyr have a similar or flatter slope. This indicates that their PDMF [Present day mass function] may have been changed by the dynamical evolution which has had more time to operate. A dynamical model of each of these clusters is necessary to confirm this interpretation”

    The problem, as I understand it, is the end of the main sequence and this MSK kink, is more difficult to interpret.

    As to the binary formation, this same paper of Prisinzano;

    “Among others, a very important result is that the cluster evolution crucially depends on the Initial Mass Function (IMF), and on the cluster richness. De la Fuente Marcos (1997) found that power-law (Salpeter 1955) like IMFs accelerate the cluster disruption if the initial population is small, and that at the end of the evolution, a huge number of binary systems with the same mass is formed.”

    As far as I know,, the IMF of globulars is different – as the initial population is huge – and therefore the number of binary system is fairly small. (Besides creation of binaries and blue stragglers are created in globulars, acting like the small gravity sinks, are few in number and are more distinct from the area of the MSK as they are so evolutionary different?)

    Thanks for the response earlier!

  6. Lawrence B. Crowell said;

    “It is my understanding that main sequence stars evolve by peeling off the HR diagram and moving along a path more or less heading to the upper right. This puts a lot of ordinary stars in the red giant region.”

    Yes it does, but in globulars, most of the star are lighter than 0.8 solar masses. They solar-like star evolve more slowly than the sun, and mostly occupy the main sequence. Only stars around one solar mass in globulars will occupy the AGB portion of the HR diagram.

    Of course a former F through K class star that end up there is different from a big Antares-like star that can be 20 or more times the mass of the sun.

    More to the point >1.2 Solar mass stars are already dead and buried, existing as white-dwarfs. The 20+ Solar mass stars were all extinguished in the first 5% of the globular star age.


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