The hundreds of thousands of stars orbiting inside the globular cluster M13 (HST/NASA)

Dating a Cluster – A New Trick

Article Updated: 24 Dec , 2015

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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.



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The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
December 7, 2009 6:38 PM
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… Read more »
The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
December 7, 2009 6:40 PM

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!

Lawrence B. Crowell
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Lawrence B. Crowell
December 7, 2009 8:41 PM

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.

LC

The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
December 7, 2009 8:45 PM
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=”http://adsabs.harvard.edu/abs/1994A%26AS..106..573C”>”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… Read more »
The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
December 7, 2009 8:52 PM
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… Read more »
The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
December 7, 2009 8:53 PM

The first referenced paper should be ; Carraro, G. and Ortolani, S. “Deep CCD BV photometry of the poorly studied NGC 4815” , A&A.Sup.Ser., 106, 573 (1994)

The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
December 7, 2009 8:55 PM

The second is;
Prisinzano, L. et.al. “Luminosity and mass function of galactic open clusters I. NGC 4815” A&A. 369, 851 (2001)

(Wish there was editing or a preview before you post feature here!)

Ivan3man_At_Large
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Ivan3man_At_Large
December 8, 2009 9:24 AM

@ Jon Voisey,

Those two links of yours, above, to your blog don’t work; however, I searched your blog and found the correct URL links:
* angryastronomer.blogspot.com/search?q=astronomy+internship+day
* http://angryastronomer.blogspot.com/search?q=stellar+20+evolution

Ivan3man_At_Large
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Ivan3man_At_Large
December 8, 2009 9:32 AM

P.S. Nice blog, Jon; I’ve now added it to my RSS Feed.

Ivan3man_At_Large
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Ivan3man_At_Large
December 8, 2009 9:40 AM

ERRATUM: Actually, the first link should be this one:
* http://angryastronomer.blogspot.com/search?q=astronomy+internship+day+40

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