Widdekind Posted December 22, 2009 Share Posted December 22, 2009 NGC 6287 is the oldest Globular Cluster in the Milky Way Galaxy. And, according to this article, the cluster's Main-Sequence Turnoff happens at a V-I Color Index of ~1.3. Then, according to this site, that V-I Color Index corresponds, according to the formula: [math]T_{eff} = \frac{9385}{0.95 + (V-I)} K[/math] to an effective surface temperature of roughly Teff = 4200 K. Finally, according to the appendix of Carroll & Ostlie's Introduction to Modern Astrophysics (1st ed.), that effective surface temperature corresponds most closely with Spectral Class K5, which has [math]M \approx 0.67 M{\odot}, L \approx 0.15 L_{\odot}[/math]. Such a star should have a stellar lifetime longer than our Sun's by a factor of roughly (M / L) = 4.5. CONCLUSION (?!?): Since our Universe is purportedly under 14 billion years old, and then even if NGC 6287 is nearly as old, our Sun should have a lifetime of at most 14 Gyr / 4.5 = 3.1 Gyr. And, conversely, if our Sun really has a lifetime of roughly 10 Gyr, then NGC 6287 (and the Universe) must be at least 45 Gyr old. What went wrong ?? Link to comment Share on other sites More sharing options...
swansont Posted December 22, 2009 Share Posted December 22, 2009 What went wrong ?? Lifetime ≠ age Link to comment Share on other sites More sharing options...
Widdekind Posted December 23, 2009 Author Share Posted December 23, 2009 Shouldn't a K5-class (orange) dwarf live for roughly 4-5 times as long as the Sun ? And, if the K5-class stars in NGC 6287 are already beginning to turn off from the Main Sequence ... ?? Link to comment Share on other sites More sharing options...
D H Posted December 23, 2009 Share Posted December 23, 2009 What justifies your assumption of orange dwarfs, Widdekind? The age of those old globular clusters is determined by red giants that have turned off the main sequence, not orange dwarfs. Link to comment Share on other sites More sharing options...
Widdekind Posted December 23, 2009 Author Share Posted December 23, 2009 What am I missing ? Isn't the "Main Sequence Turnoff" that group of stars that are "still just barely" on the Main Sequence ? According to the cited sources, the last stars still on the MS have a V-I CI of ~1.3, which theoretically corresponds to a Teff ~ 4200 K... which would be, as a MS star, a K-6 or K-5 class orange dwarf... yes?? So, stars smaller than K-6 are still on MS in NGC 6287, while stars bigger than K-5 have already begun (to varying degrees) to "peel off" the MS... yes ?? Link to comment Share on other sites More sharing options...
swansont Posted December 23, 2009 Share Posted December 23, 2009 I misunderstood what you were claiming before. —— Why are you using 1.3? http://venus.mporzio.astro.it/~marco/gc/cluster_4.php?ggc=NGC%206287 V-I is given as 1.74. Link to comment Share on other sites More sharing options...
Widdekind Posted December 27, 2009 Author Share Posted December 27, 2009 I misunderstood what you were claiming before. —— Why are you using 1.3? http://venus.mporzio.astro.it/~marco/gc/cluster_4.php?ggc=NGC%206287 V-I is given as 1.74. Thanks for the link. Is that V-I for the complete cluster, or for the Main Sequence Cutoff ? (Are they the same, or different ?) Link to comment Share on other sites More sharing options...
Widdekind Posted May 29, 2011 Author Share Posted May 29, 2011 I misunderstood what you were claiming before. —— Why are you using 1.3? http://venus.mporzio.astro.it/~marco/gc/cluster_4.php?ggc=NGC%206287 V-I is given as 1.74. Wouldn't V-I = 1.74 > 1.3 "make the problem worse", since that would correspond to an effective temperature of 3500 K ? I'm still confused. Link to comment Share on other sites More sharing options...
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