Globular clusters are fascinating and mysterious. What formed them? What was the universe like when they were born?
In any case, globular clusters come in basically two kinds, very low-metal ones with numerous so called RR Lyrae variables (which are exellent, if slightly faint, standard candles for distance estimates), and higher metal-ones with very few, if any, RR Lyrae variables. The globulars that are rich in RR Lyrae variables also have a rich population of blue horizontal stars, whereas the higher-metal, RR-Lyrae-poor globulars have few or no blue horzontal stars.
This picture shows the horizontal branch (HB) of globular clusters very well. The blue horizontal branch is the blue part of the horizontal branch. The RR Lyrae stars are located in the "gap" of the horizontal branch. Most of the bright globulars of the Milky Way are very metal-poor and have blue horizontal branches and RR Lyrae stars. One example is Omega Centauri, which contains at least 74 RR Lyrae stars
. Compare that with more metal-rich globular cluster 47 Tuc, which may have only one RR Lyrae star
altogether! 47 Tuc has a very short horizontal branch
and lacks the blue part of it completely.
What sort of beast is NGC 2419, then?
We present a new, deep (V ~ 26) study of the Galactic globular cluster NGC 2419 based on B, V, I time-series CCD photometry over about 10 years and extending beyond the cluster published tidal radius. We have identified 101 variable stars, of which 60 are new discoveries, doubling the known RR Lyrae stars and including 12 SX Phoenicis stars. The average period of the RR Lyrae stars (Pab = 0.662 days and Pc = 0.366 days, for fundamental-mode—RRab—and first-overtone pulsators, respectively) and the position in the period-amplitude diagram both confirm that NGC 2419 is an Oosterhoff II cluster. The average apparent magnitude of the RR Lyrae stars is V = 20.31 ± 0.01 (σ = 0.06, 67 stars) and leads to the distance modulus μ0 = 19.60 ± 0.05. The color-magnitude diagram, reaching about 2.6 mag below the cluster turnoff, does not show clear evidence of multiple stellar populations. Cluster stars are found until r ~ 10.5', and possibly as far as r ~ 15', suggesting that the literature tidal radius might be underestimated. No extratidal structures are clearly detected in the data. NGC 2419 has many blue stragglers and a well-populated horizontal branch extending from the RR Lyrae stars down to an extremely blue tail ending with the "blue hook
," for the first time recognized in this cluster. The red giant branch is narrow, ruling out significant metallicity spreads. Our results seem to disfavor the interpretation of NGC 2419 as either having an extragalactic origin or being the relict of a dwarf galaxy tidally disrupted by the Milky Way.
On the Remote Galactic Globular Cluster NGC 2419 - ResearchGate. Available from: http://www.researchgate.net/publication ... r_NGC_2419
[accessed Jun 4, 2015].
The colors, larger font size and use of bold text are my additions. But according to this article, NGC 2419 has probably been born in the Milky Way, or at least it appears to be more associated with the Milky Way than with any other galaxy.