Universe Today
Description:
"The post--core-collapse globular cluster NGC 6681 (M70) is characterized by an intermediate metallicity ([Fe/H] = --1.5), a low reddening (E(B--V) = 0.06), and a blue horizontal branch (HB). Photometry of the blue HB stars in the far-ultraviolet ( ~ 1600 Angstroms) images is in good agreement with the WFPC2 Woods filter photometry of Watson et al. (1994, ApJL, 435, L55). The F25CN182 -- F25CN270 color-magnitude diagram shows a tight cluster of blue HB stars, a pronounced blue straggler sequence, and several white dwarf candidates."
"We propose to carry out 70 ksec X-ray imaging of the globular cluster NGC 6681 with Chandra. This cluster was extensively observed in the FUV with HST, yielding the deepest FUV survey of a globular cluster to date. Our X-ray observations will allow us to (i) find X-ray counterparts to the white dwarf - main sequence star binaries in our ultra-deep FUV survey, thus identifying and confirming the cataclysmic variables amongst them; (ii) detect the X-ray faintest interacting binaries (IBs) in this cluster; (iii) classify all X-ray sources based on their X-ray, FUV and optical properties; (iv) and finally, using all the information obtained, test models for IB formation and evolution and verify the empirical results emerging from previous work on other clusters."
"The determination of globular cluster (GC) ages rests on the fact that colour-magnitude-diagrams (CMD) of single-age single-composition stellar populations exhibit specific time-dependent features. Most importantly, this is the location of the turn-off (TO), which – together with the cluster’s distance – serves as the most straightforward and widely used age indicator. However, there are other parts of the CMD, which change their colour or brightness with age, too. Since the sensitivity to time is different for the various parts of the cluster CMD, it is possible to either use various indicators independently, or to use the differences in colour and brightness between pairs of them; these latter methods have the advantage of being distance independent."
"Less than a decade ago the age of the oldest globular clusters appeared to be much higher than that of the expanding universe. But at the end of the last millennium significant improvements both in models and in observational data, notably in the determination of cluster distances by virtue of Hipparcos-based distances, lead to a reduction of cluster ages. Presently, most determinations scatter around a typical age of the oldest objects of 12–14 Gyr. With the growing confidence in the absolute age determinations and an increasing number of extensive homogeneous and high-quality photometric cluster data, the interest has shifted to questions concerning relative ages in order to learn about the formation of the galaxy and its halo and disk components."
History of Observation:
"Nebula without star, near the preceding [M69], & on the same parallel: near it is a star of the ninth magnitude & four small telescopic stars, almost on the same straight line, very close to one another, & [they] are situated above the nebula, as seen in a reversing telescope; the [position of the] nebula was determined from the same star Epsilon Sagittarii.' (diam 2')".