Universe Today
Description:
"By combining high-resolution and wide-field ground based and space observations, we have collected a multi-wavelength photometric data base (ranging from the far UV to the near infrared) of the galactic globular cluster NGC1904 (M79). The sample covers the entire cluster extension, from the very central regions up to the tidal radius. In the present paper such a data set is used to study the BSS population and its radial distribution. A total number of 39 bright (m218 ? 19.5) BSS has been detected, and they have been found to be highly segregated in the cluster core. No significant upturn in the BSS frequency has been observed in the outskirts of NGC 1904, in contrast to other clusters (M 3, 47 Tuc, NGC 6752, M 5) studied with the same technique. Such evidences, coupled with the large radius of avoidance estimated for NGC 1904, indicate that the vast majority of the cluster heavy stars (binaries) has already sunk to the core. Accordingly, extensive dynamical simulations suggest that BSS formed by mass transfer activity in primordial binaries evolving in isolation in the cluster outskirts represent only a negligible (0--10%) fraction of the overall population."
"Ultra-dense cores of Galactic Globular Clusters (GCs) are very efficient ”furnaces” for generating exotic objects, such as low-mass X-ray binaries, cataclysmic variables, millisecond pulsars (MSP), blue stragglers (BSS), etc. Most of these stars are thought to be the by-products of the evolution of binary systems, possibly originated and/or hardened by stellar interactions. Thus, studying the nature of these exotic objects and the properties of artificial sequences, as that of BSS, in the color magnitude diagrams (CMDs) of GCs can serve as a powerful diagnostic of the dynamical evolution of clusters, and of its effects on the evolution of their stellar population and binary systems. Recent high-resolution observations of the central region of Galactic globular clusters have shown the presence of a large variety of exotic stellar objects whose formation and evolution may be strongly affected by dynamical interactions. The existence of such a population has been a puzzle for many years, and even now its formation mechanism is not completely understood, yet. At present, the leading explanations involve mass transfer between binary companions, the merger of a binary star system or the collision of stars (whether or not in a binary system). Direct measurements and indirect evidence show that BSS are more massive than the normal MS stars, pointing again towards collision or merger of stars."
History of Observation:
"Nebula without star, situated below Lepus, and on the same parallel as a star of sixth magnitude: seen by M. Mechain on October 26, 1780. M. Messier looked for it on the following December 17: this nebula is beautiful; the center brilliant, the nebulosity a little diffuse; its position was determined from the star Epsilon Leporis, of fourth magnitude."
"The 79th of the Connoiss. is a cluster of stars of a globular construction, and certainly extremely rich. Towards the centre the stars are extremely compressed, and even a good way from it. With 171 the diameter is a little less than 1/3 of the field, and with 220 a little more; the field of one being 9'0", and of the other 8'0", a mean of both gives the diameter of the cluster 2'50", but I suppose that the lowness of the situation prevents my seeing the tiny scattered stars, so that this cluster is probably larger than it appears."