Despite the shorter nights, summer is great time to pursue globular clusters. Messier objects 4 and 80 are in Scorpius. M22 is in Sagittarius. The Hercules cluster M13 is high in the sky overhead at 11:00 PM about July 15. Under truly dark skies, M13 can be a naked eye object. On the other hand, from the city, even with a moderate telescope, M4 can be hard to obtain as will be the less well known M92 in Hercules. Even so, with the milder nights, if you want to stay up or go out early, you can catch M15 in Pegasus on the meridian about 4:00 AM on July 15. Whether easy or challenging to enter in your observation logs, better appreciation for the success of finding them comes from understanding what you are looking at. (This entry is from my notes collected for an article to appear in the June 9 issue of Sidereal Times of the Austin Astronomical Society.)
“Though the clusters show in their arrangement, a definite relation to the galactic plane, they are not concentrated close to it; indeed, there is a conspicuous absence of globular clusters near the central line of the Milky Way in the sky, and within about 2000 parsecs from the galactic plane in space. There is no known reason why globular clusters should not exist near this plane, and it is probable that those which are there are hidden from us.” [6]
“The globular clusters in the Milky Way are all estimated to be at least 10 billion years old and therefore contain some of the oldest stars in the galaxy. They contain an abundance of low-mass red stars and intermediate-mass yellow stars, but none greater than 0.8 solar masses. There are about 150 known globular clusters in the Milky Way. It is thought that globular clusters formed very early in the vast halo surrounding the nascent galaxy before it flattened to form the spiral disc. Star formation would have stopped in these clusters maybe 13 billion years ago, so only old stars are expected to be found there.” http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/globular.html)
Excellent introductions to the hobby of observational astronomy such as NightWatch by Terence Dickinson all tend to give the same easy explanations. Of 150 or so globular clusters orbiting the Milky Way, about 30 are readily found by dedicated observers with telescopes of larger aperture under darker skies. When it was accepted that the Milky Way is just another galaxy the examination of other galaxies led to new understanding of the nature of globular clusters and to new questions about them.
Globular clusters are far more prevalent in elliptical galaxies than in spirals and it is accepted as likely that many are in fact the cores of elliptical galaxies. [3.b] Elliptical galaxy M87 (Virgo A) is often available to 60-mm refractors, given dark skies suited to its 8.6 magnitude, second brightest in the Virgo Cluster. It has thousands of globular clusters.[3.b] [3.c] perhaps 15,000 [https://en.wikipedia.org/wiki/Messier_87]. Close studies of the motions of globular clusters have revealed tidal trails of stars being left behind. [3.b] Globular clusters are also known “in the field,” meaning in the otherwise unorganized free space between galaxies where they are detected by instruments seeking the farthest (oldest) galaxies. [3.c]
Blue Stragglers
In the dense cores of globular clusters, a typical star has a significant chance to undergo a collision. The velocity gradient within a globular cluster is one to two orders of magnitude less than escape velocity. Therefore, almost all of the mass in a collision is retained. Moreover, the velocities of the materials are less than the escape velocities from stellar bodies of the original masses. Therefore, the mass is retained to form the merged product. If the sum of the products is significantly large (more than 1 solar mass), the merged product will remain on the H-R main sequence but will be positioned bluewards on the turnoff from the Main Sequence. These have been labeled “blue stragglers.”[5]
Blue Stragglers were first identified in 1953 by Alan R. Sandage. His data came from examination of Messier 3 and Messier 95, which are often available to small telescopes and M71 which usually is not. So far, over 400 Blue Stragglers have been identified in 20 globular clusters. They still do have a completely integrated theoretical explanation. Available data strongly suggests a causal link between the unperturbed evolution of binary stars and the existence of Blue Stragglers. [3]
REFERENCES
Book Review - Seeing in the Dark: Your Front Row Seat to the Universe
Measuring Your Universe: Alan Hirschfeld’s Astronomy Activity Manual
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