Messier 70
Written by Tammy Plotner
Object Name: Messier 70
Alternative Designations: M70
Object Type: Class V Globular Cluster
Constellation: Sagittarius
Right Ascension: 18 : 43.2 (h:m)
Declination: -32 : 18 (deg:m)
Distance: 29.3 (kly)
Visual Brightness: 7.9 (mag)
Apparent Dimension: 8.0 (arc min)
Locating Messier 70: Because the constellation of Sagittarius is so low for the northern hemisphere, it is best to wait until it is at culmination (its highest point) before trying for this small globular cluster. Begin by identifying the familiar teapot asterism and draw a mental line between its southernmost stars – Zeta and Epsilon. About halfway the distance between Epsilon and Zeta (and due south of Lambda) is the location of M70.
In binoculars, M70 will appear almost stellar and very faint – like a hairy star that won't quite resolve. To a small telescope it will appear cometary and begin resolution in apertures around 8". It requires dark, transparent skies and is not well suited to moonlight or urban lighting situations.
What You Are Looking At: At around 29,300 light years from Earth and running away from us at a speed of 200 kilometers per second, this 68 light year diameter ball of stars is only slightly brighter than its neighboring globular cluster – M69. Even though it is being ripped apart by tidal forces from being so near to the center of our galaxy, M70 still has a super dense core region, perhaps the product of a core collapse at one point in its evolution. "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)." says W. Landman (et al), "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."
But what else lay within this unique globular cluster? Try interacting binary stars! "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." says Andrea Dieball, "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."
By closely studying globular clusters such as M70 within our own galaxy, we can get a much better feel for how they age and evolve – giving us a basis for which we can use to study the evolution of other galactic structures. "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." says F. Meissner and A. Weiss, "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."
Is this age and dating thing important? You bet. "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." says Solaris and Weiss, "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: M70 was discovered by Charles Messier and added to his catalog on August 31, 1780, the same night he found M69. In his notes he states: "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')".
On July 13, 1784, Sir William Herschel would be the first to resolve M70 into stars, but his private notes include a very odd entry: "A very faint red perceivable." No where else in historic observations does this occur again! M70 would go on to be observed many times by W. Herschel and to be cataloged by son John as "Bright; round; gradually much brighter toward the middle."
Enjoy your observations!
Top M70 image credit, Palomar Observatory courtesy of Caltech, M70 Hubble image, M70 Chandra Image, Messier 70 2MASS image and M70 color image courtesy of REU program/NOAO/AURA/NSF.
Filed under: Astronomy
