M3 - Globular Cluster in Canes Venatici


M3 - Cluster in Canes Venatici
Mag 6.2, 18 arcminutes across
RA 13:42.2 Dec 28 23'
This from SEDS.ORG:

Discovered 1764 by Charles Messier. Messier 3 (M3, NGC 5272) is one of the most outstanding globular clusters, containing an estimated half million stars. It is famous for the large number of variable stars discovered in it.

This cluster was the first 'original' discovery by Charles Messier when he logged it on May 3rd, 1764. At that time it was the 75th deep sky object ever observed by human eyes (and apparatus), although at that time, it was only the 54th known nebulous object, while 21 objects had been forgotten again, according to the sources and current knowledge of the present author (see the Deep Sky Object Discovery Table). Perhaps the discovery of this object eventually caused Charles Messier to start a systematical search for these comet resembling objects, and not just catalog chance findings as in the previous cases, M1 and M2. Alternatively, Messier may have started this endeavor due to other reasons, and it was just his first discovery - anyway, the search which started with M3 lead him to catalog the objects up to M40 during this year 1764.

M3 was first resolved into stars and recognized as a cluster by William Herschel around 1784.

At a distance of about 33,900 light years, M3 is further away than the center of our Galaxy, the Milky Way, but still shines at magnitude 6.2, as its absolute magnitude is about -8.93, corresponding to a luminosity of about 300,000 times that of our sun. M3 is thus visible to the naked eye under very good conditions - and a superb object with the slightest optical aid. Its apparent diameter of 18.0 arc minutes corresponds to a linear extension of about 180 light years; Kenneth Glyn Jones mentions an estimate of even 20 arc minutes from deep photographic plates, corresponding to about 200 light years linear diameter. It appears somewhat smaller in amateur instruments, perhaps about 10 minutes of arc. But its tidal radius, beyond which member stars would be torn away by the tidal gravitational force of the Milky Way Galaxy, is even larger: About 38.19 minutes of arc. Thus, this cluster gravitationally dominates a shperical volume 760 light years in diameter.

On the other hand, M3 has a compressed, dense core measuring 1.1' in diameter, or linearly, 11 light years, comparatively large for a globular. Its half-mass radius is 1.12', or about 11.2 light years, so that half of this clusters mass is contained in a volume of only 22 light years in diameter.

Situated in the Galactic halo, out about 40,000 light-years from the Galactic Center, M3 is moving on a box-type orbit of approximate excentricity 0.55, which takes it out up to 66,000 light-years apogalactic distance and up to 49,000 light-years above and below the Galactic plane (currently it is about 33,000 light-years above - i.e., north of - that plane). On the other hand, its perigalactic distance is only 22,000 light-years - at that distance, the tidal radius of M3 will go down to below 200 light-years, so that the outermost stars may easily escape from this globular cluster.

Globular cluster M3 is extremely rich in variable stars: According to B. Madore (in Hanes/Madore, Globular Clusters, 1978), 212 variables have been found, 186 periods determined, more than in every other globular cluster in our Milky Way galaxy (and thus the most ever observed); at least 170 RR Lyrae variables (sometimes called "cluster variables") were discovered. These stars have served as "standard candles" to determine the cluster's distance. The first variable star was discovered by E.C. Pickering in 1889, the next 87 were found by S.I. Bailey in 1895 (see Pickering and Bailey 1895), and to 1913, 138 by Bailey. Christine Clement (2009) lists 268 variables, 11 suspects and 24 formerly designated non-variables, most of them (253) RR Lyra's.

M3 contains a relatively large number of so-called Blue Stragglers, blue main-sequence stars which appear to be rather young, much younger than the rest of the globular's stellar population would suggest. These were first discovered by Alan Sandage (1953) on photographic plates taken with the 200-inch Hale telescope on Mt. Palomar. A mystery for a long time, these stars are now thought to have undergone dramatic changes in stellar interactions, getting their cooler outer layers stripped away in close encounters, which occasionally occur when stars are passing through the dense central regions of globular clusters.

To find M3, either prolong the line from Gamma Comae Berenices near the Comae Berenices Cluster over Beta Comae by about 2/3 and look slightly north to have M3 in the low-power field: it is about 6 degrees north-northeast of Beta Comae.

While M3 is visible to the naked eye only under very good conditions and stays just below the limit of visibility under more average conditions, it can be easily seen with the smallest instrument. In binoculars, it appears just like a hazy, nebulous patch. A 4-inch shows its bright compact core within a round and mottled, grainy glow, which fades slowly and uniformly to the outer edges; it doesn't resolve the cluster, but shows just some of the brightest stars under good conditions. A 6-inch resolves the about outer two thirds into faint stars on a background glow formed by the unresolved fainter member stars of the cluster. An 8-inch shows stars throughout the cluster but in the very core, which is resolved into stars by larger telescopes (about 12-inch).

Camera: Apogee U16M w Astrodon Gen II LRGB
Scope: 12.5" Planewave CDK
Mount: Paramount ME guided with an SBIG ST-402ME and Astrodon MMOAG
LRGB 6 x 10 min
Acquired with CCDSoft5 and CCD-Commander
All images reduced aligned and combined in Maxim
Colour combines, Levels and Curves in PS CS4 and Russ Croman's GradX was needed to remove terrible gradients (thanks Russ!)

Flesherton, Ontario
April 24 2014