M3, also known as NGC 5272, GCl 25, C 1339+286, is a Class VI Globular Cluster located in the northern constellation Canes Venatici, the Hunting Dogs, and since it was first observed, this globular star cluster has gone on to become one of the best-studied objects in the night sky. It is one of the brightest, largest globular clusters in the sky with an apparent magnitude of 6.2 and is approximately 33,900 light-years distant from Earth. It has an absolute magnitude of about -8.93 and a luminosity about 300,000 times that of the Sun. The cluster is approaching us at 147.6 km/s. Messier 3 is the prototype for the Oosterhoff type I cluster, which is to say a metal-rich globular cluster or one with a high abundance of elements other than hydrogen and helium compared to other globular clusters. M3 contains an estimated half a million stars. The brightest stars in the cluster are of magnitude 12.7 and the average brightness of the 25 brightest stars is 14.23 mag. The overall spectral type is F2. The cluster has a total mass of about 450,000 solar masses. Messier 3 can be found at right ascension 13h 42m 11.62s and declination +28°22’38.2” halfway from the bright star Arcturus in Boötes constellation to Cor Caroli in Canes Venatici. It lies about 6 degrees north-northeast of Beta Comae Berenices, near the border between the constellations Canes Venatici and Boötes. The best time of year to observe the cluster from northern latitudes is during the months of March, April and May.
Messier 3 is one of the 250 or so known globular clusters in the Milky Way Galaxy. The cluster lies 38,800 light-years or 11,900 parsecs from the galactic centre and 31,600 light-years or 9,700 parsecs above the plane of the Milky Way, in the galaxy’s halo. When observed from Earth, the cluster lies in the direction of intergalactic space, opposite to the galactic centre. The dense core of M3 measures 1.1′ in diameter, corresponding to 11 light-years, while the entire cluster spans about 180 light-years, corresponding to an apparent diameter of 18 arc minutes. Messier 3 also commands about 760 light-years of space – meaning that it keeps all stars within that distance tied to its rich core. The Cluster is believed to be between 8 and 11.4 billion years old. It contains mostly old, red stars.
The cluster is also home to an unusually large number of variable stars than any other known cluster. The brightness of a variable star fluctuates with time. For some variable stars, their period relates to their intrinsic luminosity, so astronomers can use those stars’ brightness fluctuations to estimate their distances. This makes them extremely useful for measuring distances to deep-sky objects. M3 contains at least 274 variable stars. The first variable star in the cluster was discovered by the American astronomer and physicist Edward Charles Pickering in 1889. The American astronomer Solon Irving Bailey identified the next 87 in 1895 and another 138 by 1913. New variable stars continue to be discovered in the cluster to this day. Of these, at least 170 stars are RR Lyrae variables, of which about a third display the Blazhko effect of long-period modulation. The overall abundance of elements other than hydrogen and helium, what astronomers term the metallicity, is in the range of –1.34 to –1.50 dex. Messier 3 also contains a relatively high number of blue stragglers, blue main-sequence stars that appear to be young and are bluer and more luminous than other stars in the cluster. These stars are now believed to form as a result of stellar interactions.
The cluster was discovered by Charles Messier on May 3, 1764. It was the 75th deep-sky object ever observed at the time of discovery and the first object in the Messier catalogue discovered by Messier himself, who noted: “On May 3, 1764, when working on a catalogue of the nebulae, I have discovered one between Bootes & one of the Hunting Dogs of Hevelius, the southern more of the two, exactly between the tail & the paws of this Dog, according to the charts of Flamsteed. I have observed that nebula on the meridian, & I compared with Mu Bootis; its right ascension has been found as 202d 51′ 19″, & its declination as 29d 32′ 57″ north. That nebula which I have examined with a Gregorian telescope of 30 pouces focal length, which magnifies 104 times, doesn’t contain any star; the centre is brilliant, & the light gets lost fading [outward]; it is round, & could have 3 minutes of arc in diameter. One can see it in a good sky with an ordinary [nonachromatic] refractor of one foot [FL].” William Herschel was the first to resolve Messier 3 into individual stars and recognise it as a cluster in 1784. He observed M3 using a 20-foot long reflector and described it as “one of the globular clusters; very brilliant and beautiful. The compression of the stars begins to increase pretty suddenly from the outside at 3/4 of the radius, and continues gradually up to its centre, its diameter taking in the outside is fully half of the field of the glass magnifying 171 times, giving 4’30”.” In 1832, John Herschel (William Herschel’s son) observed the cluster and made the following entry: “A most superb object, diam = 10s.0 time in RA. Not less than 1000 stars 11m and under. They run into a blaze at the centre, and form as it was radiating lines and pointed projections from the mass, with many stragglers.” Credits: Messier Objects, NASA, Universe Today, Wikipedia.