M3, a globular cluster in Canes Venatici

Image by Danny Flippo
M3 is one of the most outstanding globular clusters, containing an estimated half million stars. At a distance of about 33,900 light years, it 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 16.2 arc minutes corresponds to a linear extension of about 160 light years; Kenneth Glyn Jones mentions an estimate of even 20 arc minutes from deep photographic plates, correspondint to about 200 light years linear diameter. It appears somewhat smaller in amateur instruments, perhaps about 10 minutes of arc. The cluster's brightest stars are of mag 12.7, while the so-called Horizontal Branch giants are of mag 15.7, and the 25 brightest stars have an average brightness of 14.23 mag. The age of globular cluster M3 has been estimated from its color-magnitude diagram on various occasions; historically, early values have been given at 5 billion years (Baade), 11.4 billion years (Woolf), 20 billion years (Arp) and 26 billion years (Sandage). Sandage (1954) counted 44,500 stars brighter than mag 22.5 within a radius of 8 arc min; the total mass has been estimated at 245,000 solar masses (Sandage and Johnson). Helen Sawyer Hogg has given M3's overall spectral type as F2, and a color index -0.05, rather blue for a globular, while the Sky Catalogue 2000.0 gives its spectral type at F7. This stellar swarm is approaching us at 148.5 km/sec.