## Thursday, April 12, 2012

### Globular Cluster Photo Series (Part 17): M3

Today I have a picture of one of the biggest and brightest globular clusters in the Northern Hemisphere, narrowly beaten out by the likes of M13. This globular cluster, Messier 3, is found in the northerly constellation of Canes Venatici, the Hunting Dogs, just below the handle of the Big Dipper asterism.

M3 is, as its name indicates, the 3rd object in Messier's popular list of bright non-comets, and as such it may have the distinction of being the object that prompted him to begin a systematic search for these objects instead of simply cataloging ones that he came across by chance.

M3 is a bright cluster that currently stands as the record-holder for largest number of variable stars – 274 – found in a globular cluster to date. It has an angular size on the sky of 18.0 arcminutes, a bit less than two-thirds of the width of the full Moon and ever-so-slightly smaller than M13's 20.0. It is actually a bit larger than M13 physically at 180 light-years across (10 more than M13),  but is also a bit further away from us at 33,900 light-years compared to M13's 25,000. Even this far away from us, further away than the center of our own galaxy, it still shines just bright enough to possibly be seen with the naked eye under pristine conditions at apparent magnitude 6.2. With any sort of magnification, of course, it looks quite nice. (For comparison, the distance to the supermassive black hole Sagittarius A* at the center of our galaxy is 25,900 $$\pm$$ 1,400 light-years.)

 Messier 3 in Canes Venatici. Click for larger picture.
M3 also beats out M13 in the number-of-stars department, having perhaps 500,000 compared to M13's 300,000 which puts it at about half the number of Omega Centauri, and in the top few percent for Milky Way globulars overall. This unusually large population is probably one reason so many variable stars have been discovered in it.

Currently M3 is cruising at about 40,000 light-years away from the galactic core, which is quite respectable when you consider that the Milky Way is only about 100,000 light-years across, and which puts M13 even further from the core than we are from M13. (You know you're an astronomer when you find yourself saying things like “only one-hundred-thousand light-years” – that's only about six-hundred-thousand trillion miles, for you curious.) Despite this distance, it probably has a better view of the core than we do as it is about 33,000 light years above the plane of the disk, while we have to look through all the dust and gas in the plane of the galaxy between us and the core.