Globular clusters are almost spherical, compact, gravitationally bound collections of stars, containing around ten thousand to a million members. They range in diameter from a few tens to over 300 light-years and lie round the edge of our Milky Way Galaxy. Over one hundred are known, but all are very remote; Messier listed 28 of them.
On a clear evening, look for the brilliant star Arcturus (Alpha Bootis) and draw a line up to the northwest until you hit Cor Caroli (Alpha Canum Venaticorum), the lone star visible to the naked eye just under the end of the Dipper’s handle.
Now, aim your binoculars near the midpoint of this line and then look for a right triangle of three dim stars that points toward the southeast. Spot it? If you look carefully, you will notice that the star marking the right angle is not a perfect point of light. Instead, it looks like a tiny, fuzzy “star blob”. You are actually seeing a distant globe of more than 100,000 stars – a globular cluster called M3.
The French astronomer Charles Messier discovered M3 on the evening of May 3, 1764, and this marked an important point in the history of astronomy. This accidental sighting, along with his earlier discoveries of a globular cluster in Aquarius and the Crab Nebula (M1) in Taurus sparked his interest in compiling his now-famous catalog of 110 deep sky objects.
M3 looks great even through the smallest backyard telescope. The cluster has a wide, bright center that accounts for about half of its apparent width. Surrounding the center are dozens of stars whose density gradually decreases with their distance from M3’s core. Larger telescopes with a high power will reveal hundreds of stellar points, massing to a central blaze, with glittering streams of stars running out on all sides.
M3 has a mass of nearly 800,000 suns and its stars range from magnitude +11 or so to the limit of detectability. It orbits the Galaxy on a elliptical path that is highly inclined with the galactic plane and it takes about three hundred million years to make one revolution of the Galaxy, never straying farther than about 50,000 light years from the Galactic center.
Finder map – field width 15°, stars to magnitude +8.