Astronomy: Messier 3 a globular cluster of stars

M3 is the second by Charles Messier observed globular cluster of stars. It is one of the largest and brightest clusters in the Northern hemisphere, easy visible with binoculars. It contains about 500.000 stars, most of which are old and red. At a distance of 35.000 light years from Earth it orbits the center of our Milky Way Galaxy and spans about 150 light years across. Like many star clusters, this star cluster appears constant because photographs of them are frozen in time. In reality, though, cluster stars swarm the center and frequently fluctuate in brightness. Although the time it takes for stars to cross a cluster is about 100.000 years, the time it takes for a star to fluctuate noticeably can be less than one night (!). In fact, the following time lapse movie of bright globular cluster M3 was taken over a single night.	M3 pictured here as a composite of blue and red images

Most of the variable stars visible above are RR Lyrae stars, stars that can quickly double their brightness while becoming noticeably bluer. Furthermore, RR Lyrae stars vary their light in a distinctive pattern that allows unique identification. Lastly, since RR Lyrae stars all have the same intrinsic brightness, identifying them and measuring how dim they appear tells how far they are, since faintness means farness. These distances, in turn, help calibrate the scale of the entire universe. Related subjects >> Messier catalog of 110 deep sky objects >> Charles Messier >> Milky Way Galaxy