Let’s continue our survey of Messier objects. Check some basic info if you’re joining late here.
The star cluster marked M5 is very old. How do we know? Stars age at different rates. Large, bright stars burn out very quickly. Smaller, less massive stars have longer lifespans. Our sun, a smallish yellow and quite average star, has an expected lifetime of about ten billion years, according to scientific actuarial tables.
Blue stars are larger and more massive than the sun. The largest of them might last significantly less than a tenth of one percent of the age of the sun. A red dwarf the size of Jupiter will shine with a fraction of the sun’s energy for a trillion years or more. Looking at stars in the M5 cluster, reds, oranges, and yellows tend to indicate age. We think these clusters are older than the sun. Much older. But there are also some blues here, see:
Image credit, ESA and Hubble. Description here.
If M5 were really old, how did the blue ones get there? It could be that with the tight packing of a hundred-thousand stars in a sphere about a quadrillion miles across, some collisions have occurred. When two yellow stars collide or merge, the resulting star will burn hotter, more blue, and more quickly. Seem to me M5 must have been more populated in the past for so many to be seen here. Another explanation, perhaps? Or maybe one of many causes.
By the way, if you are looking for this object, about the same size to the Earthbound eye as one of the largest craters on the moon, check the constellation Serpens. Summertime is good; sorry. Map here.