The southern summer Milky Way is rich in globular clusters. Several are within about a binocular field-width of bright, orange-red Antares. M80 isn’t so prominent, but can be located easily halfway between Graffias (Beta Scorpii) and Antares.
The discovery of M80 has been credited to both Messier and Mechain, both of whom recorded the cluster in January 1781; Messier’s observation seems to have preceded that of his friendly rival by about three weeks. Messier describes it as “A nebula without a star in the Scorpion, between the stars g [now Rho Ophiuchi] and Delta. It was compared with g to determine its position. The nebula is round, its centre brilliant and it resembles the nucleus of a small comet, surrounded by nebulosity.”
At magnitude +7.3 you’ll easily sweep up M80 through a 3-inch telescope. Its stars appear tightly packed, so at 70x it looks more like a tiny, glowing cloud with a bright core. Under excellent conditions (and fairly high power), one can begin to resolve delicate dark rifts in the central core, but even then there’s no real resolution of the cluster into individual stars.
When observing M80, you’ll notice the magnitude +8.5 star SAO 184288. It sits only 4’ northeast of the cluster’s center, but has nothing to do with M80.
M80 has a mass of nearly five hundred thousand Suns and a diameter of about 50 light years. It’s a bulge cluster (meaning it orbits inside the central bulge of our Galaxy) and takes about 70 million years to complete one revolution about the Milky Way. The cluster is home to the largest known concentration of “blue stragglers”, hot stars that have remained in their hydrogen-burning stage longer than they should because stellar mergers have boosted their fuel supplies.
M80 is also the first globular cluster to have a nova discovered in it. The new star (known as T Scorpii) appeared in 1860 and for a few days outshone the globular itself. It soon faded away, but in case it’s a recurrent nova, M80 is worth monitoring.
Finder map – field width 15°, stars to magnitude +8.
