Globular Cluster Photo Series (Part 12): M80

Today for your perusal I have an image of the globular cluster Messier 80. This is a very populous cluster with several hundred thousand stars and also one of the more densely populated ones, as all those stars are contained in a sphere only about 95 light-years across. Given M80's appreciable distance of about 32,600 light-years from us that translates to a somewhat smaller angular diameter of 10.0 arcminutes.

Messier 80 in Scorpius.

One curious incident in M80's history started around the time of the outbreak of the Civil War on May 21, 1860, when a nova (dubbed T Scorpii) was observed that briefly and spectacularly outshone the rest of the cluster (although it was still invisible to all but the most sensitive eyes with ideal dark sky conditions). Like all novae, this one was most likely the result of a white dwarf accreting mass from a larger binary companion star which eventually builds up to the point that it flash fuses extremely rapidly -- think an H-bomb with the mass of a small planet distributed across the surface of the star -- and blows away any gas that didn't fuse, leading to a massive brightening of the star. Novae in globular clusters are rather rare as a rule, with only a few others known. Interestingly, observations of M80 with the Hubble Space Telescope have only revealed two candidate binary systems for novae, a lot fewer than were expected based on models given the density of M80's core.