M80

This stellar swarm is M80 (NGC 6093), one of the densest of the 147
known globular star clusters in the Milky Way galaxy. Located about
28,000 light-years from Earth, M80 contains hundreds of thousands of
stars, all held together by their mutual gravitational
attraction. Globular clusters are particularly useful for studying
stellar evolution, since all of the stars in the cluster have the same
age (about 15 billion years), but cover a range of stellar
masses. Every star visible in this image is either more highly evolved
than, or in a few rare cases more massive than, our own
Sun. Especially obvious are the bright red giants, which are stars
similar to the Sun in mass that are nearing the ends of their lives.

By analyzing the Wide Field and Planetary Camera 2 (WFPC2) images,
including images taken through an ultraviolet filter, astronomers have
found a large population of "blue stragglers" in the core of the
cluster. These stars appear to be unusually young and more massive
than the other stars in a globular cluster. However, stellar
collisions can occur in dense stellar regions like the core of M80
and, in some cases, the collisions can result in the merger of two
stars. This produces an unusually massive single star, which mimics a
normal, young star. M80 was previously unknown to contain blue
stragglers, but is now known to contain more than twice as many as any
other globular cluster surveyed with NASA's Hubble Space Telescope
(HST). Based on the number of blue stragglers, the stellar collision
rate in the core of M80 appears to be exceptionally high.

M80 is also unusual because it was the site of a nova explosion in the
year 1860. Nova outbursts occur when a close companion star transfers
fresh hydrogen fuel to a burned-out white dwarf.  Eventually the
hydrogen ignites a thermonuclear explosion on the surface of the white
dwarf, giving rise to the nova outburst. The ultraviolet Hubble
observations have revealed the hot, faint remnant of this exploding
star, which was named T Scorpii in the 19th century. Curiously,
however, the WFPC2 observations have revealed only two other nova-like
close binary stars in M80, far fewer than expected theoretically based
on the stellar collision rate.

So the blue stragglers in M80 seem to indicate that there are lots of
collisions, yet the nova-like stars suggest only a few. Sometimes life
for astronomers isn't so simple, but it is from exploring
discrepancies like this that our understanding eventually deepens.

This high-resolution image was created from 2 separate pointings of
HST. One WFPC2 data set was obtained by Francesco R. Ferraro (ESO,
Bologna Obs.), Barbara Paltrinieri (U. La Sapienza), Robert T. Rood
(U. Virginia), and Ben Dorman (Raytheon/STX), who study blue
stragglers. The other data set was acquired by Michael Shara (STScI,
AMNH), David Zurek (STScI), and Laurent Drissen (U. Laval) to search
for dwarf novae.

Image Credit: Hubble Heritage Team (AURA/ STScI/ NASA)