Things that go bump in the night
The idea of collisions in space is nothing new - if you've seen the moon at any magnification you've seen craters. There are even a few here on Earth. As
we all know, these are the result of meteor impact - collisions between the moon and wandering space debris. 1994, astronomers were granted an opportunity
to see another level of collision when Shoemaker-Levy 9 assaulted Jupiter. 20 comets in a tight row did a strafing run on the gas giant, causing enormous
planetary destruction and leaving a scar larger than the Earth. It made us take such events more seriously - what if Shoemaker-Levy 9, a comet capable of
leaving earth-sized craters, had hit us instead? 
Such an event would be catasrophic
beyond imagination - certainly ending life on Earth. Can collisions in outer space get any worse?
Raising the bar, the standing theory explaining the formation of the moon involves another kind of collision greater than moon and meteor or planet and comet. 4.5 billion years ago the orbit of Earth crossed that of another proto-planet roughly the size of Mars. The two planets crashed violently, creating enough debris to form a temporary ring around the Earth that aggregated into the moon. This event - which would unquestionably end all life if it happened today - is, ironically, partially attributable for development of Terrian life. But there are levels of collision that surpass even a two-planet pile-up.
Rex Saffer of Villanova University and colleagues have made observations that dwarf such a concept. They have theorized that violent collisions between
small stars (as shown to the right) may explain the existence of massive young stars in old globular clusters. Such collisions between stars -- which
have never been observed and have been assumed to be very rare -- explain the mystery of the blue straggler stars (BSS) contained in cluster NGC 6397.
Clusters like NGC 6397 are very old, and since more massive stars have shorter lives than smaller ones, theories of star evolution predict that the cluster should have no stars more massive than about four-fifths that of the Sun. Yet for decades astronomers have seen BSS in this cluster with masses twice that of the Sun and a corresponding age of just one-tenth of the cluster.
Two explanations have been proposed for BSS formation. One calls for the gradual merger of two stars already tied together in a binary system. The other, more spectacular, explanation is the collision of two or more low-mass stars. The two explanations predict different outcomes for the mass and rotation of BSS.
That observation is backed up by theoretical simulations that favor collisions as the best way to form massive BSS. "Our result is especially exciting because
stellar collisions have long been predicted on theoretical grounds," said Saffer."To the best of our knowledge this is the first observation of the product
of these stellar smashups."
Its hard to imagine anything could exceed the cataclysm of colliding stars, until you consider that galaxies are not above collision! Images
returned from NASA's Chandra X-Ray Observatory launched nearly a year ago continue to give astronomers unprecedented views of the cosmos. Images have even
been collected of a "cannibal" galaxy at the heart of one of the largest gravitationally bound objects in the known universe, a cluster of thousands of
other galaxies. A bubble of relatively cool gas embedded in the 70-million-degree gas of gigantic Perseus A is believed to be a small galaxy of 20 billion
stars colliding with the larger behemoth.