Astronomy Journal Entry

Spectacular View of Colliding Galaxies (March 18, 2006)

NASA's Spitzer Space Telescope recently caught a view of a massive collision of galaxies. The huge interstellar crash is creating one of the biggest shock waves ever seen.

One of the galaxies in a group known as Stephan's Quintet is slamming into three neighboring galaxies at a speed of over a million miles per hour (1.6 million kilometers per hour).

Stephan's Quintet is a cluster of five galaxies found in the constellation Pegasus about 300 million light-years from Earth. A light-year is the distance light travels in one year, or about 6 trillion miles (10 trillion km).

Several years ago, astronomers noticed the collision in the galaxy cluster but couldn't see the details with regular telescopes. The Spitzer telescope uses infrared rays to see space events more clearly. The telescope observed that a galaxy known as NGC7318b is creating an enormous shock wave called "bow shock" as it races towards its neighbors. This is similar to the ripples in water made by the front or bow of a moving boat.

Shock waves occur when an object moves faster than the speed of sound through a medium. On our home planet, the medium is air or water. In outer space, the medium is interstellar gas. Waves traveling at the speed of sound have a rating of Mach 1. The shock waves from the intergalactic collision are greater than Mach 100.

In the picture above, the centers of galaxies in Stephan's Quintet are bright pink spots inside clouds of blue stars. The crash has already stripped away most of the hydrogen gas from these galaxies' interiors. The galaxy crashing into the others is the left-most of two pink spots just to the right of the picture's center.