Hubble spies colliding galaxies

Distant and distorted: Two galaxies pass each other

By BBC News Online science editor Dr David Whitehouse

The Hubble Space Telescope has captured a stunning view of galaxies in collision showing how the event can trigger a burst of star formation.

The object astronomers call NGC 6745 is actually two galaxies passing through each other.

When they started to collide a few hundred million years ago, both galaxies became distorted by the gravitational attraction of the other.

Streamers of stars were scattered into interstellar space as parts of the two galaxies were dismembered by tidal forces.

Empty space

Because galaxies are mostly empty space, when they collide the stars will almost never smash into each other. The galaxies pass freely between each other with little stellar carnage.

But the situation is quite different for the stuff between the stars. This material consists largely of clouds of atomic and molecular gases, and dust.

When the interstellar clouds of the two galaxies come into contact, they do so dramatically. The high relative velocities cause so-called ram pressures at the contact surface. This pressure causes the clouds to fragment and collapse to form new stars.

The hot blue stars in the image are evidence of this formation process. They form near the site of the collision as they live for just a short time compared with the crossing of the galaxies.