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Last Updated: Thursday, 22 April, 2004, 14:45 GMT 15:45 UK
Hubble sees glowing ring of fire
By Dr David Whitehouse
BBC News Online science editor

ring galaxy, hubble
A splash of stars on the sky
The Hubble Space Telescope has seen a brilliant circle of bright blue stars in a rare example of a "ring galaxy" - the result of a galactic collision.

The galaxy is larger than our own Milky Way and lies far away in the direction of the southern constellation Dorado.

This image was released to commemorate Hubble's 14 years of working in space.

Earlier this year, the US space agency Nasa said there would be no more manned servicing missions to Hubble, a decision that will limit its lifespan.

Galactic collision

The galaxy, catalogued as AM 0644-741, is a prime example of a ring galaxy.

These galaxies are the result of collisions that dramatically change the structure of the impactors involved while also triggering the formation of new stars.

Such galaxies are created in a particular type of collision, in which one galaxy plunges directly through the disc of another.

In the case of AM 0644-741, the galaxy that pierced through the other's disc is out of the image but visible in larger-field images.

The shock of the collision has drastically changed the orbits of stars and gas in the penetrated disc, causing them to rush outward.

Gas clouds in the expanding ring have collided and been compressed. They have then contracted under their own gravity to form brilliant, new stars.

The blue colour comes from these hot, young stars.

Another sign of robust star formation is the pink regions along the ring which are clouds of glowing hydrogen gas, fluorescing because of the strong ultraviolet light from the newly formed massive stars.

Theoretical studies indicate that the blue ring will not continue to expand forever. After about 300 million years, it will reach a maximum radius, and then begin to disintegrate.

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