Hubble sees 'wheel within wheel'

By Dr David Whitehouse BBC News Online science editor

A stellar "wheel within a wheel" is how one astronomer described it.

It is an unusual galaxy called Hoag's Object. The Hubble view of the galaxy is the most detailed image ever obtained of this object. It may help astronomers unravel how such strange objects form.

The entire galaxy is about 120,000 light-years wide, slightly larger than our Milky Way Galaxy.

The blue ring - which is dominated by clusters of young, massive stars - contrasts sharply with the yellow nucleus of mostly older stars.

This unusual galaxy was discovered in 1950 by astronomer Art Hoag. Hoag thought the smoke-ring-like object resembled a planetary nebula, the glowing remains of a Sun-like star.

But he quickly discounted that possibility, suggesting that the mysterious object was most likely a galaxy.

Big smash

It was not until the 1970s that observations confirmed this prediction, though many of the details of Hoag's galaxy remain a mystery.

The galaxy is 600 million light-years away in the constellation Serpens.

What appears to be a "gap" separating the two types of stars may not be a gap at all but contain star clusters that are almost too faint to see.

Curiously, an object that bears an uncanny resemblance to Hoag's Object can be seen in the gap. It is probably a background ring galaxy.

Ring-shaped galaxies can form in several different ways. One way is through a collision with another galaxy. Sometimes the second galaxy speeds through the first, leaving a wave of star formation in its wake.

In Hoag's Object, however, there is no sign of a second galaxy, which leads to the suspicion that the blue ring of stars may be the shredded remains of the galaxy that passed nearby.

Some astronomers estimate that the encounter occurred about two to three billion years ago.