Hubble eyes the Cartwheel.

Millions of years ago, a small, energetic galaxy plowed through the core of a large, quiescent one. Luckily for astronomers, this changed the dormant galaxy in dramatic fashion.

Like a pebble cast into a pond, the tiny intruder generated a ripple of energy that expanded outward from the center of the big galaxy. Traveling at about 320,000 kilometers per hour, the wave compressed gas and dust in front of it and ignited rings of star birth in its wake. This process has so far lasted for some 200 million years. At the forefront of the wave lies the bluest, newest batch of stars; closer to the core reside redder, older stars.

That's how astronomers believe the Cartwheel, a striking, ring-shaped galaxy complete with spokes and a brilliant, bulls-eye core, got its shape (SN: 4/18/92, p.248). Images recently taken by the Hubble Space Telescope depict with unprecedented clarity the Cartwheel's highly organized structure. The pictures also provide new clues to what the Cartwheel, located 500 million light-years from Earth, may have looked like before the fateful collision.

Hubble's pictures reveal hundreds of bright blue knots -- individual clusters of newborn stars -- that the expanding wave generated. Huge loops and bubbles indicate where massive stars, also formed in the aftermath of the collision, exploded as supernovas, hurling their contents into space. In addition, the images show in new detail the galaxy's true colors -- its red center and blue outskirts -- notes Kirk D. Borne of the Space Telescope Science Institute in Baltimore.

"This is a chronometer on the galactic scale," he says. "We see old and new stars separated in space."

Borne and his collaborators note that the galaxy's interior contains so little dust that Hubble's wide-field and planetary camera saw right through it, imaging a more distant galaxy that lies directly behind the Cartwheel.

This lack of dust suggests that before the collision, the Cartwheel was merely an immense cloud of hydrogen gas or a galaxy with a very low density of stars. If so, the collision awoke a sleeping giant, setting the Cartwheel ablaze with stars millions of years before the galaxy would otherwise have created them. "We see no evidence for a pristine population of stars underlying the pattern [of those generated by the interloper]," says Borne.

Although scientists had hoped the Hubble images might unveil the identity of the intruding galaxy, the pictures haven't narrowed the list of suspects. The lower of the two small galaxies to the right of the Cartwheel appears distorted and recently experienced a burst of star formation, features that make it a likely candidate. On the other hand, the top galaxy contains little gas, so the missing material may have been stripped as this galaxy passed through the Cartwheel.

A third galaxy, not shown in the accompanying image, also go remains a suspect.