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See Inside October 2005

Ripples in a Galactic Pond

Astronomers are coming to realize that the beautiful shapes of galaxies are not merely incidental. They are essential to the galaxies' growth and development

The elegant spiral shape of galaxies is one of the quintessential sights of astronomy. A classic example is the galaxy Messier 51: it resembles a giant cyclone, and one of the first names given to it was the "whirlpool." The brightest stars in this galaxy are confined like pearls on a coiled necklace. Running alongside the strands of stars are dark swaths of dust, which betray the presence of interstellar gas, from which stars are born. In Messier 51 and many other galaxies, the spiral pattern is anchored by an inner ring of stars, but in most the spiral begins in a bar--a long, luminous rectangle of stars. A barred galaxy looks like a spinning lawn sprinkler, where the water flows through a straight tube, emerges at right angles and then curves around.

Most people think of our own galaxy, the Milky Way, as a pure spiral, but astronomers now know it is actually a barred spiral. The evidence, at first indirect, began to accumulate in 1975: stars and gas tracked in the middle of the galaxy did not follow the orbits they would if the spiral pattern reached all the way in. Recent surveys of the sky in near-infrared light, which penetrates the dust clouds that block our view of the galactic core, have revealed the bar directly and dispelled the remaining doubts.

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