Why fewer stars are being born today.

• Position: Galaxies

"We have known for more than a decade that in the early universe--3,000,000,000 to 5,000,000,000 years after the Big Bang or 9,000,000,000 to 11,000,000,000 years before today--galaxies churned out new stars at a much faster rate than they do now," remarks Michael Cooper, a postdoctoral fellow at Arizona's Steward Observatory, who, along with other astronomers, has helped solve a mystery surrounding the birth of stars in galaxies that long has puzzled scientists.

"What we haven't known is whether this was because they somehow formed stars more efficiently or because more raw material--molecular gas and dust--was available," notes Benjamin Weiner, assistant astronomer at Steward Observatory. Compared to the average galaxy today, which produces stars at rates equaling about 10 times the mass of our sun per year, the rate of star formation in those same galaxies appears to have been up to 10 times higher when they were younger.

Because the universe is expanding, galaxies behave a bit like cosmic ambulance tracks: As they move farther away from an observer based here on Earth, the light they emit shifts to a slightly lower frequency--like the sound of a passing ambulance on the highway--toward the red in the light spectrum. "By observing those galaxies in the infrared spectrum and measuring their radio frequency emissions, we are able to make their cold gas clouds visible," explains Cooper. Adds Weiner, "What we find now is that galaxies...