New Mega Prime Numbers

The Great Internet Mersenne Prime Search (GIMPS), a volunteer-powered distributed-computing group, formally announced in September the discovery of the two largest known prime numbers—those divisible only by 1 and themselves. The bigger of the two, 2<sup>43,112,609</sup> – 1 in shorthand, has nearly 13 million digits and came out of the machine of Edson Smith of the University of California, Los Angeles. GIMPS is set to claim the $100,000 prize offered by the Electronic Frontier Foundation for the first 10-million-plus-digit prime. The smaller of the new primes, turned up by a German GIMPS member at 11.2 million digits, would also have qualified but was found two weeks later. —John Matson

Fire Breathing

Air must contain at least 12 percent oxygen for matter to burn, according to conventional wisdom. New experimental burns using pinewood, moss, paper, matches and a candle have convinced scientists at University College Dublin in Ireland that fires need at least 15 percent oxygen. (Air is typically about 21 percent oxygen.) Low oxygen levels, coupled with ancient charcoal evidence of wildfires, have been implicated in mass extinctions in the earth's history. The new findings, in the August 29 Science, suggest oxygen levels could not be as low in some eras as once thought and may help refine models of the ancient atmosphere. —Charles Q. Choi

Stem Cells Against Stroke

Injecting stem cells into the brains of mice that recently suffered a stroke can reduce damage to neurons by up to 60 percent, according to new research. But the stem cells do not simply replace damaged nerve cells as previously believed. Instead they affect the brain's immune cells, called microglia, which go into overdrive during stroke, attacking and destroying healthy tissues. In the mouse experiment the stem cells calmed down the microglia and got them to call off their assault. The treated mice performed better than their untreated peers on a battery of movement, cognitive and behavioral tests. —Nikhil Swaminathan