U.S. and British astronomers have located the youngest known remnant of an exploding star in the Milky Way. The discovery might help researchers understand why our galaxy seems to have so few supernovas and where the raw materials of planets and life came from.
The Milky Way is a perfectly ordinary spiral galaxy, except for a shortage of supernova activity. These titanic explosions, which mark the deaths of giant stars, produce the heavy elements--including oxygen and carbon--necessary to form planets, life, and people. With more than 300 billion stars in the Milky Way, astronomers expect to observe a supernova about three times every century. That should translate into about 60 supernova remnants in the galaxy that are under 2000 years old, after which they begin to fade away and become undetectable. But so far, astronomers have found fewer than 10.
The discrepancy is so large that theoretical physicists wonder if the Milky Way is unusual in some unknown respect. Another possibility is that somehow the missing supernova remnants occur at energies and wavelengths that make them difficult to detect.
The new object could help answer these questions. It's called G1.9+0.3 and is located about 26,000 light-years from Earth, near the center of the Milky Way. Astronomers first studied it in 1985, but they didn't recognize its age. Then last year, researchers using NASA's orbiting Chandra X-ray Observatory took another look at the object and realized that its boundaries had expanded by 16% in the intervening 22 years. A second viewing by an earthbound radio telescope array confirmed the size of the expansion, and team members calculated that the debris of G1.9+0.3 is moving faster than any supernova remnant ever seen before--some 15,000 kilometers per second. Based on that speed, the team is reporting jointly in upcoming editions of Astrophysical Journal Letters and the Monthly Notices of the Royal Astronomy Society that G1.9+0.3 expanded to its current size in only about 100 years.
Given its speed, G1.9+0.3 could be expelling energy at wavelengths that researchers don't usually check for, co-author David Green of Cambridge University in the U.K. said at a teleconference today. If other young supernovas do the same, he said, it might explain why they've been overlooked in previous searches. Still, the work "highlights the fact that the other [young] remnants are still missing," said lead author Stephen Reynolds of North Carolina State University in Raleigh. "It's a serious problem," he told reporters at the same teleconference. The discovery "is a step in the right direction," Reynolds added, "but there are a lot of steps."
The find is a "cause for celebration," says astronomer Shri Kulkarni of the California Institute of Technology in Pasadena. "We have what finally appears to be a genuine young supernova remnant." Researchers still don't understand where in the supernova and when the heavy elements form, so Kulkarni predicts there will be a "mad scramble" to detect them in G1.9+0.3. And astronomer James Lloyd of Cornell University says it's an "important discovery" with "profound implications" for our understanding formation of "the necessary ingredients for life throughout the universe."