Gauging star birth: spacecraft uses gamma rays as stellar tracer.

By detecting the radioactive remains of material hurled into space by dying stars, astronomers have estimated that, on average, our galaxy churns out seven new stars each year.

The researchers used the European Space Agency's INTEGRAL spacecraft to record gamma-ray light, which is high-energy radiation undetectable from Earth's surface. They collected the particular wavelength that arises from the radioactive decay of aluminum-26. The distribution of this aluminum isotope traces the location of dead massive stars in the Milky Way. These stellar heavyweights forge nearly all the galaxy's aluminum, which they expel when they die in explosions known as supernovas.

The INTEGRAL team, led by Roland Diehl of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, confirmed that aluminum-26 is found primarily in star-forming regions of the galaxy. In the Jan. 5 Nature, the researchers conclude that over the past few million years, an average of two massive stars per century have died as supernovas in the galaxy.

Using theoretical models of the number of massive stars in relation to the total number of stars in the Milky Way, the team also calculated that seven new stars appear each year and that their total mass is about four times that of the sun.

That star-formation rate agrees with those derived from other methods of estimating star birth, notes study coauthor Dieter Hartmann of Clemson University in South Carolina.

Determining star-formation rates in the Milky Way galaxy is a tricky business, he adds. Astronomers have previously used visible and ultraviolet light emitted by newborn stars. However, such radiation is obscured by gas and dust clouds that tend to concentrate in the Milky Way's spiral arms, where most new stars form. In contrast, gamma rays easily penetrate these clouds.

Aluminum-26's relatively long half-life of 750,000 years also aided in the new estimate, says Hartmann. That longevity enabled INTEGRAL to record the emissions from stars that perished during the past several million years.

Diehl and other researchers had previously constructed maps of the galaxy's aluminum-26 by using less-sensitive instruments, such as a detector on the now-defunct Compton Gamma Ray Observatory (SN: 1/25/,92, p. 53). But in those older maps, researchers were concerned that a significant amount of the gamma-ray emission might be coming from the sun's neighborhood or star formation at a few localized sources rather than from throughout the galaxy.

The spectrometer on INTEGRAL, launched in 2002, has a critical advantage over previous detectors. It's sensitive enough to record a variety of tiny shifts in the wavelength of gamma-ray light that arise from the rotation of objects spread across the Milky Way.

The shift "is telling us that the aluminum-26 is almost certainly associated with the [entire] galaxy," rather than just a few locations within it, according to James Kurfess of the Naval Research Laboratory in Washington, D.C. The new map therefore validates the use of aluminum-26 as a highly precise gauge of the recent history of supernovas and star birth in the Milky Way, he adds.