6 January 2006
Gamma rays from a rare aluminium isotope produced by exploding stars appear to permeate our galaxy, the Milky Way.
The new measurement has allowed astronomers to predict a supernova rate of two explosions per century in our galaxy, confirming the supernova rate seen in other galaxies. The researchers also calculated that about 7.5 stars are born in our galaxy every year.
Massive stars and supernova explosions create a radioactive isotope of aluminium – aluminium-26. The decay of the isotope to magnesium creates gamma rays that astronomers are able to observe.
The team used ESA’s INTEGRAL spacecraft, launched in 2002, to examine the distribution of gamma rays in the disc of the Milky Way. Astronomers could see that the gamma rays were coming from the central region of the galaxy and not simply from local hot spots in the foreground, near Earth.
Matter closer the centre of the galaxy rotates faster than matter farther out, counterbalancing the increased gravitational tug at the heart of the Milky Way. This means the gamma rays are affected differently by the Doppler shift, and this can be detected from Earth: gamma-rays produced in regions closer to Earth would be shifted less.
Gamma rays penetrate the dust and gas that obscures many of the star-forming regions of the galaxy. “This observation of aluminium-26 creates an instant snapshot of the galaxy for us,” says Dieter Hartmann, an astronomer at Clemson University, South Carolina, US, and a member of the research team. “It has drawn us an accurate picture that we haven’t had before.”
Based on the distribution of gamma rays, Roland Diehl, of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and colleagues estimated that the aluminium-26 in the galaxy has a mass equivalent to that of three suns.
Aluminium-26 has a half life of 750,000 years, allowing researchers to track the number of recent supernovae. They inferred that one supernova explosion about every 50 years would be needed to generate the amount of aluminum-26 seen in the galaxy, though none have been recorded for a while: “We’ve actually been overdue now for 300 years,” Hartmann told New Scientist.
“Our galaxy isn’t the biggest producer of stars and supernovae in the universe, but there’s still plenty of activity,” Diehl notes. “A sustained star formation rate of this magnitude is just what one needs to drive its chemical and dynamical evolution, which has led to life on Earth.”
Journal reference: Nature (vol 439, p 45)