An analysis of the most energetic light in the galaxy has revealed that we may be wrong about star formation rates in the Milky Way.
Gamma rays produced by the radioactive decay of isotopes produced during star formation reveal that stars form at a rate of four to eight times the mass of the Sun per year. That might not seem like much, but it’s two to four times more than current estimates, suggesting that our home galaxy isn’t quite as quiet as we’d thought.
And this has important implications for our understanding of the evolution of our galaxy, and those around us, since the rate at which stars are born and die can change a galaxy’s overall chemical composition.
A paper describing the discovery, led by astrophysicist Thomas Siegert of the University of Würzburg in Germany, has been accepted for publication in Astronomy and astrophysicsand is available on the preprint server arXiv.
Stars are the factories that produce the universe’s more complex elements. Their cores are nuclear furnaces, which smash atoms together to forge them into ever larger atoms. When they die, their violent death struggles spew these heavier elements into interstellar space, to drift in clouds or be absorbed into new stars that form. Their supernova explosions are also energetic, creating even heavier elements that their cores could not support.
Like their deaths, star births are also energetic. They form from dense clumps in clouds of interstellar dust and gas, collapse under gravity and greedily gobble up material from the space around them until there is enough pressure and heat in their cores to ignite fusion. As they do so, they begin to emit powerful stellar winds that blow particles into space, and jets launched from their particle poles accelerate along the baby star’s magnetic field.
One element known to result from stellar death is a radioactive isotope of aluminum called aluminum-26. Aluminum-26 doesn’t last long, cosmically speaking; it has a half-life of 717,000 years. And when it decays, it produces gamma radiation at a specific wavelength.
But aluminum-26 is also present in significant amounts in the clouds of material surrounding newly forming stars. If the speed at which material falls into a star exceeds the speed of sound, a shock wave is formed that generates cosmic rays. As the rays collide with isotopes in the dust, such as aluminum-27 and silicon-28, they can produce the isotope aluminum-26.
So, by looking at the budget of gamma radiation in the universe produced by the radioactive decay of aluminum-26, astronomers can estimate the rate at which stars generating the isotope form and die in the Milky Way, and use that to determine an overall rate of star generation.
Current estimates for the star formation rate of the Milky Way Galaxy are around two suns worth converted to stars each year. Since most of the stars in the Milky Way are much less massive than the Sun, it is estimated to average around six or seven stars annually.
Siegert and his colleagues took a census of the aluminum-26 gamma radiation in the galaxy, and performed modeling to see the most likely production mechanism for the observed abundance of this light. They found that the best fit is a star formation rate of about four to eight solar masses per year; or up to about 55 stars per year.
There is still room for improvement on this estimate; the models did not fully reproduce the Milky Way’s gamma radiation as it is now observed; and the distance to the gamma-ray source may change the final estimate, but it is difficult to measure. This is why the researchers could only provide a range for the star formation rate, rather than a specific mass.
However, the team’s method shows promise for a better understanding of how the Milky Way creates new stars. Star formation is usually shrouded in thick gas and dust that is difficult to see into; counting the gamma radiation it produces can be an effective way to peek behind the curtain.
The team’s research has been accepted for publication in Astronomy and astrophysicsand is available on arXiv.