An analysis of the most energetic light in the galaxy has revealed that we may be wrong about the Milky Way’s star formation rates.
Gamma rays, produced by the radioactive decay of isotopes produced during star formation, show that stars are forming at a rate of four to eight times the mass of our Sun per year. That may not seem like a lot, but it’s two to four times more than current estimates, suggesting our home galaxy isn’t quite as calm as we thought.
And this has important implications for our understanding of the evolution of our galaxy and the people around us, since the rate at which stars are born and die can alter a galaxy’s overall chemical composition.
An article describing the finding, led by astrophysicist Thomas Siegert of the University of Würzburg in Germany, has been accepted for publication in Astronomy & Astrophysicsand is available on the preprint server arXiv.
Stars are the factories that produce the more complex elements of our universe. Their cores are atomic furnaces that bang atoms together to forge them into larger and larger atoms. As they die, their violent death spasms spew these heavier elements into interstellar space to drift in clouds or be picked up by newly forming stars. Also their supernova explosions are energetic, to forge even heavier elements who could not carry their cores.
Like their death, star births are energetic. They form from dense clumps in clouds of interstellar dust and gas that collapse under gravity, slurping up material from the space around them greedily 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 fired from their particle poles are accelerated along the baby star’s magnetic field.
One element known to result from the death of stars is a radioactive isotope of aluminum called aluminum-26. Aluminum-26 doesn’t last long cosmically; it has a half-life of 717,000 Years. And as it decays, it produces gamma rays at a specific wavelength.
But aluminum-26 is also present in significant amounts in the clouds of material surrounding newly forming stars. When the speed at which material falls into a star exceeds the speed of sound, a shock wave forms, producing cosmic rays. When the beams collide with isotopes in the dust like aluminum-27 and silicon-28, they can produce the isotope aluminum-26.
So by looking at the universe’s budget of gamma-rays produced by the radioactive decay of aluminum-26, astronomers can estimate the rate at which stars that produce the isotope are forming and dying in the Milky Way, and use this, to determine an overall rate of star formation.
Current estimates for the Milky Way’s star formation rate are obvious Material worth about two suns converted into stars every year. Because most of the stars in the Milky Way are much less massive than the Sun, it’s estimated that it averages around six or seven stars annually.
Siegert and his colleagues made a census of aluminum-26 gamma rays in the galaxy and performed modeling to determine the most likely production mechanism for the observed abundance of this light. They found that a star formation rate of about four to eight solar masses per year works best; or up to about 55 stars per year.
There is still room for improvement on this estimate; the models have not fully reproduced the gamma rays of the Milky Way as currently observed; and the distance of the gamma ray source could change the final estimate but is difficult to estimate. For this reason, the researchers could only give a range for the star formation rate and not a pinpoint mass.
However, the team’s method promises a better understanding of how the Milky Way produces new stars. Star formation is usually shrouded in thick gas and dust that is difficult to see inside; Counting the gamma rays it produces could be an effective way to see behind the curtain.
The team’s research has been accepted for publication in Astronomy & Astrophysicsand is available at arXiv.
