An analysis of the galaxy’s most energetic light has revealed that we can 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 a lot, but it’s two to four times higher than current estimates, suggesting that our home galaxy isn’t as quiet as we thought.
And this has important implications for our understanding of the evolution of our galaxy and those around us, because the rate at which stars are born and die can change the overall chemical composition of a galaxy.
A paper describing the discovery, 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 most complex elements of our Universe. Their cores are nuclear furnaces, smashing atoms together to forge them into ever larger atoms. When they die, their violent death pangs throw these heavier elements back into interstellar space, to drift into the clouds or be absorbed by new forming stars. Their supernova explosions, too, are energetic, forge even heavier elements that their cores could not bear.
Like their deaths, star births are also energetic. They form from dense clumps in clouds of interstellar dust and gas, collapsing under the effect of gravity and greedily sucking up material from the space around them until they collapse. there is enough pressure and heat in their cores to ignite the fusion. As they do so, they begin to emit powerful stellar winds blowing particles out into space, and jets launched from their poles of particles accelerating along the small 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 does not last long, cosmically speaking; it has a half-life of 717,000 years. And as it decays, it produces gamma radiation at a specific wavelength.
But aluminum-26 is also present in significant amounts in the clouds of matter that surround forming stars. If the rate at which matter falls into a star exceeds the speed of sound, a shock wave is formed, generating cosmic rays. When the rays collide with isotopes in the dust, such as aluminum-27 and silicon-28, they can produce the aluminum-26 isotope.
Thus, by examining the gamma radiation budget in the Universe produced by the radioactive decay of aluminium-26, astronomers can estimate the rate at which stars that generate the isotope form and die in the Milky Way, and l are used to determine an overall star generation rate.
Current estimates of the Milky Way galaxy’s star formation rate are at about two suns of material converted into stars each year. Since most stars in the Milky Way are much less massive than the Sun, this is estimated to average around six or seven stars a year.
Siegert and his colleagues mapped aluminum-26 gamma radiation in the galaxy and performed modeling to see the most likely mechanism for producing 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 approximately 55 stars per year.
There is still room for improvement on this estimate; the models did not quite reproduce the gamma radiation of the Milky Way as it is currently observed; and the distance from the gamma ray source could alter the final estimate, but is difficult to gauge. That’s why the researchers were only able to give a range for the rate of star formation, rather than an accurate mass.
However, the team’s method shows promise for better understanding how the Milky Way creates new stars. Star formation is usually shrouded in thick gas and hard-to-see dust; counting the gamma radiation it produces could be an effective way to look behind the curtain.
The team’s research has been accepted for publication in Astronomy & Astrophysicsand is available at arXiv.