According to a new estimate of the Milky Way’s star-forming rate, the galaxy is producing much more stars than previously thought.
Researchers say in a paper posted to arXiv.org on January 24 that gamma rays from aluminum-26, a radioactive isotope produced predominantly by huge stars, suggest that the Milky Way transforms four to eight solar masses of interstellar gas and dust into new stars each year. Because most stars are less massive than the sun, this range is two to four times the usual estimate and corresponds to an annual birthrate of around 10 to 20 stars in our galaxy.
At this rate, our galaxy generates 10 million to 20 million new stars every million years – a blink of an eye in cosmic terms. That’s enough to fill around 10,000-star clusters, such as the lovely Pleiades cluster in the constellation Taurus. In contrast, many galaxies, including the vast majority of those that orbit the Milky Way, produce no new stars at all.
“The pace of star formation is really crucial to understand for galaxy development,” says Thomas Siegert, an astronomer at the Germany’s University of Würzburg. The more stars a galaxy produces, the faster it enriches itself with oxygen, iron, and other elements produced by stars. These elements then influence star-forming gas clouds, affecting the number of massive and tiny stars.
Siegert and his colleagues investigated the measured intensity and geographical distribution of aluminum-26 emission in our galaxy. This isotope is produced by a large star throughout both its life and death. Throughout its lifetime, the star generates a tremendous wind that propels the metal into space. When a star dies and bursts, the resultant supernova creates other stars. With a half-life of 700,000 years, the isotope decays and emits gamma rays.
Gamma rays, like X-rays, but unlike visible light, penetrate the dust that envelops the youngest stars. “We’re scanning the entire cosmos,” Siegert adds. “We’re not doing an X-ray; we’re doing a gamma-ray.”
The more stars our galaxy produces, the more gamma rays it produces. The researchers discover that a star creation rate of four to eight solar masses per year best fits the findings. This is far greater than the usual estimate for the Milky Way, which is around two solar masses every year.
According to Pavel Kroupa, an astronomer from the University of Bonn in Germany who was not involved in the research, the revised pace is extremely probable. “I’m quite amazed by how comprehensive their simulation of the star creation process is,” he says. “It’s a very lovely piece of work. I can see some ways to improve it, but this is a significant start in the right direction.”
The more stars our galaxy produces, the more gamma rays it produces. According to Siegert, it is impossible to determine how far the gamma rays have traveled before reaching humans. If any of the observed emissions originates close to us — within a few hundred light-years — the galaxy has less aluminum-26 than the researchers estimated, implying that the star formation rate is on the lower side of the revised estimate. Nonetheless, he believes it will be less than the typical two solar masses every year.
In any case, the Milky Way is the most active star-creator in the Local Group, a collection of more than 100 neighboring galaxies. Andromeda, the largest Local Group galaxy, converts only a fraction of a solar mass of gas and dust into new stars per year. The Milky Way is the second largest galaxy in the Local Group, but its high star production rate requires us to work much harder.
Reference: Ken Croswell@www.sciencenews.org
