Gaze into a clear night sky, and there’s the Milky Way, that pale band arching across the black. Its light emanates from billions of stars being born and dying along the multiple spirals, or arms, of our disk-shaped galaxy. Just how many arms flare around its core has been in dispute for decades.
In the 1950s, astronomers mapped clouds of gases using the latest technology—radio telescopes—and detected four major arms. But in 2008, NASA’s orbiting Spitzer Space Telescope detected only two, based on the infrared light emitted by 110 million stars. Now, astronomers have targeted much larger, younger suns, 1,650 in all, and by calculating their distances and luminosities, have again traced four spirals.
“It isn’t a case of our results being right and those from Spitzer’s data being wrong. Both surveys were looking for different things,” says Melvin Hoare, a physicist at the University of Leeds who contributed to the latest study. The work of Leeds scientists was described in the January issue of Monthly Notices of the Royal Astronomical Society.
Why care about such disputes? “The Milky Way is our galactic home,” Hoare says, “and studying its structure gives us a unique opportunity to understand … where stars are born and why.”
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