The Milky Way has twice the number of spiral arms than recent observations suggested. That’s according to a new analysis—part of the biggest census of star-forming regions to date—that focused on stars eight times the mass of our sun or larger (the size that eventually explode as supernovae) at a very early stage in their lifetime, when they’d still be inside the clouds of gas and dust where they formed. The researchers identified about 1650 such stars and then estimated the distance to those stars (red dots in image; the black dot denotes the position of our solar system). For the most part, the stars lie along four spiral arms (artist’s representation of the Milky Way, background image), the researchers report online today in the Monthly Notices of the Royal Astronomical Society. Only about 40% of these young, massive stars were located in previously identified star-forming regions, the researchers say. In 2008, a different team that had mapped about 110 million stars found evidence for only two spiral arms—but that census of stars included many smaller, cooler, and older stars that had had ample time to migrate out of the regions of space where they had formed, the new study’s authors contend. While the new analysis finds that two of the spiral arms contain far more stars than the other two structures, the overall rates at which young, massive stars are forming are approximately the same in all four of the newly recognized spiral arms. In the 1950s, a low-resolution survey of our galaxy’s star-forming regions had hinted at four arms, a configuration the new study confirms. While the new study adds to the Milky Way’s tally of star-forming regions, it may not substantially boost our galaxy’s star total because the young, massive stars focused on in this study make up only a small percentage of the overall population.

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