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Milky Way may indeed have four spiral arms

By mapping the the distribution of massive stars (red) in the Milky Way, astronomers have found evidence to reaffirm that the galaxy has four arms instead of two. The location of the solar system is circled in black.

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In 2008, the Milky Way lost two of its four major spiral arms, according to some analyses. But the galaxy may be getting them back.

Radio telescope data from the 1950s first suggested that the galaxy had four arms. But observations from NASA's Spitzer Space Telescope reported in 2008 knocked the number of major arms down to two. Spitzer looked at cooler, longer-living, lower-mass stars like the sun that can be tugged toward the center of the galaxy.

But a new survey of massive stars throughout the Milky Way seems to reaffirm the decades-old observations that the galaxy has four major spiral arms. The massive stars surveyed burn for a shorter period of time, don’t move far from where they first begin to develop and appear distributed across four distinct spiral arms, researchers report in the January 11 Monthly Notices of the Royal Astronomical Society.

The difference in location of low- versus high-mass stars may explain the inconsistent picture of the structure of the galaxy’s spiral arms, the authors suggest.

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