It was four all along: More arms in Milky Way than recent studies showed

By Ben Sullivan

A 12-year study of large stars has reaffirmed that our Galaxy has four spiral arms, following years of discussion sparked by pictures taken by NASA’s Spitzer Telescope that showed 2 arms. The new research, published December 17, is part of the RMS Survey, launched by University of Leeds.

The new study appears in the Monthly Notices of the Royal Astronomical Society,

Astronomers can’t see how the Milky Way looks, as we’re in it, peering outward. They deduce the galaxy’s shape by looking at its constituent stars and their relative distances from Earth and its orbiting observatories.

“The Milky Way is our galactic home,” according to Melvin Hoare, a member of the RMS Survey Team. “Studying its structure gives us a unique opportunity to understand how a very typical spiral galaxy works in terms of where stars are born and why.”

Sixty years ago, astronomers used radio telescopes to try to map the Milky Way,  and they focused on the massive clouds of gas in which new stars are hatched. This showed the galaxy had four major arms. The NASA Spitzer Space Telescope later hunted for infrared light emitted by stars. In 2008 NASA announced Spitzer had more than 100 million stars, but just two spiral arms.

In the new study, wonks combined data from telescopes in the U.S., China and Australia to look at about 1,600 massive stars that had been ID’d by the RMS Survey. The distances and brightnesses they turned up showed stars distributed across four spiral arms.

Touched_by_His_Noodly_Appendage“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 Hoare. “Spitzer only sees much cooler, lower mass stars – stars like our Sun – which are much more numerous than the massive stars that we were targeting.”

Massive stars are rarer than their lower mass counterparts, mainly due to their shorter lifespans of roughly 10 million years. This means they’re only found in the arms in which they formed, possibly explaining the discrepancy in the number of galactic arms found in the different studies.

“Lower mass stars live much longer than massive stars and rotate around our Galaxy many times, spreading out in the disc. The gravitational pull in the two stellar arms that Spitzer revealed is enough to pile up the majority of stars in those arms, but not in the other two,” explains Professor Hoare. “However, the gas is compressed enough in all four arms to lead to massive star formation.”

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