Massive Stars Reveal Milky Way's 'Missing' Four Arms

Astronomers have debated exactly how many arms our Milky Way Galaxy has. While some were certain that our galaxy had four spiral arms, images taken by NASA's Spitzer Space Telescope showed only two arms. Now, scientists have reaffirmed that our galaxy has four spiral arms, paving the way to future studies.

Determining the shape of the Milky Way is a tricky process. We can't see it from the outside, because we're on the inside of it looking out. However, astronomers can deduce its shape by careful observation of the galaxy's stars and their distances from us.

"The Milky Way is our galactic home and 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," said Melvin Hoare, one of the researchers, in a news release.

NASA's Spitzer Space Telescope scoured the galaxy for infrared light emitted by stars. In 2008, researchers announced that the telescope had found about 110 million stars, but only evidence of two spiral arms, despite previous findings that revealed four major arms. In order to test what the true shape of our galaxy might be, the researchers then used several radio telescopes to observe 1,650 massive stars. In the end, the astronomers found evidence of four arms.

"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," said Hoare in a news release. "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 actually much less common than their lower mass counterparts because they only live for a short time-about 10 million years. The shorter lifetimes of massive stars means that they're only found in the arms in which they're formed.

"Lower mass stars live much longer than massive stars and rotate around our Galaxy many times, spreading out in the disc," said Hoare. "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. However, the gas is compressed enough in all four arms to lead to massive star formation."

The findings reveal a little bit more about the shape and form of our galaxy. This, in turn, helps inform future studies about the Milky Way.

The findings are published in the journal Monthly Notices of the Royal Astronomical Society.