New Research Affirms That Milky Way Has Four Spiral Arms

A debate has been raging about whether our galaxy has four spiral arms, or two. A 12-year study of massive stars suggests that it has four.

 

Every picture you’ve ever seen of our Milky Way galaxy has been an artist’s illustration, like this one. After all, we can’t get outside the galaxy to photograph it. Illustration via NASA/JPL-Caltech.

 

Our galaxy has four spiral arms, not two, says astronomer James Urquhart at the Max Planck Institute for Radio Astronomy in Bonn, Germany. Urquhart is lead author of new research, published today (December 17, 2013) in the Monthly Notices of the Royal Astronomical Society. He and his team conducted a 12-year survey of massive stars in our galaxy, and, Urquhart says, these stars trace out the galaxy’s four spiral arms.

An investigation into the precise structure of the Milky Way has ongoing, perhaps, for as long as we’ve known we live inside a galaxy, one of billions of islands of stars in space. That awareness hasn’t been around as long as you might think, less than a century. We can’t step outside the Milky Way to get perspective. Every picture you’ve ever seen of it has been an artist’s concept.

For the new study, astronomers used several radio telescopes in Australia, the U.S. and China to observe about 1,650 massive stars. From their observations, the distances and luminosities of the massive stars were calculated, revealing, they say, a distribution across four spiral arms.
These astronomers point out that massive stars are much less common than the lower-mass star seen by Spitzer, because they only live for a short time – about 10 million years. According to the press release:

The shorter lifetimes of massive stars means that they are only found in the arms in which they formed, which could explain the discrepancy in the number of galactic arms that different research teams have claimed.

So, if Spitzer caused two of the galaxies spiral arms to go missing for awhile, at least in the minds of astronomers … now they’re back. Will other astronomers agree with this new result? We’ll see. In the meantime, astronomer Hoare said:

Star formation researchers, like me, grew up with the idea that our galaxy has four spiral arms. It’s great that we have been able to reaffirm that picture.

Looking at the Milky Way from the inside out. Photo taken over Joshua Tree National Park by EarthSky Facebook friend Manish Mamtani. Thank you, Manish. From our perspective inside the galaxy, it’s tough to know the Milky Way’s exact structure. But new research suggests the Milky Way has four spiral arms, not two.