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EarthSky // Science Wire, Space Release Date: Dec 17, 2013

Milky Way galaxy has four arms again

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.

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.

So … two arms or four for the Milky Way? Images taken by NASA’s Spitzer Space Telescope in 2008 showed two arms. That’s when the current debate began raging.

In Dr. Urquhart’s study, astronomers are attempting to deduce the shape of our Milky Way galaxy by careful observation of its stars and their distances from us. Massive stars are an obvious choice because they shine so brightly.

Astronomers say their study of massive stars in our Milky Way galaxy suggests the galaxy has four spiral arms.  Can you trace four arms in the red dots here, which represent those stars?  Me neither, but astronomers say this study confirms four Milky Way arms.  We'll see if other astronomers agree.  Image via University of Leeds.

A group of astronomers says its study of massive stars in our Milky Way galaxy suggests the galaxy has four spiral arms. Can you trace four arms in the red dots here, which represent those stars? Me neither, but astronomers say this study confirms four Milky Way arms. We’ll see if other astronomers agree. Image via University of Leeds.

From our vantage point on a planet, orbiting a single star among billions in the galaxy, how can we possibly know our galaxy’s true shape? In the 1950s, astronomers used radio telescopes to map the Milky Way’s structure. They focused on the vast clouds of gas in which new stars are born, and, indeed, the result was that the Milky Way has four major arms. Many of today’s astronomers grew up with this idea as conventional wisdom.

In 2008, NASA’s Spitzer Space Telescope got a different result, in part because it looked at different things. Spitzer scoured the galaxy for infrared light emitted by stars. It found about 110 million stars, but only evidence of two spiral arms.

Astronomer Melvin Hoare at the University of Leeds, a co-author of the research paper, said in a press release issued today:

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. 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.

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.

Starlit trail of Milky Way - edgewise view into our own galaxy - over Joshua Tree National Park via EarthSky Facebook friend Manish Mamtani.  Thank you, Manish!

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.

Bottom line: A new 12-year study of massive stars in the Milky Way suggests our galaxy has four spiral arms, not two.

Via University of Leeds