Mapping the Milky Way’s Arms

Using a 12-year survey of massive stars, a research team concludes that four major arms, not two, are wrapped around our home galaxy's central bar.

Two arms, or two more arms — that is the question.

With apologies to Prince Hamlet, astronomers can't agree on whether the Milky Way boasts two dominant spiral arms (and some smaller appendages) or four of them. The evidence is murky, because observers are deducing details of the spiral shape from within the disk.

Milky Way as seen by Spitzer
Artist's concept of the Milky Way's structure, with only two major arms, based on Spitzer Space Telescope data. Click on the image for a larger version.
NASA / JPL-Caltech / R. Hurt
Everyone agrees that our galaxy has an elongated central region — a fat bar crowded with stars — that's several thousand light-year long, though its true length is a matter of debate. And they agree that two big arcs of stars spiral out from the bar's ends: the Perseus Arm and Scutum-Centaurus Arm. These names correspond to the constellations in which the arms are tangent to our line of sight, creating an apparent pileup of stars.

Several decades ago, radio-wavelength observations suggested the existence of two other major appendages dubbed the Sagittarius and Norma arms. But by 2008, astronomers had a potent new observational tool: the Spitzer Space Telescope. Its detailed infrared survey of the galactic plane, involving 800,000 images recording more than 110 million stars, confirmed the Perseus and Scutum-Centaurus arms.

But strong evidence for the other two arms was absent, leading Robert Benjamin (University of Wisconsin) and others to conclude that they'd "gone missing" and were probably just minor appendages like the Orion Spur, on which our Sun lies.

Not everyone accepted the Spitzer-based result, however. Other researchers, led by Thomas Steiman-Cameron (Indiana University), mapped pockets of interstellar gas using far-infrared observations from the Cosmic Background Explorer (COBE) satellite. Their results backed the four-arc model.

Now a new shot has been fired in the Great Arm War. James Urquhart (Max Planck Institute for Radio Astronomy) and others have analyzed 12 years of observations collected by radio telescopes in Australia, the United States, and China that determined the distance and brightness of massive stars just emerging from their natal cocoons of gas and dust. These stars, each with at least eight times the Sun's mass and 10,000 times its luminosity, last only about 10 million years. So they don't survive long enough to meander to new locations as they orbit the galactic center. (Our Sun, for example, has already circled around 10 times during its 4½-billion-year existence.)

A four-armed Milky Way
A new survey of young, massive stars (red dots) throughout our galaxy reveals that the Milky Way has four prominent arms after all — not just two (Perseus and Scutum-Centaurus), as earlier observations had suggested. No observations were made in the gray area, due to confusion with the galactic center.
J. Urquhart / MNRAS; background: NASA / JPL / R. Hurt
In January 11th's Monthly Notices of the Royal Astronomical Society, Urquhart's team reports that a 3-D map of 1,650 such massive stars clearly delineates all four spiral arms. In fact, the Sagittarius Arm stands out in the new survey more prominently than the better-known Perseus Arm does. The new result appeared online last week.

"It isn't a case of our results being right and those from Spitzer's data being wrong," comments co-author Melvin Hoare (University of Leeds) in a press 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."

5 thoughts on “Mapping the Milky Way’s Arms

  1. Anthony Barreiro

    It is sobering to reflect on the fact that we can see the shapes of galaxies hundreds of millions of light years distant in much more detail and with much greater confidence than we know our own galaxy. Self-awareness takes patience and skill! Thanks Kelly for reporting these interesting findings and giving us the background and context to understand them. I wonder if the ALMA telescope will give us a better mirror in which to see the Milky Way.

  2. Rod

    My observation – the massive stars used in the survey suggest the four spiral arms are all very young and have not rotated over millions or hundreds of millions of years. This is the windup problem all spiral galaxies have.

  3. Peter WilsonPeter

    It would be nice to see a map with only the red dots. Then we could all form an opinion about how many arms there are…

  4. Gregg-WeberGregg Weber

    Every once in a while I would put the following on the return address of a letter, just to make sure that they knew where I was;

    Seattle, Wa, USA, Earth, Sun, Orion Spur

    Is that last one (Orion Spur) accurate?

  5. Andrew D Wilson

    I don’t think either apply. I’ve studied structures and have read many research "conclusive results" and have found neither 2 or 4 "arms" exist. I started studying quantum physics in 4th grade and have a photographic memory (right brained, not left). Try looking at all sides of a Rubik Cube by considering many more variables, compounding new variables that create a completely dynamic system like an ocean in space. What I see is looking out from the center with the nucleus of the galaxy having a north and south polar emission spinning on a side access creating gravity waves resulting in crests and troughs that change the spectral wavelengths giving an illusion that one area is more "visible" than other areas, hence the change in "dominant arms". I also see dark matter filaments that emit outward from the side axial polar regions that create alignments of all bodies rotating on the side axis’. This produces a…

    Please do not plagiarize what I have reveled here today or pass off any part of this statement of intellectual property belonging to me and the years of study I’ve devoted to advance the current state of knowledge. If you have serious interest in publication please contact me via this post and I will respond accordingly.

    Andrew D. Wilson
    Senior Electrical Engineer
    Amateur Theoretical Physicist

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