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.
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.)
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."