This might come as a surprise, but every portrayal ever made of how the Milky Way galaxy looks from afar is more imagined than factual. That's because we sit squarely inside our galaxy's turgid disk, which together with its bloated central bulge make it impossible to see most of what lies on the side opposite the Sun. We're only getting half the picture.

Oh, astronomers have certainly tried to piece together the true shape of its spiral. "For years, people created maps of the whole galaxy based on studying just one section of it, or using only one method," says Robert Benjamin (University of Wisconsin). "Unfortunately, when the models from various groups were compared, they didn't always agree."

Milky Way's structure

The Milky Way's basic structure involves two large spiral arms believed to originate at either end of an elongated central bar. But only parts of the arms can be seen — gray segments indicate portions not yet detected.

Thomas Dame

A few years ago, Benjamin and others used NASA's Spitzer Space Telescope to deduce that our galaxy really has just two main spiral arms, not four as had been thought. Called the Scutum-Centaurus and Perseus arms, these appear to connect up nicely with the ends of the galaxy's central bar. (Our Sun lies along a minor offshoot, about halfway from the center to the outer edge, known as the Orion Spur.)

But there's been little hard evidence to prove that the galaxy's majestically sweeping arms continue around to its far side — until now. As Thomas Dame (Harvard-Smithsonian Center for Astrophysics) described this week at a meeting of the American Astronomical Society, he and CfA colleague Patrick Thaddeus have identified a distant arm on the disk's outer margin, on the far side, that's roughly 50,000 light-years from its center.

Rooftop radio telescope

This modest rooftop radio dish, only 4 feet (1.2 m) across, has mapped the presence of carbon monoxide in giant clouds throughout the galaxy.

Center for Astrophysics

They detected the arm's presence not with some enormous globe-spanning array of radio telescopes, but instead with a very modest 4-foot (1.2-m) dish atop one of the CfA's buildings in Cambridge, Massachusetts. The telescope was tuned to the 115-gigahertz emission of carbon monoxide (CO) in giant molecular clouds, then pointed at hundreds of target fields near the galactic plane.

The telltale carbon-monoxide emission turned up in a string of 10 locations — hardly a comprehensive sampling, but enough to sketch in the distant arm's location and extent. One of the CO-rich clouds turns out to be about 150 light-years across and has a mass of at least 50,000 Suns.

Radio emissions in Milky Way's new arm

This color-coded plot of radio emission from neutral hydrogen traces out a long portion of a spiral arm located on the far side of our galaxy. Black dots indicate where radio astronomers detected strong emissions from carbon monoxide in giant molecular clouds along the arm.

T. Dame & P. Thaddeus

The arm also shows up in prior radio surveys that mapped the abundance of neutral hydrogen throughout the disk. It largely escaped detection, Dame speculates, because it's both displaced from the galactic plane and tilted with respect to it. Also, Dame adds, "The main problem with hydrogen is that there is too much of it." In any case, the CO radio hits match strongest hydrogen concentrations along the distant arm, assuring that it's real.

The new find is an isolated segment that's roughly 60,000 light-years long. Dame and Thaddeus believe it marks the distant end of the Scutum-Centaurus arm, which would mean that the entire arc is more than 200,000 light-years long and that it wraps more than 300° around the galactic center.

The two researchers published their results in the May 10th issue of Astrophysical Journal Letters. To know for certain that the new arm is really an extension of the Scutum-Centaurus Arm, they hope to use their little radio dish to map it more completely in the years ahead.

Even if they don't or can't, it's reassuring to know that the Milky Way really does have the gracefully sweeping arms and beautiful symmetry that befits a grand spiral galaxy.


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Anthony Barreiro

May 25, 2011 at 3:56 pm

This is a fascinating story. It's particularly satisfying to know that this research was done with a relatively small instrument from a light-polluted city. Unfortunately I now have to replace all my astronomy reference books!

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May 27, 2011 at 11:29 am

I wonder when they found the new arm .. It's great to discover things like that and the chance increases when you are somewhere dark.

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Michael Welsh

May 27, 2011 at 12:30 pm

Why do the galactic arms appear to spiral outward? Are the stars in the arms moving outward? In the image associated with this article, is the galaxy spinning clockwise? Such movement, if it is occurring suggests that there is an attachment of the arms to the ends of the central bar. Alternatively, is the central bar contracting? What accounts for the morphology of the arms, and the relationship of the arms to the central bar? Thanks.

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Kelly Beatty

May 27, 2011 at 2:13 pm

Michael: as viewed from "above" (over the galactic north pole), the Milky Way would appear to spin counterclockwise. It takes about 225 million years to rotate. But the spiral arms aren't associated with that rotation. Instead, they're formed by waves of energy (called spiral density waves) propagating outward. It's a little like the wave motion in a pond when you toss a rock in: the surface water isn't really moving outward, but the wave energy is. As they move outward, the density waves trigger bursts of star formation, and these stars and bright nebulas are what give the Milky Way and other such galaxies their spiral appearance.

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Peter Wilson

May 27, 2011 at 2:23 pm

The best analogy is a standing wave, like in a river, where the water is moving, but the peaks and troughs may remain fixed. The stars move in elliptical orbits, while the gas moves in a clockwise, circular orbit. Somehow, the interaction of billions of stars in elliptical orbits with the gas in circular orbit creates the bar/spiral pattern. The pattern itself, however, moves very slowly, or not at all.

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Björn Gimle

May 28, 2011 at 4:16 am

Are only young stars contributing to the increased density of stars in the arms? If not, could it be that the stability of planetary orbits is a lot higher where the Sun is, than for the majority of galactic stars? This should have a large influence on one or two factors in the Drake formula.

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Rick Shaffer

May 28, 2011 at 2:56 pm

That antenna might seem too small to do useful work. But the "gain" of the antenna (its "radio light-gathering power, if you will) is actually rather high, because the frequency at which the observations were made was so high.

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Jon Zander

June 2, 2011 at 7:38 pm

Must be the geographer in me.

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Kurt Meyer

June 6, 2011 at 10:16 am

To my eye, arms of regular spiral galaxies as well as irregulars make sense if viewed as ejecta from a rotating core. Just look at pictures of M51, M101, etc. It seems so plainly simple and obvious just from visual inspection.
The thing that made this 'click' for me was an image of the failed Russian missile launch widely seen over Norway a couple years ago; the booster was tumbling. It looked like a perfect spiral galaxy. Here is a link to an animation of that event:
The simple model I visualize is a road flare, shooting smoke and gas out of each end. Envision rotating it slowly about the middle and there is your spiral. Now break it in half and have both pieces rotating and spitting out material and you get interacting spirals. Uneven flow, intermittent output, a pinched end, etc. could produce irregular morphology.
This model is consistent with the arms not rotating much or at all. It's also no stretch to then envision perpendicular jets or ejections from the nucleus along the axis of rotation, such as have been observed.
I'm not thumping a drum for or against any particular theory of cosmology or physics, but ever since I saw that image of the tumbling rocket I see it in every galaxy, it just seems so obvious.

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June 17, 2013 at 3:02 am

How is neutral H emitting radio waves with energies of a hundred GHz?? Is it just thermal?

-faye kane

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Faye Kane

June 17, 2013 at 3:56 am

So this week's astounding discovery is another arm for the galaxy. Slow down! I'm still trying to absorb the fact that black holes are real and you can see stars whip around the one in the milky way, Pluto is really a huge comet, we've soft-landed on Titan, I've seen pictures of planets in other solar systems, there's liquid water on Mars, astronomers never take pictures with film, the milky way is a *barred* spiral, the 200 inch Palomar is a small telescope, Andromeda is going to crash into us, there's a real space station that you can look up and see, we've floated next to asteroids in the asteroid belt, the moon happened when two planets collided, we can see the neutrino shadow of the earth's core, you can get sharper astrophotographs from the ground than Hubble gets from space, but only if you bang the large primary mirror with rubber hammers during the exposure, Jupiter has rings, the CMBR isn't smooth, and Iapetus isn't rectangular but
there's a huge hexagon on saturn!

What have you people got lined up for next month's issue, time travel?

Sometimes I ask myself, "Say, just what hath god wrought, anyway?"

-- faye kane, homeless brain

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