The Milky Way Has Only Two Spiral Arms

by Fraser Cain on June 3, 2008

If you were stuck inside your house, you’d never know what it looks like from the outside. That’s the situation with the Milky Way. We’re inside it, so we don’t really know what its structure looks like. There are other examples of grand spirals that we can see, but this is like seeing other houses outside your window; you just can’t be sure. Astronomers have developed a detailed map of the Milky Way, and realized that they were giving our home galaxy too many arms; it’s only got 2, and not 4 like astronomers originally thought.

The new revelation was made possible thanks to NASA’s Spitzer Space Telescope, which sees in the infrared spectrum, and can peer though the gas and dust that obscures the plane of the Milky Way.

Previous maps of the Milky Way were first developed in the 1950s, when astronomers used radio telescopes to trace out the spiral arms of our home galaxy. They focused on gas clouds, and revealed what they thought were 4 major star-forming arms: Norma, Scutum-Centaurus, Sagittarius and Perseus.

We live in minor arm called the Orion Arm, or the Orion Spur, located between the Sagittarius and Perseus Arms.

And then in 2005, astronomers used infrared telescopes to pierce through the clouds of gas and dust to see that the central bar in the middle of the Milky Way extends much further than previously believed.

In a new survey by Spitzer, astronomers merged together 800,000 photographs containing over 110 million stars. Software counted up the number of stars and measured their density.

As expected, astronomers found an increase in density in stars towards the Scutum-Centaurus Arm, but no increase towards the Sagittarius and Norma arms. The Perseus arm wraps around the outer portion of our galaxy and can’t be seen in the Spitzer images.

This helps make the case that the Milky Way only has two spiral arms; a commonly seen situation where a galaxy has a long central bar.

Original Source: NASA/JPL News Release


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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