Apparently our galaxy has only 50% of the spiral arms we originally thought it did.

Early models of the Milky Way gave it a spiral structure, with four arms named Norma, Scutum-Centaurus, Sagittarius and Perseus.

“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 of the University of Wisconsin, who is presenting the galactic limb surgery at the current American Astronomical Society meeting in St. Louis (press release). “Unfortunately, when the models from various groups were compared, they didn’t always agree. It’s a bit like studying an elephant blind-folded.”

Using a mosaic of images from the Spitzer telescope Benjamin counted the stars in various areas. While there were more in the Scutum-Centaurus Arm direction, there were no more where there should have been in the Sagittarius and Norma regions. Perseus got camera shy and can’t been seen in the images used.

As a result of this, Benjamin believes the Milky Way actually has only two major arms, Scutum-Centaurus and Perseus, and that Sagittarius and Norma are minor arms.

According to Space.com, “The findings confirm an earlier observation by a team of astronomers, making a strong case that the Milky Way has two major spiral arms, a common structure for galaxies with bars. These major arms have the greatest densities of both young, bright stars and older, so-called red-giant stars.”

Not to worry though, another researcher in St Louis is presenting evidence of a new arm. Looking at the smaller arms close to the centre of the galaxy Tom Dame found an arm nearly a twin of another one on the other side.

“Our galaxy isn’t as messy as many thought,” says Dame, of the Harvard-Smithsonian Center for Astrophysics. “What we have found is evidence of some balance and order, like the yin and yang of Chinese philosophy.”

Image: NASA/JPL-Caltech