Since the days of radio observations, astronomers believed that the Milky Way galaxy, in which our solar system is located, has four spiral arms wrapped around the central galactic nucleus. But radio observations are only able to detect concentrations of gas in the galaxy and what previously appeared to be two spiral arms can't seem to show up at all in a recent investigation carried out in the infrared spectrum with NASA's Spitzer Space Telescope.

Simply put, the new infrared map of the Milky Way may have to be drawn without two of the arms, Sagittarius and Norma. The images provided by Spitzer and studied by astronomer Robert Benjamin from the University of Wisconsin reveal that the two arms may be in fact just accumulations of gas and dust filled with a handful of young stars, unlike the Scutum-Centaurus and Perseus arms, which are not quite rich in gas, but they do seem to have a considerable population of both young and old stars.

"These major arms could be the things that would really stand out if you were looking at the Milky Way galaxy from Andromeda", said Benjamin.

"I think it could be right, but I think we have a lot of work to do to shore this up", said Thomas Dame from the Harvard-Smithsonian Center for Astrophysics, who did not participate in Benjamin's study.

Benjamin reckons that there are still many things unknown about the structure of the Milky Way and trying to construct a map of the galaxy is more a matter of imagination than actual science. Scutum-Centaurus, Perseus, Sagittarius and Norma are the largest arms, but the galaxy also contains some partial smaller ones, among which the Orion Spur located between Sagittarius and Perseus, where the solar system in located.

The second image featured here represents a mosaic of the galaxy in the infrared spectrum created with the Spitzer Space Telescope. The results of the study led by Benjamin have been detailed yesterday at the American Astronomical Society in St. Louis where Sean Carey from Caltech also presented a poster of the second image, measuring some 55 meters in length.