London, June 4 : An astronomer is calling for demoting two entire arms of our galaxy, after they failed to turn up in a sensitive new map of the Milky Way's stars.
According to a report in New Scientist, the astronomer in question is Robert Benjamin of the University of Wisconsin in Whitewater, US.
Astronomers have long believed that our galaxy possesses four spiral arms, since radio observations show concentrations of gas that trace such a spiral structure.
But now, two of the Milky Way's arms have failed to turn up in a sensitive new survey that used the Spitzer Space Telescope to map the distribution of millions of stars.
Benjamin has determined that these two arms, called Sagittarius and Norma, may be mostly concentrations of gas, perhaps sprinkled with pockets of young stars.
By contrast, the other two arms, called Scutum-Centaurus and Perseus, appear rich not only in gas, but in stars both young and old.
"These major arms could be the things that would really stand out if you were looking at the Milky Way galaxy from Andromeda (a nearby galaxy)," said Benjamin.
According to Thomas Dame of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Masssachusetts, US, the major-minor arm idea is interesting.
"I think it could be right, but I think we have a lot of work to do to shore this up," he told New Scientist.
In addition to the four large arms, the Milky Way has some smaller, partial arms. The Sun is located in one such stub called the Orion Spur, which is wedged between the Sagittarius and Perseus arms.
Another team of astronomers unveiled a vast mosaic image of the Milky Way - the most sensitive ever made in infrared light - created from Spitzer Space Telescope observations.
The team, led by Sean Carey of Caltech in Pasadena, California, US, displayed a 55-metre-long poster version of the image.