Milky Way galaxy has four spiral arms

Our Milky Way galaxy has four spiral arms, a 12-year study of massive stars has confirmed.
The study ends years of debate sparked by images taken by NASA's Spitzer Space Telescope that only showed two arms of the galaxy.
The new research is part of the Red MSX Source (RMS) Survey, launched by academics at the University of Leeds.
"The Milky Way is our galactic home and studying its structure gives us a unique opportunity to understand how a very typical spiral galaxy works in terms of where stars are born and why," said Professor Melvin Hoare, a member of the RMS Survey Team in the School of Physics & Astronomy at the University of Leeds and a co-author of the research paper.

In the 1950s astronomers used radio telescopes to map our Galaxy. They focused on clouds of gas in the Milky Way in which new stars are born, revealing four major arms.
NASA's Spitzer Space Telescope, on the other hand, scoured the Galaxy for infrared light emitted by stars. It was announced in 2008 that Spitzer had found about 110 million stars, but only evidence of two spiral arms.

The astronomers behind the new study used several radio telescopes in Australia, US and China to individually observe about 1,650 massive stars identified by the RMS Survey.
From their observations, the distances and luminosities of the massive stars were calculated, revealing a distribution across four spiral arms.

"It isn't a case of our results being right and those from Spitzer's data being wrong - both surveys were looking for different things," said Hoare.
"Spitzer only sees much cooler, lower mass stars - stars like our Sun - which are much more numerous than the massive stars that we were targeting," Hoare said.
Massive stars are much less common than their lower mass counterparts because they only live for a short time - about 10 million years.
The shorter lifetimes of massive stars means that they are only found in the arms in which they formed, which could explain the discrepancy in the number of galactic arms that different research teams have claimed.

"Lower mass stars live much longer than massive stars and rotate around our Galaxy many times, spreading out in the disc," Hoare said.
"The gravitational pull in the two stellar arms that Spitzer revealed is enough to pile up the majority of stars in those arms, but not in the other two.
"However, the gas is compressed enough in all four arms to lead to massive star formation," he said.
"It's exciting that we are able to use the distribution of young massive stars to probe the structure of the Milky Way and match the most intense region of star formation with a model with four spiral arms," said Dr James Urquhart from the Max Planck Institute for Radio Astronomy in Bonn, Germany, and lead author of the paper in the Monthly Notices of the Royal Astronomical Society.