Well, half of an octopus, at least. Because we’re on the inside looking out, it’s difficult for astronomers to get an idea of what our Galaxy looks like as a whole. Fortunately, scientists (being scientists) have found a way to approximate its shape: In a 12-year study by researchers at the University of Leeds, the observation of massive stars reveal that the Galaxy has four distinct “arms.” Previous research speculated that there were only two.

“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.

The newer findings actually support data collected in the 1950s, when radio telescopes were used to map the Milky Way. Focusing on clouds of gas, they also found there to be four distinct arms. It wasn’t until 2008, when NASA’s Spitzer Space Telescope used infrared light to count about 110 million stars and find evidence of two arms, that scientists doubted the previous theory.

The new research, which is part of the RMS Survey, once again uses multiple radio telescopes across the globe to observe around 1650 huge stars first identified in the RMS Survey. Several calculations later, it was revealed that the stars’ locations pointed to four arms, not two.

“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 Professor 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.”

Massive stars burn hotter and live for shorter periods than their lower mass counterparts, making them more difficult to track and less common. They’re likely to develop and burn out only within the arm they started in, which might be why different projects returned different findings.

“Lower mass stars live much longer than massive stars and rotate around our Galaxy many times, spreading out in the disc. 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,” explains Professor Hoare. “However, the gas is compressed enough in all four arms to lead to massive star formation.”

The good news is, the Milky Way Galaxy now looks even cooler than we originally thought.