Our home in the universe NASA/JPL

It’s tricky to map an entire galaxy when you live in one of its arms. But astronomers have made the clearest map yet of the Milky Way – and it turns out that the arm that hosts our solar system is even bigger than previously thought.

The idea that the Milky Way is a spiral was first proposed more than 150 years ago, but we only started identifying its limbs in the 1950s. Details about the galaxy’s exact structure are still hotly debated, such as the number of arms, their length and the size of the bar of hot gas and dust that stretches across its middle.

The star-filled arms are densely packed with gas and dust, where new stars are born. That dust can obscure stars we use to measure distances, complicating the mapping process.


Two of the arms, called Perseus and Scutum-Centaurus, are larger and filled with more stars, while the Sagittarius and Outer arms have fewer stars but just as much gas. The solar system has been thought to lie in a structure called the Orion Spur, or Local Arm, which is smaller than the nearby Perseus Arm.

Just as grand

Now, Ye Xu and colleagues from the Purple Mountain Observatory in Nanjing, China, say the Local Arm is just as grand as the others.

The team used the Very Long Baseline Array in New Mexico to make extremely accurate measurements of high-mass gas clouds in the arms, and used a star-measuring trigonometry trick called parallax to measure their distances.

“Radio telescopes can ‘see’ through the galactic plane to massive star forming regions that trace spiral structure, while optical wavelengths will be hidden by dust,” Xe says. “Achieving a highly accurate parallax is not easy.”

The new measurements suggest the Milky Way is not a grand design spiral with well-defined arms, but a spiral with many branches and subtle spurs.

However, Xu and colleagues say the Orion Spur is not a spur at all, but more in line with the galaxy’s other spectacular arms. The team also discovered a spur connecting the Local and Sagittarius arms.

“This lane has received little attention in the past because it does not correspond with any of the major spiral arm features of the inner galaxy,” the authors of the study write.

Future measurements with other radio telescopes will shed more light on the galaxy’s shape. The European Space Agency’s Gaia spacecraft is in the midst of a mission to make a three-dimensional map of our galaxy, too. More measurements of the high-mass gas regions will help astronomers determine what our galaxy looks like, from the inside out.

Journal reference: Science Advances, DOI: 10.1126/sciadv.1600878