The Most Accurate View of the Milky Way

We can judge the value of any scientific endeavour based on how much of our knowledge it overturns or transforms. By that metric, the ESA’s Gaia mission is a resounding success. The spacecraft gave us a precise, 3D map of our Milky Way galaxy and has forced us to abandon old ideas and replace them with compelling new ones.

Currently, we’re marking the end of the Gaia mission, our best effort to understand the Milky Way. Gaia is an astrometry mission that’s built an impressive map of the Milky Way by taking three trillion observations of two billion individual objects in the galaxy, most of them stars, over an 11-year period. Measuring the same objects repeatedly means Gaia’s map is 3D and shows the proper motion of stars throughout the galaxy. Rather than a static map, it reveals the galaxy’s kinetic history and some of the changes it’s gone through.

Gaia showed us our galaxy's turbulent history, including the streams of stars stemming from past disruptive events. Image Credit: ESA/Gaia/DPAC, Stefan Payne-Wardenaar — Gaia showed us our galaxy’s turbulent history, including the streams of stars stemming from past disruptive events. Image Credit: ESA/Gaia/DPAC, Stefan Payne-Wardenaar

We’ve waited a long time for such a detailed look at our galaxy.

Radio astronomy, which gained momentum in the 1950s, helped us understand the structure of the Milky Way. Radio telescopes could see through intervening dust clouds and detect the distribution of hydrogen in the galaxy. In 1952, astronomers began the first major radio survey of the Milky Way. Astronomers theorized that the galaxy had a spiral structure, and finally, they detected the spiral arms, revealing the Milky Way’s basic structure.

In a 1958 paper, the authors wrote that “The distribution of the hydrogen evidently shows great irregularities. Nevertheless, several arms can be followed over considerable lengths.”

This figure shows the hydrogen distribution in the plane of the Milky Way's disk. Though it appears outdated to our modern eyes, it was exciting at the time. Image Credit: From "The galactic system as a spiral nebula" by Oort et al. 1958. — This figure shows the hydrogen distribution in the plane of the Milky Way’s disk. Though it appears outdated to our modern eyes and isn’t visually intuitive, it was exciting at the time. Image Credit: From “The galactic system as a spiral nebula” by Oort et al. 1958.

Astronomers also used RR Lyrae and Cepheids, two types of variable stars with known intrinsic brightnesses (standard candles), to calculate their distances. This allowed them to trace the Milky Way’s structure. Globular clusters also helped astronomers map the Milky Way.

In the 1980s, infrared telescopes like NASA’s IRAS peered through cosmic dust to help find features like the Milky Way’s central bar. Then, in 1989, the ESA’s Hipparcos mission was launched. Hipparcos was an astrometry mission and was Gaia’s predecessor. Though not nearly as precise, and though it only measured 100,000 stars, it was finally able to measure their proper motions. It revealed more details of the Milky Way and helped confirm its barred spiral form. It also provided some insights into our galaxy’s history and evolution.

But astronomers craved more detailed knowledge. Gaia was launched in 2013 to meet this need, and it’s been a total success.

Gaia is a tribute to ingenuity. We’re effectively trapped inside the Milky Way, and no spacecraft can get beyond it to capture an external view of the galaxy. Gaia has given us that view without ever leaving L2.

While many prior efforts to trace the Milky Way’s structure depended on sampling select stellar populations, Gaia precisely measured the position and motion of almost two billion stars throughout the galaxy.

Gaia's map of the Milky Way has become iconic. This image is constructed from Gaia data that's mapping two billion of the galaxy's stars. It also mapped stars in the Large and Small Magellanic clouds. Image Credit: ESA/Gaia/DPAC — Gaia’s map of the Milky Way has become iconic. This image is constructed from Gaia data that’s mapping two billion of the galaxy’s stars. It also mapped stars in the Large and Small Magellanic clouds. Image Credit: ESA/Gaia/DPAC

Gaia’s work has culminated in artist impressions of the Milky Way based on its voluminous data. These impressions show that the Milky Way has multiple arms and that they’re not as prominent as we thought.

Gaia’s observations have given us a much more detailed and precise look at the Milky Way’s spiral arms. It has identified previously unknown structures in the arms, including fossil arms in the outer disk. These could be remnants of past tidal arms or distortions in the disk, or remnants of ancient interactions with other galaxies. Gaia has also found many previously unknown filamentary structures at the disk’s edge.

The Gaia mission has also allowed us to finally see our galaxy from the side. We’ve learned that the galactic disk has a slight wave to it. Astronomers think this was caused by a smaller galaxy interacting with the Milky Way. The Sagittarius Dwarf Spheroidal galaxy could be responsible for it.

The Sagittarius Dwarf Spheroidal Galaxy has been orbiting the Milky Way for billions of years. According to astronomers, the three known collisions between this dwarf galaxy and the Milky Way have triggered major episodes of star formation, one of which may have given rise to our Solar System. Image Credit: ESA/Gaia

Alongside the compelling science, artists have created illustrations based on Gaia data that really hit home. The stunning side view of our galaxy is one of the most accurate views of the Milky Way we’ve ever seen.

This artist's reconstruction of Gaia data shows the Milky Way's central bulge, galactic disk, and outer reaches. Image Credit: ESA/Gaia/DPAC, Stefan Payne-Wardenaar — This artist’s reconstruction of Gaia data shows the Milky Way’s central bulge, galactic disk, and outer reaches. Image Credit: ESA/Gaia/DPAC, Stefan Payne-Wardenaar

Gaia has updated our understanding of the galaxy we live in and brought its history to life. Even if it had no more to offer beyond today, it would still be an outstanding, successful mission. But even though its mission is over, we still don’t have all of its data.

Its final data release, DR5, will be available by the end of 2030.

Who knows what else the mission will show us about our home, the Milky Way galaxy.