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Gaia Mission Reveals History Of The Milky Way

The European Agency launched the Gaia mission to discover the history of the Milky Way by mapping out the positions of one billion stars.

The European Space Agency launched the Gaia mission to discover the history of the Milky Way by mapping out the positions of one billion stars.

European Union – The European Space Agency launched the Gaia mission in 2013. The mission’s overall goal was to discover the history of the Milky Way by mapping out the positions and velocities of one billion stars. The result is kind of like a movie that shows the past and the future of our galaxy.

The mission has released two separate, massive data sets for researchers to work through, with a third data release expected soon. All that data has spawned a stream of studies into our home galaxy.

Recently, the ESA drew attention to five new insights into the Milky Way galaxy. Allof these discoveries directly stemmed from the Gaia spacecraft.

Gaia’s job was to create the largest, most precise, catalog of stars in the Milky Way. It’s gathered data on one billion objects, mostly stars but also some quasars, comets, and other objects. Gaia monitored each of its target objects 70 separate times, which accounts for the data’s precision. Its mission was originally planned for five years, but it’s been extended because it has enough fuel to operate until about November 2024.

In a new press release, the ESA outlined five important results of its Gaia mission.

Cannibalistic Spiral

Astronomers have thought for a long time that the Milky Way has grown by consuming other smaller galaxies that get caught up in its gravitational pull. But Gaia’s data gave astronomers an unprecedented look at how this has happened in the past, and how it’s still happening now.

A team of researchers working with Gaia data found a family of 30,000 stars moving through the Milky Way. These stars are all around us, interspersed with other stars, and they’re all moving the same speed and direction. But their motion is separate from the rest of the Milky Way. They’re moving in “elongated trajectories in the opposite direction to the majority of the Galaxy’s other hundred billion stars, including the Sun,” according to a press release.

They also stood out from other stars on the Hertzsprung-Russell Diagram. The team behind that research concluded that this was a separate population of stars. This group was the result of a galactic merger some time in the past. “The collection of stars we found with Gaia has all the properties of what you would expect from the debris of a galactic merger,” said Amina Helmi, lead author of the paper published in Nature.

Gaia data not only allowed researchers to find this merger remnant, it allowed them to piece together what happened. About 10 billion years ago, the Milky Way collided with a galaxy about the size of one of the Magellanic Clouds. The lost galaxy, called Gaia-Enceladus, was consumed by the Milky Way. The Milky Way was much smaller then, only about four times larger than Gaia-Enceladus, so the collision must have created enormous upheaval.

We now know that there’ve been other collisions, and we also know that the Milky Way is in the process of consuming the Magellanic Clouds, starting with their halo of gas.

A Galactic Collision Formed the Sun?

Collisions and mergers play a huge role in the Milky Way, and possibly in our very existence.

One of the Milky Way’s neighbours is the Sagittarius Dwarf Galaxy (SDG). It’s been orbiting the much more massive Milky Way for billions of years. While the Milky Way has a few hundred billion stars, its little neighbour has only a few tens of millions of them. So the Milky Way is something like 10,000 times more massive.

But even though the SGD is tiny compared to the Milky Way, it’s had a huge effect on it, especially on our little corner.

“The galaxy was relatively quiet. Suddenly, Sagittarius fell in and disrupted the equilibrium…”

Tomás Ruiz-Lara, Instituto de Astrofísica de Canarias | IAC

Each time the SGD orbits the Milky Way, it slams into it. Of course, there’s no actual slamming. There’s too much space between all the stars for any to actual physical encounters. The slam is more of a gravitational slam; an interaction.

Astronomers think that the SGD has struck the Milky Way at least three times already: five or six billion years ago, two billion years ago, and one billion years ago. Each time it does so, the Milky Way steals some of its stars, and the SDG becomes less massive after each encounter. But the encounter also triggers star formation in the Milky Way.

A paper published in 2020, and based on Gaia data, showed that these encounters led to episodes of increased star formation in the Milky Way. That paper called the SDG the “main dynamical architect of the Milky Way disk.” Each time the SDG passed through the Milky Way, it created ripples and compressions in the gas, which lead to accelerated star formation.

“After an initial violent epoch of star formation, partly triggered by an earlier merger, the Milky Way had reached a balanced state in which stars were forming steadily,” says Tomás Ruiz-Lara, the lead author of the 2020 study. “The galaxy was relatively quiet. Suddenly, Sagittarius fell in and disrupted the equilibrium, causing all the previously still gas and dust inside the larger galaxy to slosh around like ripples in water.”

One of those encounters took place about 4.7 billion years ago, the same time the Sun and the Solar System formed. While scientists stop short of saying that the Sun was definitely formed via collision with the SDG, the idea is there. It’s possible that our very existence stems from one of these encounters. Future studies will likely confirm or rule it out.

Living Arms, Living Disc

Prior to Gaia, astronomers knew a lot about the Milky Way. The trouble is, it’s difficult to observe from inside, and so some of what we know about the Milky Way is based on observing other galaxies like it.

For example, when we look at other galaxies, the spiral arms can appear bluer than other parts of the galaxy. That blue indicates stars burning at higher temperatures. Hot stars are massive, and massive stars are young. So researchers concluded that the spiral arms are areas of intense star formation. But astronomers weren’t certain if the Milky Way had two arms, or four.

Gaia allowed astronomers to examine individual stars in the arms directly.

“Before Gaia, we didn’t know whether there were two or four spiral arms in the Milky Way,” says Sergey Khoperskov, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Germany. “Now we have clear evidence that there are four of them. With Gaia, we can measure the distance to the stars and see where they are more densely packed together, which is an indication of a spiral arm.”

Another question around the spiral arms concerns their longevity. Some researchers say that the arms are actually manifestations of a travelling density wave, and that they’re short-lived phenomena—in astronomical terms. The arms can disappear and then reform later. “Many astronomers believe that spiral arms are short-lived structures caused by some sort of gravitational instability and that they disappear within a couple of rotations and then re-emerge with some different pattern,” said Sergey Khoperskov, an astrophysicist…

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