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.
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.
This graphic shows the distribution of the Gaia-Enceladus stars across the Milky Way. The stars of Gaia-Enceladus are represented with different colours depending on their parallax – a measure of their distance – with purple hues indicating the most nearby stars and yellow hues the most distant ones. White circles indicate globular clusters that were observed to follow similar trajectories as the stars from Gaia-Enceladus, indicating that they were originally part of that system; cyan star symbols indicate variable stars that are also associated as Gaia-Enceladus debris. Image Credit: ESA/Gaia/DPAC; A. Helmi et al 2018
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.
The SGD has collides with the Milky Way at least three times, triggering accelerated star formation. One of those encounters may have led to the birth of the Sun. Image Credit: ESA
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.”
The anatomy of the Milky Way. Image Credit: ESA
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 at the Max Planck Institute for Extraterrestrial Physics.
Jos de Bruijne is a deputy project scientist with the Gaia mission. He says that the arms don’t contain the same stars over their billions of years history. Instead, he says, the spiral arms are kind of like a traffic jam.
“Stars are moving out at the front but the traffic jam stays because stars are piling up at the back,” he says. “We still don’t know exactly why these traffic jams form and I am convinced that Gaia will shed light on this.”
Gaia also showed us that the entire galactic disk is a busy, active place. There are a variety of forces acting on the parts of the Milky Way, and they create all kinds of movement. Its satellite galaxies are constantly interacting with it, creating movement, tension, and shaping the galaxy.
Some astronomers even think that collisions with the SDG gave the Milky Way its spiral shape.
The Milky Way is Like a Magnet, Stealing Stars from Other Galaxies
A bully and a thief. That’s what the Milky Way is. But the niceties and rules of human behaviour are lost amongst the vastness of the Universe.
The Milky Way is not a nice neighbour. It’s constantly taking stars from other star clusters and smaller galaxies. Gaia has shown us this in great detail, and has found streams of stars torn form other galaxies, stretching through space. Some of these streams stretch for thousands of light years. Gaia observations of these streams can tell astronomers a lot about the Milky Way’s gravitational power, and its mass.
Gais has also found stars travelling through the Milky Way at very high speeds.
Gaia has found stars that are sprinting at high speeds through the Milky Way. Some of them might have enough velocity to eventually escape the galaxy. Image Credit: ESA
Some of these fast-moving stars have enough velocity to escape the Milky Way. If they do, they won’t be reunited with their former homes. Instead, they’ll likely spend an eternity travelling through intergalactic space. They’re a bit of a puzzle for astronomers.
“Of the seven million Gaia stars with full 3D velocity measurements, we found twenty that could be travelling fast enough to eventually escape from the Milky Way,” explains Elena Maria Rossi, from Leiden University, the Netherlands, one of the authors of a recent study. “But rather than flying away from the galactic centre, most of the high-velocity stars we spotted seem to be racing towards it.”
There’s one thing at the center of the the Milky Way that’s so massive it could be drawing these stars towards it: the supermassive black hole Sagittarius A-Star (Sgr A*). Conversely, a supermassive black hole in another galaxy could’ve flung these stars outwards.
“Stars can be accelerated to high velocities when they interact with a supermassive black hole,” Elena explains. “The presence of these stars might be a sign of such black holes in nearby galaxies.”
Another possibility is that these stars were in binary pairs. If their partner exploded as a supernova, that could’ve propelled them outward at high velocities.
Or, astronomers admit, these sprinting stars could have a more prosaic explanation. They could be from the Milky Way’s own halo. Interactions between the Milky Way and its satellites could’ve could’ve plucked these stars from more stable gravitational relationships and sent them speeding through space.
Better data on these stars might constrain their age and composition, providing important clues to their origins. “A star from the Milky Way halo is likely to be fairly old and mostly made of hydrogen, whereas stars from other galaxies could contain lots of heavier elements,” says the study’s co-author Tommaso Marchetti. “Looking at the colours of stars tells us more about what they are made of.”
Our Sun is a Solar Surfer
In our Solar System’s neighbourhood there are clouds of interstellar gas. A 2019 study showed that they form an undulating wave 9000 light years long. Its undulations take it up to 500 light years above and below the Milky Way’s disk. The wave is about 400 light years wide, and astronomers have named it the Local Arm. They say it’s a small spiral arm of the Milky Way.
Astronomers like to keep a keen eye on gas clouds, because that’s where new stars form. Prior to the 2019 study, astronomers thought that gas clouds in the Sun’s neighbourhood were concentrated in a feature called the Gould Belt. The Gould Belt is a ring-shaped structure of stars, dust, and gas about 3000 light years long that rises above and falls below the galactic plane.
But Gaia showed that the Gould Belt is not the dominant gas structure. It showed scientists that the massive, newly-discovered wave structure is dominant.
“Instead, what we have observed is the largest coherent gas structure we know of in the galaxy, organised not in a ring but in a massive, undulating, narrow and straight filament,” says João Alves, a professor of Stellar Astrophysics at the University of Vienna, a 2018-2019 Radcliffe fellow, and one of the three scientists who discovered the structure. The structure is now named the Radcliffe Wave, after the Radcliffe Institute.
Our Sun is only 500 light years from the Radcliffe Wave at its closest point. It almost looks like its surfing on the wave. In fact, it was only 13 million years ago—about the time that apes were getting going on Earth—that the Sun last crossed the wave, and it’s likely to surf across it again in the future.
The discovery of this wave demonstrates Gaia’s power. The wave has always been there, but until got busy mapping stars, there was no way to see it. “The wave has been right in front of our eyes all the time, but we couldn’t see it until now, João adds.
“We don’t know what causes this shape but it could be like a ripple in a pond as if something extraordinarily massive landed in our galaxy,” said Alves in a press release. “What we do know is that our Sun interacts with this structure. It passed by a festival of supernovae as it crossed Orion 13 million years ago, and in another 13 million years it will cross the structure again, sort of like we are ‘surfing the wave’.”
Researchers don’t know what caused this wave structure. The unexpected undulating structure is not revealing its secrets, yet. It’s possible that, like a lot of features of the Milky Way, interactions with another small galaxy created it. Some researchers suggest that dark matter might be involved. Some say the Radcliffe Wave spawned the Sun. It’ll take more study to come up with an explanation.
Whatever its cause, the discovery of the Radcliffe Wave was a shock to some astronomers.
“We were completely shocked when we first realised how long and straight the Radcliffe Wave is when looking down on it from above in 3D, but also how sinusoidal it is when viewed from Earth,” said Alyssa Goodman, Professor of Applied Astronomy and co-director of the Science Program at the Radcliffe Institute of Advanced Study. “The Wave’s existence is forcing us to rethink our understanding of the Milky Way’s 3D structure.”
Forcing us to rethink things. That’s science’s role throughout history. And it’s something that missions like Gaia continuously do.
The third Gaia data release is coming soon. An early portion will be released by the end of this year, with the full third data release in the first half of 2021.
What will all of that data force us to rethink?