HUMANS have always been captivated by the Milky Way. To the ancient Greeks, it was a squirt of breast milk, while the Mayan farmers of Mesoamerica saw it as a growing maize stalk. Today, astronomers understand that what we see when we gaze up at the hazy streak of light stretching across the night sky is actually just one part of our host galaxy – one of four spiral arms emanating from an enormous disc of dust and gas that together contain at least 100 billion stars.
But even as we have traced our cosmic origins on the grandest scales, from the big bang to the growth of trillions of other galaxies entangled in vast webs, the history of our galactic backyard has remained opaque. It isn’t for want of trying. The problem is that when you are inside something, it can be hard to see what is really going on.
Now, a clearer picture of how our galaxy, the Milky Way, came to look the way it does is beginning to emerge thanks to data gathered by the European Space Agency’s Gaia satellite. Hidden patterns in the movements of stars have taken astronomers by surprise, unveiling in rich detail a turbulent history of cataclysmic collisions and aftershocks.
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Some say that soon we will even be able to pinpoint the specific events that led to the formation of our sun – the reason we are here to ponder such things. “It’s our heritage, a way of knowing ourselves,” says , an astronomer at the Institute of Astrophysics of the Canary Islands, Spain.
Over the past century, astronomers have sketched an outline of the Milky Way. We now know that it formed some 14 billion years ago, probably from clouds of gas and dust and smaller star clusters pulled together under gravity into a thin disc. We know that this disc is warped, with a thick, bar-shaped bulge at its centre. We also know that all of this is encapsulated in a spherical halo of stars and an ill-defined, even bigger halo of dark matter – the mysterious stuff that seems to hold galaxies together.
We know that we aren’t alone, too. There are dozens of smaller dwarf galaxies orbiting the Milky Way, entangled in intricate and occasionally violent cosmic dances. Our best computer simulations suggest that mergers with bigger galaxies, several of them on a par with the Milky Way, were formative in our galaxy’s youth. “But we don’t know when or how or precisely how many,” says , an astronomer at the University of Groningen in the Netherlands.
The Gaia satellite is now pulling this blurry history into focus by measuring the distance and motion of billions of stars up to several thousand light years away from the sun with exquisite precision. The information it is sending back to Earth amounts to a “huge treasure chest” for astronomers trying to piece together the details of the Milky Way’s evolution, says , a theoretical astrophysicist at University College London.
In April 2018, when Gaia released its second tranche of data, Helmi and her colleagues threw themselves into investigating a particular subset: the fine motions of some 7 million stars as they wobble up and down in the plane of the Milky Way’s disc. This is roughly akin to watching a human hair an arm’s length away grow in 5 million times slow motion. Walking to her car one night, Helmi glanced at a data plot sent to her by a colleague, who was searching for patterns in “phase space” – an abstract, six-dimensional world that combines the 3D positions and velocities of stars. “I was like: ‘Oh my god! What is this?’ That plot was absolutely stunning,” says Helmi.
The team had uncovered . Based on existing computer simulations and observations of star streams, , which was known to have plunged through the Milky Way three times in the past. But no one expected an imprint of such an ancient collision to still be encoded in the motions of stars today. “That was genuinely a surprise,” says James Binney at the University of Oxford.
Astronomers are still wrangling over the precise origin of the snail shell. T and , but it was , says Joss Bland-Hawthorn at the University of Sydney. “Sagittarius has been shedding a lot of mass along its orbit. Today, it’s nowhere near massive enough to do any of this,” he says.
Indeed, these days Sagittarius is a strung-out shadow of its former self, hurtling back towards the Milky Way as it traces the third leaf of its . Eventually, like many galaxies before it, Sagittarius will become part of us, having profoundly shaped the Milky Way on its journey.
Regardless, the snail shell is one among many previously hidden patterns that show Gaia is ushering in a new age of “galactic seismology”, says Bland-Hawthorn. No longer restricted to fossil records, we can now observe the faint tremors left behind by the impacts of intruders past – in the same way that we learn about Earth’s hidden interior by watching how earthquakes propagate. Deciphering these tremors, astronomers are sketching a new panorama of the Milky Way and the riotous saga of how it came to be.
A decade ago, it was proposed that an impact with Sagittarius was . There might even be clues to the true identity of dark matter lurking in this surprisingly dynamic galactic dance. The type of particles that make up dark matter – whether they are heavy or light, for example – would influence the orbits of dwarf galaxies like Sagittarius around the Milky Way and the patterns left in their wake. “Two years ago, if you had asked anyone in the field, they would not have thought that anything like that was possible,” says Schoenrich.
As if all that wasn’t enough, Ruiz-Lara suspects that Sagittarius may even be responsible for giving birth to the star that warms Earth to create just the right conditions for liquid water – and life – today.
Stars in a galaxy are a bit like people in a city, says Ruiz-Lara. From year to year, the number of people born fluctuates, and so does the city’s population. “This is what we are trying to characterise with the star formation history,” he says. “How many stars were born 10 billion years ago? How many stars are being formed right now?”
Ruiz-Lara and his colleagues were searching Gaia data for answers when another unexpected pattern emerged: three . Tantalisingly, one of these explosions of star births was calculated to happen between 4 and 6 billion years ago. “The age of the sun is 4.6 billion years,” says Ruiz-Lara. “Can we say for sure the formation of the sun was triggered by this event? We cannot, but it’s compatible. It could be that we are talking now because of this collision with Sagittarius.”
When galaxies clash, shock waves emanate outwards, creating high-pressure belts of gas and dust that collapse into stars. Although we know that some groups of stars are formed in these violent collisions, it is much harder to say why specific stars appear where they do. The way to figure out the unique life stories of stars is to look for their families, says Ruiz-Lara. “We might find that our sun has a lot in common with a particular family of stars that was caused by a certain event.”
Joining up the dots in this way requires huge data sets, which is why Gaia measurements are now being combined with data from Earth-based telescopes that have been collecting detailed information on the chemical fingerprints and brightnesses of tens of millions of individual stars.
Similar techniques are revealing previously obscure primordial galaxies such as , which is a million times more massive than the sun and is thought to have merged with the Milky Way in its youth about 10 billion years ago. We are just beginning to untangle these mergers, like archaeologists uncovering cultural artefacts from the Roman and Viking invasions of Britain, says Ruiz-Lara.
Yet the trove of intriguing patterns that the Gaia satellite is unearthing is also raising difficult questions. Few thought that the Milky Way would still be recoiling so blatantly from its collisions with minor galaxies billions of years ago. “We have to proceed more cautiously,” says Binney, because we don’t yet have the tools to model this responsive and dynamic galactic ecosystem. “You have to think holistically in terms of the whole beast.”
“Data from the Gaia satellite is ushering in a new era of ‘galactic seismology’”
Indeed, Gaia is a stark reminder of how hazy our underlying picture of the Milky Way is. Until this develops, it is hard to be sure what the new data glut is telling us, and these notions of primordial mergers, spiral arms and the birth of the sun are by no means certain.
Other ideas are certainly conceivable. at the Leibniz Institute for Astrophysics Potsdam in Germany and his colleagues, for example, have proposed that the snail shell pattern is an . “I wouldn’t be surprised if somebody comes up with a new model tomorrow that says this Sagittarius thing is junk and there’s another explanation,” says Chervin Laporte at the University of Barcelona in Spain.
Astronomers are eagerly awaiting Gaia’s next data release in the first half of 2022 for more clues. Pushing deeper inside the Milky Way, the data will offer a much bigger and more representative sample of our galaxy’s stars, offering new information about their temperature, size and chemical composition, alongside their precise positions and movements. The phase-space data – in which the snail shell was found – will also increase five-fold, revealing patterns in star movements in finer detail. Helmi expects there will be many more patterns hidden in the ebb and flow of the Milky Way.
No one expects definitive answers any time soon. Theoreticians are only beginning to grapple with the mind-boggling complexity of our shape-shifting galaxy. “We’ll need 20 years to understand the ESA Gaia data,” says Schoenrich. But at least now we can begin to piece together the finer details of how our home galaxy came to be.
