Did you know that the history of the Milky Way has been rewritten?

In the past two years, astronomers have rewritten the history of the Milky Way with newer and more accurate data.

The boundless galaxy and stars divide the night sky in two. When the Khoisan hunters in sub-Saharan Africa stare at this scene, they will think of the wood blocks in the bonfire. Polynesian sailors will eat clouds when they see a shark. The ancient Greeks thought it was the river of milk and called it gala, which was the source of the modern English Galaxy.

In the 20th century, astronomers discovered that our Milky Way is only a part of a huge galaxy, so they also wrote their own stories about the origin of the Milky Way. To put it simply, about 654.38+0.4 billion years ago, under the action of gravity, huge clouds of gas and dust converged, bringing our galaxies together. With the passage of time, there are two kinds of structures in the Milky Way: the first is a huge spherical halo (silver heart), and then a bright and dense disk (silver disk). Billions of years later, our own solar system began to rotate into this dish, so when we look at the sky from the side of the dish at night, we can find spilled milk splashing into the sky.

However, in the past two years, researchers have rewritten almost every major link in the history of the Milky Way. What the hell happened? In fact, they got better data.

On April 25th, 20 18, a European spaceship named Gaia released a lot of information about the sky. Surprisingly, Gaia collected data sets for several years, which clearly described the motion details of about 654.38 billion stars. Past research can only draw the motion map of thousands of stars at most. These new data have brought the previously motionless Milky Way to life. "Gaia gave birth to a new revolution," said Federico Sestito, an astronomer at the Strasbourg Observatory.

As a result, astronomers rushed to download the dynamic celestial map, and then a series of new discoveries were made. They found that some parts of the sky were not as old as before. They also found evidence of large-scale collisions that led to violent activities shortly after the formation of the Milky Way, and new signs that the Milky Way kept turning in unexpected ways.

Since the launch of 20 13 and 13 in February, the information collected by Gaia satellite has completely changed people's understanding of the Milky Way. Photocredit: ESA /ATG Media Lab

In a word, these results open a new story for the turbulent past and developing future of our galaxy. "Our view of the Milky Way is changing rapidly," said astronomer Michael Peterson of the University of Edinburgh. "The point is that the Milky Way is not a static individual as we used to think, but a rapidly changing system. 」

In order to trace back to the early days of the formation of the Milky Way, astronomers began to look for the stars that existed at that time. These stars are only composed of hydrogen and helium (the most primitive substances in the early universe). Fortunately, the smaller early stars are still burning slowly, so many stars are still shining in the sky.

After decades of investigation, the researchers sorted out an ancient star containing 42 stars with this feature and called it "ultra-poor metal star" (for astronomers, any atom larger than helium is called a metal atom). In the past, people thought that these stars should gather in the center of the Milky Way, the first part of the Milky Way. In contrast, scientists believe that the stars in the second layer of the Milky Way may take billions of years to rotate and will be polluted by heavy metals such as carbon and oxygen.

At the end of 20 17, Sestito began to analyze Gaia's new data by coding to study the movement of this super metal-deficient star. He thinks that perhaps their trajectories can provide some clues to the mystery of the formation of the Milky Way.

A few days after the publication of the Gaia database, he extracted the data of 42 ancient stars from the complete data set and analyzed their trajectories. As he expected, most ancient stars will flow through the galactic center belt; But about 1/4 didn't move that much, but stayed in the youngest region of the Milky Way-the Silver Disk. "What the hell is this?" Sestito expressed it again in four different words. "What is this? 」

Subsequent research confirmed that these stars did exist in the silver belt for a long time, not just passers-by. According to two recent studies, Sestito and his colleagues collected about 5,000 stars lacking metals, of which hundreds seem to have existed in the silver belt for a long time. Another team screened 500 stars from another study and found that about110 was flat in a sun-like orbit. The third research team found that stars of different metals (also representing different ages) move in flat disk orbits. "This is a brand-new discovery," said Paula, an astronomer at the Paris Observatory and the first author of the paper. Di？ Paula di Matteo pointed out.

Why on earth do these wrong-dated stars appear in unexpected places? Sestito speculated that perhaps the original gas could avoid all the metals released by supernovae for a long time and then collapse on a star that looks very old. Or about 10 billion years ago, when the silver heart began to gather, the silver plate was already formed.

In order to know which hypothesis is more likely, he contacted Tobias Buck, a researcher at the Leibniz Institute of Astrophysics in Potsdam, Germany, who specializes in analog digital galaxies. From the beginning, as expected, previous studies have always believed that the silver plate appeared after the silver heart. But these are the results obtained in relatively low-resolution images.

Subsequently, Barker improved the resolution of the simulated galaxy image by about 10 times. At this resolution, the computer will consume a lot of computing resources every time it runs. Even if he can use the resources of the Leibniz Supercomputer Center in Germany, a single simulation will take three months to calculate, and he will repeat this process six times.

During these six times, five times were very close to the birth of the Milky Way, and two of them also appeared a large number of metal-deficient stars with silver belts.

How on earth did those ancient stars get into the silver belt? In short, they are all immigrants from the stars. Some of these stars were born in the clouds before the appearance of the Milky Way. Then these clouds just let some stars deposit in orbit, and these orbits eventually become part of the silver belt. Other stars come from small "dwarf galaxies". These dwarf galaxies were thrown into the Milky Way and entered the new silver disk belt.

The results released by the research team in May 438+065438+10 show that the human model simulating the formation of the Milky Way in the past is incomplete. As expected, the gas cloud will indeed collapse in the spherical silver psychology. But a star that reaches the spherical silver center at a right angle can also give birth to some silver halos. "Theorists are right, they just don't see the other part of the picture," Barker said.

The complexity is not limited to this. With the help of Gaia, astronomers have found a lot of direct evidence of catastrophic collisions, and they can make the assumption that the Milky Way galaxy moved frequently in its early days. But Helmo, an astronomer at the Institute for Advanced Studies in Princeton, New Jersey, USA? Helmer Koppelman used Gaia's data to accurately point out the residue after the collision of large galaxies.

Copeman still remembers that Gaia released data on a Wednesday in 20 18, and astronomers scrambled to download it, and the website was down for a time. Nevertheless, he began to process the data on Thursday and knew that he would make an amazing discovery before Friday. No matter which direction he used to process the data, he saw a large number of stars with silver halos jumping back and forth in the center of the Milky Way in the same strange way, which indicated that they came from a dwarf galaxy. Copeman and his colleagues wrote these findings in a short paper before that Sunday, and immediately made a more detailed analysis in June of that year.

The wreckage of the Milky Way is everywhere. Maybe all the stars within 60,000 light-years (hundreds of thousands of light-years in all directions) inside the center of the Milky Way were born in a collision, which increased the mass of the Milky Way by 10%. "This discovery has changed the rules of the game of previous related research," Copeman pointed out. "I expect more different little stars to explain this phenomenon. 」

This is a simulation of the formation of the Milky Way since about 10 billion years ago. Many smaller dwarf galaxies will gather in the main galaxy and become part of it. Image source: tobias Barker

The research team named the newly discovered galaxy "Gaia-Enceladus". The origin of this name is Gaia, the goddess of the Greek primitive god, and her son Enceladus. Another research team at Cambridge University discovered the same galaxy alone at the same time. They named it "sausage" because it looks like sausage on some track maps.

About 10 billion years ago, when the Milky Way collided with the Gaia-Ensladas galaxy, the fragile silver disk belt of the Milky Way galaxy might be seriously damaged. Astronomers argue why our Milky Way astrolabe seems to have two parts: a thin plate and a thick plate. When the latter rotates around the center of the galaxy, the stars above will bounce up and down. Yu Di? The research led by Mato shows that the Gaia-Enderla Das galaxy will blow up most thick disks and make them expand when colliding. Copeman explained, "The first ancient disk was formed very quickly, and we think that the Gaia-Ndeladas galaxy destroyed it. 」

In clusters called globular clusters, more evidence of collision damage has emerged. Diederik Kruijssen, an astronomer at the University of Heidelberg in Germany, introduced the training system of artificial intelligence neural network into galaxy simulation technology to observe globular clusters carefully. He asked the neural network to analyze the age, composition and orbit of globular clusters. Neural network can reconstruct the collision that constituted the Milky Way from these data, and then it will no longer pay close attention to the data of the real Milky Way. This research project subverts our previous understanding of galaxies, including the Gaia-Enderla Das galaxy, and a galaxy with a longer history and more violent collisions, named Kraken Tentacles Of The Deep.

In August, Cruisen's team published the genealogy of the Milky Way and dwarf galaxies that make up the Milky Way. They also hope to confirm the remaining 10 past collisions through independent observation. "We haven't found these 65,438+00 collisions yet, but we will," Cruisen said.

All collisions and mergers have led astronomers to infer that the halo of the Milky Way may only be composed of younger immigrant stars, which is quite different from the general speculation in 1960 and 1970, that is, the halo of the Milky Way existed long before the arrival of immigrant stars. Di？ However, when more and more stars are regarded as invaders of the Milky Way, astronomers may no longer need to assume that many or any stars originally existed there.

The Milky Way is relatively calm in modern times, but there are still many planets coming one after another. Astronomers in the southern hemisphere can see a pair of dwarf galaxies-Magellanic Cloud with naked eyes. Astronomers have always believed that these two stars are our unswerving orbital partners, just like the satellites of the Milky Way.

Then, from 2006 to 20 13, astronomers made a series of observations on Magellanic clouds with Hubble Space Telescope and found that they were more like new meteorites. Nitya Kaliva, an astronomer at the University of Virginia, estimates that the clouds in the Magellanic Cloud release heat at a speed of about 330 kilometers per second, almost twice the expected speed.

Jorge Penarpia, an astronomer at the Royal Observatory in Edinburgh? A research team led by Arubia calculated the Magellanic Cloud a few years later. They come to the conclusion that the quality of fast-moving clouds must be huge, which may be 10 times that of the past. "This is really surprising," he said.

Different research groups predict that unexpected thick dwarfs may drag some parts of the Milky Way. This year, Penarpia and Peterson collaborated to find evidence to prove this argument.

The problem when observing the movement of the Milky Way is that the Milky Way is like a snowstorm, a turbulent and chaotic star, and astronomers can only look out from one of the snowflakes. Therefore, Penaru Pia and Peterson spent most of their time studying how to counteract the use of the earth and the sun, and how to average the motion of the stars in the halo, so that the outer edge of the halo can be used as a static background for easy observation.

When they adjusted the data in this way, they found that the earth, the sun and the silver belt all tilted in the same direction, not towards the current position of the Damager Nebula, but towards the position about 1 100 million years ago. (Peterson explained that galaxies are like clumsy beasts, and their reaction speed is very slow. ). They were recently published in Nature? Their findings were published in detail in Nature Astronomy.

The silver disk slides near the silver center, which destroys the basic assumption of studying the Milky Way in the past: the Milky Way was considered as a balanced object in the past, and it did rotate and slide in space; But most astronomers assume that after billions of years, with the gradual development and maturity of the silver-heart silver disk, the state will tend to be stable.

But the analysis of Penaru Pia and Peterson proves that the above assumption may be wrong. Even after1400 million years, the Milky Way continues to collide and reshape the overall shape of the galaxy. This understanding is the latest change in our understanding of the milk river in the sky.

"We think everything we know about the Milky Way is related to its future and history," Peterson said. "We need a new model to describe these phenomena. 」