When we have the chance to fall on it, far from artificial lights, we stay baba! What? This is our venerable house, some 13 billion years old. Yes, it is she, the Milky Way, this splendid milky trail composed of billions of stars, which is discovered at night, when the Moon is hiding and no lamppost comes to distract our retina.
“It's our galaxy, it's where we live,” recalls François Hammer of the Paris Observatory. And as from Earth, we observe it from the inside, we have a side view, but all the other stars that twinkle are also part of it. And at what address are we precisely? “In one of his arms, Orion's little arm, this is where our sun is housed,” says the astronomer.
But before we realized that the Milky Way was just one galaxy among many, as in the Star Wars saga, we fumbled for a long time. Under ancient Egypt, it was considered a kind of celestial double of the Nile; the Chinese called it "the silver river"; the Japanese, "river of paradise". The Mayas, more mystical, evoke it as a path of souls. It was the Greeks who ended up giving it its Milky Way name after a scene worthy of a Brazilian drama.
From Heracles… to the Gaia satellite
One day Zeus had a mortal son with one of his countless human mistresses. His name was Heracles, and as Zeus wanted him to become immortal, the baby was sneaked into the bed of his sleeping wife Hera. Heracles suckled her breast a little too vigorously. Hera woke up, looked at this child who was not hers and pushed him away. The milk of the goddess spurts in the sky ...
But what was this Milky Way. A simple cluster of gas and dust (nebula) or a real stellar world? From Antiquity to Galileo, we hesitated. It was finally in 1609 that we finally decided: it was indeed a myriad of stars. But from there to understanding its geography ... In 1750, the English astronomer Thomas Wright set to work. Immanuel Kant, the German philosopher, also examines his case. But to no avail. Finally, it was in 1785 that the British astronomer of German origin William Herschel offered the first menu, a sort of runny omelet obtained by meticulously counting the visible stars.
Three centuries later, this culinary image remains rather relevant since the Gaia satellite, launched at the end of 2013 by the European Space Agency (ESA), left for a precise inspection to measure its age, dimensions, structure. and draw up the first 3D map. Posted at the point called "Lagrange L2" 1.5 million kilometers from our planet, this satellite has made it possible to make giant leaps in our knowledge of the Milky Way by sifting through more than a billion of the most common objects. brighter in our sky.
The mystery of the dark mass
Now he is trying to understand the mystery of the dark mass, this invisible matter, which would make up ... 85% of the Milky Way, which we are trying to prove the existence. For now, very schematically, our galaxy is a bit like a fried egg: there is a ball in the middle with white around it ...
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"In the center, there is a bulb," explains scientist François Hammer. It contains a kind of nucleus, a small, super-massive black hole located 26,000 light years from Earth, called Sagittarius A, which is not very active. "Around this bulb," there is the disc populated with young stars, gas and dust, then further on the halo made up of isolated, aged stars, further describes the expert. Our sun is located between this edge and this center and it turns around the latter like a merry-go-round at a speed of about 230 km / s. "
Our sun is of course not the only one to participate in this incredible dance. "In total, the number of stars existing in the Milky Way alone is estimated at 200 billion," he adds. It's very crowded! Is there not risk of shock between all these small people? “No,” replies the scientist, “these billions of stars are very far apart from each other and it is rather with other galaxies that there can be some collisions. "
Twice the size of the Andromeda galaxy
For now, on this front, it's calm. Unlike our big neighbor the Andromeda galaxy, no major collision known for at least 10 billion years with our neighbors. This did not prevent some meetings all the same: two billion years ago, the Milky Way began to swallow the stars of the dwarf galaxy of Sagittarius.
“But it was like meeting a hornet on the windshield of a car,” says our expert. At the moment, it is the Magellanic Clouds, another small neighboring galaxy, which are approaching the Milky Way, "either they will be eaten like the small galaxy of Sagittarius, or they will pass through", explains François Hammer .
Will our milky cocoon always be so serene? “Perhaps one day, he will meet the Andromeda galaxy which is twice as large as ours and which is a bit our anti-twin. The two in any case are approaching each other at 124 km / s at the moment, ”he explains. Brrrr! ? So what ? "There, if there is a collision, which is not 100% certain, it can give rise to a great hullabaloo, notes François Hammer, and our sun could end up, be ejected somewhere in intergalactic space , either degummed from its current place to join the bulb or the disc. What a bowling game! But let's be reassured, it's not for tomorrow.
The history of natural phenomena in 6 episodes
1. The shooting stars
2. The tides
3. The foreshore
4. The Milky Way
5. The lightning
6. The sunsets
