We found an old star that gives the Milky Way a very old appearance

NASA

Our own galaxy could be a lot, much older than previously thought – from three billion years ago, to be exact.

Astronomers have drawn this conclusion after discovering what appears to be one of the oldest stars ever found in the universe. He is small, very poor in metal – which indicates that he was born right after the Big Bang – and lives in the Milky Way.

It is estimated that our Sun is about 4.6 billion years old, making it a newcomer to our galaxy, which is currently 13.5 billion years old. The new discovery, however, would make our galaxy much older. In all galaxies, stars are born and die all the time, although their death tends to be much more spectacular than birth – at least for stars large enough to become supernovae. Do not worry about our sun, though; he is far too small for such a spectacular disappearance. It will die quietly in about 10 billion years, although it will destroy the Earth with it.

Scientists believe that the stars of the early universe were entirely composed of hydrogen, helium and a tiny bit of lithium, so a lot of gas and very little metal. These stars then became massive, much larger than the Sun, and produced heavier elements in their nuclei; when they went supernova, they seeded the universe with these elements. This newly discovered star is however very different.

So, how can astronomers know the age of a star? In the end, they rely on models to help them estimate the evolution of stars over time. When they observe a star for the first time, they look for the best model that fits the data, based on its age and composition.

Galaxies, in turn, come in four distinct forms: spiral, elliptical, lenticular and irregular. Our natal galaxy is a spiral that looks like a reel – which consists of a nucleus, or "bulge" with lots of stars, gas and dust, surrounded by a thin disc that contains spiraling "arms" outward, plus a halo. There are billions of spiral galaxies in the universe and each of them has billions of stars of different ages. In our galaxy alone, there are about 250 billion stars.

The bulge is a particularly crowded place. Not only do we find many more stars than on the periphery, but at its center is also the supermassive black hole of the Milky Way, which is considered the oldest component of any galaxy. Most astronomers believe that galaxies are formed when gas flows in all directions and becomes very dense in the center, which then triggers the formation of the first stars. "Big chunks – maybe small, full-fledged galaxies – have continued to dive into the young Milky Way, turning like water when it sinks into a chasm," says Jacco van Loon, astronomer at Keele University in Newcastle. . This naturally flattens the Milky Way and turned it into a disc.

The old stars were supposed to move perpendicular to the galactic disk, while only the youngest stars – like the Sun – would get stuck inside the rotating disk around the Milky Way. The recently spotted old star, however, lives inside the disc, moving in our galaxy as if it were a younger star – although this is absolutely not the case. This suggests that the Milky Way disk has been around for much longer than expected and that some of its components must be very old. Van Loon's conclusion: "The formation of galaxies must have been a very fast process."

In other words, when the supermassive black hole of the Milky Way was young and accumulating the bulge all around, a thin disk of stars was already there – and has since grown into the much larger disk we see today. "The oldest part of the thin disc should be at least as old as its oldest member," says lead author Kevin Schwartfman, Johns Hopkins University astronomer at the University of Toronto. Baltimore, Maryland.

To find the star, the team used a technique called Doppler spectroscopy, also called Doppler oscillation method – in the same way that many exoplanets are detected in orbit around other stars .

Here is how it works. First, scientists analyze the spectrum of a star or the way in which starlight splits into integral wavelengths. A planet orbiting a star pulls gravitationally on the star, so the star moves toward us or away from us – and the wavelength of its light becomes shorter or longer. Astronomers analyze the amplitude and periodicity of these movements to estimate the orbit and mass of the planet. This technique has been used successfully over the past two decades to detect more than 700 exoplanet candidates orbiting nearby stars.

The Schlaufman team, which published its results in The astrophysical journal, found the star using a similar method, observing a larger and brighter star. The researchers were intrigued when they saw the data obtained by another group of star-gazing scientists and noticed that there had to be a very dense object causing it to flicker, causing it to move towards them and away from it: maybe a black hole.

They observed the star for a year, waiting to find the black hole, regularly analyzing the spectra and noting minimal changes in the speed of oscillation. The star was moving in a regular pattern, which suggested that she was actually in orbit. But then Schlaufman and his colleagues realized that the previous group's calculations were based on an error. the movement was not caused by a black hole, but by a star of very low mass. "That turned out to be more interesting than a black hole," says Andrew Casey, astronomer at Monash University and co-author of the study.

The shooting star was found to have a mass slightly lower than that of our Sun. To determine his age, the team then used various computer simulations showing how stars changed their color and brightness during their lifetime. The farthest stars are weaker, so you need to know the distance to be able to say how bright it is. Fortunately, we have the appropriate technology to cover this distance. The Gaia satellite, launched in 2013, can measure with very high precision the angle of vision of stars by astronomers. this angle varies as the Earth rotates around the Sun and the magnitude of the variation gives the distance. And the model that best matched the brightness and color was that of a very old star, just a little younger than the age of the universe itself – it 's only the same. is probably formed barely 300 million years after the Big Bang. It's "at the earliest we think the stars could form," says Casey.

Since the new star is so small and contains so few metallic elements, the discovery could also enlighten us about the size and composition of the very first stars in the universe. For now, scientists think that the first metal-poor stars were massive, not small, and that they are all already extinct. "The stars more massive than the Sun, which were born at that time, are no longer alive and have evolved into remains," says Avi Loeb, astronomer at Harvard University.

But the discovery suggests that there could also be stars totally devoid of metal – what astronomers call the "Population III" stars, the very first generation of stars – who survived until These days. "It's amazing, because astronomers once thought that metal-free stars must be 10 or 1000 times more massive than our Sun, which means that these metal-free stars would have died billions of years ago," says Casey. . Computer simulations suggest that metal-free stars could have a low mass – and this discovery appears to be "an indicator of nature: metal-free stars can have a low mass, which means that there could be true first-generation stars – relics of the Big Bang – still around us today, "says Casey.

Loeb says that the existence of stars like the recently discovered "opens the window to study very old stars that still burn their nuclear fuel."

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