The Milky Way, everyone now knows, is a large spiral galaxy. Over the millennia, it has not stopped swirling. Thus mixing the gases that compose it. At least that’s what astronomers thought until then. But they are now discovering that the interstellar medium is not that homogeneous.
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[EN VIDÉO] Milky Way: simulating the formation of a galaxy This film shows a computer simulation of a galaxy like the Milky Way. The film advances rapidly through simulated time, from 13 billion years in the past to today. The main galaxy expands as many smaller galaxies merge with it. Heracles looks like one of the small galaxies that merged with the Milky Way early in the process. © Ted Mackereth, EAGLE
At the very beginning of the history of Milky Way, there was a gas from the Big Bang. Mainly fromhydrogen scattered in intergalactic space. But also a little helium. That was over 10 billion years ago. Then this gas condensed and formed stars. These burned the hydrogen and created new elements by nucleosynthesis. The astronomers talk about metals. Understand here, anything heavier than helium. Metals that are ejected into the interstellar medium when the stars die. They can then condense into dust. Especially in the coldest and densest regions of our Galaxy. The bricks which will then constitute the planets.
Over the millennia, the process continues to be fueled by gas ” Virgin “ came from outside the Milky Way. And the whole thing is mixed by the rotational movement of our galaxy spiral to come up with something rather homogeneous. At least that’s what models and astronomers imagined until then. That the level of metal enrichment – what researchers call the metallicity – in the region surrounding the Soleil should therefore be similar to what we observe in the very heart of our star.
But a team of researchers led by the University of Geneva (Switzerland) argues today that all these elements are not as well mixed as it was thought. This could have an impact on how astronomers understand how galaxies evolve. Until perhaps questioning the simulations of the evolution of the Milky Way.
Pockets of low metallicity
To arrive at these conclusions, the researchers focused on 25 bright stars that they observed for 25 hours using the spectrographe ultraviolet du télescope spatial Hubble and using the Very Large Telescope (VLT) installed in Chile. Their objective: to estimate the enrichment of metals in the vicinity of our star.
The problem is that the Hubble spectrograph does not take into account the dust component. So astronomers had to develop a new technique of observation. They simultaneously noticed several elements like the do, zinc, titanium, the silicon and theoxygen. In order to be able to trace the quantity of metals present in the dust and to add it to that already quantified by the other observations.
Researchers at the University of Geneva have thus shown the existence of pockets in the interstellar medium where the metallicity is only 10% of that which we find in the Sun. What push astronomers to consider refining their simulations by increasing their resolution. Especially since metals play a fundamental role in the formation of stars, cosmic dust, molecules and planets. Thus, stars and planets could form today from gases with very different compositions.
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