Friday, September 10, 2021

Contrary to popular belief, the composition of the Milky Way is not homogeneous

 Astronomers have discovered that the galaxy's mixture of gases is heterogeneous and have overturned the hypothesis that it would have chemical enrichment similar to that of the Sun's atmosphere.

 Cosmic gas clouds and fluxes accumulate in the Milky Way, but do not mix homogeneously in the galactic disk.

 The cosmic gases that make up a Milky Way do not mix homogeneously as scientists have until now supposed.  The discovery, made by astronomers at the University of Geneva (UNIGE), in Switzerland, could change what is known about the evolution of our and other galaxies.

 The news is in a study published in the scientific journal Nature on Wednesday.  Until then, theoretical models wrongly said that the homogeneous mixture of gases from the Milky Way reached solar metality, that is, a level of chemical enrichment similar to that of the Sun's atmosphere.

 This gaseous mix that makes up our galaxy, in fact, does not mix equally, as it is heterogeneous.  It is made up of three main elements: gas coming from outside the Milky Way;  the gas between the stars within the Galaxy, composed of hydrogen and helium;  and the dust generated by the condensation of metals present in this last gas.

 Material from outside the Galaxy forms stars, which burn the hydrogen that makes them up and generate other elements.  When these stars explode, they expel the metals they've produced, such as iron, zinc, carbon and silicon.  This feeds the intergalactic gas, so atoms condense like dust, especially in cooler, denser areas of the Milky Way.

 “Initially, when the Milky Way was formed, over 10 billion years ago, it had no metals.  From then on, the stars gradually enriched the environment with the metals they produced”, explains Annalisa De Cia, professor at the Department of Astronomy at the Faculty of Sciences at UNIGE and leader of the study, in a statement.

 To better understand the metallicity present in the Milky Way, scientists observed for 25 hours the atmosphere of 25 stars with the Hubble Telescope and the Very Large Telescope (VLT) in Chile.

 Using an unprecedented observational technique, the team tracked the amount of metals present in the galaxy's dust to compare the result with previous data.  “It's about taking into account the total composition of the gas and dust while simultaneously observing various elements such as iron, zinc, titanium, silicon and oxygen”, says De Cia, describing the new technique.

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