28 juin 2015

Les premières étoiles et la vie


Extrait de la fascinante nouvelle:

A team of astronomers this week announced the discovery of a galaxy that contains stars from the very first generation after the Big Bang—the ones we have to thank for life on Earth.

When the first stars in our universe died and exploded, they would gush hydrogen and helium into space, causing thermonuclear reactions. These reactions eventually formed elements that are now abundant in the universe, including oxygen, carbon, and iron, the key ingredients to life and the reason for our existence today.

This ancient galaxy is three times as luminous of any other known galaxy, according to their paper published in The Astrophysical Journal. Since it came into existence only 800 million years after the Big Bang, the light from the galaxy has been travelling for 12.9 billions years to reach Earth. As far as astronomers can tell, the galaxy is a bright blue cloud consisting of only hydrogen and helium, and since the earliest stars were only made up as hydrogen, helium, and small amounts of lithium, it’s likely they were among the first stars in the universe.

David Sobral, a professor at Portugal’s University of Lisbon, led the team of astronomers and decided to name this galaxy CR7, or Cosmos Redshift 7, after the universe dating technique. (...)

The stars in CR7 contain the building blocks for most of the stars that exist today, including our solar system’s sun. Today’s younger stars contain a mix of heavier elements, called metals and are in the spiral arms and younger parts of galaxies.

In the mid-twentieth century, astronomers first noticed that in the oldest parts of galaxies stars are older and have fewer metals. Because of this, they realized that the earliest stars must have consisted almost entirely of hydrogen and helium. Stars in CR7 fit this profile, what astronomers call Population III stars.

(...) So while the stars in CR7 are truly old, they’re not quite the absolute oldest stars in the universe.

The science isn’t quite settled, though. There is a remote possibility that the light is coming from a primordial cloud—containing only hydrogen and helium and created by the Big Bang—that skipped becoming a star and then collapsed into a black hole. But that’s never been seen before, so to rule it out, they’ll be training the Hubble and James Webb space telescopes on that part of the sky.



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