You may be able to thank the Milky Way鈥檚 spiral arms for supplying Earth with a fair share of the vital element at our planet鈥檚 birth.
Oxygen is the third most abundant chemical element in the universe, after hydrogen and helium. It arises mainly in massive stars, which forge the element during their brief lives and then cast it into space when they explode.
But a galaxy鈥檚 spiral arms also help spread the wealth, says听听at the Max Planck Institute for Astronomy in Heidelberg, Germany.
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He and his colleagues have measured oxygen levels throughout a galaxy named NGC 1365 located 60 million light years away in the constellation Fornax. It has two gargantuan spiral arms joined together by a bar of older stars, all embedded in a huge disc of stars and interstellar gas and dust. As viewed from Earth, the galaxy rotates clockwise, but most of the stars and gas revolve faster than the spiral arms themselves.
Ho鈥檚 team has discovered that oxygen is 60 per cent more abundant in the spiral arms than in gas that has just passed through them.
听鈥淭o the best of our knowledge, this is the most extreme case so far,鈥� Ho says.听In contrast, using older instruments, astronomers have failed to detect any variations in oxygen levels as one proceeds clockwise around most spiral galaxies.
We can鈥檛 see our galaxy from the outside, so it鈥檚 harder to do this in the Milky Way. Nevertheless, there are hints of such variation in our galaxy too.
In a spin
鈥淚t鈥檚 certainly a fascinating observation,鈥� says at IBM Research in Yorktown Heights, New York.听 鈥淚t would not have been predicted, I think, and that鈥檚 why it鈥檚 so interesting.鈥澨� Understanding the result, he says, will require simulations that incorporate spiral arms, star formation, supernova explosions and oxygen enrichment.
Ho thinks that as gas that has just passed through a spiral arm revolves toward the next arm, massive stars within the gas explode, raising the oxygen level. The next generation of newborn stars inherit this oxygen and raise the level further when they explode.听The abundance rises until the gas levels reach a peak in the next spiral arm before turbulence again begins to dilute it.
Then turbulence in the spiral arm mixes the oxygen-rich gas with other gas, diluting it.听That explains why gas that has just passed through a spiral arm is lowest in oxygen.
By spreading the oxygen around, however, the spiral arms help distribute the element far and wide so that oxygen eventually makes its way into future generations of stars and planets elsewhere in the galaxy.
Reference:听ArXiv:听听
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