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Scientists have calculated the most exact age of the Milky Way galaxy ever by using stars as clocks.
Scientists
have determined the exact age of our galaxy, with some regions of the Milky Way
forming only 800 million years after the Big Bang.
Astronomers
Maosheng Xiang and Hans-Walter Rix of the Max-Planck Institute for Astronomy
used a survey of nearly a quarter of a million stars to clock the development
of the Milky Way in a new paper published in the journal Nature. They
used existing knowledge of stellar lifecycles to better understand the longer
galactic lifecycle.
The
Milky Way's spiral disc can be divided into two populations: a thin inner disc
of younger stars, to which our Sun belongs, and a thick outer disc of slightly
older stars that extends further out from the galactic spiral plane. The halo,
a sparse concentration of older stars, surrounds the galaxy as a whole.
The
thick disc developed roughly 13 billion years ago, or 800 million years after
the Big Bang, according to Xiang and Six, whereas the inner galaxy halo formed
two billion years later. The old Milky Way combined with the Gaia-Enceladus
galaxy, a dwarf galaxy that primarily merged with our galaxy between 8 and 11
billion years ago, to form the inner halo.
The researchers used stellar clocks, which are low mass stars in the "sub-giant" phase, to achieve the discovery. When a low-mass star, such as the Sun, runs out of hydrogen, its core compresses as a counterweight to gravity's pressure driving it toward collapse. Later, gas in a shell around the star will ignite and swell the star into its giant phase, although a star's brilliance is intimately connected with its age while it is still a sub giant.
Unfortunately,
stars only spend a few million years in the subgiant phase, thus finding enough
of these stars to collect relevant data requires a vast survey of stars. So
Xiang and Six surveyed 247,104 subgiant stars using data from China's Large Sky
Area Multi-Object Fibre Spectroscopic Telescope (Lamost) and the European Space
Agency's Global Astrometric Interferometer for Astrophysics (Gaia) mission.
When
paired with future surveys using the same technique, the findings could help
astronomers learn more about how galaxies originate, how our galaxy formed, and
how we got to be here to consider either subject.
In
a commentary published in Nature, University of Notre Dame physicist Timothy
Beers commented, "With a unique approach to estimate the birth dates of
stars, Xiang and Rix have succeeded in assisting us to better understand how
our Galaxy began."
"And
because the method is scalable, this image will become much clearer as data for
larger samples of stars in the Milky Way becomes accessible."
References:
Xiang, M., Rix, HW. A time-resolved picture of our Milky Way’s early formation history. Nature 603, 599–603 (2022). https://doi.org/10.1038/s41586-022-04496-5
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