2MASS J18082002-5104378 B, an ultra metal-poor star located some 1,950 light-years away from Earth, is approximately 13.53 billion years old, making it one of the most ancient stars in our Milky Way Galaxy.
This image shows the binary star system 2MASS J18082002-5104378. Image credit: Centre de Données astronomiques de Strasbourg / SIMBAD / DSS2.
The Universe’s first stars after the Big Bang would have consisted entirely of elements like hydrogen, helium, and small amounts of lithium.
Those stars then produced elements heavier than helium in their cores and seeded the Universe with them when they exploded as supernovae.
The next generation of stars formed from clouds of material laced with those metals, incorporating them into their makeup.
The metal content, or metallicity, of stars in the Universe increased as the cycle of star birth and death continued.
2MASS J18082002-5104378 B, also known as Gaia DR2 6702907209758894848 B, is unusual because unlike other stars with very low metal content, it is part of the Milky Way’s ‘thin disk’ — the part of the Galaxy in which our own Sun resides.
“This star is maybe one in 10 million. It tells us something very important about the first generations of stars,” said lead author Dr. Kevin Schlaufman, a researcher at the Johns Hopkins University.
The extremely low metallicity of 2MASS J18082002-5104378 B indicates that, in a cosmic family tree, it could be as little as one generation removed from the Big Bang.
Indeed, it is the new record holder for the star with the smallest complement of heavy elements — it has about the same heavy element content as the planet Mercury. In contrast, our Sun is generations down that line and has a heavy element content equal to 14 Jupiters.
Astronomers have found around 30 ancient ‘ultra metal-poor’ stars with the approximate mass of our Sun. 2MASS J18082002-5104378 B, however, is only 14% the mass of the Sun.
The star is a tiny, almost invisibly faint secondary member in the binary system 2MASS J18082002-5104378.
Dr. Schlaufma and colleagues found it after another group of astronomers discovered the much brighter ‘primary’ star.
That team, led by Dr. Jorge Meléndez of the Universidade de São Paulo, measured the primary’s composition by studying a high-resolution optical spectrum of its light.
The presence or absence of dark lines in a star’s spectrum can identify the elements it contains, such as carbon, oxygen, hydrogen, iron, and more. In this case, the star had extremely low metallicity.
Dr. Meléndez and co-authors also identified unusual behavior in the star system that implied the presence of a neutron star or black hole.
Dr. Schlaufman’s team found that to be incorrect, but in doing so, they discovered the visible star’s much smaller companion.
The existence of the smaller companion star turned out to be the big discovery.
Dr. Schlaufman and colleagues were able to infer its mass by studying the primary star’s slight ‘wobble’ as the little star’s gravity tugged at it.
As recently as the 1990s, astronomers believed that only massive stars could have formed in the earliest stages of the Universe. But as astronomical simulations became more sophisticated, they began to hint that in certain situations, a star from this time period with particularly low mass could still exist, even more than 13 billion years since the Big Bang.
Unlike huge stars, low-mass ones can live for exceedingly long times. Red dwarfs, for instance, with a fraction of the mass of the Sun, are thought to live to trillions of years.
The discovery of 2MASS J18082002-5104378 B opens up the possibility of observing even older stars.
“If our inference is correct, then low-mass stars that have a composition exclusively the outcome of the Big Bang can exist. Even though we have not yet found an object like that in our Galaxy, it can exist,” Dr. Schlaufman said.
A report on the discovery is published in the Astrophysical Journal (arXiv.org preprint).
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Kevin C. Schlaufman et al. 2018. An Ultra Metal-poor Star Near the Hydrogen-burning Limit. ApJ 867, 98; doi: 10.3847/1538-4357/aadd97
