In the halo of the Milky Way, a tiny, ancient star called J0023+0307 drifts about, minding its business. It’s very old indeed, which isn’t uncommon for stars in the halo – but it contains no detectable carbon, which is pretty danged weird. So weird that the astronomy team who found it even said the star “should not exist.”
Now, they have found something else peculiar in J0023+0307. It apparently has a whole butt-load of lithium.
This isn’t actually that uncommon for very old stars, but it was certainly unexpected in J0023+0307, which formed in the first 300 million years after the Big Bang, just after the first generation of stars were starting to die.
“This primitive star surprises us for its high lithium content, and its possible relation to the primordial lithium formed in the Big Bang,” said astronomer David Aguado of Cambridge University.
When the Universe somehow came into being 13.8 billion years ago, only the very lightest elements were forged. That means hydrogen and helium, of course, as well as very small trace amounts of lithium and beryllium.
The process whereby these elements formed from the primordial sea of neutrons, protons, electrons, positrons, photons, and neutrinos that came into being immediately following the Big Bang is called Big Bang nucleosynthesis.
Heavier elements came a bit later, created inside the first generations of stars. When these stars died, spewing their contents out into the Universe, the elements they had forged were caught up in the formation of new stars.
That’s how the process of spectroscopy actually identifies very old stars. If they don’t have a lot of heavier elements – what we call extremely metal-poor stars, or EMPs – then it’s a sign they formed at a time before these elements existed in the Universe. J0023+0307 has less than a thousandth of the metallicity of the Sun; it is one of the most iron-poor stars known.
Yet, in spite of its unusual absence of carbon, J0023+0307 has lithium in roughly the same proportions as other EMPs.
“The lithium content of this primitive star is similar to that of other metal-poor stars in the halo of our galaxy, and they define, roughly, a constant value, independent of the value of the metal content of the star,” said astronomer Jonay González Hernández of the Instituto de Astrofísica de Canarias in Spain.
In regular stars, at the temperature of 2.5 million Kelvin that is necessary for stellar hydrogen fusion, lithium is destroyed. Larger stars can retain lithium in their cooler outer atmospheres, but generally speaking, smaller stars don’t contain lithium at all.
But metal-poor stars don’t burn as hot as later stars with higher metal content. This means that their lithium content is the lithium they started out with – so it’s possible that J0023+0307 contains lithium from Big Bang nucleosynthesis, and could shed light on that mysterious event.
The paper has been published in the Astrophysical Journal Letters.