The discovery of a star exceptionally low in most elements, but abundant in carbon, has thrown into question ideas about the nature of the first stars, suggesting not all were the giants we have imagined.
The early universe was a very hot place. So hot, it is thought, the processes by which stars form today were not possible. Instead, astrophysicists think stars could only form 100 million years after the Big Bang through the collapse of protogalaxies to create stars with masses hundreds of times that of the Sun.
Large stars have short lifespans, so these pioneers are long gone. However, their debris seeded a second generation of stars. By finding and examining the characteristics of these star-children, Dr Thomas Nordlander of the Australian National University hopes to learn about the generation before them.
Such ancient stars should be low in metals, so Norlander seeks those that are bright at 422.7 nanometers, indicating an absence of calcium, which absorbs this wavelength of light. Having examined the spectra of 3,000 low-calcium stars, he found one, J160540.18-144323.1, that is lower in iron than any star previously measured. Iron provides a proxy for other metals in stars.
"In this star, just one atom in every 50 billion is iron – that's like one drop of water in an Olympic swimming pool," Nordlander said in a statement.
In Monthly Notices of the Royal Astronomical Society, Nordlander reports J160540.18-144323.1's concentration of most other elements besides hydrogen and helium is similarly low.
Carbon, which J160540.18-144323.1 has in abundance, is the exception. J160540.18-144323.1 is clearly the right age to be a second-generation star, but its carbon anomaly contradicts what we would expect from a giant star's offspring.
Instead, J160540.18-144323.1 looks like what we'd expect if it was seeded by a star just above the 8 solar mass threshold, at which stars end their lives as supernovae. Nordlander and colleagues think the star that fed J160540.18-144323.1 its carbon was probably 10 solar masses, and certainly no more than 20.
"We think the supernova energy of the ancestral star was so low most of the heavier elements fell back into a very dense remnant created by the explosion,” Nordlander said. "Only a tiny fraction of the elements heavier than carbon escaped into space and helped to form the very old star that we found."
Co-author Professor Martin Asplund said: "The good news is we can study the first stars through their children – the stars that came after them like the one we've discovered."
The finding was not a total surprise, following the discovery of a similarly low-iron/high-carbon star. However, Norlander told IFLScience that star's iron levels could not even be measured and this made it harder to constrain the size of the supernova that preceded it, leaving a much wider range of possibilities than for J160540.18-144323.1.
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