Long ago, billions of years ago, the Universe was covered in darkness. It wasn’t until the first stars began to shine that space became transparent and light flowed.
Surprisingly, none of those early stars, known as Population III, have ever been found.
Now, astronomers have found the next best thing: A star so chemically depleted that it must have formed so hard in the generation that changed the Universe.
Such stars are known as Population II stars, and are extremely rare. This anemic one, called PicII-503, is very exciting – it’s the most iron-poor star ever found outside the Milky Way, in a young galaxy more than 10 billion years old.
Astronomer Anirudh Chiti of Stanford University says: “Discovering a star that clearly harbors heavy metals from the first stars was at the end of what we thought was possible, because of the extreme scarcity of these elements.”
“With the most metal abundances ever found in any very young galaxy, PicII-503 provides an unprecedented window into the production of early objects in the early system.”
The universe has no center, so if the first stars of the universe still existed, they would be distributed evenly throughout space.
However, scientists think that Population III stars were much more numerous than any stars today, and as a result had a shorter lifespan.
Now, the interesting thing about stars is that back when the Universe was just born, there weren’t many types of things available to make stars. There was a lot of hydrogen and helium.
However, as soon as the stars exploded, they began to break apart the atoms in their cores to form heavier elements like iron.
When the stars ran out of fusion, they would have exploded in spectacular fashion, releasing and scattering all those fused elements into space. In addition, Supernova explosions are violent furnaces whose components are heavier than metal.
These heavier elements – which astronomers call ‘metals’ – then fuse into the gas that will form the next generation of stars, and so on. The smaller the star, the brighter its metal. An older star, on the other hand, has less metal.
PicII-503 is a star located 150,000 light-years away, in a small, dim galaxy orbiting the Milky Way called Pictor II. Pictor II is what is known as a fossil galaxy – all the stars in it are very old, and have not formed stars or acquired new stars for billions of years.
This makes it a good place to look for stars that may have emerged from the bowels of Population III stars, as Chiti and his colleagues have done.
They used data from the Mapping the Ancient Galaxy in CaHK (MAGIC) Survey, collected using the Dark Energy Camera on the US National Science Foundation’s Víctor M. Blanco 4-m Telescope, to search for stars with very low metal abundances.
PicII-503 stood out. According to analysis of its light, the star has 43,000 times less iron than the Sun, and 160,000 times less calcium. However, its carbon content was large – about 3,000 times higher compared to those parts.
The mass is so low that researchers say the star best fits our understanding of the Universe if it is a Population II star. It suggests that the star is made of gas accreted by the first stars.
That imbalance is a clue: It suggests that the star was formed from the debris of an unusually weak supernova, where heavier elements like iron and calcium fell into the remnants while lighter ones like carbon escaped.
Related: We May Finally Have Sighted the Universe’s First Stars
If the supernova had been high-energy, the material would have exploded at a speed greater than the escape velocity for a small galaxy like Pictor II, and PicII-503 would not have been able to form.
This may also tell us something about the ancient stars lurking in the glow of our galaxy. The Milky Way has absorbed many young galaxies throughout its life, and continues to do so. Finally, Pictor II may fall into the same situation.
“What makes me even more excited is that we saw the result of the creation of the first objects in an ancient galaxy, which is a fundamental observation!” Chiti said.
“It also ties in neatly with the signature we’ve seen in low-metal halo stars in the Milky Way, linking their origin to the evolved stellar nature of these objects.”
Research published in The Nature of Astronomy.
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