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SkyMapper turns up oldest star ever found

There's no iron in this 13.6 billion-year-old fire

Spectrograph of SMSS J031300.36−670839.3

It's a one-in-60-million search: a group of astronomers has turned up a “second-generation” star, the oldest yet discovered.

The star, while given an age of 13.6-plus billion years (more on this later), is quite nearby at just 6,000 light years distant, and is in the Milky Way. What's special about SMSS (SkyMapper Southern Survey) J031300.36−670839.3 is its chemical composition, as revealed in its spectrum lines. These show that the star has very little iron – at a maximum of 10-7.1 the concentration in our Sun, it's the most iron-poor star ever characterised.

In this case, to paraphrase an old saw, “absence is evidence”. All the spectrum reveals is hydrogen, helium, carbon, calcium, and magnesium. And that means J030300.36-670839.3 is believed to be a second-generation star, formed from the remnants left when the universe's very first stars began exploding.

The lack of iron revises our understanding of the early universe, Stefan Keller, lead author of the paper in Nature (abstract here) that announces the discovery, told The Register.

Spectrograph of SMSS J031300.36−670839.3

Spectrum image posted to Twitter by Anna Frebel, MIT. Lots of blue means little iron.

That's because in current Big Bang modelling, those first-generation stars should have thrown off iron as well as other elements – and that should be observed in a second-generation star. The discovery of J-etcetera, with a lack of iron, suggests another process could be at work.

Instead of the kind of high-energy supernovae we observe now in late-generation stars (which would throw heavy elements into space along with the reset of the ejecta), “what this seems to suggest is these first stars have these “wimpy” explosions, and subsequent generations have more energetic explosions,” Keller explained.

Instead of a star like our own sun, which could carry the ejecta of as many as 1,000 predecessors, this discovery represents just one parent supernova.

Because the first generation stars had weaker explosions, “They ended up consuming all their iron, by ending in a low-energy explosion. Most of the material falls into a black hole.”

The Curious Case of the Missing Lithium

The discovery also points to a possible resolution of another way in which our models of the early post-Big-Bang universe is at odds with what we observe today: the abundance of lithium.

Keller explains that the lithium we see today is primordial, Big Bang stuff, because stars remove lithium from the universe. However, there's not enough of it: “We would expect to find about three times as much lithium in the present day universe than what we see – there's a huge chunk of lithium that's unaccounted for.”

Evidence that super-large primordial stars had weak explosions offers a mechanism by which that lithium could go missing: since the first stars didn't leave enough heavy elements behind, the second generation of stars could also grow to be immense giants like their predecessors.

“What we've found in this study, that these first stars don't emit large amounts of iron, means means the iron only increases slowly in the universe.

“If the universe is only slowly enriching in iron, it goes on forming massive stars for a longer period, incorporating a large amount of the mass of the universe. They burn the lithium, then they blow up, and don't emit much iron. This helps us bring the lithium abundance into line with what the big bang theory predicts.”

To spot these primordial stars is a multi-stage process. Keller told The Register it stars with very simple, (relatively) low-resolution measurements at SkyMapper: “We just measure the colours, at this stage. You're taking out chunks of the blue light, comparing that to a chunk of the red light.

“The ones that are the most iron-poor are the bluest. We can then go, and very efficiently select the ones that are of the most interest to us, on the basis of their heavy metal content. Once have a good list of candidates, we take that to the ANU's 2.3 metre telescope and take low-to-intermediate resolution spectrographs there.

“That gives us a snapshot of what the star is doing, and how much iron is in it. Once we've confirmed the most interesting ones, we can then go off to a much larger telescope [one of the 6.5 metre Magellan telescopes in Chile – El Reg], and get a very detailed, deep spectrum of the stars. Because the lines are so weak, we have to spend a whole night staring at one object.”

While “13.6 billion years” has been bandied about for J031300.36−670839.3, Keller was at pains to say that an exact date is conjectural at this stage.

“The only way you can ascribe an accurate age … is to measure the radioactive decay within stars. These are extremely difficult measurements to make,” he said.

“What we can do is age things on a relative basis – that this star is older than any other, by the amount of iron in it. We can say that this is the oldest known star so far discovered.” ®

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