An international team of astronomers has identified the earliest known star in the Universe. Considered a "second generation star," it formed shortly after the Big Bang — some 13.7 billion years ago.

The star, SM0313, was discovered using the ANU SkyMapper telescope at the Siding Spring Observatory and later confirmed by the Magellan telescope in Chile.

It's the fuzzy blob at center (via Digital Sky Survey):


Somewhat surprisingly, it's located a "mere" 6,000 light-years away, but its origins date back to the beginning of time itself. The chemical composition of the second-gen star shows that it likely formed soon after the very first stars flickered to life.


After the Big Bang (about 13.798 billion years ago), the Universe was saturated by particles composed primarily of hydrogen and helium. Eventually, this mixture collapsed to form the first stars — massive and extremely bright objects with a short lifespan, exploding as supernovae soon after. In turn, these explosions seeded the early universe with the basic ingredients that formed the second generation of stars.


Indeed, the chemicals that lace the surface of a star can tell astronomers about what the Universe was like before. Today, for example, the youngest stars formed from virtually every element listed in the period table. But back then, there wasn't much to work with.

Prior to this discovery, astronomers thought the first stars in the universe were highly energetic objects that produced the first heavy elements, including carbon, iron, and oxygen. But the latest research now suggests not all of these first-gen stars were forceful exploders.

The spectrum of SM031 barely contains any absorption lines. The strong lines are from hydrogen, and carbon (at 4300A), and from the Earth's atmosphere (at 5800 and 6300A; not from the star itself). For comparison, the young metal-rich Sun would have thousands and thousands of dark lines. Via Anna Frebel.

The chemical composition of the gas cloud where SM0313 formed tells a different story. The chemical abundances within it were so low that only one first-gen star was needed to provide the required elements to the cloud after it exploded.

In fact, the iron in this star is less than 0.01% of the mass of Earth's iron core, approximately 1% of our moon's mass. But incredibly, the star is one million times bigger than the Earth. It's an absolutely iron-starved star, and now the official record-holder for the most iron-poor star we know of.

Think of it this way: if the Moon was made from iron, and you squeezed all that iron into a hydrogen-helium gas ball a bit smaller than the Sun, you've got an object very similar to SM0313. Or putting it yet another way, you would need no more than an Australia-sized asteroid of iron and lots of carbon.

Because SM0313 contains the lowest level of iron ever detected in a star, the astronomers are confident it's a second-generation object.

The Oldest Known Star?

"Is SM0313 the oldest star we know?," asks MIT astronomer and study co-author Anna Frebel in a blog post. "Truthfully, we don't actually know how old SM013 is. This is because, sadly, we can't determine a specific age of these kinds of objects. However, the chemical composition of SM0313 tells us that it is a second-generation star in the Universe which naturally makes this star nearly as old as the universe itself."

Indeed, early last year, astronomers detected a star that appeared to be older than the universe itself.

What's more, the discovery is also shedding light on this early period of the Universe's history.

For example, the star, while having low iron content, has high levels of carbon. The scientists theorize that the star arose from a low-energy, first-gen star whose explosion spewed the contents of its outer layers, but was not forceful enough to release chemicals such as iron from its inner layers. This carbon-rich, iron-poor, cloud would eventually coalesce to form SM013.

"One very central question for all of us is, 'How did the first stars and galaxies get started?'" says Frebel. "This star had a lower-than-expected explosion energy, and also lower than today's regular supernovae, which was really an unexpected finding. That tells us that, to some extent, we have to go back to the drawing board, because there is more variety amongst this very first generation of stars than we have assumed so far."

Read the entire study at Nature: "A single low-energy, iron-poor supernova as the source of metals in the star SMSS J031300.36−670839.3." Other sources via MIT and ANU.

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