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An ancient star casts new light on the birth of the universe

A distant star may be one of the oldest astronomers have  seen, and its discovery reveals  details about the very first stars.
Image: L2 Puppis
Region of sky in the southern constellation of Puppis surrounding the red giant star L2 Puppis.ESO

Astronomers have discovered an exceedingly old star at the edge of our galaxy that seems to have formed only a few million years after the Big Bang – and what they are learning from it could  affect their understanding of the birth of the universe.

In a study published last week, researchers found the star during an astronomical survey of the southern sky with a technique called narrowband photometry, which measures the brightness of distant stars in different wavelengths of light and can reveal stars that have low levels of heavy elements.

They then studied the star – known by its survey number as SPLUS J210428.01−004934.2, or SPLUS J2104−0049 for short – with high-resolution spectroscopy to determine its chemical makeup.

They’ve now determined it is one of a very few “ultra metal-poor” stars, or UMP, signifying that it is one of the oldest stars ever seen.

“They are very rare – we only know of about 35 of them after looking for decades,” said astronomer Vinicius Placco of the National Science Foundation’s astronomical research laboratory NOIRLab in Tucson, Arizona.

He said SPLUS J2104−0049 – a red giant star with about 80 percent of the mass of the sun – is at least 10 billion years old and possibly just a few million years younger than the universe itself, which astronomers estimate is 13.8 billion years old.

Placco is the lead author of  the study published in Astrophysical Journal Letters about the distant star.

The researchers used data from an astronomical survey conducted by a telescope at Cerro Tololo in northern Chile. It revealed the star in the halo of our galaxy, far beyond the main disk of the Milky Way and about 16,000 light years from Earth – much too far away to be seen with the eye.

Placco said the initial survey covered about 20 million stars, from which he selected about 200 to be investigated with medium-resolution spectroscopy using NOIRLab’s Gemini South telescope, a few miles away on Cerro Pachon in the Chilean Andes.

SPLUS J2104−0049 stood out as particularly interesting, and so was investigated further with high-resolution spectroscopy using the U.S.-operated Magellan telescopes in Chile’s Atacama desert, about 100 miles further north, he said.

The observations show that SPLUS J2104−0049 is extremely poor in heavy elements and that it has one of the lowest levels of carbon recorded. That implies that it is a very early “Population II'' star that formed from the remnants of exploded “Population III” stars – the very first population of pristine stars, containing only hydrogen and helium, that formed only a few million years after matter was created in the Big Bang.

So far, no one has found a Population III star. The larger a star’s mass, the more quickly it burns out, and it’s thought most Population III stars were extremely large and  burned out  long ago.

Most stars, such as the sun, are third-generation “Population I” stars that contain relatively heavy elements  such as iron, nickel, carbon and oxygen. Those heavy elements were created by fusion within Population II stars that exploded as supernovas and seeded them into interstellar clouds.

Our sun, which contains around 2 percent of its mass in the form of heavier elements, is estimated to be 4.6 billion years old. Astronomers think it has another 5 billion years to go before it swells into a red giant star that will engulf the Earth and then  shrink into a white dwarf star.

Placco said  modeling of the conditions that SPLUS J2104−0049 formed under suggest it coalesced from an interstellar cloud polluted by the supernova of a single Population III star with about 30 times the mass of our sun.

The models also suggest that the Population III star that it formed from had a different fusion process than expected, which could lead to a greater understanding of interstellar conditions in the early universe. 

The discovery shows the value of the narrowband photometry surveys for identifying ultra metal-poor stars and suggests that even more could be found, he said.

It’s even possible that searching in this way could lead to the discovery of a genuine Population III star that formed soon after the Big Bang, although it would need to have the mass of the sun or smaller to have survived so long without burning up all its fuel, Placco said.

Astronomer Howard Bond of Pennsylvania State University said the new method is a development of an early technique for identifying metal-poor stars.

The Methuselah Star is the oldest known star in our galaxy.NASA

Bond has led studies of the oldest-known Population II star – dubbed HD 140283, or the “Methuselah Star,” after an extremely long-lived patriarch in the Bible – which is about 200 light years from Earth and estimated to be more than 13.5 billion years old.

He noted that while a star’s composition can be determined by spectroscopy, determining a star’s age requires knowing its distance from Earth with very high precision.

SPLUS J2104−0049 was likely to be very old indeed, and might even be older than HD 140283, but “it will be very difficult to actually determine its age because it is at a relatively large distance,” he said.

Meanwhile, the search for the original Population III stars continues: “Nobody has found a truly pristine star made only of hydrogen and helium,” he said.