The nebula surrounding HD 148937 as seen in visible light. Image: ESO/VPHAS/CASU
After collecting data for a decade, European scientists have stumbled upon a cosmic discovery that can explain how some massive stars get their magnetic fields.
Some of the most beautiful sights can often have violent pasts, and space is no exception.
New data from the European Southern Observatory (ESO) shows that two massive stars cushioned in a cloud of gas 3,800 light years away from Earth do not exhibit typical characteristics associated with stellar pairs: the larger of the two appears to be much younger and, surprisingly, magnetic.
According to Dr Abigail Frost, an ESO astronomer, this observation – made using the Very Large Telescope Interferometer (VLTI) based in Chile’s Atacama Desert – makes no sense because star systems functioning in a pair are supposed to have formed at roughly the same time.
“When doing background reading, I was struck by how special this system seemed,” said Frost, who is the lead author of a study published in Science today (11 April) that details how there must have been a third star that collided with the one that is bigger and younger.
The system, HD 148937, is located about 3,800 light years away from Earth in the direction of the Norma constellation. It is made up of two stars much more massive than the sun and surrounded by a beautiful nebula, a cloud of gas and dust.
“A nebula surrounding two massive stars is a rarity, and it really made us feel like something cool had to have happened in this system. When looking at the data, the coolness only increased,” she said. “After a detailed analysis, we could determine that the more massive star appears much younger than its companion.”
The team knew that there had to be a reason behind the striking 1.5m-year age gap between the two stars. What helped Frost and the team to unravel the violent stellar collision is the surrounding nebula, known as NGC 6164/6165, which is only 7,500 years old.
Hundreds of times younger than the stars, the nebula was also observed to contain high amounts of nitrogen, carbon and oxygen – all elements normally expected deep inside a star – which points to a violent event.
“We think this system had at least three stars originally; two of them had to be close together at one point in the orbit while another star was much more distant,” explained Dr Hugues Sana, a professor at KU Leuven in Belgium and the principal investigator of the observations.
“The two inner stars merged in a violent manner, creating a magnetic star and throwing out some material, which created the nebula. The more distant star formed a new orbit with the newly merged, now-magnetic star, creating the binary we see today at the centre of the nebula.”
The latest study is based on nine years’ worth of data from the PIONIER and Gravity instruments on the VLTI and archival data from the FEROS instrument at ESO’s La Silla Observatory. It also explains why one of the stars in the system is magnetic and the other is not — another peculiar feature of HD 148937.
“Magnetism in massive stars isn’t expected to last very long compared to the lifetime of the star, so it seems we have observed this rare event very soon after it happened,” Frost added, noting that the study helps solve a long-standing mystery in astronomy: how massive stars get their magnetic fields.
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