Astronomers at the European Southern Observatory have discovered that the strange features of the HD 148937 star system, including a magnetic star and its youthful appearance, were caused by the merger of two stars in a group of three stars. The event also formed the surrounding nebula, providing important evidence of how massive stars create their magnetic fields.

This image, taken by the European Southern Observatory's Paranal Observatory's VLT Survey Telescope, shows the beautiful nebula NGC 6164/6165, also known as the Dragon's Egg. The nebula is a cloud of gas and dust surrounding a pair of stars named HD 148937. In a new study using data from the European Southern Observatory (ESO), astronomers discovered that the two stars have unusual differences - one appears to be much younger and, unlike the other, is magnetic. Additionally, the center of the nebula is much younger than both stars and is composed of gas typically found deep within stars rather than outside. These clues could help unravel the mystery of the HD148937 system - which likely contained three stars until two of them collided and merged, creating a new, larger magnetic star. This violent event also created the spectacular nebula that now surrounds the remaining stars. Source: ESO/VPHAS+ team. Acknowledgments: CASU University of Chinese Academy of Sciences

Astronomers were in for a surprise when they observed a pair of stars at the center of a stunning cloud of gas and dust. Pairs of stars are usually very similar, like twins, but in HD 148937, one star appears younger and, unlike the other, has magnetic properties. New data from the European Southern Observatory (ESO) shows that there were originally three stars in the galaxy until two of them collided and merged. This violent event created the surrounding clouds and forever changed the fate of the galaxy.

Abigail Frost is an astronomer at the European Southern Observatory (ESO) in Chile and the first author of the study published in the journal Science. The system, named HD148937, is located in the direction of the Norma constellation, about 3,800 light-years from Earth. It consists of two stars much more massive than the Sun, surrounded by beautiful nebulae - clouds of gas and dust. "It's very rare for a nebula to surround two massive stars, which made us think something cool must be happening in this galaxy. This feeling was reinforced when we looked at the data. After detailed analysis, we were able to determine that the more massive star seemed to be much younger than its companion, which didn't make sense at all, since they should have formed at the same time!" said Frost. The difference in age—one star appears to be at least 1.5 million years younger than the other—suggests that something must be rejuvenating the more massive star.

Another piece of the puzzle is the nebula surrounding the star, known as NGC 6164/6165. It is 7,500 years old, hundreds of times younger than these two stars. The nebula also contains large amounts of nitrogen, carbon and oxygen. This is surprising because these elements are usually found deep within stars rather than outside; it's as if some violent event unleashed them.

To unravel the mystery, the team collected nine years of data from the PIONIER and GRAVITY instruments on the European Southern Observatory's Very Large Telescope Interferometer (VLTI) in Chile's Atacama Desert. They also used archival data from the FEROS instrument at the European Southern Observatory's La Silla Observatory.

"We think that the system originally had at least three stars; two of the stars had to be very close together at a certain point in their orbit, while the other star was much further away," explains Hugues Sana, a professor at the University of Leuven in Belgium and the main researcher of the observation. "The two inner stars merged violently to form a magnetic star and ejected some material, creating the nebula. The more distant star formed a new orbit with the newly merged, now magnetic star, forming the binary star we see today at the center of the nebula."

"I had already thought about the merger scenario as early as 2017, when I was studying nebula observations obtained by the European Space Agency's Herschel Space Telescope," added co-author Laurent Mahy, now a senior researcher at the Royal Observatory of Belgium. "The discovery of the age differences between the stars suggests that this scenario is the most plausible and has only been possible to show with new ESO data."

This situation also explains why one star in this galaxy is magnetic and another star is not - another peculiar feature of HD148937 discovered in the VLTI data.

It could also help solve a long-standing mystery in astronomy: how massive stars acquire their magnetic fields. Magnetic fields are a common feature of low-mass stars such as the Sun, and more massive stars are unable to maintain magnetic fields in the same way. However, some massive stars do have magnetism.

Astronomers have suspected for some time that massive stars produce magnetic fields when two stars merge. But this is the first time researchers have found direct evidence of this happening. In the case of HD148937, the merger must have occurred recently. "Magnetic fields in massive stars are not expected to last very long compared to the lifespan of the star, so we appear to have observed this rare event shortly after it occurred," Frost added. "

Compiled from /ScitechDaily