Clash of stars solves stellar mystery

This breathtaking image of nebula NGC 6164/6165, also known as the Dragon's Egg, is captured by the VLT Survey Telescope. The nebula is a cloud of gas and dust surrounding a pair of stars called HD 148937, which lies about 3800 light-years away from Earth towards the Norma constellation.

In a new study using data from the European Southern Observatory (ESO), astronomers have shown that the two stars are unusually different from each other - one appears younger and, unlike the other, is magnetic. This is a surprise finding as stars in such proximity typically exhibit more similar characteristics.

One star appears to be at least 1.5 million years younger than the other, with the age difference suggesting something must have rejuvenated the more massive star.

NGC 6164/6165 is 7500 years old, hundreds of times younger than both stars. Its composition includes unusually high levels of nitrogen, carbon, and oxygen - elements typically found deep within a star rather than outside. This anomaly suggests a violent historical event that expelled these materials.

These clues together helped solve the mystery of the HD 148937 system - originally there were three stars in the system until two of them clashed and merged. This violent event also created the spectacular nebula that now surrounds the remaining stars.

To unravel the mystery, the team used archival data from the PIONIER and GRAVITY instruments, both on ESO's Very Large Telescope Interferometer (VLTI), located in Chile's Atacama Desert, and the FEROS instrument at ESO's La Silla Observatory.

"We think this system had at least three stars originally; two of them had to be close together at one point in the orbit whilst another star was much more distant. 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," explains Hugues Sana, a professor at KU Leuven in Belgium and the principal investigator of the observations. 

This explains why one star is magnetic and the other is not - a peculiarity of HD 148937 observed in VLTI data. It also provides crucial insights into the formation of magnetic fields in massive stars - a process distinctly different from that seen in smaller, low-mass stars like our own Sun.

Astronomers had long suspected that when two stars merge, massive stars could acquire magnetic fields. However, this is the first time that researchers have found direct evidence of this phenomenon. In the case of HD 148937, the merger must have occurred recently.