Two Stars in a Binary System are Very Different. It's Because There Used to be Three

A beautiful nebula in the southern hemisphere with a binary star at it’s center seems to break our standard models of stellar evolution. But new data from the European Southern Observatory (ESO) suggests that there may once have been three stars, and that one was destroyed in a catastrophic collision.

About 3800 light years away, in the Southern constellation of Norma, you can find an object called the Dragon’s Egg Nebula (catalogue number NGC 6164). In the heart of this nebula lies a double star known as HD 148937. The pair are bright enough to be seen through binoculars and small telescopes but are far enough away that they only appear as a single star. Both of the stars that make up the pair are hot young blue giants, but the nebula surrounding them is quite unusual, which is why astronomers have been studying them for a long time.

Dr Abigail Frost is an astronomer at the European Southern Observatory (ESO) in Chile, and she has been paying attention to this system for the past nine years.

“When doing background reading, I was struck by how special this system seemed,” she says. “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.”

Frost, like other astronomers before her, have noticed many strange features about the nebula. Most obviously, hot young stars like these aren’t usually found in nebulae, as their intense radiation tends to disperse surrounding dust and gas quite efficiently. But beyond that, the nebula itself has an unusual composition. If this nebula were the remains of the gas cloud that birthed these stars, it would be composed almost entirely of molecular hydrogen. But instead, it contains heavier elements like oxygen, nitrogen and carbon. Old stars create these elements by fusing Helium, and they eject them in their final stages of life. But that cannot be the source of this nebula, as the stars are still young.

The stars themselves have their own mysteries. The larger of the two has a strong magnetic field. Magnetic fields in stars like our Sun are formed when the thick central shell of super-heated plasma circulates. Much of the heat from the Sun’s core is transferred to the surface by convection: hot plasma near the core bubbles up towards the surface, where it cools and then sinks back down. Plasma is electrically charged, and all that charge moving generates a magnetic field, in what scientists call a dynamo effect.

But truly massive stars, like those in HD 148937, are so big that heat can simply radiate out from the core. There is such a large distance from the core to the surface that the temperature gradient is very gradual. There is nowhere inside the star with a high enough temperature differential to start convection, so there is no flow of material to generate a magnetic field. Nevertheless, the star has a magnetic field, which leads to the next oddity: magnetic stars experience a braking effect, causing their spin to gradually slow. So, this star, with its strong magnetic field which it should not have, spins rapidly, which the magnetic field should have prevented.

But that’s not all! The primary star is at least 1.5 million years younger than its companion. According to Dr Frost, this shouldn’t be possible: “After a detailed analysis, we could determine that the more massive star appears much younger than its companion, which doesn’t make any sense since they should have formed at the same time”

If this system of stars and nebula doesn’t match what our models of stellar evolution tell us to expect, then how do we explain all these anomalies?

“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,” explains 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.”

In other words, the system was originally a triple star, not a double. Triple systems tend to be quite unstable, and usually end up ejecting one of their members. But sometimes the third star will smash dramatically into one of its companions instead. Nobody has ever seen a stellar collision, but computer modelling predicts a number of things, which we see in NGC 6164. A star is, essentially, a vast and massive cloud of gas, so big and heavy that its central regions are compressed to an enormous temperature and pressure. So, when two stars collide, these masses of gas merge chaotically. The different layers mix, dredging nuclear ash (like helium, nitrogen, carbon and oxygen) from the core to the surface. A lot of the gas, including the heavier elements, is ejected to create a vast new nebula. What’s left will collapse back inwards, settling down into a new star, with a rapid spin to match. And finally, the turbulence of the collision generates and sustains a powerful magnetic field.

This sequence of events has long been predicted by astronomers trying to model stellar mergers, and the nine years of work by Dr Frost could well provide the evidence to confirm that they are right. The metal-rich gas of NGC 6164, the youthful appearance of the primary star, it’s rapid spin and strong magnetic field all seem to confirm that this was indeed once a three body system that ended with a collision between two stars.

Read the original press release at https://www.eso.org/public/news/eso2407/