Massive stars may get magnetic fields when two stars merge together - study

Scientists studying the mystery behind magnetic stars have found that these magnetic fields seem to form through stellar mergers, when two stars form together, according to a recent study.

The findings of this study were published in the peer-reviewed academic journal Science.

The result of this study carried out after nine years of observations, sheds light on one of the strange qualities of magnetic stars - something scientists never knew how to explain before.

A polarizing find? How magnetic stars form

Magnetic fields are a sort of physical area that has magnetic influence. This can be seen in how they impact electrical currents and charges and how they work on magnetic materials. 

Contrary to popular belief, magnetism goes far beyond metals—that's actually something known as ferromagnetism, and it's just one kind. Magnetism itself is a force formed when things are either attracted to or repelled by each other, and it is far more widespread than just in metals. 

Everything is, to an extent, magnetic. This is because everything is made of atoms, and atoms have electrons, which have electric charges. When electrons move around, they start to make an electric current, and each electron becomes a very tiny magnet on its own right. Normally, most things have the same amount of electrons spinning in different directions, which cancels out magnetism. Ferromagnetic metals like iron don't have this equilibrium, so they are more obviously magnetic.

But magnets aren't just magnetic. To be a magnet, you need a magnetic field, which has north and south poles—the opposite poles attract each other, while the same type of poles repel each other. 

So, how do you have a magnetic field? Well, on a smaller scale, you can run electricity through it, and a magnetic field will temporarily form. But what about on a bigger scale?

Let's look, for example, at the single largest magnet on Earth: the Earth itself. 

The Earth's magnetic field starts inside the planet and goes out into space. This magnetic field is incredibly vital for life on Earth because it can repel certain electrically charged particles from cosmic rays and solar winds. If the magnetic field wasn't in place, these rays and winds could destroy parts of the atmosphere, essentially leaving the planet and everything on it vulnerable to harmful radiation. 

While scientists don't entirely understand it, the general consensus is that the Earth's magnetic field is formed in the Earth's outer core. A process caused geodynamo causes heat to escape the core, which causes iron and nickel in the core to heat up, becoming molten, and move around. This movement is known as a convection current, and it ends up creating electrical currents. 

That all makes sense for Earth, but it doesn't make any sense when it comes to stars. 

Now, to clarify, there is a type of star known as a magnetar. Specifically, these are neutron stars, meaning they are dead stars who already went supernova and became incredibly dense, but did not become black holes. The fact that these stars have magnetic fields is not debated - in fact, their magnetic fields are incredibly strong and are the most powerful magnetic objects known to exist. Their magnetic fields are thought to form either through certain circumstances when a neutron star forms after a supernova, or possibly when a star with an existing magnetic field turns into a neutron star.

But here is where the problem begins: How do regular stars, specifically massive ones, have magnetic fields at all? 

Smaller stars have magnetic fields and we know why. Like the Earth, their interiors can have convective currents, which can produce magnetic fields. But the more massive stars, either or more times the mass of our Sun, don't have this option. Their interiors can't undergo convection, so it shouldn't be possible for them to have magnetic fields. 

But around 7% of all known massive stars have them anyway. How is this possible?

This is the question the researchers, led by Abigail Frost of the European Southern Observatory (ESO), sought to answer. 

Their study centered on HD 148937, a binary star system around 3,800 light years away. SInce it's a binary star system, it means there are two stars there, both of them massive in this case. But only one of the two is magnetic. So how could this happen? 

After studying the binary star system for nine years, learning everything they could about it, they have made a rather astronomical conclusion: Stellar mergers.

One possibility for why massive stars could be magnetic is that stellar material could mix together. But this wouldn't happen in a star by itself since it doesn't have convection. But it could happen if that stellar material came from somewhere else.

The two stars in the system were found to have a discrepancy in age. One of them, the primary star, which is also the magnetic one, was found to be considerably younger, hotter, faster, more massive, and less evolved than the secondary star. This age discrepancy doesn't make sense since both stars should have formed around the same time. 

Surrounding both stars is a nebula rich in nitrogen and carbon, alongside other elements. These elements are on the outskirts of the nebula, far from the stars themselves. This shouldn't be possible since something like nitrogen and carbon could only be formed in stellar interiors. The only explanation - after ruling out several others - was that something happened that disrupted a stellar interior violently when the nebula was forming. 

Consider also the mystery of the primary star. Why is it moving so fast despite having a magnetic field? That should cause its rotation to slow down, a process known as magnetic braking. 

But every single one of these things could be explained by a stellar merger.

Essentially, this binary star system would have once had three stars instead of two. At some point, while the nebula was forming, two of those stars ended up merging together. This would have caused stellar material to mix, forming a magnetic field, and would have caused other stellar materials like nitrogen to be ejected out into the nebula. 

This would have had to be fairly recent before magnetic breaking could have taken effect. But this makes sense when considering the nebula's age. A nebula like this is estimated to be around 3,000-7,500 years old. It would take 1.5 million years for the magnetic breaking to take effect. The amount of stellar mass lost during the merger would also account for the estimated mass of the nebula itself. 

But what are the odds of this happening? 

Researchers have noted that around 8% or more of these massive stars are thought to experience mergers with one another. This lines up very well with the fact that around 7% of these massive stars have magnetic fields. 

But this also means that the magnetic field is only temporary. Eventually, the interior of the star will be fully mixed between the different stellar materials. It will settle down, the dynamo fueling the magnetic field will stop, and the magnetic field will die. 

The researchers theorize that this may be the primary cause of magnetic stars in the universe. However, more research may be needed to confirm this.