The Strangest Star: A Neutron Star Inside a Red Giant

Artist's impression of a neutron star, which may sit at the heart of a red supergiant in a newly discovered Thorne-Zytkow object.  Image Credit: NASA

Some of the stuff in space is truly weird. Last month a group of astronomers announced the discovery of what might be one of the strangest: A star inside another star.

The star, which has the exciting name HV 2112, is thought to be a Thorne-Zytkow Object (TZO). This type of star, the existence of which was first suggested in 1977, is made up of a red supergiant, a vast dying star many times the size of the Sun, with a neutron star sitting at its core.

Neutron stars are some of the most extreme objects in the universe. The remains of the explosive deaths of massive stars, neutron stars are the densest objects we know of (black holes excepted, because they're just odd).

Most matter, including you, me and everything we see around us, is made of atoms, comprised of a tiny nucleus surrounded by a cloud of electrons. While the common statement that atoms are mostly empty space isn't quite true (there's all sorts of fields and virtual particles flying around in there), the vast majority of the mass of an atom is concentrated into a tiny area relative to it's overall size.

Not so for neutron stars. They, as the name suggests, are made almost entirely out of neutrons, one of the particles found in atomic nuclei. The neutrons are packed in literally as tightly as it is possible to be, making the star ridiculously dense.

Though they have a mass of between one to two times the mass of the Sun, neutron stars never get bigger than about twenty kilometres across. This means that a sugar cube-sized lump of neutron star weighs around a billion tons.
 
(To use the accepted measurement standards of this blog, a 10km radius gives it a surface area of roughly 0.06 Wales)

The TZO would have started off as a binary, two massive stars closely orbiting each other. When the larger of the two stars ran out of hydrogen fuel in its core, it blew up in a supernova explosion, its core collapsing to form the neutron star.

From that point there are two ways by which the neutron star could have fallen into the core of its companion. The physics of supernovae  are still not fully understood, but it is thought that in many cases the explosion may not be symmetric. The explosion could have given the neutron star a "kick", slamming it into its nearby companion.

Alternatively the neutron star may have remained as it was until its companion also reached the end of its life and swelled up into a red giant. The forming red giant would have engulfed the neutron star, slowing it down until it spiralled down into the core of the red giant.

However once the neutron star reaches the core of the companion it begins to look just like every other red supergiant. Without being able to see the buried neutron star, how can we tell that HV2112 is a TZO and not just a (far more common) red supergiant?

There are two main ways to move heat around inside a star. Heat transfer in the Sun is dominated by radiation, where material that gets hot shines out light, heating up material around it and so on. In the Sun energy is transferred by radiation for about 70% of the distance from the core to the surface.

For the last 30% this changes, and energy is transferred by convection currents, where the material itself moves. Hot material expands and becomes less dense, so rises up through the cooler materiel around it until it reaches the surface. It then radiates its heat in to space, cooling down and sinking back into the Sun to be heated up again, and so on. This mix of rising hot material and sinking cooler stuff creates a granulated pattern of convection cells on the Sun's surface (a structure I studied as part of my Masters degree).

Granulation on the surface of the Sun, caused by convection cells formed of hot (light) and cool (dark) material. This phenomena can reveal the presence of a neutron star at the heart of a red supergiant. Image Credit: NASA

It's these convection currents that allow us to distinguish between a normal red supergiant and a neutron star-cored TZO. Unlike the Sun, in a red supergiant the convection zone extends all of the way to the core. This means that material is being regularly transferred from the core to the surface: material that in a TZO has recently been in contact with the neutron star core.

The interaction between the surface of the neutron star and the mass of hydrogen around it ignites a unique nuclear reaction, the rapid-proton process. This creates large amounts of otherwise uncommon elements, such as rubidium, strontium, yttrium, zirconium and molybdenum. The convection currents then carry these material up to the surface, where we can see them with out telescopes.     

By looking at the the spectrum of HV 2112 the discoverers were able to observe all of these elements and compare them with the material they expected to see in the atmosphere of the neutron star. They found much higher levels of rubidium, lithium and molybdenum than they expected, leading them to suggest that HV 2112 had a neutron star hidden at its core. The 37 year search for a TZO may well be over.

The team behind the discovery were careful to point out that, although the signs of HV 2112 being a TZO are all there, there were other signs in its atmosphere that didn't quite fit. Some of the ratios of different metals were different from what we'd expect, and the way the light from the star varied over time was unusual. However none of these anomalies are showstoppers, although they show us just how much more we have to lean about these giant stars.

If it is a TZO, HV 2112 is certainly one of the strangest, most interesting objects around. We'll just have to see what weird and wonderful things we'll find next...

The discovery paper is here. Follow me on Twitter for new blogs.