The interesting thing about the Universe is that it’s big. Really big.★ With enough space and time, even the rarest events can find time to happen, with bizarre and unusual things happening as a result. And some rather good evidence for one such unusual thing was found earlier this year.
A Thorne-Żytkow object is a bizarre and seemingly paradoxical kind of star. Named after the two astrophysicists who first hypothesised its existence, Kip Thorne and Anna Żytkow, a strange star like one of these is a hybrid, of sorts. It’s believed to be a red giant star with a neutron star hiding at its core.
Earlier this year, the star HV 2112 was found to behave… not quite as expected. It didn’t quite match with models made to predict its behaviour, suggesting that there was something odd about it. While the astronomers involved stress that their idea is a cautious one, a possible explanation is that this star is precisely one of these curious stellar cocktails.
Red giants are astonishingly large. A typical sized red giant star could engulf our entire inner solar system without much difficulty. The largest known star, VY Canis Majoris, is a red giant – and it’s large enough to be difficult to fully appreciate. The human brain wasn’t engineered to encompass the immense scales which we encounter in astronomy. Neutron stars, on the other hand, are stellar corpses. The still-hot cinders from once mighty stars long ago burned out. Neutron stars are quite mindbending little things in their own right. The core of a massive star, containing more material than the Sun, compressed down to the size of a large city here on Earth, neutron stars are so unimaginably dense that their gravity can bend light and warp the fabric of spacetime.
So if you have a densely populated region of space, such as a globular cluster, there are enough stars in such a tight space, that every so often, a couple of them may crash into each other. If the two stars colliding with each other happen to be a neutron star and a red giant, the end result is a Thorne-Żytkow object. After colliding, the neutron star spirals inwards, sinking to the core of the ill-fated red giant. Once there, it proceeds to violently consume the larger star. As material from the red giant crashes onto the surface of the neutron star, it makes things incredibly hot. Hotter than the cores of most normal stars. Huge gravitational forces compress and heat up anything which lands on the neutron star, eventually mashing together individual atoms and driving nuclear fusion at hugely accelerated rates. Unusual chemical isotopes may form, driven by the high temperatures, and this bizarre type of fusion.
Stars like these may at first appear to us as red supergiants, or possibly as more violent Wolf-Rayet stars. Eventually though, one of two things will happen. The most extreme outcome is if the neutron star can accrete enough mass to collapse into a black hole. If this happens, the energy released will result in a supernova, blowing away any outer layers of stellar material not yet accreted. Otherwise, the two will eventually merge into a single object, resulting in one massive neutron star. Leftover stellar material will most likely end up as a massive accretion disk surrounding the neutron star. Interestingly enough, this disk works in the same way as the disks around young stars. It may even be massive enough to form additional companion stars of its own…
Whether or not HV 2112 is actually one of these objects remains to be confirmed. But it’ll be very interesting if it is. I haven’t read the actual study yet, but it’s available on arXiv if you’re curious.
★ You just won’t believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space…