The binge-worthy TV series Game Of Thrones (which I recently watched, years after everyone else) contains zombies. A lot of zombies! Zombies are dead and they eat the living. This article is, however, not about the zombie cast in Game Of Thrones, but instead it is about the zombie cast in space. It is about astronomy, or more precisely: about compact objects! But quite not the way we looked into them previously.
The recent attention on the neutron star merger GW170817 has made many people, who would not otherwise come across astronomy online, curious about space and specifically gravitational waves sources like neutron stars. I love this type of star so what is more fitting than to spread the love. I previously presented to you three crazy features in neutron stars, one of them being the accreting nature of compact objects in binary systems – and these are exactly the systems we will look into here.
Coming Back From The Dead
Neutron stars, white dwarfs and black holes are dead stars. They are a part of the compact object family, and any compact object is a dead star. Dead means they no longer fuse elements in the periodic system. But just because they die, does not mean we no longer can see or hear them. In fact, sometimes dead stars make more noise than the ones still alive, especially if they find someone to eat!
Binary Star Systems
A binary star system is a system where two stars are bound to each other via gravity. Like the Earth is bound to our own star, the Sun, stars can also be bound to each other. It does not have to be a planet/star-system, it can also be a star/star system. Dead stars can appear in these systems just as well as the living ones, because even though they are dead, they still have mass and hence gravity (just like when humans die we stop breathing, but we still weigh something).
A system with a compact object and a normal star is called an X-ray binary system, if the compact object accretes (i.e. eats!) the other star. The reason for the name is because the system emits a lot of X-ray radiation, when matter from the normal star, the donor star, falls onto the surface of the accreting compact object, the accreting star.
So, what does this have to do with zombie stars? This is where it gets interesting! Take for instance a binary system, where the white dwarf accretes from its companion star. The white dwarf is already dead, as white dwarfs can only form when lowmass stars such as the Sun dies. But in this binary system, the white dwarf stars to eat again, sucking the matter out of its companion donor star. As the matter is transferred from the donor to the white dwarf, its mass increases.
White dwarfs have a natural upper limit for their masses and this limit is called the Chandrasekhar limit. It is named after an Indian astronomer, who knew what he was doing as he improved the math behind these objects – at the age of 20. This mass limit is around 1.4 solar masses, i.e. almost one and a half Sun squeezed into the size of the Earth.
Should the mass exceed this limit the accreting white dwarf will explode in a supernova explosion. Astronomers call it a type Ia supernova, when this happens.
Wait. Did we not establish earlier that supernova explosions happen, when highmass stars collapse and die? Yes, we did. But there exists several types of supernova, one being the so-called core collapse (or type II) when a highmass star dies. The other type is when a white dwarf explodes after eating too much companion star – this type is called a type Ia supernova and is observed for instance in SN2011fe.
The remnant from this supernova explosion will be a neutron star or black hole. What started out as a lowmass star dying and leaving a dead white dwarf, actually re-lived after eating a fellow star – only to die in a violent supernova explosion like the highmass stars do!
A dead eating a living – that is a zombie right there!
Coverphoto credit: NASA/JPL-Caltech