What is a zombie star?


In the usual sense, the term «zombie» means a creature that has survived death. We are all used to seeing such creatures in horror films, but it turns out that there are also zombie stars in astrophysics. In short: these are stars that have twice experienced death. To understand how this is possible, first you need to figure out what “death” means for a star.

The stars are extremely massive, which is why a huge force of gravity is constantly directed towards their core, which tends to crush the star. The collapse of the star does not occur because the compression is opposed by the forces of gas pressure and radiation, which draw energy for this from thermonuclear reactions taking place in the star’s core.

But since the star’s fuel reserves are not infinite, sooner or later the intensity of nuclear reactions decreases, gravitational forces begin to prevail and a supernova explosion occurs, which is accompanied by the release of a huge amount of energy. As a result, the star dies, and in its place remains a white dwarf or neutron star, which are simply the cooling remains of the star.

Now let’s imagine the following picture: there is a binary star system, in which one of the stars is a white dwarf, and the other is the most common and still alive. If the distance between them is small, then the white dwarf will pull on the matter of a living star and, as a result, gain mass. Over time, when approaching a certain mass limit, the white dwarf heats up to such a temperature that thermonuclear reactions begin again in its core, which are accompanied by another supernova explosion. In fact, the star dies for the second time and in its place again remains the same white dwarf, albeit with a lower mass and high rotation speed, which is called a zombie star.

One of the theories why white dwarfs experience a similar outbreak and turn into zombies is as follows: when enough hydrogen accumulates around the white dwarf, extremely intense combustion reactions begin in it, which increases the pressure in the core, which cannot be balanced by gravity and the star explodes … At the moment, this theory has not yet been fully confirmed and alternative reasons for the transformation of a star into a zombie are being considered.

Author: Vladislav Kigim. Edited by Fedor Karasenko.

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