A Side of Science: Why do dead stars shine in X-rays?

In my latest video, I talk about ring galaxies and how black holes and neutron stars in those rings shine in X-rays. But where do those X-rays come from?

Most stars form as binary systems – that is, most stars are found in pairs that orbit each other. Our Sun is unusual in this respect. In those binary systems, both stars can be massive (several times the mass of our Sun), both can be small (similar to our Sun), or a combination.

Credit: NASA/JPL-Caltech

Massive stars live and die much faster than their smaller counterparts. This is because they burn fuel so much faster that despite having more fuel to begin with, they run through it very, very fast (astronomically speaking). In a binary system with a high mass and a low mass star, that high mass star will evolve and die before its companion.

Credit: NASA, ESA, Hubble Heritage Team (STScI/AURA), J. Hughes (Rutgers University)

High mass stars end their lives in a supernova explosion as their core collapses into either a neutron star or black hole. Both are incredibly dense objects – astronomers call them “compact objects” – with black holes even more dense than neutron stars.

In general, it’s not terribly remarkable to find a binary system with a small star orbiting a neutron star or black hole.

Credit: NASA’s Goddard Space Flight Center

However, it’s possible for the neutron star or black hole to start pulling material off of its companion. This can happen if the stars are in a close orbit or when the ordinary star evolves into a red giant (when its core runs low on hydrogen). Essentially, the outer layers of the small star get so close to the collapsed companion that the material gets siphoned off.

The material falling into the neutron star or black hole forms a disk – an accretion disk – to lose angular momentum as it spirals in. This disk gets very hot from friction and other forces – so hot that it emits X-rays. A lot of them. These systems are bright enough in X-rays to be seen in distant galaxies.

And, in case you missed my ring galaxy video, be sure to check it out.

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