# What impact would a white dwarf-neutron star collision have on a civilization on a planet orbiting the white dwarf?

An advanced civilization lives on an Earth-like planet orbiting a white dwarf. Their star is on a collision course with a neutron star. I imagine this collision will cause a supernova, which will obliterate the planet. But what other effects will this situation have on the civilization/planet, prior to the supernova event (e.g., increased radiation)? And at what point is the civilization likely to die out (probably much earlier than the supernova itself)? If it helps, I imagine the aliens to be an avian species, like large owls.

• A Type 1 supernova is created when a superdense star, like a white dwarf, draws hydrogen from a companion star onto itself. The hydrogen "atmosphere" is compressed by the intense gravitational field until sufficient hydrogen is piled on to spark a thermonuclear reaction. If a neutron star collides with the white dwarf, all kinds of bad things will happen, but not a supernova. – Thucydides Jun 3 at 2:35
• @Thucydides The detonation is usually carbon or oxygen fusion, which makes sense, given the composition of white dwarfs. Hydrogen fusion isn't particularly relevant - a white dwarf/neutron star merger would also presumably result in a supernova from runaway carbon or oxygen fusion. – HDE 226868 Jun 3 at 2:37
• meanwhile scientists there: "finger crossed this is the historic moment to prove gold is produce from kilonova... don't blink or eternal regret!" – user6760 Jun 3 at 4:02
• @Marmel. I think that a civilization existing on a planet of a whit dwarf star would be considered highly improbable. Quite possibly the planet would have had to be moved from another star system into orbit around the white dwarf by a highly advanced civilization. Presumably the natives of the planet would detect the oncoming neutron star and take some measures to escape, especially if they were the ones who moved their planet there in the first place. – M. A. Golding Jun 3 at 18:46

## Gravitational waves

Presumably, there would be gravitational waves emitted as the two compact objects slowly came close to merging. Given their small cross-sections, a direct, head-on collision is highly unlikely. A more plausible scenario involves an interaction with a third massive body.$$^{\dagger}$$ The white dwarf and neutron star would be inserted into a tight orbit around each other, while that third object would be ejected from the system.

Any two bodies orbiting each other will emit gravitational waves. Usually, these are fairly insignificant up until the bodies merge. In particular, at a distance $$r$$ from the binary, the measured strain would be $$h\sim\frac{GM}{c^2}\frac{1}{r}\left(\frac{v}{c}\right)^2$$ If we take $$M\approx3M_{\odot}$$, $$v/c\approx0.6$$ (realistic shortly before the merger) and $$r=1\text{ AU}$$, we would find $$h\sim10^{-8}$$. This is roughly 13 orders of magnitude higher than the typical black hole-black hole merger detected by LIGO as measured on Earth - not negligible, but not enough to cause serious damage.

## Thermal x-rays from an accretion disk

If we assume that the neutron star arrived with a companion, losing it in the three-body interaction, it's quite possible that it was accreting mass and thereby formed an accretion disk. This disk is presumably quite hot, with temperatures in the millions of Kelvin. This in turn would lead to thermal x-ray emission. Depending on the size and orientation of the disk, this could prove dangerous for the planet. (This statement is either quite conservative or the understatement of the year!) I'd assume there would also be non-thermal emission of some sort, but I don't know enough about the relevant processes to say anything intelligent on the matter. I'm certainly worried about the thermal x-rays, though.

$$^{\dagger}$$ This third body could be either a then-ejected companion to the neutron star or else one of the planets in the system, which would then be tossed out. This also brings up another point: The odds of the system retaining the planets up to the merger seem quite low.