The recent detection of a binary neutron star inspiraling and colliding raises an interesting question in my mind. Type Ia supernovae are believed to be caused by white dwarf/regular star pairs and/or white dwarf pairs. Short gamma ray bursts are now known to be produced by neutron star/neutron star collisions. Black hole/black hole collisions haven't produced any verifiably detected electromagnetic radiation. What about some of the other mixed type collissions? Especially white dwarf/neutron star or neutron star/black hole?

My understanding is that stellar binaries tend to have similar mass, so mixed type remnants will likely be rather rare. Hence, why I didn't ask about white dwarf/black hole binaries. Even so, I'm curious how the large difference in density would affect things. Especially if a neutron star would be tidally shredded into an accretion disk by a black hole before they can collide, making a relatively slow event, or will the process be more rapid and violent, akin to the events we've seen so far?

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    $\begingroup$ EM counterparts to NS+BH mergers are definitely expected. $\endgroup$
    – ProfRob
    Commented Oct 18, 2017 at 10:04
  • $\begingroup$ Are you asking about interesting products (elements, high-energy photons, etc) or about the resulting artifact (new star, big black hole, etc)? $\endgroup$ Commented Oct 18, 2017 at 13:32
  • $\begingroup$ @CarlWitthoft Mostly observable characteristics of the event, less about element production, or resulting artifact. If there's a black hole involved, for example, I'd expect the result to contain a black hole. While dwarf/neutron star collision seems likely to be like a type Ia supernova and leave behind nothing (?). $\endgroup$
    – Sean Lake
    Commented Oct 18, 2017 at 17:33

1 Answer 1


WD+NS collision cannot leave nothing behind. Gravitational mass defect of NS is ~10% of its rest mass -- thus whatever energy released in the collision cannot disrupt the NS. The energy released is basically the nuclear energy of the WD material burning into heavier elements as it gets heated in the process of collision. That amounts to at most ~0.2% of the WD rest mass (assuming the super-idealistic situation of helium WD, completely transformed into 56Ni). Not much of the material will be ejected; the phenomenon would be dimmer than SNIa [Double degenerate SNIa scenario is the disruption of the colliding pair of white dwarfs, their matter transformed into 56Ni and lighter elements; without the deep gravitational well of the NS.]

A separate question is whether/how such a NS-WD collision could be realized with appreciable probability. NS must have formed from the heavier star in the initial binary system; that formation must be through the core-collapse supernova event. It is unlikely the binary would survive ccSN. So the needed WD must arrive to the vicinity of the already formed NS from far away... Pretty exotic circumstances are required.

  • $\begingroup$ Did you mean "gravitational mass deficit" not defect? If not, please add a a quick explanation of the term "gravitational mass defect", or a link to an explanation. $\endgroup$
    – Sean Lake
    Commented Nov 26, 2021 at 19:21
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    $\begingroup$ I meant mass defect, as in periodic-table.org/what-is-mass-defect-definition , physics.stackexchange.com/questions/220945/… . That is, Δm= minus difference between the rest mass of the composite object and the sum of the rest masses of the constituent particles. That difference comes from the binding energy in the composite object, E=Δm×c² as per Einstein's relation. $\endgroup$ Commented Nov 26, 2021 at 20:00

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