Let's take a massive star that has zero velocity. When this star goes supernova, the resulting supernova remnant will expand in a sphere. But, let's take a runaway star travelling at 200 km/s. I am predicting that the remnant will form a teardrop shape, with the rounded edge compressed a little with the rear end elongated.

My reasoning is that the material ejected "forwards" relative to the star's velocity vector will gain some velocity, leaving the dead star behind. However, the material ejected backward will lose velocity, meaning that it will stream out over a longer distance. So to a viewer it would look like a "cosmic fireball" in some sense.

Is this correct, and would there be anything that could change this result? If I am wrong, what would be the actual way that the remnant would expand?

  • 1
    $\begingroup$ By "fast-moving" I assume you mean "relative to the surrounding interstellar medium". My guess would be that the velocity of the expanding gas (10000km/s?) is so much faster that the initial velocity of the star is relatively insignificant. (only a guess) $\endgroup$
    – James K
    Feb 13, 2021 at 23:29
  • $\begingroup$ Also note that supernova explosions have a high probability of being quite asymmetrical, not neatly spherical explosions, and the remnant can receive a sizeable kick. Eg, Pulsar B1508+55 is moving at ~1000 km/s & it's possible that some pulsars have speeds around 1500 km/s. See arxiv.org/abs/astro-ph/0509031 $\endgroup$
    – PM 2Ring
    Feb 14, 2021 at 3:52
  • $\begingroup$ Also, assuming the supernova is spherically symmetric, the supernova remanent will remain symmetric wrt its star. An anisotrpy can only happen due to interaction with the ISM which might slow down the ejected gas somewhat in one side or when the supernova is not symmetric, thus net momentum is transferred to the stellar remanent $\endgroup$ Feb 14, 2021 at 12:46

1 Answer 1


A spherical supernova in free space will be a sphere no matter the velocity of the star (ignoring relativistic flattening). Just consider the explosion in the rest frame of the star.

If there is a medium a lot depends on the actual fluid dynamics, which can get rather involved (especially since there is plasma and magnetic effects). The initial shock front moves faster (>10,000 km/s) than the local speed of sound and would be spherical, but soon Rayleigh-Taylor instability sets in and things curl up. Still, momentum conservation means that a particle ejected "upwind" will be slowed a bit more than a particle going "downwind".

However, the relative velocity difference is small (400 km/s) compared to the ejecta velocity so the effect is going to be small in the early stages. It will begin to matter once the ejecta has slowed to such velocities, about the time it has swept up its own mass ahead of it. However, at this time the hydrodynamic instabilities will also have turned the sphere into a fractal. It might well be roughly elliptic with the star at one foci, but the overall shape is still likely close to spherical.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .