Supernova 1987A is a supernova remnant in the Milky Way’s largest satellite galaxy, the Large Magellanic Cloud. The remnant was formed when a massive star exploded, which we witnessed here on Earth in February 1987. Since then, the remnant has evolved as the exploding star’s energy has spread outward, slamming into nearby gas and dust.
But, if the explosion spread matter and energy in all directions, why does the remnant appear to have three rings instead of a sphere? The answer lies in what happened before the supernova, in the behavior of the doomed progenitor star. As it neared the end of its life, that star changed from a red supergiant to a blue supergiant. Simultaneously, the star’s winds shifted from dense and slow to thin and fast. When the fast-moving wind collided with the slower-moving wind ejected before it, material accumulated around the star.
Furthermore, astronomers believe that, based on factors such as the star’s magnetic field and its rotation during the red supergiant phase, more material piled up around its equator and in regions around its poles. When faster winds slammed into those areas later, they formed three distinct rings.
When the star finally exploded, the blast’s energy spread outward in all directions. The rings we see now are caused by shocks that occur when that energy smacks into the existing rings, exciting the gas and lighting them up. The other areas around the now-dead star aren’t glowing in a sphere because there isn’t much material there, which is mostly concentrated in the rings left behind by the explosion. As the shocks move through the rings, they appear to change and expand, illuminating new regions as the material closer to the star fades once the shock has passed.
To make matters more complicated, astronomers believe the supernova explosion was not symmetric, but instead directed more energy in one direction than others, exaggerating the nonspherical shape of the remnant we see.
