Supernovae glow for decades thanks to radioactivity

The flash lasts seconds, the brightness dies down in months. Yet centuries later, the remnant still shines.

“When we recall the past, we usually find that it is the simplest things — not the great occasions — that in retrospect give off the greatest glow of happiness.” –Bob Hope

In 1987, the closest supernova was observed since humans last saw one with our naked eyes in 1604.

*Image credit: NASA/ESA/JHU/R.Sankrit & W.Blair, of an optical/IR/X-ray composite of the 1604 supernova remnant, the last naked-eye supernova to occur within our galaxy.*

A rush of neutrinos and a flash of light illuminated the heavens as a stellar core imploded, blowing off the massive star’s outer layers.

This image shows the remnant of Supernova 1987A seen in light of very different wavelengths. ALMA data (in red) shows newly formed dust in the centre of the remnant. Hubble (in green) and Chandra (in blue) data show the expanding shock wave. Image credit: ALMA (ESO/NAOJ/NRAO)/A. Angelich. Visible light image: the NASA/ESA Hubble Space Telescope. X-Ray image: The NASA Chandra X-Ray Observatory.

While a supernova brightens, reaches a peak and fades away over a few months, SN 1987A has been visible for decades.

With glittering beads in a circular pattern brightening around the center, the intricate pattern belies a tremendous story.

The ‘supernova impostor’ of the 19th century precipitated a gigantic eruption, spewing many Suns’ worth of material into the interstellar medium from Eta Carinae. Image credit: Nathan Smith (University of California, Berkeley), and NASA.

Previous ejecta from similarly massive stars, like Eta Carinae, show that the surrounding interstellar medium is rich in material.

*The Carina Nebula, with Eta Carina, the brightest star inside it, on the left. Image credit: ESO/IDA/Danish 1.5 m/R.Gendler, J-E. Ovaldsen, C. Thöne, and C. Feron.*

As the supernova radiation smacks into this gas and plasma, electrons ionize and recombine to form atoms, emitting light in the process.

An animation sequence of the 17th century supernova in the constellation of Cassiopeia. Surrounding material plus continued emission of EM radiation both play a role in the remnant’s continued illumination. Image credit: NASA, ESA, and the Hubble Heritage STScI/AURA)-ESA/Hubble Collaboration. Acknowledgement: Robert A. Fesen (Dartmouth College, USA) and James Long (ESA/Hubble).

As energy propagates outwards, it smacks into the prior ejecta, lighting it up.

*A supernova explosion enriches the surrounding interstellar medium with heavy elements. Image credit: ESO / L. Calçada, of the remnant of SN 1987a.*

But instead of fading away entirely, radioactive material produced in the explosion — like Cobalt-56 and Cobalt-57 — keep emitting energy.

The neutron capture process builds elements up the periodic table, creating a lasting energy source from a rapid release.

*SN 1987A is located at the edge of the Tarantula nebula, within the Large Magellanic Cloud. Image credit: ESO, annotation by E. Siegel.*

Supernova remnants like this can remain with visible components for centuries, and can continue to be spotted in the radio and X-ray for thousands of years.

*Supernova remnants, like Cassiopeia A (L) and G1.9+0.3 (R), will shine in the radio and/or X-ray for thousands of years.*

Mostly Mute Monday tells the story of a single astronomical phenomenon or object in mostly visuals, limited to no more than 200 words.

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