Supernova: Finally Found: The remains of the most famous supernova

Anyone who sees what appears to be a star in the sky where there was no star shortly before should be skeptical — at least as an astronomer. Canadian Ian Shelton knew this too when he developed a newly exposed photographic plate at Chile's Las Campanas Observatory on the evening of February 23, 1987. Then he saw an unusual new bright spot suddenly appear in the Large Magellanic Cloud, a neighboring galaxy to our own Milky Way. Maybe something wrong with the telescope? To rule this out, Shelton ran outside and checked with the naked eye: indeed. Something shone that wasn't there before.

This thing quickly went down in history under the name SN 1987A. Its existence was the result of a cosmic catastrophe: a star exploded 166,000 light-years from Earth, an event called a supernova. The last such explosion that could be observed with the naked eye was seen by Johannes Kepler in 1604. Such near-Earth supernovae are rare, and SN 1987A was a stroke of luck for modern astrophysics.

Today, SN 1987A is one of the most studied cosmic objects. We know what exploded (the star Sanduleak -69°202, a massive blue giant twenty times the mass of the Sun) and why (because the star exhausted its fuel and then collapsed under its own weight). Only one question cannot be answered conclusively: What is left of Sanduleak -69°202? Last week, astronomers reported in the journal Sciences: There is an answer!

In principle, scientists knew what to look for. When massive stars collapse under their own weight at the end of their lives, their material at the center of the explosion is so compressed that neutrons are formed from electrons and protons: a neutron star is formed, an object only about 20 kilometers across that is roughly one and a half times the mass of the Sun. This process also produces neutrinos, which are weakly interacting elementary particles. Some of them already reached Earth in 1987.

Since then, astronomers have been searching the opposite region of the sky for traces of a neutron star. In 2019, they noticed unusually warm dust there. Was it heated by the star they were looking for? This was not conclusive evidence. In the new study, the researchers used the best telescope available for this purpose. “We had already looked at the supernova using the Herschel Space Telescope and the Chilean ALMA Observatory, so it made sense to look for the compact object using the James Webb Space Telescope,” says Thomas Henning, director of the Heidelberg Max Planck Institute. Planck Institute for Astronomy, which participated in the study.

The James Webb Telescope provided observations of the spectral lines of argon and sulfur, their chemical fingerprints, so to speak. From this, the researchers were able to deduce that the observed gas must be exposed to intense radiation, such as that emitted by a neutron star. They were able to prove that the gas is located at the center of the supernova – that is, near the potential object. Henning explains that the results clearly indicate the existence of a neutron star. Never before has a neutron star been found in such a young supernova remnant.

There may still be some residual dissatisfaction: the neutron star itself has not yet been seen; That's why we'll need better observations with radio telescopes. But it is possible, believes Thomas Hennig. The search continues.