The explosive death of a star—a supernova—is among the most violent cosmic events in the universe, but exactly what this cataclysm looked like as it unfolded remained mysterious. Now, scientists have observed for the first time the early stages of a supernova, with a massive star exploding into an olive-like shape.
The researchers used the European Southern Observatory’s Very Large Telescope, or VLT, based in Chile, to observe the supernova, which involved a star approximately 15 times the mass of our sun, residing in a galaxy called NGC 3621, about 22 million light-years from Earth, in the constellation Hydra. A light year is the distance that light travels in one year, that is, 9.5 trillion kilometers.
The shape of these explosions had been difficult to determine until then, due to the speed at which they occur, so it was necessary to act quickly with this supernova. The explosion was detected on April 10, 2024, when astrophysicist Yi Yang, from Tsinghua University in China, was disembarking from a long flight to San Francisco. Yang’s formal request to point the VLT at the supernova was granted a few hours later.
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Thus, researchers were able to observe the explosion just 26 hours after initial detection and 29 hours after material from the star’s interior first broke through the stellar surface.
What they saw was the doomed star surrounded at its equator by a pre-existing disk of gas and dust, with the explosion pushing material out of the stellar core, distorting the star’s shape, causing it to resemble an upright olive. The explosion, remarkably, did not cause the star to disintegrate into a spherical shape. Instead, the explosion advanced violently outward on opposite sides of the star.
“The geometry of a supernova explosion provides fundamental information about stellar evolution and the physical processes that lead to these cosmic fireworks,” said Yang, lead author of the study published Wednesday in the journal Science Advances.
“The exact mechanisms behind supernova explosions of massive stars, those more than eight times the mass of the Sun, are still debated and are one of the fundamental questions scientists want to address,” Yang said.
Large stars of this type are relatively short-lived. This one, a red supergiant, was about 25 million years old at the time of its death. In comparison, the Sun is over 4.5 billion years old and still has a few more billion years to go.
At the time of the explosion, the diameter of the star was 600 times greater than that of the Sun. Part of the star’s mass was expelled into space. The remainder is believed to have turned into a neutron star, a highly compact stellar remnant, according to study co-author Dietrich Baade, an astrophysicist at the Germany-based European Southern Observatory.
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When a star runs out of hydrogen fuel for nuclear fusion occurring at its center, its core collapses, causing material to be ejected outward, penetrating the stellar surface and space.
“The first VLT observations captured the phase during which matter accelerated by the explosion near the center of the star passed through the star’s surface, the photosphere,” Yang said.
“Once the shock breaks the surface, it releases immense amounts of energy. The supernova then brightens dramatically and becomes observable. During a short-lived phase, the initial shape of the supernova can be studied before the explosion interacts with the material surrounding the dying star,” Yang explained.
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That shape, Yang said, offers clues about how the explosion was triggered in the star’s heart. The new observations appear to rule out some current scientific models of the explosion process, according to Yang, as scientists refine their understanding of the death of massive stars.
