Supernovae are enormous explosions that mark the final stages of a star’s life. As they are sudden and unpredictable, they have long been difficult to study, but today, thanks to high-cadence sky surveys, astronomers can discover new ones almost daily. A new study led by the Institute of Space Sciences (ICE-CSIC), published in the Journal of Cosmology and Astroparticle Physics (JCAP), presents a new method to detect supernovae hours after they erupt.

The pilot study focuses on a sample of ten supernovae using observations from the Gran Telescopio de Canarias (GTC). It shows how targeted protocols and fast telescope follow-up can capture the earliest spectra of these stellar explosions, ideally within 48 hours, or even 24 hours of their first light. This breakthrough offers an unprecedented opportunity to study the moments immediately following a star’s death and makes rapid detection essential for understanding their origins and evolution.

Supernovae fall into two broad categories, determined by the mass of the progenitor star. Thermonuclear supernovae involve stars whose initial mass did not exceed eight solar masses. “The most advanced evolutionary stage of these stars before the supernova is the white dwarf—very old objects that no longer have an active core producing heat. White dwarfs can remain in equilibrium for a long time, supported by a quantum effect called electron-degeneracy pressure”, explains Lluís Galbany, an astrophysicist at the Institute of Space Sciences (ICE-CSIC) and Institut d'Estudis Espacials de Catalunya (IEEC) and first author of the study.

If such a star is located in a binary system, it can siphon matter from its companion. The extra mass raises the internal pressure until the white dwarf explodes as a supernova.

The second supernovae major category involves very massive stars, above eight solar masses, “They shine thanks to nuclear fusion in their cores, but once the star has burned through progressively heavier atoms—right up to the point where further fusion no longer yields energy—the core collapses. At that point, the star collapses because gravity is no longer counterbalanced; the rapid contraction raises the internal pressure dramatically and triggers the explosion”, Galbany explains.

Adapted image of a host galaxy of the supernovae sample of the study. Credits: Galbany et al (2025).Rapid follow-up observations of infant supernovae with the Gran Telescopio Canarias, JCAP. 

Early detection

The first hours and days after a supernova blast preserve direct clues to the progenitor system—information that helps distinguish competing explosion models, estimate critical parameters, and study the local environment. Historically, obtaining such early data was difficult because most supernovae were discovered days or weeks after the explosion. Modern wide-field, high-cadence surveys—covering large swaths of sky and revisiting them frequently—are changing that picture and allowing discoveries within mere hours or days.

Protocols and criteria are still needed to exploit these surveys fully, and the team tested such rules using observations from the GTC. Their study reports on ten supernovae: half thermonuclear, half core-collapse. Most of them were observed within six days of the estimated explosion, and in two cases within 48 hours.

The protocol begins with a rapid search for candidates based on two criteria: the light signal must have been absent in the previous night’s images, and the new source must lie within a galaxy. When both conditions are met, the team triggers the OSIRIS instrument  mounted on the GTC to obtain a spectrum.

“The supernova’s spectrum tells us, for instance, whether the star contained hydrogen—meaning we are looking at a core-collapse supernova,” Galbany explains. “Knowing about the supernova in its very earliest moments also lets us seek other kinds of data on the same object, such as photometry from the Zwicky Transient Facility (ZTF) and the Asteroid Terrestrial-impact Last Alert System (ATLAS) that we used in the study. Those light-curves show how brightness rises in the initial phase; if we see small bumps, it may mean another star in a binary system was swallowed by the explosion”, he adds. Additional checks cross-match data on the same patch of sky from other observatories.

Because this first study managed to gather data within 48 hours, the authors conclude that even faster observations are within reach. “We now know that a rapid-response spectroscopic program, well coordinated with deep photometric surveys, can realistically collect spectra within a day of the explosion, paving the way for systematic studies of the very earliest phases in forthcoming large surveys such as the La Silla Southern Supernova Survey (LS4) and the Legacy Survey of Space and Time (LSST), both in Chile”, Galbany concludes.