The annihilating explosion of a star 400 times larger than the Sun

In May 2023, different telescopes distributed throughout the planet recorded the explosion of a red supergiant star located 21 million light-years away. This is the earliest detection of a supernova phenomenon caused by the collapse of the star's core moments before it dies. Scientists have analyzed all the observations, obtaining new and revealing data.

The analysis was carried out by a team from Tsinghua University, in China. Among the observations analyzed are those made by the BOOTES-4/MET telescope managed by the Institute of Astrophysics of Andalusia (IAA), dependent on the Higher Council for Scientific Research (CSIC) in Spain.

The results of the study reveal that the star most likely expelled large amounts of dust in the years before its death, creating a dense cloud around it that dimmed the first flash of light from the explosion.

The supernova, known as SN2023ixf, was triggered when the rate of fusion reactions in the core of the red supergiant star (with a diameter four hundred times that of the Sun) decreased to the point where the pressure could not sustain its own mass, which caused the core to begin to collapse in on itself. This process generated a shock wave that crossed the different layers of the star, causing an uncontrolled reaction accompanied by an intense flash of light.

“The temporal evolution of the first light curves after the explosion offers key information about the nature of the red supergiant, the propagation of the shock wave through its envelope, as well as the material that surrounded the star before the explosion “That is why it is so crucial to detect this type of phenomenon in its first moments,” explains Alberto Castro-Tirado, researcher at the IAA and the University of Malaga (UMA) in Spain, and one of the signatories of the work.

Amateur observations and robotic telescopes

Despite being 21 million light-years from Earth (in the galaxy M101), SN 2023ixf is one of the closest supernovae observed so far. It was first detected on May 19, 2023 by amateur astronomer Kōichi Itagaki, in Japan, and during the following days it increased in brightness before beginning to fade from June 10. This proximity allowed many amateur astronomers to monitor it during the first hours after the explosion.

According to Xiaofeng Wang, a researcher at Tsinghua University who led the research, these observations represent “the earliest detected signal of this type of supernovae caused by the collapse of the star's core.” The researcher assures that “these observations represent a tremendously valuable set of data for the scientific community about the first moments after the phenomenon.”

After receiving news of its detection, Wang's team immediately contacted many amateur observatories located in China, Spain, Russia and the United States. “We requested the raw images obtained by a multitude of amateur telescopes to be able to process them and extract the separate signals in the green, blue and red light bands,” explains Gaici Li, a predoctoral student in Wang's group and a member of the research team.

In addition to the observations provided by the amateur astronomical community, SN 2023ixf could be monitored in its first hours by the professional robotic telescope BOOTES-4/MET, located at the Lijiang Astronomical Observatory, belonging to the observatory complex of the Kunming National Astronomical Observatory in China. This astronomical station is part of the global network of BOOTES robotic telescopes, the first of its kind deployed by a country on all five continents. BOOTES is managed by the IAA, with strong involvement from the University of Malaga and other national and international entities.

“Observations with BOOTES-4/MET began approximately 1.4 hours after the explosion, highlighting the importance of having a global network of telescopes deployed on all continents to be able to respond immediately and effectively to both this and to other types of transient phenomena,” explains Alberto Castro-Tirado, leading researcher of the network since its creation. “In this case, we were able to make very fast and high-quality observations of the supernova SN 2023ixf, which turned out to be key in the subsequent analysis.”

Image of the supernova SN 2023ixf, obtained at the Lijiang Astronomical Observatory (China). The red arrow indicates its location. (Image: D.-R. Xiong (YNAO) / AJ Castro-Tirado / IAA / CSIC / ISA / UMA)

A reddish and dusty halo

The first light curves of SN 2023ixf evolved rapidly, on time scales of one to two hours, showing a weaker, redder glow than anticipated by theoretical models. For the research team, this attenuation and reddening in the supernova signal during its first hours suggests the previous existence of a thick layer of dust surrounding the red supergiant. About four hours later, the radiation from the supernova showed the bluish color spectrum typical of such an energetic and hot shock wave, suggesting that the explosion finally destroyed this dust envelope. “This rapid evolution from red to blue in the light curve of a supernova is unusual and has never been seen before,” says Wang. For the authors of the work, in the stages prior to the explosion, the star must have been expelling dust up to generate a shell of particles several billion kilometers in diameter and a mass tens of thousands of times the mass of the Earth.

Analysis of the observations also suggests that the explosion may not have been symmetrical or that the dust cloud was not distributed uniformly in space. This last hypothesis suggests that the star could expel the material in a series of eruptions or irregular pulses.

Wang says his team is conducting a more detailed study of observations of SN 2023ixf, which will help provide a more accurate estimate of the progenitor star's mass. Additionally, he will potentially reveal more details about the dust cloud that surrounded it, as well as the synthesis of chemical elements generated during the explosion.

The study is titled “A shock flash breaking out of a dusty red supergiant.” And it has been published in the academic journal Nature. (Source: IAA / CSIC)