What happens just before a massive star explodes as a supernova? To figure that out, astronomers need to look at very “young” supernovae across multiple wavelengths of light. That’s what happened when SN 2023ixf occurred in May 2023. It turns out its aging progenitor star blew off a solar mass worth of material just before it died. Now, the big question is: why?
SN 2023ixf lies some 20 million light-years away in the Pinwheel Galaxy. That puts it pretty close in cosmic terms and allowed a team of astronomers from Harvard’s Center for Astrophysics to use a wide range of telescopes to study it very soon after the explosion. And, it’s a good thing they did. Not long after the first observations of the supernova, made by Japanese amateur astronomer Koichi Itagaki, the supernova brightened considerably. That was expected. It happens when the shock wave from the explosion reaches the outer edge of the star. That’s called the “shock breakout.”
Composite KeplerCam griz image of SN 2023ixf. Captured using the 1.2m telescope at CfA’s Fred Lawrence Whipple Observatory on June 27, 2023, just over a month after SN 2023ixf’s progenitor star exploded, the image in this composite combines together green, red, near-infrared, and infrared light to highlight both SN 2023ixf and the Pinwheel Galaxy. SN 2023ixf is located in one of the spiral arms of the galaxy, as expected for the explosions of massive stars.Credit: S. Gomez/STScI
However, the characteristics of the breakout weren’t what the astronomers expected from a typical core-collapse (Type II) supernova. It appeared to be delayed, and it wasn’t clear why. The multi-wavelength observations revealed a sun’s worth of matter that was ejected prior to the explosion.
“The delayed shock breakout is direct evidence for the presence of dense material from recent mass loss,” said CfA postdoctoral fellow Daichi Hiramatsu Hiramatsu, who led the observations. He pointed out that such extreme mass loss is atypical of Type II supernovae. “Our new observations revealed a significant and unexpected amount of mass loss—close to the mass of the Sun—in the final year prior to the explosion.”
When stars get very old, they lose mass. It happens as the star goes through various stages of nuclear fusion (often referred to as “burning”) in its core. Hydrogen gets fused to become helium, and so on. This will happen to the Sun in about 5 billion years. As a result, it will expand and begin to lose mass through a stellar wind. Essentially, each element becomes the fuel for the next stage of core activity and each stage heats the star up, causing it to expand and lose mass.
In stars destined to end in core-collapse supernovae, they go through all the heavier elements in successive stages of helium-burning, carbon-burning, and silicon-burning processes. The fusion process continues in the star up to the point where iron (Fe) is consumed. It takes more energy to process the iron, and the nuclear fusion process stops. The core collapses and all the layers of the star collapse with it. Then, the material collides with the core and rebounds. That sends a shock wave through the star and creates the final explosion. That’s what happened with the red supergiant star that was the progenitor for SN 2023ixf and its fiery explosion.
That’s the general theory behind the activities in core-collapse supernovae. However, there’s a lot that astronomers still don’t understand about the process leading up to the final catastrophic event. And, SN 2023ixf’s strange light curve for its shock blowout is challenging that conventional theory. Observations of this young supernova seem to point to some strange instabilities inside the star during the last year of its life. Those instabilities led to what astronomers call “extreme mass loss”. It’s extreme because generally, a supernova progenitor star is going to experience some loss of its outer shells of material. But, losing a Sun’s worth is quite a lot.
Artist’s conception of pre-explosion mass loss by the progenitor star of SN 2023ixf. In the year prior to going supernova, the red supergiant star shed an amount of mass equivalent to the mass of the Sun. This artist’s conception illustrates what the final stages of mass loss might have looked like.Credit: Melissa Weiss/CfA
It could be that the proposed instabilities began in the core when it started burning higher-mass elements (such as silicon). Silicon burning requires higher temperatures in the core, but can also occur explosively in regions outside the core. That might lead to the instabilities spurring a higher-than-expected period of mass loss just before the star ultimately died.
Whatever caused the mass loss, that sun’s worth of material appears to be much denser than expected. The clue is in the light curve produced as the ejecta from the supernova collided with it. Observations made by Harvard astronomer Edo Berger using the CfA’s Submillimeter Array tracked the collision between the supernova ejecta and the dense cloud of material. They found that it’s a complex, probably non-spherical shape and they plan to continue observing the site to see how it evolves.
“The only way to understand how massive stars behave in the final years of their lives up to the point of explosion is to discover supernovae when they are very young, and preferably nearby, and then to study them across multiple wavelengths,” said Berger. “Using both optical and millimeter telescopes we effectively turned SN 2023ixf into a time machine to reconstruct what its progenitor star was doing up to the moment of its death.”
The discovery of this supernova and ongoing studies also illuminate a good partnership between amateur and professional astronomy communities. Itagaki discovered the supernova on May 19, 2023, from his private observatory in Okayama, Japan. Itagaki’s data, along with observations from other amateurs helped pinpoint the time of the explosion to a small window of about two hours. That allowed astronomers at CfA and other observatories a head start in capturing the events almost as soon as they began. CfA astronomers continue to work with Itagaki on ongoing optical observations.
“The partnership between amateur and professional astronomers has a long-standing tradition of success in the supernova field,” said Hiramatsu. “In the case of SN 2023ixf, I received an urgent email from Koichi Itagaki as soon as he discovered SN 2023ixf. Without this relationship, and Itagaki’s work and dedication, we would have missed the opportunity to gain a critical understanding of the evolution of massive stars and their supernova explosions.”
In the future, astronomers hope to be able to study supernova progenitor stars before the final conflagration. That should reveal more about activities inside the supermassive stars that produce supernovae like SN 2023ixf.
Extreme Weight Loss: Star Sheds Unexpected Amounts of Mass Just Before Going Supernova
From Discovery to the First Month of the Type II Supernova 2023ixf: High and Variable Mass Loss in the Final Year Before Explosion
Millimeter Observations of the Type II SN 2023ixf: Constraints on the Proximate Circumstellar Medium
