Astronomers have made a groundbreaking discovery by observing a new type of supernova, which offers an unprecedented glimpse into the processes occurring deep within a star right before it undergoes a catastrophic explosion. This significant finding was detailed in a study published on Wednesday in the journal Nature.
Massive stars, often described as celestial onions, are composed of multiple layers. The outermost layers consist of lightweight elements such as hydrogen and helium, while heavier elements lie beneath. These colossal stars, which can weigh between 10 to 100 times that of our sun, are sustained by a process called nuclear fusion. In this process, lighter elements fuse to create heavier ones. According to Adam Miller, a coauthor of the study and assistant professor of physics and astronomy at Northwestern University, stars typically begin their life with a composition of approximately 75% hydrogen and 25% helium, along with trace amounts of carbon, nitrogen, silicon, and other elements.
Fusion occurs at the star's core, where temperatures and densities are highest, converting hydrogen into helium and forming the star's onion-like structure. Over time, this fusion process continues, fusing lighter elements and creating heavier ones, thus adding internal layers of silicon, sulfur, oxygen, neon, magnesium, and carbon beneath the helium and hydrogen.
As a massive star approaches the end of its life, it develops an iron core. Miller explains that while fusion generates energy that creates pressure to counteract gravitational collapse, the fusion of iron into heavier elements does not produce enough energy to maintain this pressure. Consequently, the core collapses under the immense gravitational force, ultimately resulting in a stellar explosion.
The recent observations surrounding a unique supernova, identified as SN2021yfj, defy conventional expectations. Notably, this star had lost its outer hydrogen, helium, and carbon layers prior to its explosion. Just before detonating, it expelled a typically hidden layer of heavier elements—silicon, sulfur, and argon—that are rarely observed in dying stars. This unprecedented event allowed astronomers to witness a layer of these elements that had never been seen before, according to Miller.
Lead study author Steve Schulze, a research associate at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics, remarked, “This is the first time we have seen a star that was essentially stripped to the bone.” He further noted that this discovery illustrates the intricate structure of stars and confirms that they can lose significant amounts of material before exploding. “Not only can they lose their outermost layers, but they can be completely stripped down and still produce a brilliant explosion observable from vast distances,” Schulze added.
This discovery offers direct evidence of the long-theorized, yet challenging-to-observe internal structure of massive stars, prompting a reevaluation of existing theories on stellar evolution. Miller stated, “This event quite literally looks like nothing anyone has ever seen before.” He expressed that the peculiar nature of this star suggests that current understanding of stellar evolution may be too limited. “Our textbooks are not incorrect, but they clearly do not fully capture everything produced in nature,” he explained.
While the exact type of star that preceded the supernova remains unclear, Schulze and Miller estimate its mass to be approximately 60 times that of the sun. However, the star likely had a smaller mass at the time of its explosion due to the loss of its outer hydrogen layer. Massive stars are known to shed layers before their demise, but the degree to which this star lost material exceeds prior observations.
As Schulze noted, “Stars experience very strong instabilities that can lead to a sudden release of energy, causing them to shed their outermost layers.” Such shedding can occur multiple times throughout a star's lifetime. In previous supernova events, elements like silicon and sulfur have been detected mixed with other elements in the ejected material, but they had never been seen prior to a supernova explosion.
The team first discovered SN2021yfj in September 2021 using the Zwicky Transient Facility at the Palomar Observatory in Southern California. Zwicky is known for its ability to detect fleeting cosmic phenomena, including supernovas. While reviewing data for signs of supernovas, Schulze noticed an object that rapidly increased in brightness, located 2.2 billion light-years from Earth.
To analyze this phenomenon further, the team sought to capture a spectrum of the object, which would reveal the wavelengths of colored light and identify the elements present. However, Zwicky only measures brightness changes. Fortunately, Yi Yang, now an assistant professor at Tsinghua University in China, observed the object using the W. M. Keck Observatory in Hawaii and successfully captured its spectrum.
With the spectrum in hand, the team collaborated with Avishay Gal-Yam from Israel’s Weizmann Institute of Science, who identified the spectrum's unusual features as silicon, sulfur, and argon. This identification marked a significant milestone in their research.
The team is still investigating the triggers behind the star's release of the silicon and sulfur shell. They are considering various hypotheses, including interactions with a potential companion star, exceptionally strong stellar winds, or a massive pre-supernova outburst. However, they lean towards the idea that the star may have torn itself apart.
This discovery has led to the designation of a new type of supernova known as a type Ien supernova. Supernova classifications are determined by the presence of different elements; for example, type II supernovas contain hydrogen, while type Ib and Ic lack hydrogen and helium, respectively. The new type Ien is characterized by its deep inner layers containing silicon, sulfur, and argon.
As noted by Miller, the rarity of this supernova type underscores the need to identify additional examples to enhance understanding of their nature. Despite the Vera C. Rubin Observatory’s potential to detect over a million supernovas, it does not measure their spectra. Miller emphasized that a simple machine learning model would not have classified this supernova as unusual based solely on its brightness.
In conclusion, the discovery of SN2021yfj invites further exploration into the mysteries of massive stars and their explosive end stages. “The big open question is—how often do such explosions occur in the Universe?” Miller pondered, highlighting the significance of ongoing research in the field of astrophysics.
