In the vast cosmos, supernova explosions are widely regarded as the most brilliant fireworks in the universe. Among them, Type Ia supernovae (SNe Ia) are hailed as "standard candles" due to their highly consistent peak luminosity, making them a key tool for measuring cosmic distances.

However, a recent study has found that even SNe Ia that appear nearly identical may hide small "genetic" differences that could affect the accuracy of distance measurements.

Dr. Abdusamatjan Iskandar from the Xinjiang Astronomical Observatory of the Chinese Academy of Sciences, in collaboration with researchers from Tsinghua University and other institutes, conducted a systematic analysis of SN 2021pfs, a SN Ia that exploded in June 2021 in the galaxy NGC 5427, using observational data obtained from the Las Cumbres Observatory global telescope network.

The researchers carried out a detailed study of its light and spectra, from optical brightness measurements to spectral analysis. The results were published in Science China: Physics, Mechanics & Astronomy.

The researchers found that this supernova is very similar to the famous SN Ia SN 2011fe discovered a decade earlier. The B-band light curve width and peak absolute luminosity, as well as the evolution of its spectra over time, all looked almost the same.

However, through more detailed comparison, slight differences appeared. SN 2021pfs brightened faster in blue and violet light, and its peak brightness in the "bolometric" light curve—a measure of total energy output—was slightly dimmer than that of SN 2011fe. It also produced slightly less radioactive nickel, which is the key energy source that powers a supernova's glow.

By analyzing the environment in its host galaxy, the researchers found that SN 2021pfs was born in a region with higher metallicity. This small difference in "family background"—a higher metallicity in the progenitor star—may explain the slight difference in its light curve compared with that of SN 2011fe.

Comparing this pair of nearly "twin" SNe Ia, the study reveals that metallicity is a hidden factor that can affect the uniformity of SNe Ia. It shows that scientists need to explore the complex and subtle physics behind the diversity of SNe Ia. According to the researchers, this will help improve future cosmic distance measurements and increase their accuracy.

Absolute-magnitude UBVgr-band light curves of SN 2021pfs and SN 2011fe, together with the UBV-bandlight curves of SN 2011by. The bottom panel plots the magnitude difference (SN 2011fe-SN 2021pfs) in each band. (Image by XAO)