Most Magnified Supernova Ever Found: Astronomers Detect Stellar Explosion Amplified 250 Times by Cosmic Gravity

An international team of astronomers has discovered the most magnified supernova ever recorded: a stellar explosion located approximately 9 billion light-years from Earth whose light has been amplified at least 100-fold — and possibly 250-fold — by the gravitational field of an entire intervening galaxy acting as a cosmic magnifying glass. The discovery, detailed in a preprint published this week by a collaboration led by researchers at Stockholm University's Oskar Klein Centre, opens a new window onto the deaths of massive stars in the early universe that would otherwise be completely invisible to existing telescopes.

The supernova, designated SN 2025mkn, was first flagged by the Zwicky Transient Facility (ZTF), an automated sky survey telescope at the Palomar Observatory in California, which detected an unusually bright blue point of light far more luminous than expected for its apparent position. Follow-up spectroscopy quickly revealed the source was located at a redshift of z=1.371 — corresponding to a lookback time of roughly 9 billion years, meaning astronomers are observing this stellar explosion as it appeared when the universe was less than 5 billion years old. A foreground galaxy at z=0.42 lies almost precisely in the line of sight, acting as a gravitational lens.

Gravitational lensing occurs when the mass of a foreground object bends light passing around it, much as a glass lens refracts light. When the alignment is particularly precise, the effect can be dramatic: the lensing galaxy not only bends the background source's light but amplifies it, producing multiple distinct images of the same object. Subsequent observations with the James Webb Space Telescope's NIRCam instrument revealed that SN 2025mkn is accompanied by a secondary image roughly 30 times fainter, located on the opposite side of the lensing galaxy — confirming the gravitational lensing interpretation.

The key question for the team was the magnification factor. Type II supernovae — the class SN 2025mkn belongs to — are caused by the collapse and explosion of massive stars and follow relatively predictable brightness patterns. By comparing SN 2025mkn's apparent brightness to the known luminosity of similar nearby events, including SN 2023ixf, a well-characterized Type II supernova observed in 2023, the team calculated that the lens must be magnifying the source by approximately 100 times at minimum, and as much as 250 times to fully account for the discrepancy. At 250x magnification, the effect is comparable to reading a newspaper from a mile away.

Lead author Cameron Lemon, a researcher at the Oskar Klein Centre, said the discovery amounts to a natural telescope more powerful than anything humanity could build. The discovery also offers a rare opportunity for what astronomers call time-delay cosmography: because the two images of the supernova travel along paths of different lengths around the lensing galaxy, they arrive at Earth at slightly different times. By measuring that delay and combining it with a model of the lens, astronomers can independently constrain the Hubble constant — the rate at which the universe is currently expanding, a figure that has been at the center of a major unresolved disagreement in cosmology for nearly a decade.

The discovery builds on growing interest in strongly lensed supernovae as cosmological tools. A related object, SN 2025wny — the first spatially resolved, gravitationally lensed superluminous supernova ever observed — was reported by a Stockholm University team in late 2025. The Vera C. Rubin Observatory in Chile, expected to begin its full Legacy Survey of Space and Time in 2026, is projected to detect thousands of lensed supernovae over the course of its decade-long survey, turning what has been a one-off rarity into a statistical tool for cosmology.

For cosmologists wrestling with the Hubble tension — a persistent disagreement between measurements of the universe's expansion rate using different methods that has resisted explanation for years — SN 2025mkn offers a potential new data point from an entirely independent technique. Each new measurement constrains the competing models. Whether it ultimately supports the values from the cosmic microwave background or from local distance ladder measurements remains to be seen, but the existence of a supernova amplified 250 times by the universe itself represents a gift to the science of cosmic cartography.