Astronomers have announced the discovery of the smallest and most massive white dwarf observed to date. This rapidly rotating object, with an extraordinarily powerful magnetic field, formed from the merger of two white dwarfs in a binary system.
Low- to intermediate-mass stars—those up to about eight times the Sun's mass—expand dramatically as they age, evolving into red giants that engulf nearby planets and disperse more distant ones. These stars ultimately shed their outer layers, leaving behind a dense, hot core: a white dwarf. Roughly 97% of stars, including our Sun, will follow this path.
While our Sun evolves solo, many stars orbit in binary pairs, aging together. If neither exceeds eight solar masses, both become white dwarfs. Orbiting each other, they lose energy through gravitational waves and spiral inward. If their combined mass surpasses a threshold, they trigger a Type Ia supernova. Below that limit, they merge into a single, heavier white dwarf. This process amplifies the magnetic field and speeds up rotation.
A new study in Nature details one such merger product from a binary white dwarf pair.
Observed using the Zwicky Transient Facility at Caltech's Palomar Observatory, ZTF J1901+1458 has a 2,140-kilometer radius and 1.327 solar masses. As lead author Ilaria Caiazzo notes, it packs "more mass than the Sun into a body the size of the Moon."
Pushing the limits with its mass, this white dwarf nears the Chandrasekhar limit, beyond which it would collapse and explode as a supernova. It spins on its axis every 6 minutes and 57 seconds, boasts a tremendously strong magnetic field, lies just 130 light-years from Earth, and is remarkably young at less than 100 million years old.
Future observations could reveal more. "There are so many questions: What's the white dwarf merger rate in the galaxy? Does it account for Type Ia supernovae? How do magnetic fields form in these events, and why the diversity in strengths?" asks Dr. Caiazzo. "A larger sample will help answer these and more."
