The Pulsating White Dwarf G117-B15A: Still the Most Stable Optical Clock Known
S. O. Kepler et al 2021 ApJ 906 7
The pulsating hydrogen atmosphere white dwarf star G 117-B15A, observed since 1974, has a stable pulsation period that varies by only 1 second in 6.2 million years, making it an exceptional optical clock.