Why Can a White Dwarf Be Heavier Than the Sun but Smaller Than Earth?

Why can a white dwarf weigh more than the Sun and still be smaller than Earth? A calm science story for sleep, told beside Sirius, the brightest star in the night sky, where a companion almost nobody can see packs slightly more mass than the Sun into a ball a little smaller than Earth. This white dwarf episode asks how that is possible, and why, among white dwarfs, the heavier ones are always the smaller ones. A sugar cube of white dwarf material would weigh as much as a large car, and the surface gravity is around four hundred thousand times Earth's. Nothing about it behaves the way matter behaves here: add mass to a white dwarf, and it does not grow, it shrinks. Along the way, we compare what holds up the Sun, what holds up a planet, and what holds up a dead star; follow the electrons that refuse to share their place; and walk the curve that runs from asteroids and giant planets to brown dwarfs and white dwarfs. We revisit the century-long effort to weigh and measure Sirius B, from a wobble noticed in the 1840s to orbits traced with the Hubble Space Telescope and light stretched by gravity, along with the mistakes that had to be corrected on the way. We also meet a white dwarf barely wider than the Moon, look at what happens near the mass limit Chandrasekhar calculated, and follow white dwarfs as they cool and slowly crystallize over billions of years. The episode ends in a quiet place: a star that will be exactly the same size tomorrow night. ━━━━━━━━━━━━━━━━━━━━ 📚 MAIN SOURCES ━━━━━━━━━━━━━━━━━━━━ 1. W. M. Keck Observatory. Extreme White Dwarf Sets Cosmic Records for Small Size, Huge Mass (press release on ZTF J1901+1458). 2021-06-30. 2. Caiazzo, I. et al. A highly magnetized and rapidly rotating white dwarf as small as the Moon (Nature 595, 39-42). 2021-06-30. 3. NASA Goddard Space Flight Center, Imagine the Universe. White Dwarf Stars. 2010-12; updated 2021-09-23. 4. Bond, H. E. et al. The Sirius System and Its Astrophysical Puzzles: Hubble Space Telescope and Ground-Based Astrometry (Astrophysical Journal 840, 70). 2017. 5. Joyce, S. R. G. et al. The gravitational redshift of Sirius B (Monthly Notices of the Royal Astronomical Society 481, 2361-2370). 2018. 6. Trimble, V. and Barstow, M. Gravitational redshift and White Dwarf stars (Einstein Online, Max Planck Institute for Gravitational Physics). 2020-11. 7. Blouin, S. White dwarf fundamentals (chapter for the Encyclopedia of Astrophysics). 2024-09-05. 8. Eddington, A. S. The Internal Constitution of the Stars. 1926. 9. MacTutor History of Mathematics, University of St Andrews. The Chandrasekhar-Eddington dispute. Undated. 10. Chandrasekhar, S. The Maximum Mass of Ideal White Dwarfs (Astrophysical Journal 74, 81). 1931. 11. Fields, B. D. (University of Illinois). Astro 404 Lecture 23: degenerate electron gas and white dwarfs. 2021-10-18. 12. NASA HEASARC. Another Rung on the Ladder (High Energy Astrophysics Picture of the Week). 2013-04-08. 13. European Space Agency. Gaia reveals how Sun-like stars turn solid after their demise. 2019-01-09. 14. Tremblay, P.-E. et al. Core crystallization and pile-up in the cooling sequence of evolving white dwarfs (Nature 565, 202-205). 2019. 15. NASA Goddard Space Flight Center, Imagine the Universe. Neutron Stars. 2017-03; updated 2021-09-23. 16. Lieb, E. H. Quantum Mechanics, The Stability of Matter and Quantum Electrodynamics. 2004. 17. Bond, H. E., Bergeron, P., Bédard, A. et al. Astrophysical Implications of a New Dynamical Mass for the Nearby White Dwarf 40 Eridani B (Astrophysical Journal). 2017. ━━━━━━━━━━━━━━━━━━━━ 🎬 CREDITS ━━━━━━━━━━━━━━━━━━━━ Visual material: 105 clips and stills in total. • 101 stock clips from Pexels under the Pexels License. • 4 clips from NAc domain: SA in the publi Space Science SDO: Year 4 Simulations Create New Insights Into Pulsars Artemis I Launch from NASA Causeway (slow motion) Imagery courtesy of NASA. NASA does not endorse this channel. Scientists quoted in the narration are cited in the Main Sources above.