A star in the constellation Centaurus has a diamond core 4,000 km wide

BPM 37093 was first cataloged in the early 1940s as an unremarkable white dwarf in the constellation Centaurus, identified through the Bruce Proper Motion survey. Decades later, a team led by Travis Metcalfe at the Harvard-Smithsonian Center for Astrophysics announced something unexpected: the star was ringing.

White dwarfs are the burned-out cores of sun-like stars that have exhausted their nuclear fuel. When they cool below a certain temperature, theoretical models predicted their carbon and oxygen interiors should crystallize — atoms locking into a rigid lattice the same way water becomes ice, except under pressure measured in millions of atmospheres. The problem was proving it. You cannot drill into a white dwarf.

The confirmation came in two stages. A team led by Antonio Kanaan used the Whole Earth Telescope — a coordinated global network — to observe BPM 37093 across campaigns in 1998 and 1999, mapping the star's oscillations in detail. White dwarfs pulse in measurable frequencies, and those frequencies depend on the internal composition and structure of the star. Metcalfe's team then applied asteroseismology to that data in their 2004 analysis, comparing the oscillation patterns to theoretical models. The match required that approximately 90 percent of the star's mass had already crystallized into a diamond-structured carbon-oxygen lattice.

The star itself is about 1.1 solar masses compressed into a body roughly the size of Earth. The crystallized core, by the team's 2004 analysis, was estimated at around 10 to the 34th carats. That is a 1 followed by 34 zeros. For context, the Cullinan Diamond, the largest gem-quality diamond ever mined, weighs 3,106 carats.

The Beatles connection was deliberate. The team named the star Lucy after 'Lucy in the Sky with Diamonds,' and the press release that accompanied their 2004 American Astronomical Society presentation made the analogy explicit. Astronomers tend to resist whimsy. They made an exception.

The physics is stranger than the metaphor suggests. The carbon lattice inside BPM 37093 is not arranged like a terrestrial diamond. Terrestrial diamonds form under pressures around 5 gigapascals at temperatures around 1,300 Celsius. The interior of a white dwarf operates at pressures roughly a million times higher and temperatures around 6,500 Celsius. The resulting structure is a body-centered cubic lattice — closer in arrangement to crystalline iron than to jewelry-store diamond. Calling it a diamond is correct in the chemical sense and misleading in the visual one. It would not sparkle. It would not refract light. It would, however, conduct sound at velocities far exceeding anything attainable on Earth.

A 2019 paper in Nature, led by Pier-Emmanuel Tremblay at the University of Warwick, used Gaia satellite data on roughly 15,000 white dwarfs to confirm that crystallization is a universal stage in white dwarf cooling. The process releases latent heat — the same way freezing water releases energy — which delays cooling by approximately one billion years. Many of the white dwarfs in the Milky Way are, by mass, mostly crystallized carbon. The galaxy is full of diamond cores.

The sun will become one of them. In approximately 5 billion years, after the red giant phase, what remains of the solar core will collapse into a white dwarf. Over the following several billion years it will cool, contract, and crystallize. Whatever is left of human civilization, or whatever has replaced it, will orbit a dead star slowly turning into the largest gemstone in the solar system.

The sky has been full of jewelers all along.