The biggest diamond ever found on Earth, known as the Cullinan diamond, weighed three thousand one hundred and six carats before it was cut, or about one and three-tenths pounds. The biggest diamond ever found in the universe, whose discovery was announced this week, has no name, will never be cut, and weighs approximately a million trillion trillion pounds. This makes it as massive as the sun, and no wonder: it’s the corpse of a star that once looked very much like the sun, lying nine hundred or so light-years from Earth.

Nobody has actually seen this gigantic diamond, not even through a telescope. David Kaplan, an astrophysicist at the University of Wisconsin-Milwaukee and the lead author of the study in this week’s Astrophysical Journal, told me that the star’s invisibility is a key part of the circumstantial case for its existence.

The first step to the proof, Kaplan explained, was the discovery of a neutron star, which itself is the husk of a star that burned up all its fuel, exploded, and then collapsed to form an object about as far across as Manhattan is long. A neutron star is so dense that a teaspoonful of its substance would weigh ten million tons, and this one rotates so fast that its “day” lasts just a few thousandths of a second.

Every time it goes around, it emits a pulse of radio waves that astronomers can detect with radio telescopes, making it a pulsar, a subcategory of neutron star. The first pulsar was found in 1968, and hundreds have been discovered since then. A normal pulsar’s radio bleeps are so perfectly regular that at first astronomers considered that the signal might be artificial; until they figured out what was going on, scientists called the phenomenon “L.G.M.,” for “little green men.”

The bleeps from this pulsar, known as PSR J2222-0137, aren’t absolutely regular; they speed up and slow down with a regular rhythm, which told Kaplan and his colleagues that something was orbiting it. The gravitational tug from that something was pulling PSR J2222-0137 slightly toward the Earth, then slightly away, then toward again, forcing the beeps to pile up on each other, then pull back, and so on.

By measuring the changes in pulses very carefully, the astronomers could calculate how massive the invisible something must be (about five per cent more massive than the Sun, it turned out). The object could have been an ordinary star, but then the astronomers would have seen it easily. It could have been a second neutron star, but, in that case, the explosion that created it would have kicked it into an elongated orbit around its partner. “We were able to show the orbit is circular, so it couldn’t be a neutron star,” Kaplan told me.

The only reasonable answer, they concluded, was that the invisible object is a white dwarf star. Like a neutron star, it’s the remnant of what was once an ordinary star, but not one massive enough to explode. Instead, it simply puffed off its outer layers and shrank—exactly the same thing that will happen to the sun in about five billion years. In this case, the star died eleven billion years ago, less than three billion years after the birth of the universe.

At a distance of just nine hundred light-years, however, even a dim white dwarf should have been visible. The fact that they still can’t see it suggests to Kaplan that it must be unusually cool—no more than two thousand degrees, as opposed to about six thousand for the sun. “Most white dwarfs are made mostly of carbon and oxygen,” he said, and, at that temperature, given the density of a white dwarf, those elements should have crystallized. Another name for crystallized carbon is “diamond.”

The oxygen would have crystallized, too, so this object wouldn’t necessarily be pure diamond. But, when you’re talking about a sun’s worth of diamond, that hardly matters.

Kaplan and his team haven’t given up on actually seeing the still-unnamed object. “We’re going to try with the Hubble Space Telescope,” he said. If they can manage it, they might be able to say more about the thing’s structure and precise chemical composition. If they can’t, they may be out of luck. “These things are probably very rare,” he said. “There are probably others out there, but they’re almost certainly too far away for us ever to see.”

Photograph by Antonio Zambardino/Contrasto/Redux.