Moon-size White Dwarf Is the Smallest Ever Found

Not far from us is a faint, hot cinder of a star, a white dwarf still smoldering from its formation less than 100 million years ago. Most such objects are the collapsed cores of low-mass stars. But this one is different.

In the July 1st Nature, Ilaria Caiazzo, Kevin Burdge (both at Caltech), and their colleagues report evidence that this white dwarf was born out of the union of two punier siblings. As a result, it’s the smallest white dwarf known — and just this side of collapse.

Small Yet Massive

White dwarfs owe their existence to quantum mechanics. While stars fuse elements to release thermal pressure and counteract gravity, white dwarfs can’t muster the conditions for fusion. Instead, gravity compacts their cores until electrons are forced practically next to each other. But according to the Pauli Exclusion Principle, electrons won’t stand for that — they physically cannot share the same energy state. So electrons that don’t fit in the lowest energy levels, where they’d prefer to be, instead go to higher and higher levels, whizzing around the core at greater and greater speeds.

These fluttering electrons provide their own kind of pressure that supports the core against gravitational collapse. The result is counterintuitive: The more mass a white dwarf has, the smaller it becomes in order to generate the necessary pressure to stave off collapse.

That works until there’s too much mass, and the fastest electrons are forced to flit about at near the speed of light, at which point they can’t generate enough pressure to forestall destruction. When a pair of white dwarfs merges, the scale often tips over the Chandrasekhar limit, beyond which runaway nuclear reaction ensues and the stellar cinder detonates.

But if the white dwarf parents are runts, their child might not tip that scale. That’s what Caiazzo thinks her team has found: a white dwarf born out of a white dwarf-white dwarf union that’s just this side of the Chandrasekhar limit that would mean its collapse or destruction.

Burdge found the object in data collected by the Zwicky Transient Facility in California. A megapixel camera at the Palomar Observatory scans the entire night sky every other night. Using a special computer algorithm to sort through frames taken 48 hours apart, Burdge was able to search for things varying on minutes-long timescales.

“There aren’t that many things that brighten or fade over minutes,” Burdge says. “Pretty much anything that does that is interesting.” And among the interesting finds was the white dwarf, which follow-up observations confirmed was rotating every 7 minutes. (The record-holder, EPIC 228939929, spins every 5.3 minutes.)

The rotation was the first hint that something about this one was different. Most white dwarfs rotate over hours, not minutes. But any white dwarf that’s the product of a merger would spin more quickly. And a fast spin should produce an extreme magnetic field. Indeed, Caiazzo and her colleagues obtained spectroscopy using the W. M. Keck I Telescope on Mauna Kea, Hawai‘i, that showed the white dwarf’s magnetic field is on the extreme end — a billion times stronger than Earth’s.  

Most intriguingly, though, Caiazzo and her colleagues found the white dwarf was exceedingly small, just slightly bigger than the Moon at 4,300 kilometers (2,700 miles) across. And because more massive white dwarfs are more compact, the small size suggests this is also the most massive white dwarf known. While the exact mass depends on what the white dwarf is made of, the researchers estimate it’s about a third more massive than the Sun: between 1.327 and 1.365 solar masses, depending on its composition.

(The white dwarf in the T Coronae Borealis system was measured to be even more massive at 1.37 solar masses, but hot gas streaming off its stellar companion and other complicating factors make that measurement more uncertain, Burdge says.)

However, because the white dwarf’s composition and mass are not exactly known, a question remains. Is the white dwarf so small simply because it’s so massive? Or is it actually in the process of collapsing?

Close to Collapse?

The answer depends on exactly where the Chandrasekhar limit lies. That limit is approximately 1.37 solar masses, but the exact definition actually depends on what the white dwarf is made of. And, if a white dwarf is near collapse, then the composition — and therefore the limit — is changing.

What’s more, the nuclei of a white dwarf’s heaviest elements can capture electrons, depriving the white dwarf of their outward pressure and thus speeding gravitational collapse. And while the heaviest elements are initially spread out, like sediment that drifts to the bottom of a wine glass, they settle into the white dwarf’s core over a few hundred million years and speed up electron capture.

However, this process competes against the crystallization of the white dwarf’s core as it cools. The same heavy atoms that can capture electrons can also engage in reactions that produce neutrinos. These ghostlike particles easily escape the white dwarf’s gravity, carrying energy away with them. If a white dwarf cools enough, its core solidifies, freezing into a lattice that is stable against collapse.

Caiazzo and her team estimate that this particular object could crystallize anywhere between 10 or 100 million years from now — a rough estimate because the cooling process isn’t yet well understood in massive white dwarfs. (Less massive white dwarfs can take billions of years to cool down into crystals.)

Even if it did collapse, it’s still unclear if it would explode as a Type Ia supernova, the fate it has so far avoided, or if it would instead implode to form a neutron star.

“It’s not happening tomorrow,” Caiazzo notes. Collapse could take on the order of 100 million years.

Intriguingly, though, the object is so nearby (134 light-years) that it’s probably not rare. Indeed, one other fast-spinning, highly magnetic (though not quite as small) white dwarf has previously been found. As facilities such as the under-construction Vera Rubin Observatory reach deeper for their time-lapse movies of the night sky, astronomers may well discover many more white dwarfs with such unusual births.

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