Researchers report finding the smallest white dwarf — and likely, smallest star in general — we’ve ever seen. And it’s just a tad smaller than our Moon.
White dwarves are dead stars, the leftover cores of stars which reached the red giant stage but petered out. They’re extremely dense things, usually composed mainly of carbon and oxygen. This particular one, named ZTF J1901+1458, has a radius of approximately 1,700 kilometers — just shy of the Moon’s radius of 1,737 — and sits some 130 light-years away from us.
Despite its size, however, the dwarf has around 1.3 times the mass of the Sun.
Small but mighty
“That’s not the only very amazing characteristic of this white dwarf,” astrophysicist Ilaria Caiazzo of Caltech said June 28 in an online news conference. “It is also rapidly rotating.”
White dwarfs are typically similar in size to the Earth, which has a radius of around 6,300 kilometers. But one of their interesting properties is that they tend to be smaller the more mass they contain. This has to do with how they maintain stability. White dwarfs can’t generate the same physical processes that keep other stars from collapse, as they have no fuel to ‘burn’. Instead, their shape is maintained by the electrons in their atoms being physically pushed into one another to their limit. The tighter the squeeze, the more these electrons push back through quantum processes (electrons hate being near other electrons). So higher mass white dwarves, which have a stronger gravitational pull trying to make them collapse, need to become smaller in order to squeeze their electrons that much harder and counteract the pull.
Given its small size, then, ZTF J1901+1458 is one of the most dense objects of its kind.
It’s also quite restless, making a full spin once every seven minutes or so. The Earth makes a full rotation once every day. All this motion means that ZTF J1901+1458 produces quite the impressive magnetic field, estimated to be at least a billion times stronger than our planet’s. Needless to say, this is not a peaceful place to visit.
The stellar remnant was discovered using the Zwicky Transient Facility at Palomar Observatory in California, which scours the sky for objects with variable brightness. Given that they’re basically stellar corpses with no internal source of energy, white dwarves start out bright and incandescent but slowly cool and dim over time, eventually becoming an extinguished black dwarf.
As for how it came to be, we’re still unsure — but its mass provides a solid hint. The team’s working hypothesis is that ZTF J1901+1458 was born from the merger of two white dwarves that orbited one another and eventually merged into a single, extra-chunky, dwarfier white dwarf. This would also explain why it’s spinning so fast and why its magnetic field is so powerful.
All things considered, this merging could have easily ended badly. If ZTF J1901+1458 was more massive, it wouldn’t have been able to support its own weight and would have exploded. Finding a body so close to the edge of what’s possible will help us better understand what we’re going to run into once we eventually start trekking through space.
The paper “A highly magnetized and rapidly rotating white dwarf as small as the Moon” has been published in the journal Nature.