There’s Quite the Story Being Told in the Stars

Astronomers have discovered the smallest white dwarf star ever seen, which could have big implications for our knowledge of the cosmos.


A long-standing theory about the lives and deaths of stars has received new evidence in the form of the smallest white dwarf star ever discovered. At only 4,300 kilometres in diameter — or roughly the distance from Vancouver to Halifax — the discovery will help astronomers learn more about the most extreme objects in our Universe.

The study included contributions from UBC professors Jeremy Heyl and Harvey Richer, and was published in June in Nature.

Astronomers know that different types of stars lead very different lives. Stars much bigger than our Sun, for example, will eventually explode into supernovae before collapsing under their own weight to form extremely dense black holes. Smaller stars will collapse down to white dwarfs before slowly cooling off over time.

Yet while these different classes of stars have been studied in great detail, astronomers still aren’t entirely sure about the boundaries between them. In the case of white dwarfs, mathematical arguments dating back to the 1930s have been used to determine just how massive they can get. Any bigger than this so-called Chandrasekhar Limit (named for Indian-American astrophysicist and Nobel laureate Subramanyan Chandrasekhar) and the white dwarf will collapse under its own weight to become an ultra-compact neutron star.

Interestingly, the heaviest white dwarfs are also expected to be packed into the smallest spaces. This is because white dwarfs are supported by their own pressure, and heavier masses require greater pressures — which means that the electrons that make up the white dwarf have to squeeze even closer together.

Despite knowing about these theoretical size boundaries for many years, astronomers had never previously discovered a white dwarf at the cusp of the Chandrasekhar Limit. Thanks to new observations from the Zwicky Transient Facility (ZTF) at Caltech’s Palomar Observatory, however, the astronomers behind this study were able to spot the most massive white dwarf yet — a star known as ZTF J1901+1458 that weighs more than our Sun, yet which is packed into a body as small as our Moon.

“Because it is so small, we know ZTF J1901+1458 lies within a percent or so of the maximum mass that a white dwarf can have, the Chandrasekhar mass,” said Heyl in a press release.

“If it manages to accrete even a small amount of matter, it would collapse, perhaps producing a supernova or a neutron star.”

“This is a very important test of the theory of white dwarf stars,” added Richer.

The discovery was made possible by the ZTF’s wide field of view, which allowed the team to search through large areas of the sky. They believe that the star formed when two smaller white dwarfs merged, creating a white dwarf right on the edge of what’s possible.

Going forward, the white dwarf will not only help confirm previous theories, but will also provide astronomers with insights into how supernovae and neutron stars form.

The team will also continue to search for similar objects with the ZTF. By uncovering more white dwarfs living on the edge, they’ll be able to shed further light on the Universe’s most extreme environments.

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Emily Deibert is a PhD student in the Department of Astronomy & Astrophysics at the University of Toronto with a passion for science outreach and communication. She earned her HBSc (Astronomy, English, and Mathematics) at the University of Toronto. She is excited about turning scientific research into stories and sharing these stories with the public.