Moon-sized “zombie” star challenges what we know about celestial evolution

As a star around the size of the Sun nears the end of its life, it runs out of fuel and turns into a stellar zombie known as a white dwarf star. White dwarfs may be small, but don’t be fooled by their size: some of them still pack a mean punch.

While most white dwarfs are around the size of Earth and the mass of the Sun, but a newly-discovered one is confounding astronomers: It’s the size of the Moon, and so massive it almost became an entirely different type of zombie star called a neutron star — normally the kind of thing formed in the aftermath of a supernova.

Their discovery is detailed in a study published Wednesday in the journal Nature, and could hold clues to how another mysterious type of star forms.

WHAT’S NEW — The white dwarf star, dubbed ZTF J1901+1458, was discovered by the Zwicky Transient Facility in California.

The radius of the star is only 4,300 kilometers, compared to the Sun’s 696,000 kilometers.

But this tiny white dwarf has a mass that’s about 1.35 times the mass of the Sun. That may not seem like much, but it’s gargantuan for a white dwarf.

The size and mass of the newly discovered white dwarf star may seem odd, but white dwarfs are, by nature, counterintuitive. The smaller the white dwarf is, the larger its mass.

ZTF J1901+1458 is so massive in fact that it is just below the verge of a giant explosion.

"We caught this very interesting object that wasn't quite massive enough to explode," Ilaria Caiazzo, the Sherman Fairchild Postdoctoral Scholar Research Associate in Theoretical Astrophysics at Caltech and lead author of the new study, said in a statement. "We are truly probing how massive a white dwarf can be."

Since white dwarf stars have no fuel to burn, they cannot resist their own self-gravity. The more massive a white dwarf is the less its radius, since the power of its gravitational force increases and shrinks the star.

Because of its high mass, the team behind the study believes it may have been the result of the merger between two white dwarfs creating one small but mighty cosmic monster.

What might have happened:

• Many stars come in pairs called binary stars, and the two orbit each other
• Stars like the Sun expand outward, leaving behind only a core of electron matter
• The parent bodies of ZTF J1901+1458 may have both become white dwarfs, then became drawn together gravitationally
• Most times, this results in a supernova called a Type Ia
• However, in this case, the mass was just enough to cause the white dwarfs to settle into a merged body

The stellar merger would also bring together the stars’ magnetic field and speed up its rotation. The newly discovered white dwarf has a magnetic field almost one billion times stronger than that of the Sun and is going around on its axis at a dizzying speed of one revolution per seven minutes.

WHY IT MATTERS — The white dwarf in question not only presents an alternative outcome for zombie star companions, but it could also lead to the formation of another type of mysterious star.

The scientists behind the new study suggest that the white dwarf may be massive enough to evolve into a neutron star.

Neutron stars are the collapsed cores of a massive supergiant star with a mass that’s 10 to 25 times that of the Sun’s.

"This is highly speculative, but it's possible that the white dwarf is massive enough to further collapse into a neutron star," says Caiazzo. "It is so massive and dense that, in its core, electrons are being captured by protons in nuclei to form neutrons. Because the pressure from electrons pushes against the force of gravity, keeping the star intact, the core collapses when a large enough number of electrons are removed."

What’s next — If the white dwarf star does eventually turn into a neutron star, then the new discovery points to a different way in which these dense, dead stars form.

The team is hoping to discover more white dwarf stars similar to the one recently observed.

“There are so many questions to address, such as what is the rate of white dwarf mergers in the galaxy, and is it enough to explain the number of type Ia supernovae? How is a magnetic field generated in these powerful events, and why is there such diversity in magnetic field strengths among white dwarfs?” Caiazzo says. “Finding a large population of white dwarfs born from mergers will help us answer all these questions and more."

Abstract — White dwarfs represent the last stage of evolution of stars with mass less than about eight times that of the Sun and, like other stars, are often found in binaries1,2 . If the orbital period of the binary is short enough, energy losses from gravitational-wave radiation can shrink the orbit until the two white dwarfs come into contact and merge3 . Depending on the component masses, the merger can lead to a supernova of type Ia or result in a massive white dwarf4 . In the latter case, the white dwarf remnant is expected to be highly magnetized5,6 because of the strong magnetic dynamo that should arise during the merger, and be rapidly spinning from the conservation of the orbital angular momentum7 . Here we report observations of a white dwarf, ZTF J190132.9+145808.7, that exhibits these properties, but to an extreme: a rotation period of 6.94 minutes, a magnetic feld ranging between 600 megagauss and 900 megagauss over its surface, and a stellar radius of 2140−230 +160 kilometres, only slightly larger than the radius of the Moon. Such a small radius implies that the star’s mass is close to the maximum white dwarf mass, or Chandrasekhar mass. ZTF J190132.9+145808.7 is likely to be cooling through the Urca processes (neutrino emission from electron capture on sodium) because of the high densities reached in its core.