Astronomers announced this week that they've discovered a star that is "packing a mass greater than that of our Sun into a body about the size of our Moon," according to the lead author of the study, which was published by the journal Nature.
The Zwicky Transient Facility at Caltech's Palomar Observatory made the discovery using two telescopes based in Hawaii: W. M. Keck Observatory and the University of Hawaii Institute for Astronomy's Panoramic Survey Telescope and Rapid Response System or Pan-STARRS. They were assisted by the Hale Telescope at Palomar, the European Gaia space observatory, and the Neil Gehrels Swift Observatory operated by NASA.
The study's abstract notes that "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 binaries. 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 merge. Depending on the component masses, the merger can lead to a supernova … or result in a massive white dwarf. In the latter case, the white dwarf remnant is expected to be highly magnetized … and be rapidly spinning from the conservation of the orbital angular momentum."
The white dwarf discovered by astronomers is located about 130 million light years from Earth and officially designated as ZTF J190132.9+145808.7. The researchers note that the star shows the "properties" of a white dwarf, "but to an extreme."
Ilaria Caiazzo, the Sherman Fairchild Postdoctoral Scholar Research Associate in Theoretical Astrophysics at Caltech who acted as lead author for the study, said in a statement that "it may seem counterintuitive, but smaller white dwarfs happen to be more massive. This is due to the fact that white dwarfs lack the nuclear burning that keep up normal stars against their own self gravity, and their size is instead regulated by quantum mechanics."
Caiazzo said, "We caught this very interesting object that wasn't quite massive enough to explode. We are truly probing how massive a white dwarf can be."
She added, "This is highly speculative, but it's possible that the white dwarf is massive enough to further collapse into a neutron star. 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."
The researchers note that if this is correct, it could mean that a large portion of neutron stars form this way, and the white dwarf's close proximity to Earth and its relatively young age of less than 100 million years old implies that the galaxy could hold many other stars like it.
"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?" Caiazzo said. "How is a magnetic field generated in these powerful events, and why is there such diversity in magnetic field strengths among white dwarfs? Finding a large population of white dwarfs born from mergers will help us answer all these questions and more."
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