Smallest, heaviest known white dwarf discovered

The newly discovered white dwarf, designated ZTF J190132.9+145808.7, has a mass 1.35 times the mass of the sun, and a radius of 2,140 km, which is just 1.23 times larger than our moon.

It is located approximately 130 light-years away from earth, in the constellation of Aquila. ZTF J1901+1458 was discovered by the Zwicky Transient Facility, which operated at Caltech’s Palomar Observatory. White dwarfs are remains of a dead star after the star goes through the phase called ‘supernova’. As soon as the Hydrogen in a star’s core gets converted into Helium, the Helium is then converted into more massive nuclei, by nuclear reactions.

Finally, after becoming a red giant, the star explodes and flushes off its outer atmosphere, thus leaving behind its core. Depending upon the mass of the parental star, the core is then categorized into a white dwarf, or a neutron star, or a black hole. Experts have said that ZTF J1901+1458 is about 100 million years old and has an extreme magnetic field, almost 1 billion times stronger than our Sun’s.

It has a rotation period of 416.4 seconds, that is, it spins with a tangential velocity of 8973.36 km/h. Such rotational period is unusually short for a white dwarf, as an average white dwarf has rotational periods upwards of hours. “ZTF J1901+1458 is heavy, packing a mass greater than that of Sun into a body about the size of our Moon,” said Dr. Ilaria Caiazzo, an astronomer in the Division of Physics, Mathematics, and Astronomy at Caltech.

The fact that one tablespoon of white dwarf weighs about 15 tons is that it’s extremely dense. White dwarfs lack the nuclear burning that keeps up the size of stars against their gravity. Heavy nuclei can’t generate much energy as the Hydrogen can, hence, the balance between gravity and nuclear energy trying to escape the star’s atmosphere breaks, and the star collapse under its gravity, thus becoming extremely dense.

“No one has systematically been able to explore short-timescale astronomical phenomena on this kind of scale until now. The results of these efforts are stunning,” said Dr. Kevin Burdge, a postdoctoral researcher in the Division of Physics, Mathematics, and Astronomy at Caltech. The strength of the magnetic field together with an approximately 7-min rotational period indicated that this dwarf is a result of the merger of two smaller white dwarfs.

“Many stars orbit around each other in pairs. They grow old together, and if they are both less than 8 solar masses, they will both evolve into white dwarfs,” Dr. Burdge said. “The pair of white dwarfs, which spirals around each other, lose energy in the form of gravitational waves and ultimately merges. This process boosted the magnetic field and speeded its rotational spin,” he added.



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