Astronomers discover a small yet massive white dwarf star

Astronomers had discovered a white dwarf star that is a bit of a contradiction. It's small, but at the same time, it's the most massive white dwarf ever seen. The star was formed when two less massive white dwarfs merged and is about 4300 kilometers across, making it slightly larger than the moon.

While it's small in size, it packs a mass greater than that of the sun into a body the size of the moon. Astronomers say that the smaller a white dwarf, the more massive they tend to be. That's because white dwarves don't have the nuclear fuel burning that keeps normal stars from collapsing under their own gravity.

Researchers discovered the star using the Zwicky Transient Facility or ZTF operating at the Caltech Palomar Observatory. The white dwarf is the collapsed remains of a star that was once about eight times the sun's mass or less. These stars show what will happen to our sun in the distant future.

In about 5 billion years, the sun will expand into a red giant star before losing its outer layers and shrinking to a small white dwarf. Scientists say that about 97 percent of every star in the universe will eventually become a white dwarf. Many stars orbit in binary pairs, and as they age, if both are less than eight solar masses, they both become white dwarf stars. The newly discovered white dwarf shows what can happen after that phase when the pair of white dwarfs spiral around each other and ultimately merge.

If the pair of stars were massive enough, it would explode into a type Ia supernova, but they simply combine below a certain mass threshold, creating a new white dwarf heavier than either of the separate stars. The merging process increases the star's magnetic field and speeds its rotation compared to the stars that formed it.

The newly discovered white dwarf is named ZTF J1901+1458, and it's interesting because the sum of masses of the stars that created it or just below the mass limit that would've caused supernova. The merger produced a white dwarf 1.35 times the sun's mass while it has a magnetic field almost a billion times stronger than the sun. It also orbits on its axis with a revolution every seven minutes.