Astronomers discovered a tiny, but powerful white dwarf star. It is both the smallest and largest known white dwarf. It spins quickly, packs a powerful magnetic field, and could eventually collapse into a smaller, denser neutron star.
This bizarre little creature has it all.
A white dwarf is the remnant of a star such as the Sun's death. The Sun is currently fusing hydrogen to helium in the core. This provides the Sun with both the energy and the pressure needed to support the octillion tons of material that are pressed down on its core.
There are a number of complicated events that occur when the hydrogen fuel is exhausted. The first step is that the all-helium core begins to fuse into carbon and oxygen. Meanwhile, the outer layers begin to swell and then start to blow away in dense winds of particles. The core eventually becomes visible to the outside world. This object is called a white dwarf because it is super-dense and hot. It is usually just a solid mass that sits in space and slowly cools down once it has formed.
Zoom In Hubble image showing one of the nearest binary stars to the Sun: Sirius A, (to the center) and its white dwarf companion (to the lower right); A is approximately 10,000 times brighter. Credit: NASA (ESA), H. Bond (STScI) and M. Barstow, University of Leicester
A white dwarf is about half the mass as the Sun. However, this matter is compressed by gravity to a compact ball that is only about the same size as the Earth. It is dense. It is so dense that quantum mechanics has to raise its strange head. The electron degeneracy tension holds it up. This weird state of matter, where electrons repel one another with incredible fervor, is far stronger than the "like charges repel" type of thing. This pressure holds the white dwarf up against its ridiculously strong gravity.
It also means that white dwarfs will shrink if they are massed up. It is normal for something to get bigger when it has more mass (ex. two clay balls that you mix together). Degeneracy pressure is the exact opposite.
This brings us to ZTF J190132.9+145808.7.
It was discovered by an astronomer team using the Zwicky Transient Facility (hence its name). This facility searches for objects that change brightness or move. The star was unusual in that it had a strange color for a white dwarf, and specifically one linked to white dwarfs with unusually high masses.
They then followed up with observations ZTF J190132.9+145808.7 (let’s call it J1901) on the Palomar Observatory's 5-meter telescope. They found that the white dwarf was dynamic, changing its brightness quickly. It became brighter and darker in 6.94 minutes. Yes, minutes. This is its rotation speed. It's amazing: A object with a diameter of thousands of kilometers spins in less than 7 minutes.
Gaia observations showed that J1901 is 134 lightyears from Earth. It also glows hot at 46,000 C, eight times more than the Sun. Once all the measurements were taken, the astronomers could determine J1901's dimensions. But this is where the strange things begin: J1901 is tiny at just 4300 km wide. It is the smallest white dwarf that has ever been seen.
Zoom In Artwork shows the ZTF J190132.9+145808.7 white dwarf, which is the smallest ever discovered, and compared to the Moon. Credit: Giuseppe Parisi
It's only about a third of the Earth's size, and a little bigger than the Moon. Even for a white dwarf, it is small. This is especially true for one. This one must be huge because more mass equals smaller stars. They calculate that it is 1.35 times as massive as the Sun.
This is the point where it becomes truly amazing. This makes it the largest white dwarf known. In fact, it is almost the biggest white dwarf that can be.
A white dwarf can only hold up to 1.44 times the Sun's mass. Even the pressure of electron decay cannot stop it. It collapsing under its own gravity. It either becomes a more dense and dangerous neutron star or explodes.
J1901 is the closest to this limit.
According to the team, J1901 began as two stars similar to the Sun in a binary orbit around one another. They both eventually became red giants and died. Each of them left behind white dwarf corpses with about 2/3rds of the Sun's mass. They merged over billions of years, probably less than 100 million years ago due to the high temperature. This created this white dwarf that is extremely massive but not explosive.
This explanation also explains other properties. Rapid spin is possible because two objects that are spiraling closer to each other have a lot angular momentum. This means that the final merged object should spin quickly. White dwarfs, on the other hand, have a spin period of many hours so this spin is quite fast.
They also measured J1901's strong magnetic field, which was about one billion times stronger than the Earth's. Two white dwarfs merging can produce tremendous magnetism according to the theoretical models.
In their paper, they note that star age could lead to isotopes such as sodium and magnesium absorbing electrons. These electrons are necessary to support the star. The star will shrink further if the electrons are absorbed. It could shrink too much and then become a neutron star.
It could also explode depending on certain properties that are hard to determine. It would be a bad thing if supernovae were to explode at its current distance. However, even if it does happen (and it's likely), it won't happen for billions upon billions of years. J1901's orbit around the galaxy will keep it far away.
This discovery is significant for several reasons. 95% of stars eventually become white dwarfs. Half of binary system stars are white dwarfs. We should therefore expect to see many white dwarfs like J1901. Its proximity to us also suggests that they are common. If they were rare, you would expect them to be thousands of light-years away. It's an excellent example of what is likely to be a common object, but only a few have been closely studied. There are a few other white dwarfs, both small and large, but J910 holds the record for its size.
Binary white dwarfs can also generate supernovae. These in turn tell us a lot more about the behavior and dynamics of the Universe, so that's pretty cool.
There are about 400,000 white dwarfs in the galaxy, but billions more should be catalogued. In a large sample, strange ones are bound to be found. Are there any other surprises yet to be discovered? What will we discover about the strange cosmos in which we live?