Space News & Blog Articles

About 97% of all stars in our Universe are destined to end their lives as white dwarf stars, which represents the final stage in their evolution. Like neutron stars, white dwarfs form after stars have exhausted their nuclear fuel and undergo gravitational collapse, shedding their outer layers to become super-compact stellar remnants. This will be the fate of our Sun billions of years from now, which will swell up to become a red giant before losing its outer layers.

Unlike neutron stars, which result from more massive stars, white dwarfs were once about eight times the mass of our Sun or lighter. For scientists, the density and gravitational force of these objects is an opportunity to study the laws of physics under some of the most extreme conditions imaginable. According to new research led by researchers from Caltech, one such object has been found that is both the smallest and most massive white dwarf ever seen.

The study that describes the research team’s findings appeared in the July 1st issue of the scientific journal Nature. The research was led by Ilaria Caiazzo, the Sherman Fairchild Postdoctoral Scholar Research Associate in Theoretical Astrophysics at Caltech, and included colleagues from Caltech, the University of British Columbia (UBC), UC Santa Cruz, and the Weizmann Institute of Science in Rehovot, Israel.

Artist’s impression of white dwarf ZTF J1901+1458 above the Moon in this artistic representation; in reality, the white dwarf lies 130 light-years away in the constellation of Aquila. Credit: Giuseppe Parisi

This white dwarf, known as ZTF J190132.9+145808.7 (aka. ZTF J1901+1458), is located about 130 light-years from Earth and is estimated to be 1.35 times as massive as our Sun. However, this white dwarf has a stellar radius of about 1810 km (1,125 mi) – slightly larger than the Moon (1,737.4 km; 1,080 mi) – which makes it the smallest and most massive white dwarf ever observed. As Caiazzo explained in a recent press statement from the W.M Keck Observatory:

It’s tough to see inside of Saturn, because the atmosphere is opaque to all wavelengths of radiation. We have to rely on computer simulations and physics-based guesswork to try to understand the interior of that giant world. But researchers are becoming more adept at a different technique: looking for the slightest motions in the rings of Saturn.

The rings of Saturn are made of trillions of small bits of water ice, most small enough to fit in your hand. Each individual bit orbits Saturn in its own way, and each one is a tiny gravitational detector.

The planet Saturn isn’t just a simple, uniform ball of gas. It has complex layers and perhaps even a rocky core. Material moves up and down within the Saturnian atmosphere, and everything is in a constant state of motion.

Every time the interior of Saturn shifts, the particles of ice that make of the rings respond, affected by the new distribution of gravity. And different internal compositions of Saturn will make themselves known in the rings, because different arrangements of material will lead to different preferred orbits for ring material.

Sometimes entire waves can spread out inside the rings, or brief ripples appear in them. The analysis of the rings depends on having excellent observations of them, which we have thanks to NASA’s historic Cassini mission. That mission took countless high-resolution pictures of the rings, which astronomers can now use to try to reconstruct the interior of Saturn.

If it were’t for an enormous halo of dark matter enveloping our galaxy, the spin-rate of our central bar should stay pretty constant. But researchers have recently inferred that it has slowed down by almost 25% since its formation, a clear sign of the presence of dark matter.

How do you measure the spin-rate of something that takes millions of years to make a single revolution?

The answer is chemistry. Stellar chemistry.

Stars near the center of the galaxy are much richer in “metals”, which is the word astronomers use to denote any element heavier than helium. Stars in the outskirts, however, are much more lacking in metals. Therefor, if you happen to come across a group of stars that is exceptionally rich in metals, then it most likely wandered away from the center of the galaxy.

That’s the case for the Hercules stream, a large group of stars observed with the Gaia satellite. The Hercules stream is a metal-rich clump of stars, but sits relatively far from the galactic center. There’s something else intriguing about the Hercules steam: it’s locked in a gravitational dance with the central bar of the Milky Way.