This article on gravitational waves from white dwarfs is a guest article by Ariana Vlad, senior at the International Computers High School of Bucharest, Romania, where she focuses on studying Physics and Mathematics.
Ever since their first detection back in 2015, gravitational waves have opened a new window of research. The detection of these waves from a pair of colliding black holes, by LIGO was a great milestone. Even though the black holes are extremely massive, the gravitational waves that reach the Earth are very weak. Detecting them is a very challenging task. It is like if you have a scale that is 1 billion trillion meters long and you had to tell if it shrunk by 5 mm. That’s the level of precision required.
After about a year, scientists detected the first gravitational waves from a pair of neutron stars. The signal of course was weaker as neutron stars are less compact than the black holes. Now the question is, when will the first gravitational waves from a pair of white dwarfs be detected? As you’ll see in this article, there are two major challenges in this task. So let us start with the very basics.
What Are Gravitational Waves?
Gravitational waves are, as predicted by Einstein’s theory of general relativity, ripples in space-time that propagate with the speed of light when energy is liberated in a process. The first proof of this prediction came more than 50 years after the theorem was developed, revealing a new era for astronomical measurements (before 14 September 2015 scientists were only detecting electromagnetic radiation and analogous data from space).
These waves carry out energy as they move in space, therefore also carrying information about the nature of the phenomenon that originally produced them. Unlike electromagnetic radiation, gravitational waves interact weakly with matter, so they can go over billions of light-years without suffering major distortions or alterations.
What Are White Dwarfs?
White dwarfs are thought to be the final state for those stars with a mass of less than 10 solar masses which are therefore unable to reach the neutron stars phase. Very dense and very small compared with other celestial objects (mass of the same order as the Sun, size of the same order as the Earth), white dwarfs are mainly composed out of electron-degenerated matter.
White dwarfs used to be the interior of a star, which explains their very hot temperature and the white shine that gives their color and name. There are not many white dwarfs discovered yet, until now scientists discovering only six white dwarfs within 18 light-years of Earth. Because of that, astronomers struggled to find a binary system with white dwarfs until 1967.
Gravitational Waves From White Dwarfs
“Binary system” is a term used to describe two celestial objects that are gravitationally linked together. It is known that, when acted upon by a Newton-like force, an object will move on a conical trajectory. Because of that, each part of a binary system will rotate around the other one, the orbital frequency being determined by the components’ masses and compactness. When the rotation period is less than one hour, the stars are so close together that none of them can be an ordinary star like the Sun – usually the components are white dwarfs.
For many years, research has predicted that there should be a binary system made up of white dwarfs. Calculations done using Einstein’s theory of general relativity show that such a system would continuously emit a substantial amount of energy in the form of gravitational waves. Although science doesn’t have (at the moment) devices sensible enough to measure the ripples coming from a binary motion of white dwarfs, such a system is discovered based on spectroscopic studies (studies based on electromagnetic radiation detection).
Astronomers have recently discovered a system of white dwarfs that could be further studied as a source of gravitational waves. The system is called J2322+0509.
To further facilitate the discovery of more binary white dwarf stars, scientists would need an observatory more precise than even LIGO (which is known to be an incredibly accurate apparatus and detector). Some of their projects are already focusing on that, as astronomers know that gravitational waves detection is the right way to find these tricky-to-spot systems.
In this case, the gravitational waves detected are not the result of a one-time phenomenon, as it was for the case of two black holes colliding. A binary system will lose energy until the two celestial objects forming it will merge, which can mean tens of million years. This emission of radiation will therefore continuously affect the relative motion of the two white dwarfs: as the energy of the system decreases, the orbits are slowly decaying.
Detection of white dwarfs binaries using gravitational waves presents another great advantage: it will solve an odd discrepancy between the number of known such systems and the number of predicted binary systems. Studies of the evolution of stars show that in our galaxy alone there might be more than 250 million of binaries. With the right detectors, said to be ready within the next two or three decades, astronomers could be able to detect and localize thousands of those.
Slowly but steadily, all the stray and unknown pieces of the universal puzzle come together, and a lot of this progress is thanks to the discovery of gravitational waves.