White Dwarf Morphs into Massive Pulsing Crystal
Astronomy lets us peer into some of the strangest corners of physics in a way that are incredibly hard (or impossible) to reproduce in a laboratory setting. For example, a recent discovery of pulsations from a massive white dwarf star has allowed astronomers to imagine a crystallized, semi-solid ball of oxygen and neon the size of our planet.
To understand where white dwarfs come from, you must first look at the evolution of “normal” or main-sequence stars. These begin their lives by fusing hydrogen into helium in their cores, powering the heat and light of the star as some of the mass of that interaction is turned into energy. For most stars, the core will eventually reach a state when there is not enough hydrogen in the core for this process to continue, and the star evolves and eventually dies.
For a star with the mass of our sun, helium can undergo nuclear fusion eventually creating carbon and oxygen for a short time. Stars more massive than the Sun will do this as well, and those with seven times the sun’s mass will even achieve stable carbon fusion to produce neon. However, for such stars, that’s the limit, and once that fusion process has run down, the nuclear power plant at the center shuts down and the outer layers of the star are lost to interstellar space. What is left behind is the former core of the star, now called a white dwarf.
A white dwarf is an extremely dense and hot ember of a star. Typical white dwarf masses are a little more than half the mass of our sun. There is great astrophysical interest, however, in the higher mass white dwarfs, since these are the ones that create novae and even Type 1a supernovae, which have become an important tool for measuring the acceleration of the Universe.
A team using the 2.1-meter telescope at McDonald Observatory in Texas set out to find and characterize these high mass white dwarf stars. They came across GD 518, which by its spectrum was shown to have a surface temperature of 12,000 degrees Celsuis, twice the temperature of the surface of our sun. The mass was determined by looking at the absorption lines in the spectrum due to hydrogen. These were wide lines, “distorted” by a high surface gravity one hundred million times that of what we feel at Earth’s surface. This indicates that it has a mass of 1.2 times the mass of the sun and, according to stellar models, should be made of oxygen and neon.
Since astronomers can’t go out and sample the interior of a star, they need other methods for understanding what is inside. In addition to the theoretical models, the remnants of bright novae, or partially exploded white dwarf stars, have shown oxygen, carbon, and other such materials left behind. But the group in Texas were looking for pulsations, or variability in these high mass white dwarfs.
So, with careful observations, they discovered that GD 518 was indeed varying in brightness on a time scale of six to ten minutes. Variable stars change brightness because they actually “pulse,” expanding and contracting ever so slightly because of some instability inside the star. Predictions of white dwarf pulsations depend also on how much of the interior has crystallized, or solidified. The scale of these pulsations of GC 518 indicate that a significant fraction of its oxygen-nitrogen interior is in this crystal state.
This careful following of clues using just the light of the star, coupled with predictions of the physics of stars, thus leads us to the conclusion that in this faint, hot white dwarf star, we are seeing the crystallized remains of what was once a large, brightly burning star, yet one not quite large enough to blow itself apart in a supernova. The continued study of these rare white dwarfs will provide insight into other types of supernovae, however, and ensure that we’re making the right measurements of the Universe on the largest scales.
The bonus? This work and much like it does not require the biggest telescopes in the world, but rather a class of middle-sized telescopes available to universities all over. There is much room for discovery with all types of instruments if only you know where to look.
Image: Top: Artist’s impression of a white dwarf star (NASA). Middle: Observations of the varying brightness of GD 518 using the 2.1-meter Otto Struve Telescope at McDonald Observatory. From preprint of Hermes et al.
This research was published in Astrophysical Journal Letters, and the preprint is available on arXiv.org.