White dwarf stars are they ghostly remnants of Sun-like stars after they have used up their necessary supply of nuclear-fusing fuel, and have perished with relative peace and great beauty. These stellar ghosts are surrounded by a multicolored shimmering shroud of glimmering gases that were once the now-dead progenitor star's outer layers. Indeed, these multicolored shining gases--called planetary nebulae--are so beautiful that astronomers frequently refer to them as the "butterflies of the Universe". The core of the progenitor star becomes the white dwarf.
Typically, a white dwarf cools down over the passage of a billion years or so, ultimately becoming a still-hypothetical stellar relic called a black dwarf that emits no light or heat at all, and is thus invisible. Even though our Sun is doomed to become one of these strange stellar ghosts when it reaches the end of that long stellar road, astronomers must solve many lingering mysteries that still exist concerning these dense inhabitants of the stellar zoo. In January 2019, astronomers from the University of Warwick in the U.K. announced that they have found the first direct evidence of these dense stellar remnants solidifying into crystals
In about 10 billiion years, our own Sun is destined to become one of these crystals in the sky. Our Star, when it reaches this stage--like all other dead stars of its kind--will contain a crystal core of metallic oxygen and carbon. The oldest white dwarfs are nearly the same age as our ancient Milky Way Galaxy, and they are probably almost entirely composed of crystal. The team of astronomers from the University of Warwick propose that our skies are literally filled with these crystal stars.
Observations have shown that the cores of white dwarf stars are composed of solid oxygen and carbon, as the result of a phase transition that occurs during their life-cycle. This phase transition is similar to the way that water turns into ice--only it occurs at much higher temperatures. This transition could make these stellar ghosts potentially billions of years older than previously believed.
The Ghosts Of Small Dead Stars
A typical white dwarf star is approximately 50% as massive as our Sun, but is only slightly larger than Earth. In fact, a white dwarf star is about 200,000 times as dense as Earth. This makes these stellar ghosts one of the densest collections of matter in the Cosmos, second only to neutron stars. Neutron stars are the relics of massive stars that have gone supernova. A teaspoon full of neutron star material can weigh as much as an ocean liner.
Because white dwarf stars are the remains of "dead" small stars that were originally like our Sun, they lack the ability to create internal radiation pressure--meaning they can no longer create energy by way of the process of nuclear fusion. All stars, regardless of their size and mass, are really immense balls of primarily hydrogen gas. From the time that a star is born, until it finally "dies", it must create internal pressure by fusing increasingly heavier and heavier atomic elements out of lighter ones (stellar nucleosynthesis), in order to counter the powerful and relentless squeeze of its own gravity. Radiation pressure and gravity must maintain a delicate balance throughout a star's entire nuclear-fusing "life", thus keeping the star bouncy. During the battle between these two ancient competitors, pressure pushes everything out and away from the star, while gravity ruthlessly tries to pull everything in towards the star. When a star runs out of its necessary supply of nuclear-fusing fuel, radiation pressure ceases, and gravity is the victor. If the progenitor star is a small star like our Sun, it dies a gentle and beautiful death, with its core enshrouded by a shimmering, glimmering planetary nebula. More massive stars do not go as gentle into that good night, and blow themselves up in a powerful, brilliant, fatal supernova blast--leaving only a neutron star or stellar mass black hole behind to tell its tragic story.
Under normal circumstances, twin electrons--which are defined as those with the same spin--are forbidden to occupy the same energy level. Alas, there are only two ways that an electron can spin, according to what is termed the Pauli Exclusion Principle in physics. In the case of a normal gas, there is no problem since there aren't enough electrons dancing around to fill up all energy levels completely. However, white dwarfs are not normal because their density is much higher, thus pushing all of the dancing electrons much, much closer together. Physicists refer to this state of affairs as a "degenerate" gas. This means that all of the energy levels in its atoms are filled up with electrons. In order for gravity to compress the white dwarf further, it must force the electrons to go where they cannot. Hence, once a star is "degenerate", gravity is unable to compress it any further. This is because, in the weird world described by quantum mechanics, there is no more space available to be taken up. For this reason, the white dwarf survives, but no longer by internal fusion reactions. Instead, the white dwarf survives as the result of quantum mechanical principles that forbid its total collapse.
Degenerate matter has some bizarre properties. For instance, the more massive the white dwarf, the smaller it is. The more mass a white dwarf contains, the more its electrons are forced to squeeze ever more tightly together in order to churn out enough outward pressure to support the additional mass. Alas, there is a limit on the amount of mass a white dwarf can possess. The Indian-American astrophysicist and Nobel Laureate Subramanyan Chandrasekhar (1910-1995) found this limit to be about 1.4 times solar-mass. This is known as the Chandrasekhar Limit.
Things get stranger still. A white dwarf's powerful and relentless surface gravity is about 100,000 times that of Earth. The heavier atoms in the white dwarf's atmosphere sink down, while the lighter ones stay at the surface. Some white dwarfs sport completely hydrogen or helium atmospheres--the two lightest atomic elements. In addition, gravity's relentless pull tugs the atmosphere close around it in a very thin layer. If this strange state of affairs occurred on Earth, the top of the atmosphere would sink beneath the top of the Empire State Building.
Many astrophysicists propose that there may be a crust about 50 kilometers thick beneath the atmosphere of many white dwarf stars. At the bottom of this crust there may be a crystalline lattice composed of carbon and oxygen atoms.
Thousands Of Crystal Stars In Space
The discovery of crystal white dwarf stars, by a team led by Dr. Pier-Emmanuel Tremblay of the University of Warwick's Department of Physics, has been published in the journal Nature. The study is mostly based on observations obtained with the European Space Agency's (ESA's) Gaia satellite.
Because white dwarfs are some of the most ancient stars in the observable Universe, they are extremely useful. This is because their predictable lifecycle makes them excellent cosmic clocks, and these stellar ghosts can provide estimates of the age of groups of neighboring stars to a high degree of accuracy.
Before a Sun-like star perishes to become a white dwarf, it must first balloon in size to become an enormous, swollen, fiery red giant--and the red giant's core becomes the white dwarf. This happens after these gigantic crimson stars have perished and shed their outer layers of gas.
The team of University of Warwick astronomers chose 15,000 white dwarf candidates, located within approximately 300 light-years of Earth, using Gaia satellite observations. The scientists then went on to analyze their newly acquired data concerning the ghostly stars' luminosities and colors.
They then detected an excess in the number of stars at specific colors and luminosities that did not correspond to any single age or mass. When the astronomers compared this to evolutionary models of stars, the excess was seen to coincide with the phase in their stellar development when latent heat is predicted to be emitted in great quantities--thus resulting in a slowing down of their cooling process. It is calculated that in some cases these dense stars have slowed down their aging by as much as 2 billion years. This amounts to 15% of the age of our Milky Way Galaxy.
Dr. Tremblay commented to the press in January 2019 that "This is the first direct evidence that white dwarfs crystallize, or transition from liquid to solid. It was predicted fifty years ago that we should observe a pile-up in the number of white dwarfs at certain luminosities and colors due to crystallization and only now this has been observed."
"All white dwarfs crystallize at some point in their evolution, although more massive white dwarfs go through the process sooner. This means that billions of white dwarfs in our Galaxy have already completed the process and are essentially crystal spheres in the sky. The Sun itself will become a crystal white dwarf in about 10 billion years," he added.
Crystallization is the process that occurs when material changes into a solid state, and its constituent atoms create an ordered structure. Under the extreme pressures existing in the cores of white dwarfs, atoms are pushed so tightly together that their clouds of electrons become unbound. This process leaves a conducting electron gas governed by the laws of quantum physics, and positively charged nuclei in fluid form. When the core eventually cools off to about 10 million degrees, enough energy has been released that the fluid starts to solidify. This creates a metallic core at its very heart with a mantle enhanced in carbon.
"Not only do we have evidence of heat release upon solidification, but considerably more energy release is needed to explain the observations. We believe this is due to the oxygen crystallizing first and then sinking to the core, a process similar to sedimentation on a river bed on Earth. This will push the carbon upwards, and that separation will release gravitational energy," Dr. Tremblay continued to explain to the press.
"We've made a large step forward in getting accurate ages for these cooler white dwarfs and therefore old stars of the Milky Way. Much of the credit for this discovery is down to the Gaia observations. Thanks to the precise measurements that it is capable of, we have understood the interior of white dwarfs in a way that we never expected. Before Gaia we had 100-200 white dwarfs with precise distances and luminosities--and now we have 200,000. This experiment on ultra-dense matter is something that simply cannote be performed in any laboratory on Earth," he added.
Because a diamond is just crystallized carbon, a comparison can readily be made between a cool carbon/oxygen white dwarf and a very big diamond.
The new research is published in the journal Nature under the title Core crystallization and pile-up in the cooling sequence of evolving white dwarfs.