When stars with masses similar to the Sun exhaust their hydrogen fuel, fusion ceases, causing them to exit the main sequence and transform into red giants. During this transition, the star’s loss of mass weakens its self-gravity, resulting in the shedding of outer layers into space and the formation of stunning planetary nebulae visible through telescopes. However, these nebulae are relatively short-lived, lasting only around 10,000 years.
According to theoretical predictions, a white dwarf can persist for an astonishing 100 quintillion years. These objects cool down gradually, and during this cooling process, their cores can crystallize, turning into solid structures. The crystallization of the core leads to an increase in the star’s heat release, which serves as a signature indicating the occurrence of crystallization. This heat signature establishes distinct sequences that white dwarfs follow on the Hertzsprung-Russell Diagram. Despite this theoretical understanding, precise temperature and age measurements of white dwarfs have been lacking, hindering the detection of crystallization phenomena.
The Hertzsprung-Russell Diagram plots stars’ luminosity against their temperature. In 2018, the European Space Agency’s Gaia spacecraft released its second dataset, offering more comprehensive information on white dwarfs and their placement on the HR Diagram. Gaia’s capability to provide precise parallax measurements for white dwarfs, which had been historically challenging to obtain, was a significant breakthrough. The updated data revealed three distinct evolutionary tracks for white dwarfs. While two of these tracks were well-understood and parallel to each other, the third track was new and did not align with any known white dwarf evolution paths.
This discovery triggered an “Ah-hah!” moment among researchers.
However, Gaia’s data does not pertain to individual observable stars but rather represents mass data derived from over one billion light sources. Nevertheless, scientists were able to extract approximately 260,000 probable white dwarfs from this vast dataset, exponentially expanding the known population of white dwarfs. This abundance of data enabled researchers to gain insights into the crystallization process.
Despite this progress, there remained a dearth of information regarding the timing of crystallization. Another group of researchers aimed to address this gap and fortuitously found a white dwarf undergoing crystallization, offering an opportunity to determine both its age and the crystallization process.
Their findings are published in a new paper titled “A Crystallizing White Dwarf in a Sirius-Like Quadruple System” in the Monthly Notices of the Royal Astronomical Society. The lead author, Alexander Venner, is a Ph.D. student at the Center for Astrophysics at the University of Southern Queensland. The paper is available on the pre-print server arxiv.org.
The significance of this research lies in the fact that the crystallizing white dwarf is part of a system containing other main-sequence stars. Determining the ages of main-sequence stars is relatively easier compared to white dwarfs, which indirectly allows for more accurate dating of the white dwarf and the crystallization stage.
According to the paper, the newly identified white dwarf, situated approximately 104 light-years away, is composed mostly of metallic oxygen and has three stellar companions. It shares similarities with another nearby white dwarf called Sirius B. The researchers focused on measuring the star’s cooling process and the delay caused by crystallization.
The team employed various methods to constrain the system’s age, aiming to detect a cooling delay caused by crystallization. However, the age estimate remains imprecise, and further efforts are needed to refine it. Nonetheless, the researchers managed to pinpoint the crystallization occurrence by determining the white dwarf’s mass and temperature, placing it within the expected parameter space for white dwarfs undergoing core crystallization. This discovery makes it the first confirmed crystallizing white dwarf in a Sirius-like system, and its location within such a system is crucial for age investigations.
Not all white dwarfs experience crystallization in their future. Many of them are part of binary systems and can undergo a Type Ia supernova when they accrete material from a companion star, surpassing the Chandrasekhar limit and resulting in a cataclysmic explosion.
The proximity of the system studied in this research, only 32 parsecs away, suggests the likelihood of more similar systems. These discoveries are opening up new avenues for studying white dwarf crystallization, as systems with both white dwarfs and main-sequence stars are likely abundant. Future findings of these systems will enable stronger tests of white dwarf crystallization models.
White dwarfs are undeniably extraordinary objects. After enduring billions of years of fusion, they undergo a remarkable transformation from blazing plasma spheres to degenerate carbon masses that eventually crystallize into diamonds, persisting for unimaginable lengths of time.
Complete crystallization of a white dwarf takes quadrillions of years, surpassing the age of the Universe itself, which is less than 14 billion years. Consequently, astronomers will never observe a fully crystallized white dwarf. Nevertheless, the ongoing research is unraveling some of the mysteries surrounding these cosmic diamonds by studying white dwarfs at the initial stages of crystallization. The continued exploration of these bizarre stellar remnants holds the promise of revealing the intricate details of this intriguing process.
