Precisely calculating the age of stars: Key to evolution of a type of white dwarf found

A team of scientists from the Universitat Politècnica de Catalunya (UPC Barcelona Tech), the Catalan Institute for Space Studies, the Institute of Space Sciences of the Spanish National Research Council (CSIC), the National University of La Plata (Argentina), and Liverpool John Moores University (UK), led by researcher Enrique García-Berro of the UPC's Department of Applied Physics, has demonstrated that the white dwarf stars in the NGC 6791 star cluster are 8 billion years old, not 6 billion as previously believed. The research opens up new opportunities for extending our knowledge of the origin of the universe.

The results will be published in the scientific journal Nature on May 13.

The researchers calculated the evolution of the white dwarfs from their birth to the present. Their calculations provide experimental confirmation of theories that have been proposed, but which up until now have not been corroborated by observational evidence. Specifically, the researchers have shown that sedimentation of the heaviest elements (under the strong gravity of the stars) and crystallization of material (due to the enormous pressure) take place in the interior of white dwarfs. These physical processes release energy inside the white dwarfs and slow their evolution. If they are properly taken into account, the age of stars of this type can be calculated with precision.

For years scientists have used the age of white dwarfs to estimate the age of the galaxy and other star systems. Their estimates of the age of these stars were based on theoretical considerations, but the level of uncertainty was very high because the occurrence of sedimentation and crystallization in the interior of white dwarfs could not be demonstrated. Up until now the proposed theories had not been independently verified based on observational data because in Earth-bound laboratories it is impossible to achieve the extremely high densities and temperatures that exist inside the stars (i.e., pressures of millions of grams per cubic centimeter and temperatures of millions of degrees). The calculations of this group of researchers were found to coincide with measurements of the age of NGC 6791 based on images taken by the Hubble Space Telescope.

White dwarfs are the most abundant stars in the universe. They are also very dense-similar to the Sun in mass but with a radius comparable to that of the Earth. In fact they are stellar remnants, the compact remains of stars that have reached their final evolutionary state, formed when stars exhaust their nuclear fuel. They emit stored thermal energy and are therefore generally stars of very low luminosity.

Most white dwarfs have cores composed of carbon and oxygen, though they have a surface layer of hydrogen and helium. When they form, white dwarfs are very hot and bright, but because they have no source of energy other than stored thermal energy, they gradually cool and become less bright until they reach a point at which they cease to radiate. White dwarfs, however, can have a lifespan of billions of years. Up until now most calculations indicated that the white dwarfs in the NGC 6791 star cluster were 6 billion years old, but the new research has shown that they were actually born 8 billion years ago.

This hypothesis was demonstrated by simulating the entire evolutionary process of the white dwarfs in a way that includes two physical processes that take place in the core of these stars but have not previously been taken into account: the effect of neon sedimentation, and phase separation of carbon and oxygen during crystallization, which occurs at lower temperatures.

During these two evolutionary stages, the stars release gravitational energy and cooling slows down. The faintest white dwarfs in the cluster are also the reddest and the coolest, so if scientists have good models for cooling, they can calculate the age of the cluster. Accordingly, the scientists measured the color and brightness of all the white dwarfs in the cluster and verified that in the faintest white dwarfs in the cluster the effects of these two physical processes slow down the cooling of the stars, such that the age of the cluster and that of its white dwarfs coincide.

Reliable chronometers

The discovery has important scientific implications because it confirms that white dwarfs can be used as independent, reliable chronometers to determine the age of many star systems and thus contribute to advancing our knowledge of the universe. The knowledge gained can also be applied in other fields such as dense plasma physics.