UBC astronomers Ryan Goldsbury, Jeremy Heyl, and Harvey Richer are studying the cooling rate of white dwarfs by looking for them in an old star cluster within our own galaxy.

White dwarfs are the dense and hot remnant cores of stars that have used up all of their fuel and lost their outer layers during the red giant phase of their evolution. When these objects form, their surface temperatures are over 100,000 degrees.

From this point onward they no longer produce their own energy through nuclear fusion, and so they radiate their limited supply of heat into space and slowly cool over billions of years.

Nature has thus provided us with a fantastic clock: the cooler a white dwarf, the older it is. However, while current models do a very good job of predicting the cooling rate of older white dwarfs, these models do not correctly reproduce the cooling rate seen in the younger white dwarfs studied.

By studying white dwarfs in a globular cluster, Goldsbury et al. have been able to determine the distance to the cluster and the mass distribution of the stars from which they formed.

Because all of the white dwarfs in their study come from a single well studied star cluster, both of these bits of information can be independently determined.

Their findings indicate that white dwarfs cool more slowly in the early stages of their life than models have previously predicted.

The cause of this difference is not yet understood, but it is clear that there is a discrepancy between the data and the models.

"There are a number of important factors that contribute to the cooling rate of white dwarfs at high temperatures," explains Goldsbury.

"These include high-energy particle production, core composition, the composition of the stellar atmosphere, and the processes by which energy diffuses from the core outward. To pinpoint exactly where the difference between our theories and observations comes about, we really need more data."

To address this, the team is currently observing the same star cluster with the Hubble Space Telescope. This new program focuses on the hottest and youngest white dwarfs in the dense central region of the star cluster.

These new observations will eventually yield data on over 5,000 white dwarfs. With this data set, the team will be able to investigate explanations for the anomalous early cooling rate.

Reference: "An Empirical Measure of the Rate of White Dwarf Cooling in 47 Tucanae," R. Goldsbury et al. 2012, ApJ, 760, 78.