Doing Physics in the Cores of Globular Star Clusters

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Stars up to about eight times the mass of our Sun end their lives as white dwarf stars (WDs). These stars have completed their nuclear evolution, converting their core of hydrogen into helium and then into carbon and oxygen. The star then contracts until the pressure of its electrons (the quantum mechanical electron degeneracy pressure) becomes high enough for the star to reach a balance between gravity (that wants to contract it) and pressure (that wants to expand it). The end point of this process is a star about half the mass of the Sun and about the radius of the Earth. The WD has depleted its potential source of nuclear fuel by this time, so it simply radiates its stored thermal energy out into space and cools slowly and predictably with time - WDS are good clocks. This provides a physical laboratory with conditions that cannot be reproduced on Earth; temperature about 100 million degrees, density of order of 1 million times that of water and a precise clock to boot. With this almost unique laboratory, it is possible to carry out exotic physical experiments. The rate of cooling of WDs Over most of its life, a WD cools by the emission of photons, but this breaks down for the case of very young and very hot WDs. Above surface temperatures of about 30,000K the dominant source of cooling is the production of neutrinos in the core of the star. These neutrinos are of very low energy and hard to produce and detect in particle accelerators. Using the Hubble Space Telescope (HST) to secure the largest samples of hot WDs ever obtained, we are testing WD cooling models and inferentially examining whether current neutrino theory is correct. Diffusion of WDs in star clusters WDs produced in star clusters are concentrated towards the cluster core as they evolve from its most massive stars. During this evolutionary process, they lose about half their mass, become too low in mass to be so centrally concentrated and thus begin to diffuse slowly outwards. Because we know the age of the star from its cooling time, we can establish the diffusion constant due to gravitational interactions in the cluster core - the first time that this has been accomplished. Determination of the masses of stars that produce supernovae. The lower mass limit of stars that explode as supernovae is a critical input into galaxy evolution models. These stars produce specific heavy elements that are eventually incorporated into later generations of stars and planets. We have developed a unique way of determining this by searching for the most luminous WDs in clusters in a nearby galaxy. Planets around WDs We have been searching for evidence of planetary systems around WDs in ancient star clusters. A positive result here could potentially herald an early rise to life in the universe. We have been using the HST (and will use its replacement JWST after its launch in 2018) discovering thousands of hot WDs in ancient star clusters and exploiting them to carry out a number of these experiments.**