Ultraviolet studies of white dwarf stars and their environments.

Tuesday 17th November, 2015

Our speaker was Prof Martin Barstow from Leicester University who is also currently the President of the Royal Astronomical Society and he came to talk to us about "Ultraviolet studies of white dwarf stars and their environments".

Prof Barstow began by explaining why in the fields of astronomy and astrophysics the study of white dwarfs was important. He said that they are the oldest objects in our galaxy and around 90 percent of stars are actually white dwarfs. Investigating the physics of white dwarfs also allows scientists to study matter under extreme conditions of pressure, density and temperature that cannot be recreated in the laboratories on Earth.

A white dwarf is the remains of a star just like our own Sun when the nuclear fusion reactions cease at its core. It still appears white but its luminosity is due to stored thermal energy being radiated away into space as it cools. It will still have the mass of something like our Sun but it will have shrunk in size to that of the Earth. As it cools its colour changes from white to red until it is predicted to become black. This process takes so long, though, that no cold black dwarfs will yet exist as it takes longer than the age of the Universe for them to reach this stage.

The first white dwarf, 40 Eridani B, was discovered by William Herschel in 1783 followed by the nearest white dwarf to Earth, Sirius B. This companion to Sirius, which itself is the brightest star in the Earth's night sky with a magnitude of -1.46, was actually predicted to exist before it was observed when accurate positional measurements showed that Sirius was periodically moving. In 1862 Alan G Clark managed to observe Sirius B which is 10,000 times dimmer shining at a mere magnitude 8.

Due to studies of their spectra we now know of six types of white dwarf depending on what elements dominate their surfaces. Three of these varieties have almost pure hydrogen or helium surfaces unlike others that have exposed cores of carbon and oxygen. All of these types have a designation that begins with the letter "d" which stands for "degenerate". They have been labelled in this way as white dwarfs are able to maintain their size due to electron degeneracy pressure preventing their gravitational collapse. This degeneracy pressure relates to the world of quantum theory where particles such as electrons cannot simultaneously be in the same quantum state. In effect the matter cannot be compressed anymore and the white dwarf reaches a stable state that can only be disturbed by the addition of more material.

As most stars form in pairs the infall of material sometimes occurs when a white dwarf's companion expands as it evolves. The companion becomes what is known as a "red giant" and expels its outer layers. This extra material can build up until the white dwarf's mass exceeds 1.44 times that of the Sun. When this "Chandrasekhar Limit" is breached the electron degeneracy pressure is no longer enough to combat the gravitational forces and the white dwarf collapses on itself and runaway nuclear reactions lead to it violently disintegrating. The resulting explosion is known as a Type 1a supernova and is used to measure out to distances that are a significant fraction of the radius of the known Universe.