Simon Joyce

2016 AFPGR Participant


White dwarfs in Sirius-like binary systems
About Simon

Simon is a 2nd year PhD student in the Department of Physics and Astronomy. His supervisors are Prof Martin Barstow and Dr Sarah Casewell.  After completing an MSc at the University of Warwick, Simon spent a year in Spain working for the European Space Agency in the young graduate trainee program which gives science and engineering graduates the opportunity to gain practical experience in the space industry before continuing their studies. Simon started his PhD in 2014 working on white dwarf stars.

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About My Research

White dwarfs provide a fascinating glimpse in to the future of the universe. Of all the stars in the sky, around 97% will end their life as a white dwarf star. The final fate of the Sun is to become a red giant and then collapse into a very hot and dense white dwarf which will be about the size of the Earth and so dense that even a teaspoon of its material would weigh tonnes. This is the material from which the mythical hammer of Thor is supposed to be made, which explains why only he can wield it.

From a more scientific point of view, white dwarfs are interesting in several ways. When a star dies, some of its material is returned to the galaxy where it goes on to form new stars and planets. This chemical enrichment is what made life on Earth possible since the Earth is made up of this material.  White dwarfs also allow us to observe material which is in an environment of extreme temperature and pressure. Material in this environment does not behave in the way we normally expect and it exhibits some of the stranger effects which can only be explained by quantum physics. For example adding more mass to a white dwarf would make it shrink rather than get larger.

My Research Findings

My research is focused on investigating this relationship between the mass and the size of a white dwarf. It has long been accepted that there should be a relationship between the size and mass of white dwarfs. However, it has been difficult to test this theory because white dwarfs are so faint and difficult to observe. Using observations from the Hubble Space Telescope, we have been able to observe white dwarfs in a way that they can't be seen from Earth. We are using these observations to make the most accurate measurements ever of the mass and radius of white dwarfs so we can finally test the mass-radius relationship. 

The first star I have looked at is one of the most famous. Sirius is the brightest star in the sky and can be seen low on the horizon during winter. There is a tiny white dwarf in orbit around this star, although it can't be seen without a telescope. This HST image shows the white dwarf as a tiny dot at the bottom left. (Image credit : Barstow 2005 taken with HST).

The gravitational pull of the white dwarf is so strong that any light escaping from it becomes stretched to longer wavelengths. We can measure how much the light has been stretched by observing absorption lines in the spectrum. These provide a kind of marker and are expected to be seen at particular wavelengths. If we compare the wavelength where the line occurs to the wavelength we would expect it to be at based on lab experiments, we can get an idea of how much the light has been stretched by the gravity of the white dwarf. This gives us a way to measure its mass.

Unfortunately, the strong gravity of the white dwarf is not the only thing that can cause this wavelength shift. It can also be caused by the movement of the star towards or away from us, the motion of the Hubble space telescope and the motion of the Earth around the Sun. It is like listening to someone playing a trumpet as they drive past you in a car. Its difficult distinguish how much the change in pitch was due to the motion of the car and how much it was due to the trumpet player playing a bad note.

My research is to continue to measure all these effects so that they can be corrected for. This will allow us to make the most accurate measurement yet of the mass of Sirius b. I am also studying the spectra of several other white dwarfs so that we can test the mass radius relationship over the full range of of white dwarf masses.

Simon Joyce_Celestial image

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