Dance of Death in the Stellar Graveyard

  Professor N C Wickramasinghe

The term “white dwarf” may conjure up images of a creature from the fantasy world of “Lord of The Rings” but sometimes the truth can be even stranger than fiction. For our November lecture we invited Dr Boris Gaesincke, an assistant professor from the University of Warwick, to talk to us about one of the strangest objects in astrophysics.

Dr Gaesincke explained that a white dwarf is a stage in the evolution of a star just like our Sun. Stars are born from the collapse of interstellar gas clouds and when they reach a certain density nuclear fusion is triggered turning the lightest element Hydrogen into Helium. This is the process that goes on in the centre of the Sun today and it is predicted that it will continue for about another 5 billion years. This period of a star’s life when Hydrogen is fused into heavier elements is referred to as the star being on the “Main Sequence”.

However, our nearest star will reach a point when it runs out of the majority of its Hydrogen and will start fusing Helium into Carbon. It then leaves the Main Sequence and enters what is termed its Red Giant phase due to the star expanding, its outer layers cooling and becoming redder in appearance. This new process makes the star inherently unstable and it repeatedly puffs up and throws off its outer layers producing (a rather misleadingly named) planetary nebula.

Eventually the Carbon fuel will be exhausted and the outward pressure provided by the core fusion processes disappears. Therefore the star shrinks and becomes what is known as a white dwarf, only emitting light due to its inherent temperature. It is very dense; having only on average 0.5 to 0.8 solar masses of material squeezed into an object the size of the Earth so that a teaspoonful of material would weigh many tonnes. Any further gravitational collapse is prevented by a fundamental property of matter known as “electron degeneracy pressure”.

Although white dwarfs are relatively bright their small size makes them very difficult to study and Dr Gaesincke struggles to obtain enough time on the world’s largest telescopes in order to record their spectra. However, his team has taken advantage of data from the Sloan Digital Sky Survey which uses a dedicated 2.5m telescope at Apache Point Observatory, New Mexico. This survey has data regarding 12,000 white dwarf stars and the high data collection rate is due to the use of specialized equipment to record many spectra at once.

One of the surprising things that his latest research has uncovered is that some of the white dwarfs’ spectra show an abundance of elements that cannot be explained purely by star evolutionary processes. The heavier elements such as Carbon and Oxygen that a white dwarf produces sink to its core and so should not be apparent in its outer layers but these have been seen in the data. Dr Gaesincke proposes that these come from planets that once orbited the star and due to the expansion and contraction in its Red Giant phase caused the planets to spiral inwards and meet their doom. The other additional factor may be that the star is being viewed through an accretion disk consisting of the rubble resulting from planetary break ups.