White Dwarfs?


No, I have not just got the name of a BBC Sci-fi comedy wrong (Red Dwarf for those of you who are interested), these are they first of three types of compact objects that you can obtain more information about on these pages ...

White dwarfs are the burned out core of a dead star, like the dying embers of a fire, slowly cooling as it fades away to become a black dwarf. White dwarfs are approximately the size of the Earth with an average mass of 0.5 - 0.6 M_sun. This causes it to be about 300,000 times more dense that the Sun, in fact a golf ball size piece of white dwarf would weigh about a ton on the Earth. Most white dwarfs are extremely hot, up to 20,000 degrees, hence the white light they emit. Due to their temperature and small surface area it can take a long time for their un-replenished energy to radiate away. This process is thought to take ~ 25 billion years meaning that the Universe has not lasted long enough for this to occur. This means that these objects can be used as `cosmic clocks', giving an estimate to the age of the Universe independently of other techniques.

The first white dwarf discovered was found (mainly) because its companion star, Sirius, is one of the brightest stars in our sky. In 1844 the astronomer Fredrick Bessel noticed a slight back and forth motion of the star but it was not until 1863 that the object causing the wobble was discovered by Alvin Grahem who labelled it a {\it dark companion} that appeared ten thousand times fainter than Sirius.

This faintness causes problems in finding more of these objects, for instance until just over a decade ago ground based and space based optical telescopes were only able to reveal a handful of WDs. That was until a study was carried out in August 1995 by an observing team led by Harvey Richer at the University of British Columbia. They used Hubble data from the Wide Field and Planetary camera to make observations of the M4 globular cluster, located 7,000 light years away (the nearest globular cluster to the Earth). After only a few hours of observation time they were able to detect more than 75 WDs in just a small area of this old cluster. One such observation is shown in the figure below. The left hand image shows a view of the globular cluster containing more than 100,000 stars. The right hand image contains a Hubble colour image of a small part of the cluster revealing 8 of these objects (each circled) (see Hubble press release 1995).

If you would like to know more about observing these and other types of compact objects, please click here.

Credit: Harvey Richer (University of British Columbia, Vancouver, Canada) and NASA