Rights: © Copyright 2009. University of Waikato. All Rights Reserved. Published 1 April 2009 Download

Professor Denis Sullivan, from Victoria University of Wellington, describes how most stars become red giants and then white dwarfs. Our Sun is following this pathway.


White Dwarfs - a lovely colourful name. White because they are very hot relative to most stars. Dwarf because they are quite small. White Dwarf is the mass of the sun squeezed into something the size of the Earth. Now that’s an incredible increase in density, the amount of material. Something like a million times the density of water – this is very different material to what we find in the sun.

So, we take a typical star like our sun. What we expect to happen is that when it’s finished burning hydrogen in the centre, the centre will collapse and squeeze into a much denser environment where it will burn helium, and when I say burn this is nuclear processes, and the outside region will expand. That is called a Red Giant. The size of the Red Giant will come out to roughly the Earth. But don’t worry because it is not predicted to happen for something like 5 billion years. So when it is in the Red Giant stage the centre will be very condensed and very hot burning helium.

And when that helium is finished, what happens is it’s left with this very, very compact core and the outer regions tend to get blasted away so the core is the White Dwarf. Something like 98% of all the stars we see will end their life as a cooling White Dwarf. As it loses energy the temperature reduces, it gets fainter and fainter and you end up with a Black Dwarf, which we can’t see because there is no energy coming out of it. But it’s static – it’s held up by the electrons - and it’s just destined to remain like that for ever more. And, interestingly, you can use that to measure how old is our galaxy. You look for the coolest White Dwarfs, you understand how the White Dwarfs cool and then you go back in time and say, “When did they start?” So that’s a measure of how old the galaxy is. We don’t know, we think there are no Black Dwarfs because the cooler they are, the slower they cool, so no White Dwarf has actually become a Black Dwarf yet, but there should be a lot on their way.

Chris Meaney (HTSI)/NASA
NASA, ESA, and K. Noll (STScI)
NASA/JPL-Caltech/ESA/J. Hora (Harvard-Smithsonian CfA), C.R. O'Dell (Vanderbilt University)