The Sun and white dwarfs
The life of stars
Professor Denis Sullivan at Victoria University of Wellington studies white dwarfs. These are some of the oldest stars in the galaxy – not characters from Lord of the Rings! To Denis, white dwarfs are laboratories for studying the evolution of stars and tell us what might happen to our own Sun in the future.
Becoming a white dwarf
Inside main sequence stars like our Sun, nuclear fusion converts hydrogen into helium. This releases huge amounts of energy. The light and heat that reaches Earth is a result of this process in our Sun. Most stars will eventually run out of hydrogen fuel – they will expand to become red giants, then shrink and cool down into white dwarfs. These stars contain a similar mass to the Sun, but squashed into the size of our Earth. A white dwarf is so dense that a teaspoonful of it on Earth would weigh as much as a car.
Studying white dwarfs
Radiation escaping from some white dwarfs makes them pulsate or quiver – a bit like an earthquake makes the surface of the Earth shake. Someone who studies the structure of the Earth using earthquakes is called a seismologist. Someone like Denis, who studies the inside of white dwarfs using star quakes, is called an asteroseismologist.
White dwarfs are small and cool, so they are faint and hard to find. Once a white dwarf has been found, very sensitive photometers attached to telescopes are used to detect very small changes in the light coming from it. Denis measures these short pulses of light from star quakes to get a picture of a white dwarf cooling down.
Denis can only measure what is happening on the surface of a white dwarf – he can’t see inside – so he uses his data to build a computer model. A possible structure of a star is put into the model and made to pulsate. The variables making up the inside of the model star are then changed until the observed pulsations match those measured from a real star. In this way, it has been possible to put together a picture of what is inside a white dwarf – a dense core made of carbon and oxygen, with a very thin envelope of hydrogen, or occasionally helium.
Nature of Science
Scientists can only collect data from the outside of stars, so they create computer models of what the insides might be like. They use their knowledge of physics and chemistry to change the variables of the model until they match their actual observations.
The story of our Sun
The study of stars that have become white dwarfs has helped Denis learn about the history and future of our own Sun. Like all stars, our Sun will go through several stages in its life. At the moment, it is only a middle-aged star, but like most stars, it will eventually become a white dwarf.
Our hot Sun has spent the last 5 billion years turning hydrogen into helium. Another 5 billion years into the future, the hydrogen will be running out. Gravity will cause the inside of the star, which will then be mostly helium, to shrink and get even hotter, while the outside layer will expand and cool down. Our Sun will have become a red giant, big enough to swallow up where the Earth is now.
As the core of the red giant gets hotter, the helium becomes the new fuel, building up heavier atoms of oxygen and carbon. The outer layers will be lost into space as a planetary nebula, leaving the core so densely packed together that gravity will be about a million times that of Earth.
The Sun, cooling down, with most of its fuel used up, will have become a white dwarf. Eventually, it will even stop shining.
This video 'Classroom Demonstrations: Colour and Temperature of Stars' uses a light bulb to explain the relationship between colour and temperature in stars.