The other week, I explained how a dying star balloons into a Red Giant when it runs out of fuel. The next stage in this evolution is a White Dwarf.
A Red Giant will eventually blow off the material contained in its outer layers. These expelled outer layers may go on to form planets (more on that later).
All that's left behind is a small and incredibly dense and hot core.
This is a White Dwarf.
Sirius B (pictured above) is a White Dwarf. Its mass is 98 percent of our own Sun but it is only 12,000 kilometres in diameter, making it smaller than even the Earth and much denser.
Because they are small and incredibly dense, the gravity on the surface of a typical White Dwarf is around 350,000 times stronger than the gravity on Earth.
No hydrogen fusion occurs to counteract the force of gravity. So, all the matter gets squashed together until all the electrons are pushed far further together than for normal matter.
When all the electrons are squashed together, the matter is now known as a "degenerate" gas. It can't be squashed any further as quantum mechanics dictates there is no more available space for the electrons.
Do all stars become white dwarfs?
No. Smaller stars, such as Red Dwarfs, don't reach the Red Giant stage and just burn through all their hydrogen. They should then evolve into White Dwarfs - but Red Dwarfs take trillions of years to consume their hydrogen fuel (which is longer than the age of the universe) so none have gone through this transformation yet.
Massive stars, which are about eight times the mass of our Sun, will never be white dwarfs. Instead, they explode in a violent supernova and either leave behind a black hole or a neutron star. We'll look into the massive supernova explosions that form neutron stars next week...
What happens when White Dwarfs die?
Many White Dwarfs will fade and eventually become Black Dwarfs when all their energy is radiated away.
However, if a White Dwarf is part of a binary system (where two stars orbit around one another) then its gravitational pull is so large that it may start to pull material off its companion star. Adding more mass to the White Dwarf in this way may cause it to become a neutron star or cause a supernova explosion.
If it only pulls off a small amount of matter from the companion star, a smaller explosion called a "nova" may occur. This process can be repeated several times caused a small cosmic fireworks display.
But, if you have two White Dwarfs in a binary system then they may merge together and, again, cause a supernova explosion.
Extra reading and watching
The first exoplanet to be discovered is believed to have been orbiting around a White Dwarf waaaaaay back in 1917. While White Dwarf orbiting exoplanets are highly unlikely to support life, they do provide us with a wealth of information on how to analyse potentially life-harbouring exoplanets.
White Dwarf is also a rather good magazine from British games manufacturer Games Workshop.
And here are five fascinating facts about White Dwarfs and a jolly good video about them:
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