What makes astronomers think black dwarf stars exist?
“Like a cooking pot that’s been taken off the hob, all it can do is cool”
Matt Caplan
When a star like our Sun exhausts the fuel in its centre, the core will contract, reaching extreme temperatures of millions of degrees. The star swells into a red giant, but not for long. The heat of the core is so great that it overcomes gravity, lifting off the outer layers of the star in massive stellar winds and exposing the glowing ember of the core: a white dwarf.
The exposed core of a star is quite hot, but since they’ve run out of fuel they can’t stay that way forever. Like a cooking pot that’s been taken off the hob, all it can do is cool. White dwarfs fade over billions of years as the heat leaks out, carried away by starlight, first turning a dim red colour until eventually vanishing from view entirely. Now dark and frozen, it is said to be a ‘black dwarf’. But the time it takes to cool this cold is far longer than the universe has existed – even the oldest, coldest white dwarfs in our galaxy are 5,500 degrees Celsius (10,000 degrees Fahrenheit) and will take billions more years to turn black. While we don’t have any black dwarfs
yet, their time will inevitably come.