A picture of Sirius A and Sirius B taken by the Hubble Space Telescope. Sirius B, which is a white dwarf, can be seen as a faint point of light to the lower left of the much brighter Sirius A.
A white dwarf is what’s left after a star like our Sun runs out of energy.
Stars shine because they are burning fuel deep inside—kind of like how a car runs on petrol. But stars don’t last forever. After shining for billions of years, they begin to run out of fuel.
When that happens, the star swells up and becomes a big, puffy red giant. It then blows off its outer layers into space, like peeling an onion. What’s left behind is the very hot, dense core of the star. That leftover core is called a white dwarf.
Here are some cool facts about white dwarfs
• They’re small – about the size of the Earth, but way heavier. Just one spoonful would weigh as much as a car!
• They’re hot – at first, they shine very brightly because they are super hot.
• They cool slowly – over billions of years, they cool down and eventually stop glowing.
White dwarfs don’t burn fuel anymore—they just glow from leftover heat, like an oven that’s been turned off but is still warm.
One day, our own Sun will become a white dwarf too—but not for another 5 billion years.
