Sirius B

Hiding in the glare of the brightest star in our sky is a dead star's core, squeezed until a whole sun fits into something Earth's size — one spoonful of it has as much mass as a car!

🚀 Travel there

What it is

Crushed leftover core of a dead star, the first white dwarf ever discovered.

White dwarf: The bare core a Sun-like star leaves behind, still glowing. Every kind of thing there is →

Can you see it?

No. No telescope on Earth will show you this one; it is here to be understood, not seen. It is here because it is worth knowing about — and because leaving it out would make the sky look emptier than it is.

Worth knowing about Sirius B

In weight, about the same mass as the Sun, despite being crushed to the size of Earth. On that same scale model, Sirius B sits right next to Sirius A — less than a centimeter from the Sun. Light takes 8.6 Earth years to get here from Sirius B, so what you are looking at is Sirius B as it was 8.6 Earth years ago.

How bright it is: Much dimmer than the Sun even though it's very hot, simply because it's so small.
What it is made of: The crushed core of a dead star, about the Sun's mass squeezed into an Earth-sized ball; glows white-hot from leftover heat.
What makes it worth knowing: First white dwarf ever discovered; its density puzzled astronomers until quantum physics explained it.

How did anyone measure the Earth in the first place?

The numbers