Sirius B is the white dwarf companion of Sirius, about the size of the Earth yet with roughly the mass of the Sun, the dense core of a star that died.
At a glance
| type | white dwarf |
|---|---|
| distance from the Sun | 8.6 light-years (2.64 pc) |
| its light left it | 8.6 years ago |
| luminosity | 0.056 × the Sun |
| radius | 0.008 × the Sun (5,570 km) |
| surface temperature | 25,200 K |
Read off the piece’s own catalogue, the same values it draws with.
About Sirius B
Sirius B is a white dwarf: what is left when a star has used up its fuel, shed its outer layers and left its core to cool. It packs nearly the Sun’s mass into a ball about the size of the Earth, so dense that a teaspoon of its matter would outweigh a car. Its surface is far hotter than the Sun’s, but it is so small that it gives out little light.
It was predicted before it was seen. Friedrich Bessel inferred an unseen companion from the wobble of Sirius, and Alvan Graham Clark first saw it while testing a new telescope lens. When its spectrum showed a hot star that was nevertheless faint, the only explanation was an object of impossible-seeming density, and white dwarfs became a new kind of star.
Its gravity is strong enough that light climbing out of it is measurably stretched to redder wavelengths, one of the effects predicted by general relativity. The star that made it was once the more massive of the pair, which is why it has already died while Sirius A shines on.
What you see in the piece
Fly to Sirius and B appears as a tiny, intensely blue-white point beside its bright companion, visible only once you are close enough to separate it from the glare.
Questions
How big is Sirius B?
Its radius is 0.008 × the Sun (5,570 km), about the size of the Earth, though its mass is close to the Sun’s.
Why can’t you see Sirius B?
Its luminosity is only 0.056 × the Sun, and it sits right beside Sirius, the brightest star in the sky, so it is lost in the glare.