How big is Sirius B?

Sirius B is a white dwarf, the small, dense leftover core of a star that has used up its nuclear fuel. It is about 11,300 kilometers (7,020 miles) across, a little smaller than Earth, yet it has about the same mass as the Sun. Sirius B orbits Sirius A, the brightest star in the night sky.

Size

Astronomers work out Sirius B's size rather than measure its width directly. They combine the amount of light it gives off, its temperature and its distance. In 2017, a team led by Howard Bond used the Hubble Space Telescope to split its light into colors, and found a radius of 0.008098 times the Sun's. That makes Sirius B about 11,270 kilometers across.

A 2018 study by many of the same astronomers found a radius about 1% smaller. The result also depends on the distance to Sirius, so even a small change in the measured distance changes it. The mass comes from the orbit of the two stars: Sirius B has 1.018 times the mass of the Sun.

Size comparisons

Earth is about 13% wider than Sirius B, but Sirius B has about 340,000 times as much mass.
Venus, the next larger object in the explorer, is 12,104 kilometers (7,521 miles) across, about 7% wider than Sirius B.
The Sun is about 120 times as wide as Sirius B, yet the two have almost the same mass. On average, Sirius B's matter is packed about 1.9 million times as tightly as the Sun's.
Sirius A is about 2.4 million kilometers (1.5 million miles) across, about 210 times as wide as its companion.
A cube of Sirius B's average material 1 centimeter (0.4 inches) on each side would weigh about 2.7 tonnes (6,000 pounds).

What a white dwarf is

A white dwarf is what a star like the Sun becomes after it runs out of nuclear fuel. Near the end of its life, the star throws off its outer layers, and only its hot core is left. The core no longer makes energy, so it slowly cools. Gravity squeezes it until its electrons are packed as tightly as physics allows, and that is what stops it shrinking further.

A more massive white dwarf is smaller, because its stronger gravity squeezes it harder. No white dwarf can have more than about 1.4 times the Sun's mass. Sirius B is one of the most massive white dwarfs known. Astronomers estimate that it began as a star about five times as massive as the Sun, and became a white dwarf about 126 million years ago.

A companion hidden in the glare

Sirius is about 8.6 light-years away, where a light-year is the distance light travels in one year. In 1844 the astronomer Friedrich Bessel noticed that Sirius wobbled as it moved across the sky. He concluded that an unseen companion was pulling on it. Alvan G. Clark and his father first saw Sirius B through a telescope in 1862.

The two stars circle each other every 50.1 years in a long, oval orbit. The distance between them ranges from about 8 to 31.5 times the distance from Earth to the Sun. Sirius B is about 10,000 times fainter than Sirius A, so it is hard to study from the ground. As of 2026, it is the nearest white dwarf known.