Can you see Sirius B with a telescope?

It is possible but difficult. Sirius B is about 10,000 times fainter than Sirius A and sits very close to it, so you need a good telescope, steady air and a time when the two stars are near their widest separation.

How dense is Sirius B?

Its average density is about 2.4 tonnes per cubic centimetre, more than a million times that of water. A teaspoonful of its material would weigh around ten tonnes on Earth.

Will the Sun become a white dwarf?

Yes. After a red giant phase that begins in about 5 billion years, the Sun will shed its outer layers and leave behind an Earth-sized white dwarf with a little over half its current mass.

Who discovered Sirius B?

Friedrich Bessel predicted it in 1844 from the wobble in Sirius's motion. Alvan Graham Clark first saw it in 1862 while testing a new telescope lens.

What is the closest white dwarf to Earth?

Sirius B, at 8.6 light-years, is the nearest known white dwarf. The next closest is Procyon B, about 11.5 light-years away.

The Dog Star's tiny, dense and dazzling companion

Sirius B

White dwarf · 8.6 ly

Sirius B is the faint companion of Sirius, the brightest star in the night sky, 8.6 light-years from Earth. It is a white dwarf: the exposed, burnt-out core of a star that has finished fusing hydrogen. Although it is only a little smaller than Earth, it contains about as much mass as the Sun, making it one of the densest kinds of object known.

Sirius B was the first white dwarf whose extraordinary nature was recognised, and its story runs from a wobble spotted in the 1840s to precise measurements by the Hubble Space Telescope. It also offers a preview of the distant future, because one day the Sun will end its life as a white dwarf too.

Key facts

Distance: 8.6 light-years
Mass: ≈ 1.02 solar masses
Diameter: ≈ 11,700 kmSlightly smaller than Earth's 12,742 km
Surface temperature: ≈ 25,000 K
Mean density: ≈ 2.4 tonnes per cm³
Surface gravity: ≈ 400,000 × Earth's
Orbital period: ≈ 50 yearsAround Sirius A, at an average of about 20 AU
Apparent magnitude: ≈ 8.4Lost in the glare of Sirius A (−1.46)
Discovered: 1862By Alvan Graham Clark; predicted by Bessel in 1844

Predicted before it was seen

In 1844 the German astronomer Friedrich Bessel noticed that Sirius was not moving across the sky in a straight line. Instead it weaved slightly from side to side over a period of about 50 years. Bessel concluded that Sirius must be orbiting an unseen companion, and he predicted that the companion's gravity was strong enough to pull such a bright star off course.

The companion was finally spotted on 31 January 1862 by Alvan Graham Clark, son of the American telescope maker Alvan Clark, while testing a new 18.5-inch lens on Sirius. He saw a faint point of light close to the dazzling star, almost exactly where Bessel's calculations said it should be. It was an early triumph of predicting an object from gravity alone, a technique later used to find planets around other stars.

A star that should not exist

Astronomers soon worked out from the orbit that Sirius B had roughly the mass of the Sun, yet it was about 10,000 times fainter than Sirius A. At first they assumed it must be cool and dim. In 1915 Walter Adams obtained its spectrum and found that it was in fact white-hot. A hot star that gives off so little light must be extremely small, which meant its mass was crammed into a volume similar to Earth's.

The implied density, millions of times that of water, seemed absurd at the time. It made sense only after the development of quantum mechanics in the 1920s, when physicists realised that matter could be squeezed into a new state in which electrons are packed as tightly as the laws of physics allow. Sirius B became the classic example of this degenerate matter.

Weighing Sirius B with Hubble

Einstein's general relativity predicts that light climbing out of a strong gravitational field loses energy and is shifted to redder wavelengths. In 2005 astronomers used the Hubble Space Telescope to isolate the faint light of Sirius B from the glare of Sirius A and measured this gravitational redshift precisely, giving a mass of about 0.98 to 1.02 solar masses.

What white dwarfs are

A white dwarf is what remains when a star of up to about eight solar masses runs out of fuel. Near the end of its life the star sheds its outer layers, often creating a glowing planetary nebula, and leaves its hot core behind. This core, made mostly of carbon and oxygen, no longer produces energy by fusion. It is held up against gravity not by heat but by electron degeneracy pressure, the resistance of tightly packed electrons to being squeezed closer together.

White dwarfs have a strange property: the more massive they are, the smaller they become. There is also an upper limit, about 1.4 solar masses, worked out by Subrahmanyan Chandrasekhar in 1930. A white dwarf pushed beyond this Chandrasekhar limit, for example by gaining matter from a partner star, can no longer support itself and may explode as a type Ia supernova.

Slowly fading embers

With no fusion to replenish their heat, white dwarfs simply cool and fade over billions of years. A fully cold, dark white dwarf would be called a black dwarf, but the universe, at about 13.8 billion years old, is not yet old enough for any to exist.

The Sun's future as a white dwarf

In about 5 billion years the Sun will exhaust the hydrogen in its core and swell into a red giant, probably engulfing Mercury and Venus and scorching Earth. After a period of helium fusion it will puff off its outer layers into space, briefly lighting them up as a planetary nebula.

What remains will be a white dwarf with a little over half the Sun's present mass, roughly the size of Earth. It will start out extremely hot and then cool steadily for trillions of years. Sirius B, which is more massive than the Sun's remnant will be, followed a similar path much faster, because it began life as a hot, massive star.

The Sirius system

Sirius A is a white main-sequence star about twice the mass of the Sun and some 25 times as luminous. Sirius B orbits it every 50 years or so on a fairly elongated path, at an average distance of about 20 times the Earth–Sun distance. The whole system is thought to be only about 200 to 250 million years old.

That young age is a clue to Sirius B's past. Since it has already burned out while its partner has not, Sirius B must originally have been the more massive of the pair, probably around five times the Sun's mass. It raced through its life, shed most of its material and became a white dwarf roughly 100 million years ago. Its light is so overwhelmed by Sirius A that seeing it needs a good telescope and steady air.

Surprising facts about Sirius B

Sirius B packs about the mass of the Sun into a ball slightly smaller than Earth.
A teaspoon of its material would weigh around ten tonnes.
Its existence was predicted from Sirius's wobble 18 years before anyone saw it.
Sirius B was once the larger and brighter of the two stars in the Sirius system.
Its surface is hotter than Sirius A's, yet it gives off thousands of times less visible light.

Frequently asked questions

Can you see Sirius B with a telescope?: It is possible but difficult. Sirius B is about 10,000 times fainter than Sirius A and sits very close to it, so you need a good telescope, steady air and a time when the two stars are near their widest separation.
How dense is Sirius B?: Its average density is about 2.4 tonnes per cubic centimetre, more than a million times that of water. A teaspoonful of its material would weigh around ten tonnes on Earth.
Will the Sun become a white dwarf?: Yes. After a red giant phase that begins in about 5 billion years, the Sun will shed its outer layers and leave behind an Earth-sized white dwarf with a little over half its current mass.
Who discovered Sirius B?: Friedrich Bessel predicted it in 1844 from the wobble in Sirius's motion. Alvan Graham Clark first saw it in 1862 while testing a new telescope lens.
What is the closest white dwarf to Earth?: Sirius B, at 8.6 light-years, is the nearest known white dwarf. The next closest is Procyon B, about 11.5 light-years away.

Keep exploring

The 3D views in Ember’s Universe are scientific visualisations: physically based where possible, compressed or exaggerated where the real scales would hide everything.

Sources