Sirius B

Star

Sirius B — a white dwarf, the burnt-out core of a star that was once more massive than Sirius A. It crushes about a full solar mass into a ball the size of Earth (radius ~5,630 km), making it extraordinarily dense. Though small and faint, its surface is searing — about 25,000 K, hotter than Sirius A itself. No fusion burns within it; it simply radiates stored heat and cools over billions of years. The nearest and best-studied white dwarf, it is a preview of the Sun's own distant fate. Its surface has never been imaged — none of any white dwarf has.

Open in 3D viewSee Sirius B in the live, interactive 3D map

Key facts

Mutual orbit

50.1 Earth years

18,309 Earth days

Mass

2.02 × 10³⁰ kg

1.02 M☉

Radius

5,634 km

1% of the Sun's radius

Surface temperature

25,000 K (24727 °C)

Luminosity

8.84 × 10²⁴ W

Spectral type

DA1.9 (hydrogen-atmosphere white dwarf)

Science

Physical

Mass: 2.02 × 10³⁰ kg · 1.02 M☉
Radius: 5,634 km · 1% of the Sun's radius
Density: 2,702,874,965 kg/m³ · 2.7e+06× water
Spectral type: DA1.9 (hydrogen-atmosphere white dwarf)

Orbit (around the system barycenter)

Semi-major axis: 13.3 AU
Eccentricity: 0.5914
Orbital period: 50.13 years

Composition

By mass: A white dwarf — the exposed, burnt-out core left when a once-massive star sheds its outer layers. It is a ball of electron-DEGENERATE carbon and oxygen about the size of Earth, under a thin hydrogen atmosphere (type DA). No fusion burns inside; it is held up not by heat but by quantum degeneracy pressure, and simply radiates stored warmth as it cools over billions of years. Packing a full solar mass into an Earth-sized volume makes it staggeringly dense (see Physical): its surface gravity is about 380,000× Earth's and its escape velocity roughly 6,900 km/s — some 2.3% of the speed of light (both computed from the measured mass and radius). Sirius A is about 210× wider than Sirius B, yet B still holds around half of A's entire mass. In some 5–6 billion years the Sun will end its life the same way.

Temperature

Surface: 25,000 K (24727 °C)

Radiation & temperature

Luminosity: 8.84 × 10²⁴ W · 0.0231 L☉
Surface temperature: 25,000 K (24727 °C)
Peak wavelength: 116 nm · ultraviolet (invisible to the eye)

Discovery & history

Discovered: January 31, 1862
By: Alvan Graham Clark
Name: 'Sirius B' marks it as the fainter companion of Sirius A. Astronomers have long nicknamed it 'the Pup' — the little dog beside the Dog Star.

In 1844 Friedrich Bessel noticed Sirius weaving a faint wavy path across the sky and concluded an unseen, massive companion must be tugging it — a star predicted purely from gravity, before anyone saw it. On 31 January 1862 Alvan Graham Clark first glimpsed the faint 'Pup' while testing a new 18.5-inch refractor. In 1915 Walter Adams found its spectrum was that of a hot, WHITE star — yet it was extraordinarily faint, which could only mean it was tiny: the first white dwarf recognised for what it truly is. In 1925 the gravitational redshift of its light gave early support to Einstein's general relativity.

Images

In X-rays the searing-hot white dwarf Sirius B (the bright source) OUTSHINES its far larger companion Sirius A — Chandra X-ray Observatory — NASA / SAO / CXC (public domain) · Wikimedia Commons

The white dwarf Sirius B — the faint point at lower left, beside the overexposed glare of Sirius A (Hubble Space Telescope) — NASA, ESA, H. Bond (STScI) & M. Barstow (Univ. of Leicester) (public domain) · Wikimedia Commons

Images from open archives · credit shown per image