Sirius B
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.
Key facts
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
- Computed from mass and radius
- Spectral type
- DA1.9 (hydrogen-atmosphere white dwarf)
Motion through space
- Note
- The motion of the pair's common centre of mass, computed as the mass-weighted mean of its stars' catalogued motions.
- Proper motion
- 1.24 arcseconds per year · its drift across our sky
- SIMBAD — 2007A&A...474..653V, 2020yCat.1350....0G
- Radial velocity
- -5.5 km/s · approaching the Sun
- SIMBAD — 2007A&A...474..653V, 2020yCat.1350....0G
- Speed across our line of sight
- 15.5 km/s
- Computed from proper motion and parallax
- Speed through space, relative to the Sun
- 16.4 km/s
- Computed from proper motion, parallax and radial velocity
Orbit (around the system barycenter)
- Semi-major axis
- 13.3 AU
- From the visual-binary orbit of Bond et al. 2017 (P = 50.13 yr, a = 19.8 AU, e = 0.591)
- Eccentricity
- 0.5914
- From the visual-binary orbit of Bond et al. 2017 (P = 50.13 yr, a = 19.8 AU, e = 0.591)
- Orbital period
- 50.13 years
- From the visual-binary orbit of Bond et al. 2017 (P = 50.13 yr, a = 19.8 AU, e = 0.591)
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 (24,727 °C)
Radiation & temperature
- Luminosity
- 8.84 × 10²⁴ W · 0.0231 L☉
- Surface temperature
- 25,000 K (24,727 °C)
- Peak wavelength
- 116 nm · ultraviolet (invisible to the eye)
- Computed (Wien's law) from the surface temperature
Discovery & history
- Discovered
- January 31, 1862
- By
- Alvan Graham Clark
- Method
- Telescope
- 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
Images from open archives · credit shown per image