Sirius A

Star

Sirius A — the brightest star in Earth's night sky, a hot blue-white A1V star 8.6 light-years away in the constellation Canis Major. About twice the Sun's mass and 25 times its luminosity, it far outshines its tiny companion, the white dwarf Sirius B, with which it shares a 50-year orbit. Its brilliance is as much proximity as power: Sirius is one of the Sun's nearest stellar neighbours.

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

Key facts

Mutual orbit

50.1 Earth years

18,309 Earth days

Mass

4.10 × 10³⁰ kg

2.06 M☉

Radius

1,192,708 km

1.71× wider than the Sun

Surface temperature

9,845 K (9,572 °C)

Luminosity

9.46 × 10²⁷ W

Spectral type

A1V (hot blue-white main-sequence star; formally A0mA1Va)

Science

Physical

Mass: 4.10 × 10³⁰ kg · 2.06 M☉
Radius: 1,192,708 km · 1.71× wider than the Sun
Density: 577 kg/m³ · 0.58× water; Computed from mass and radius
Spectral type: A1V (hot blue-white main-sequence star; formally A0mA1Va)

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: 6.54 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)

Rotation

Note: Rotates in only a few days: its projected equatorial velocity (v sin i ≈ 16 km/s) implies a period of roughly 5.5 days — far faster than the Sun's ~25.

Composition

By mass: A hot, hydrogen-burning main-sequence star roughly twice the Sun's mass. Its spectrum shows the 'metallic-line' (Am) peculiarity — enhanced iron-group lines — but it is otherwise a normal young A-type star. At ~9,850 K its light peaks in the blue, giving Sirius its brilliant blue-white colour.

Temperature

Surface: 9,845 K (9,572 °C)

Radiation & temperature

Luminosity: 9.46 × 10²⁷ W · 24.7 L☉
Surface temperature: 9,845 K (9,572 °C)
Peak wavelength: 294 nm · ultraviolet (invisible to the eye); Computed (Wien's law) from the surface temperature

Discovery & history

Method: Naked-eye star
Name: Sirius is from the Greek Seirios, 'scorching' or 'glowing'. As the brightest star of Canis Major — the Greater Dog — it is the 'Dog Star'; its late-summer dawn rising gave the ancient world the phrase 'the dog days' of summer.

Sirius has been known and revered since prehistory. To the ancient Egyptians its heliacal rising — the first dawn on which it reappeared after weeks lost in the Sun's glare — heralded the annual flooding of the Nile and anchored their calendar. Greek, Arabic, Chinese and Polynesian traditions all singled it out as the brilliant star of the Greater Dog. In 1718 Edmond Halley found it had shifted since antiquity — one of the first stars shown to have proper motion across the sky.

Images

Sirius blazing blue-white — the brightest star in Earth's night sky, in a real ground photograph — Cristóbal Alvarado Minic (CC BY 2.0) · Wikimedia Commons

Sirius A (the brilliant, deliberately overexposed star) with the white dwarf Sirius B as the tiny point at lower left — Hubble — NASA, ESA, H. Bond (STScI) & M. Barstow (Univ. of Leicester) (public domain) · Wikimedia Commons

Sirius (Alpha Canis Majoris) marked on the IAU chart of Canis Major, the Greater Dog — IAU / Sky & Telescope, R. Sinnott & R. Fienberg (CC BY 3.0) · Wikimedia Commons

Images from open archives · credit shown per image