Fomalhaut

Star

Fomalhaut - the brightest star in Piscis Austrinus, 25 light-years away, a young white A-type star famous for the sharp, off-centre ring of dust around it, nicknamed the 'Eye of Sauron'. A point of light once thought to be a planet ('Fomalhaut b') turned out to be an expanding cloud of dust from a collision - so no planet is confirmed, but the ring itself is spectacular.

Open in 3D viewSee Fomalhaut in the live, interactive 3D map

Key facts

Mass

3.82 × 10³⁰ kg

1.92 M☉

Radius

1,280,088 km

1.84× wider than the Sun

Surface temperature

8,590 K (8,317 °C)

Luminosity

6.37 × 10²⁷ W

Spectral type

A4V (white main-sequence star)

Science

Physical

Mass: 3.82 × 10³⁰ kg · 1.92 M☉
Radius: 1,280,088 km · 1.84× wider than the Sun
Density: 435 kg/m³ · 0.43× water; Computed from mass and radius
Spectral type: A4V (white main-sequence star)

Motion through space

Proper motion: 0.368 arcseconds per year · its drift across our sky; SIMBAD — 2007A&A...474..653V, 2006AstL...32..759G
Radial velocity: +6.5 km/s · moving away from the Sun; SIMBAD — 2007A&A...474..653V, 2006AstL...32..759G
Speed across our line of sight: 13.4 km/s; Computed from proper motion and parallax
Speed through space, relative to the Sun: 14.9 km/s; Computed from proper motion, parallax and radial velocity

Composition

By mass: A young white main-sequence star (~440 million years old) about 1.9 times the Sun's mass and ~17 times its light, still surrounded by the debris of planet formation.

Temperature

Surface: 8,590 K (8,317 °C)

Radiation & temperature

Luminosity: 6.37 × 10²⁷ W · 16.6 L☉
Surface temperature: 8,590 K (8,317 °C)
Peak wavelength: 337 nm · ultraviolet (invisible to the eye); Computed (Wien's law) from the surface temperature

Discovery & history

Method: Naked-eye star
Name: Its IAU proper name Fomalhaut is from the Arabic fam al-hut, 'the mouth of the fish'. It is Alpha Piscis Austrini, the brightest star of the Southern Fish.

Known since antiquity as a lonely bright star of autumn, Fomalhaut leapt to fame in 2008 when Hubble imaged a candidate planet inside its dust ring - one of the first exoplanets seen in visible light. By 2020 the 'planet' had faded and spread out, revealing it as debris from a smash-up of icy bodies; JWST in 2023 resolved the ring into nested belts.

Images

Hubble Space Telescope coronagraph image of Fomalhaut's dust ring — a real photograph (the star's glare is blocked; a dot marks its position). The marked 'Fomalhaut b' was later reinterpreted as an expanding dust cloud. — NASA, ESA & P. Kalas, UC Berkeley (public domain) · Wikimedia Commons

Images from open archives · credit shown per image