Fomalhaut, whose name means “mouth of the fish,” is the brightest star in the constellation Piscis Austrinus and the 18th-brightest star in the night sky. Located just 25 light-years from Earth, it is a young white star surrounded by vast debris belts and belongs to an extraordinarily wide triple-star system.
Easy to Find as the Lonely Star of Autumn
Fomalhaut shines in a relatively empty region of the southern sky, earning it the nickname “the Lonely Star of Autumn.” From mid-northern latitudes, it is best seen on late-summer and autumn evenings. One way to find it is to follow the western side of the Great Square of Pegasus from Scheat through Markab and continue southward. From southern Britain, Fomalhaut climbs only about 10 degrees above the southern horizon at its highest, so a clear, unobstructed view is essential.
The 18th-Brightest Star in the Night Sky
Fomalhaut has an apparent magnitude of approximately 1.16, making it the 18th-brightest star in the night sky and by far the brightest star in Piscis Austrinus. Its prominence is partly explained by its proximity: at just over 25 light-years away, Fomalhaut is one of the nearest first-magnitude stars to Earth. Its isolated position also makes it appear especially conspicuous despite its low altitude for many Northern Hemisphere observers.
Its Three Stars Are Spread Across Light-Years
Fomalhaut is a triple-star system, but its members are separated by extraordinary distances. The bright white star visible to the naked eye is Fomalhaut A. Its orange-dwarf companion, Fomalhaut B, lies approximately 0.9 light-years away, while the red dwarf Fomalhaut C is about 2.5 light-years from A and appears in the neighboring constellation Aquarius. Despite these enormous separations, the stars share the same motion through space and appear to remain gravitationally bound. Fomalhaut C even possesses its own debris ring. Although the stars are physically connected, they appear widely separated in our sky. The familiar naked-eye star is Fomalhaut A alone; its companions do not blend with it visually.
Nearly Twice as Massive as the Sun
Fomalhaut is a white A-type main-sequence star with approximately 1.9 times the Sun’s mass and about 1.8 times its radius. Its surface temperature is close to 8,600 kelvins, compared with roughly 5,800 kelvins for the Sun. This higher temperature and greater size allow Fomalhaut to radiate around 16 times as much energy as our star.
Only Around 440 Million Years Old
Fomalhaut is estimated to be approximately 440 million years old—less than one-tenth the age of the 4.6-billion-year-old Sun. It has already lost the primordial disk of gas and dust from which it formed, but vast quantities of leftover rocky and icy material continue colliding around it. The resulting dust makes Fomalhaut an exceptional nearby laboratory for studying the later stages of planetary-system development.
Three Vast Debris Belts Surround the Star
Observations with the James Webb Space Telescope have revealed three nested belts of dust and rocky debris surrounding Fomalhaut. Together, they extend approximately 23 billion kilometers (14 billion miles) from the star—about 150 times the distance between Earth and the Sun. The largest outer ring is roughly twice the scale of the Solar System’s Kuiper Belt and gives some images of Fomalhaut their famous “Eye of Sauron” appearance. Wide gaps separate the belts, possibly carved by the gravity of planets that remain too faint to detect. Although unseen planets may be shaping these belts, no planet has yet been confirmed in the Fomalhaut system.
Its Famous Planet Appears to Have Vanished
In 2008, astronomers announced that Hubble images appeared to show a planet, Fomalhaut b, orbiting inside the star’s outer debris ring. It was celebrated as the first exoplanet candidate directly imaged in visible light, but subsequent observations revealed something unexpected: the object expanded, faded and had disappeared by 2014. Planets do not behave that way, and astronomers now think Fomalhaut b was an expanding cloud of dust created when two large icy bodies collided.
Two Enormous Collisions Appeared Within 20 Years
Fomalhaut’s surroundings are anything but quiet. Hubble images from 2023 revealed a second compact source resembling Fomalhaut b when it was first observed about two decades earlier. Astronomers interpret both objects as enormous clouds of dust created when planetesimals tens of kilometers across smashed into one another. These bodies are leftover building blocks from the system’s formation, and their orbits may be disturbed by other planetesimals or unseen planets. Hubble did not catch either impact as it happened, but it recorded their expanding aftermath. Detecting evidence of two such violent collisions within only 20 years suggests that Fomalhaut’s young planetary system is far more active and unsettled than astronomers expected.