TW Piscis Austrini
A variable star and companion to Fomalhaut in a triple system.
TW Piscis Austrini, called Fomalhaut B, is a main-sequence star located in the southern constellation Piscis Austrinus. It sits about 24.8 light-years from the Sun, making it a relatively close neighbor. From Earth, it appears two degrees away from its brighter companion, Fomalhaut (A)—a separation roughly equal to the width of four full moons.
The star carries a variable-star designation, as it belongs to the BY Draconis class. Its brightness shifts slightly between magnitudes 6.44 and 6.51 over a 10.3-day cycle, caused by surface brightness variations as the star rotates. TW Piscis Austrini lies within a light-year of Fomalhaut, and because the two share the same proper motion and an estimated age of about 440 ± 40 million years, astronomers consider them part of a multiple-star system. A third, fainter star—Fomalhaut C—lies even farther away, appearing roughly six degrees from the pair on the sky.
In 2019, astrometric measurements detected an unconfirmed exoplanet candidate orbiting Fomalhaut B.
Quick Facts
- Designation
- TW Piscis Austrini (Fomalhaut B)
- Constellation
- Piscis Austrinus
- Distance
- 24.8 ly
- Apparent magnitude range
- 6.44 to 6.51
- Variability period
- 10.3 days
- Variable star type
- BY Draconis variable
- Age
- 440 ± 40 million years
Facts from the source article.
Lore & Background
TW Piscis Austrini is a BY Draconis variable star, its brightness variations caused by surface brightness changes as the star rotates. Its apparent magnitude varies slightly from 6.44 to 6.51 over a 10.3-day period. The star lies within a light-year of Fomalhaut (A), and the two share the same proper motion and an estimated age of approximately 440 ± 40 million years, leading astronomers to consider them elements of a multiple star system. A third star, Fomalhaut C, is dimmer and more widely separated, appearing to observers on Earth at an angular separation of about 6 degrees from Fomalhaut. To an observer on Earth, TW Piscis Austrini is visually separated from Fomalhaut by 2 degrees—the width of four full moons.
Reader's Guide
TW Piscis Austrini holds significance as a component of one of the nearest multiple star systems to the Sun, at an estimated distance of 24.8 light-years. Its status as a BY Draconis variable provides insight into stellar rotation and surface activity for a main sequence star. The star's association with Fomalhaut and Fomalhaut C forms a triple system with the widest visual separation among its components, spanning about 6 degrees as seen from Earth. In 2019, an exoplanet candidate around Fomalhaut B was detected by astrometry, though this remains unconfirmed, leaving the possibility of a planetary companion open. The system's proximity and age make it a valuable target for studying stellar evolution and multiple-star dynamics. The star's variable star designation, TW Piscis Austrini, reflects its classification as a variable star, and its shared proper motion with Fomalhaut solidifies their gravitational link.
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