Tau Aquilae
A faint giant star with the proper name Tianfu.
Tau Aquilae, also known by its Bayer designation τ Aquilae (abbreviated Tau Aql or τ Aql), lies in the equatorial constellation Aquila. Its proper name, Tianfu, comes from a traditional Chinese constellation. This star shines with an apparent visual magnitude of 5.7, making it a faint object that can still be spotted without a telescope from dark suburban locations, as defined by the Bortle Dark-Sky Scale.
The Earth’s annual orbital motion produces a parallax shift that places Tau Aquilae roughly 535 light-years away. Interstellar gas and dust dim its brightness by 0.28 magnitudes due to extinction. The star is moving toward the Sun at a radial velocity of −29 km/s.
Its spectrum corresponds to a stellar classification of K0 III, where the luminosity class III indicates an evolved giant star that has left the main sequence after exhausting its core hydrogen. Estimated to be about one billion years old, stellar models suggest a 90% probability that it currently resides on the horizontal branch, fusing helium in its core. Under that assumption, it has 2.15 times the Sun’s mass and has expanded to 18 times the Sun’s diameter. With an effective temperature of 4,660 K, its outer envelope radiates an orange hue typical of a K-type star.
Tianfu (天桴) was an ancient Chinese constellation made up of four stars. Before the Tang dynasty, τ Aquilae served as the first star in an earlier version of this grouping. On May 16, 2024, the IAU Working Group on Star Names approved Tianfu as the star’s official name, and it now appears in the IAU Catalog of Star Names. (See also θ Aquilae.)
Quick Facts
- Bayer designation
- τ Aquilae
- Proper name
- Tianfu
- Apparent visual magnitude
- 5.7
- Distance
- approximately 535 ly
- Stellar classification
- K0 III
- Radial velocity
- −29 km/s
- Effective temperature
- 4,660 K
Facts from the source article.
Lore & Background
Tau Aquilae's spectrum matches a stellar classification of K0 III, with the luminosity class III indicating it is an evolved giant star that has exhausted the supply of hydrogen at its core and left the main sequence. At an estimated age of one billion years, stellar models give a 90% chance that it is currently on the horizontal branch and is fusing helium at the core. Based on this assumption, it has 2.15 times the mass of the Sun and has expanded to 18 times the Sun's girth. The outer envelope radiates energy into space with an effective temperature of 4,660 K, giving it the orange-hued glow of a K-type star.
The annual orbital motion of the Earth causes a parallax shift that places the distance to this star at approximately 535 ly. Its magnitude is diminished by 0.28 from extinction caused by interstellar gas and dust. It is drifting closer to the Sun with a radial velocity of −29 km/s.
Tianfu (天桴) was an ancient Chinese constellation consisting of four stars. τ Aquilae was the first star of an early version of this constellation, before the Tang dynasty. The IAU Working Group on Star Names approved the name Tianfu for this star on May 16, 2024, and it is now entered in the IAU Catalog of Star Names.
Reader's Guide
Tau Aquilae, known as Tianfu, holds significance as a star with a dual identity: a modern Bayer designation and a traditional Chinese proper name. Its faint visibility at magnitude 5.7 makes it a target for observers in dark suburban skies, while its classification as a K0 III giant reveals its evolved state, having left the main sequence after about one billion years. The star's distance of roughly 535 light-years, combined with a radial velocity of −29 km/s, indicates it is moving toward the Solar System. The extinction of 0.28 magnitude from interstellar dust slightly dims its apparent brightness. The star's legacy is tied to the ancient Chinese constellation Tianfu, of which it was the first star in an early version before the Tang dynasty. The formal recognition of the name Tianfu by the IAU Working Group on Star Names in 2024 ensures its cultural heritage is preserved in modern astronomical catalogs. As a horizontal branch candidate with a 90% probability of fusing helium at its core, Tau Aquilae provides a case study in stellar evolution, representing a stage that stars like the Sun may eventually reach.
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