U Aquilae
A classical Cepheid variable in a spectroscopic binary system.
U Aquilae is a binary star system in the constellation Aquila, roughly 592 parsecs (1,930 light-years) from Earth. It consists of a spectroscopic binary pair and a more distant companion. The primary star is a yellow supergiant, 55 times the Sun’s radius and 2,570 times its luminosity. The secondary is a blue main-sequence star, twice the Sun’s mass and about thirty times as luminous; it is hotter than the primary at 9,300 K, but much smaller and fainter. The two stars orbit each other every five years, with a mildly eccentric orbit that varies their separation from five to seven astronomical units. An 11th-magnitude star located 1.6 arcseconds away from the spectroscopic pair is thought to be gravitationally bound, but its orbit would take several thousand years.
The system’s variability was first reported by Edwin Forrest Sawyer in 1886. In his announcement, he referred to the star as 50 Aquilae, its designation in Uranometria Argentina. Sawyer had begun observing it in late 1882 and estimated a period of “about one week.” It received its variable star designation, U Aquilae, in Annie Jump Cannon’s 1907 Second Catalogue of Variable Stars. The primary component, U Aquilae Aa, is a classical Cepheid variable, fluctuating between magnitudes 6.08 and 6.86 over a 7.02-day cycle. It is an evolved star that has exhausted its core hydrogen and now fuses helium into carbon.
- Distance
- 592 parsecs (1,930 ly)
- Primary star type
- Yellow supergiant
- Primary radius
- 55 R☉
- Primary luminosity
- 2,570 L☉
- Secondary star type
- Blue main-sequence star
- Secondary mass
- 2 M☉
- Secondary luminosity
- 30 L☉
- Secondary temperature
- 9,300 K
- Orbital period
- 5 years
- Separation range
- 5 to 7 AU
Lore & Background
U Aquilae is a spectroscopic binary system. The primary star is a yellow supergiant with a radius of 55 R☉ and a luminosity of 2,570 L☉. The secondary is a blue main-sequence star, twice the mass of the Sun and around thirty times more luminous, hotter at 9,300 K but much smaller and fainter. The two stars orbit every five years, with separation varying from five to seven astronomical units in a mildly eccentric orbit. An 11th-magnitude star 1.6″ from the spectroscopic pair is assumed to be related but would have an orbit of several thousand years.
Discovery of the variability of U Aquilae was announced by Edwin Forrest Sawyer in 1886. In his announcement, he called the star 50 Aquilae, its designation in Uranometria Argentina. Sawyer had begun observing the star in late 1882 and derived a period of 'about one week'. It was listed with its variable star designation, U Aquilae, in Annie Jump Cannon's 1907 Second Catalogue of Variable Stars.
U Aquilae Aa is a classical Cepheid variable star, ranging between magnitudes 6.08 and 6.86 over a period of 7.02 days. It is an evolved star which has exhausted its core hydrogen and is now fusing helium into carbon.
Reader's Guide
U Aquilae is significant as a classical Cepheid variable star, a type used as standard candles for distance measurement. Its variability was discovered by Edwin Forrest Sawyer in 1886, who initially called it 50 Aquilae and derived a period of about one week. The star was later given its variable star designation by Annie Jump Cannon in her 1907 Second Catalogue of Variable Stars. The system is a spectroscopic binary, with the primary being a yellow supergiant Cepheid and the secondary a blue main-sequence star. The primary's pulsation period of 7.02 days and its evolved state—having exhausted core hydrogen and now fusing helium into carbon—provide insight into stellar evolution. The binary nature, with an orbital period of five years and separation of 5–7 AU, adds to its importance for studying mass and evolution in Cepheid systems. An 11th-magnitude companion 1.6″ away is assumed to be gravitationally bound but with an orbit of several thousand years.
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