Definition
Eta Aquilae (η Aquilae) is a Classical Cepheid variable star located in the constellation Aquila. It is designated by the Bayer letter η (Eta) and is used as a standard candle in astrophysical distance measurements due to its well‑characterized period–luminosity relationship.
Overview
Eta Aquilae is a luminous supergiant that exhibits regular pulsations, causing its apparent visual magnitude to vary between approximately 3.5 and 4.3 over a period of about 7.176 days. The star lies at a distance of roughly 1,100 light‑years (≈ 340 parsecs) from the Sun, as estimated from parallax measurements and Cepheid period‑luminosity calibrations. It is one of the brighter Cepheids visible to the naked eye and has been extensively studied to refine the cosmic distance scale.
The system is considered to be a binary, with a faint companion detected by spectroscopic and interferometric techniques. However, the companion contributes minimally to the combined brightness and does not significantly affect the Cepheid’s pulsation characteristics.
Etymology / Origin
The name “Eta Aquilae” follows the Bayer designation system introduced by Johann Bayer in his 1603 star atlas Uranometria. In this system, Greek letters are assigned to stars within a constellation roughly in order of decreasing brightness; “η” (Eta) is the seventh letter of the Greek alphabet. “Aquila” is the Latin genitive form of the constellation name Aquila, meaning “eagle.” Thus, “Eta Aquilae” literally means “the Eta star of the Eagle.”
Characteristics
| Property | Value | Notes |
|---|---|---|
| Spectral type | F6 Ib (variable) | Classical Cepheid supergiant |
| Apparent magnitude (V) | 3.5 – 4.3 | Varies with pulsation cycle |
| Pulsation period | 7.176 days | Fundamental mode Cepheid |
| Absolute magnitude (V) | ≈ – 3.5 | Derived from period–luminosity relation |
| Distance | ~1,100 ly (≈ 340 pc) | Parallax from Gaia DR3 |
| Radial velocity | ~ – 20 km s⁻¹ (average) | Varies with pulsation |
| Mass | ~5 M☉ | Model‑dependent estimate |
| Radius | ~ 64 R☉ (average) | Varies by ≈ ± 5 R☉ over the cycle |
| Effective temperature | 5,800 K (average) | Fluctuates between ≈ 5,500 K and 6,300 K |
| Companion | Likely a main‑sequence star of late‑type | Detected via spectroscopic signatures; separation ≈ 0.1″ |
The star’s pulsations are driven by the κ‑mechanism operating in partially ionised helium layers within its envelope. The regularity of the light curve makes Eta Aquilae a benchmark object for calibrating the Cepheid period‑luminosity (Leavitt) law, which underpins extragalactic distance measurements.
Related Topics
- Classical Cepheid variables – Pulsating supergiants used as standard candles.
- Bayer designation – A stellar naming system employing Greek letters and constellation names.
- Aquila (constellation) – The “Eagle,” a northern zodiacal constellation containing several notable stars.
- Period–luminosity relation (Leavitt law) – Empirical correlation between Cepheid pulsation period and intrinsic luminosity.
- Gaia mission – Space observatory providing high‑precision parallaxes for stars such as Eta Aquilae.
- Stellar evolution of intermediate‑mass stars – Pathways leading to the Cepheid instability strip.