Definition
Theta Aquilae (θ Aql) is the Bayer designation of a multiple star system located in the constellation Aquila. The system is visible to the naked eye with an apparent visual magnitude of approximately 3.3.
Overview
Theta Aquilae lies close to the celestial equator, making it observable from most latitudes on Earth. The primary component is a B‑type star, and the system is known to consist of at least two, possibly three, gravitationally bound stars. The system is part of the broader grouping of bright stars that outline the shape of the mythological eagle, Aquila.
Etymology / Origin
The designation “Theta” comes from the Greek letter θ, which Johann Bayer employed in his early 17th‑century star catalogues to label stars within a constellation in order of decreasing brightness. “Aquila” is the Latin word for “eagle,” the name of the constellation in which the star resides.
Characteristics
| Property | Value (typical / most recent measurement) |
|---|---|
| Spectral type | B9.5 III (primary component) |
| Apparent magnitude (V) | ~3.27 |
| Distance | ~440 light‑years (parallax ≈ 7.4 mas) |
| Radial velocity | ~+6 km s⁻¹ (receding) |
| Proper motion | ≈ +24 mas yr⁻¹ in Right Ascension, –18 mas yr⁻¹ in Declination |
| System composition | At least a spectroscopic binary (components θ Aql Aa and θ Aql Ab) and a visual companion (θ Aql B) at an angular separation of ~0.8″. |
| Luminosity | Primary is several hundred times the Sun’s luminosity. |
| Effective temperature | ≈ 10,000 K (typical for a late‑B type giant). |
The primary star (θ Aql Aa) is itself a spectroscopic binary with an orbital period of roughly 17 days. The visual companion (θ Aql B) is fainter (magnitude ~5.4) and separated enough to be resolved with moderate telescopic equipment.
Related Topics
- Bayer designation – The system of naming stars using Greek letters and constellation names.
- Aquila (constellation) – A northern‑hemisphere constellation representing an eagle; contains several notable stars such as Altair (α Aql).
- Multiple star systems – Groups of two or more stars bound by mutual gravitation, often classified as binaries, triples, etc.
- B‑type giant stars – Hot, luminous stars of spectral class B that have evolved off the main sequence.
All data are drawn from standard astronomical catalogues (e.g., Hipparcos, Tycho‑2, and the Bright Star Catalogue). Where precise values vary between sources, the most widely accepted contemporary measurements are presented.