Theta Eridani

θ Eridani is a visual binary formed by the components θ¹ Eridani and θ² Eridani, alternatively called θ Eridani A and θ Eridani B, respectively. They have individual apparent magnitudes of +3.18 and +4.11,^[2] spectral classes of A3IV-V and A1V,^[5] and are separated by 8.3" in the sky, corresponding to a projected separation of 425 astronomical units (au).^[8]

θ¹ is itself a double-lined spectroscopic binary,^[14] bringing the number of known stars to three. Its components take 4.107704 days to complete an orbit, with a semi-major axis of 0.083 au (12,400,000 km; 18 R_☉) and an eccentricity of 0.105.^[4] They are so close to each other that their shapes are distorted by tidal forces, and during the orbit the surfaces visible from Earth, and hence the luminosities, vary, making the system a rotating ellipsoidal variable.^[8] The primary, θ¹ A, has already exhausted the hydrogen at its core and is in the subgiant phase, with 2.33 times the mass and 4.3 times the radius of the Sun.^[4] The secondary, θ¹ B, is still approaching the end of the main sequence,^[8] with 2.19 times the mass and 3.95 times the Sun's radius. Their effective temperatures are 7,600 and 7,800 K respectively,^[4] giving them a white color typical of A-type stars.^[15] As θ¹ A evolves and becomes larger than its Roche lobe, mass exchange between the components is expected to occur within a few tens of millions of years.^[8]

The spectroscopic binary nature of θ¹ was initially uncovered by W. H. Wright in 1905.^[16] In 2025, the system was resolved directly using interferometry by the GRAVITY instrument aboard the Very Large Telescope.^[8] Additional data led to a full orbital solution in 2026.^[4]

θ² Eridani is also near the end of its main sequence lifetime, having an estimated age of 630 million years, 2.3 times the mass and 3.2 times the Sun's radius. It has an effective temperature of 8,300 K,^[8] giving a white color typical of A-type stars.^[15] It appears to be a single star itself, and interferometric observations constrain the mass of any main sequence companion between 0.001″–0.2″ to be less than 0.55 solar masses.^[8]

In the ancient era, Theta Eridani was among the thirteenth brightest stars in the sky, as indicated by Ptolemy in his book Almagest (137 AD) and al-Sufi in his The Book of Fixed Stars (964 AD), as well as possibly being noted by Hipparchus (129 BC). This could be explained as an millenium-long outburst in the θ¹ system, powered by extraction of orbital energy during a common envelope phase, and resulted the result of mass transfer from the primary, which overflew its Roche lobe, to the secondary.^[4]

Theta Eridani, Latinized from θ Eridani, is the system's Bayer designation; θ¹ and θ² Eridani those of its two components.

The system bore the traditional name Acamar, derived from the Arabic آخِر النَّهْر ^[17] Ākhir an-nahr, which means "the end of the river", via a Roman-alphabet handwriting misread "rn" to "m". In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)^[18] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.^[19] It approved the name "Acamar" for θ¹ Eridani on 20 July 2016 and it is now so entered in the IAU Catalog of Star Names.^[12]

The term "Ākhir an-nahr", or "Achr al Nahr", appeared in the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, which was translated into Latin as Postrema Fluminis.^[20]

Historically, Acamar represented the end of the constellation Eridanus.^[21][22] Now that distinction is held by the star Achernar, which shares the same Arabic etymology. Achernar is not visible from the Greek isles (latitudes > 33° North),^[23] hence the choice of Acamar as the river's end during the time of Hipparchus and, later, Ptolemy.

In Chinese, 天園 (Tiān Yuán), meaning Celestial Orchard, refers to an asterism consisting of Theta Eridani, Chi Eridani, Phi Eridani, Kappa Eridani, HD 16754, HD 23319, HD 24072, HD 24160, Upsilon⁴ Eridani, Upsilon³ Eridani, Upsilon² Eridani and Upsilon¹ Eridani.^[24] Consequently, the Chinese name for Theta Eridani itself is 天園六 (Tiān Yuán liù, English: the Sixth Star of Celestial Orchard).^[25]