Delta¹ Tauri (δ¹ Tauri, abbreviated Delta¹ Tau, δ¹ Tau) is a double star in the zodiac constellation of Taurus. Based upon an annual parallax shift of 20.96 mas as seen from Earth, it is located roughly 156 light-years distant from the Sun. The system is faintly visible to the naked eye with a combined apparent visual magnitude of +3.772. It is considered a member of the Hyades cluster.
The two constituents are designated δ¹ Tauri A and B. A is itself a binary star with components designated δ¹ Tauri Aa (officially named Secunda Hyadum, the traditional name for the entire system)[1] and Ab.
δ¹ Tauri (Latinised to Delta¹ Tauri) is the system's Bayer designation. The designations of the two constituents as Delta¹ Tauri A and B, and those of A's components - Delta¹ Tauri Aa and Ab - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[2]
The system bore the traditional name Hyadum II, which is Latin for "Second of the Hyades". In 2016, the IAU organized a Working Group on Star Names (WGSN)[3] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[4] It approved the name Secunda Hyadum for the component Delta¹ Tauri Aa on 5 September 2017 and it is now so included in the List of IAU-approved Star Names.[1]
In Chinese, Chinese: 畢宿 (Chinese: Bì Xiù), meaning Net, refers to an asterism consisting of δ¹ Tauri, Epsilon Tauri, Delta³ Tauri, Gamma Tauri, Alpha Tauri (Aldebaran), 71 Tauri and Lambda Tauri.[5] Consequently, the Chinese name for Delta¹ Tauri itself is Chinese: 畢宿三 (Chinese: Bì Xiù sān), "the Third Star of Net".[6]
Delta¹ Tauri A is a single-lined spectroscopic binary with an orbital period of 529.8 days and an eccentricity of 0.42. The visible member, component Aa, is an evolved G- or K-type giant star with a stellar classification of . The 'CN0.5' suffix indicates a mild overabundance of cyanogen in the outer atmosphere. It is chromospherically active and shows a radial velocity variation of with a period of . The primary, component Aa, has 2.8 times the mass of the Sun, while the secondary, component Ab, has 1.3 times the Sun's mass.
Delta¹ Tauri B is a magnitude 13.21 visual companion separated by 111.8 arcseconds from A. It is most likely not physically related to the main star.