Delta1 Tauri

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) and Ab.

Nomenclature
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).

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) to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems. 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.

In Chinese, 畢宿 (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. Consequently, the Chinese name for Delta¹ Tauri itself is 畢宿三 (Bì Xiù sān), "the Third Star of Net".

Properties
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 G9.5 III CN0.5. 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 $2.8$ with a period of $1.28$. 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.