# Tau Scorpii

Tau Scorpii, Latinized from τ Scorpii, formally known as Paikauhale /ˌpaɪkaʊˈhɑːleɪ/, is a star in the southern zodiac constellation of Scorpius. The apparent visual magnitude of Tau Scorpii is +2.8, which make it among the brightest stars of the Scorpius constellation. Parallax measurements yield a distance estimate of roughly 470 light-years (150 parsecs) from Earth.

## Description

Tau Scorpii is a B-type star with an early spectral classification of B0.2V. It has 15 times the Sun's mass and 6.4 times the radius of the Sun. It is radiating about 25,000 times the Sun's luminosity from its photosphere at an effective temperature of 28,860 K. This gives it the blue-white hue characteristic of B-type stars. As yet there is no evidence of a companion in orbit around τ Sco. It is a magnetic star whose surface magnetic field was mapped by means of Zeeman–Doppler imaging. Tau Scorpii is rotating relatively slowly with a period of 41 days.
This star is 5.22 million years old, being in the main sequence. Since it is more massive than the Sun, Tau Scorpii will evolve and exhaust its core hydrogen much faster. It is expected that in around 5.38 million years it will leave the main sequence, expanding to 10.9 times the Sun's radius, and in 6.4 million years (or 11.6 Myr after its formation), it will become a red supergiant with 750 times the radius of the Sun, comparable to Antares which is 680 R$_☉$.
The spectrum of this star shows triply ionized oxygen (O IV) that is being generated by X-rays and the Auger ionization effect. Observations with the ROSAT space telescope showed it has a higher energy (harder) X-ray spectrum than is usual for B0 V stars. Over the energy range 0.8–1.2 keV, its X-ray luminosity is L$_x$ = 1.8 × 10$^{31}$ erg s$^{−1}$ with a large L$_x$ to L$_{bol}$ of log L$_x$/L$_{bol}$ = –6.53 from ASCA measurements. ROSAT measurements showed a log L$_x$/L$_{bol}$ ≃ –5.93 for the range 0.1–2.4 keV. The hard component of the X-ray spectrum from τ Sco as studied with XMM-Newton supports the presence of in-falling clumps of plasma in τ Sco.
This star is a proper motion member of the Upper Scorpius subgroup of the Scorpius–Centaurus OB association, the nearest such co-moving association of massive stars to the Sun. The Upper Scorpius subgroup contains thousands of young stars with mean age 11 million years at average distance of 470 light years (145 parsecs).

## Nomenclature

τ Scorpii (Latinised to Tau Scorpii) is the star's Bayer designation.
Tau Scorpii and Sigma Scorpii together bore the traditional name Al Niyat (or Alniyat) derived from the Arabic النياط al-niyāţ "the arteries" and referring to their position flanking the star Antares, the scorpion's heart, with Tau Scorpii being the star to the south.
Paikauhale is the Hawaiian name for Tau Scorpii. In the Hawaiian dictionary by Pukuʻi & Elbert (1986), the word paikauhale is defined as to go gadding about from house to house. In another Hawaiian dictionary (H.W. Kent, 1993, "Treasury of Hawaiian Words in One Hundred and One Categories", p.367), paikauhale is defined as Vagabond owning no home; house-to-house wanderer.
In 2016, the IAU organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN approved the name Paikauhale for the star A on 10 August 2018 and it is now so included in the List of IAU-approved Star Names. The name Alniyat was given to Sigma Scorpii.

## Cultural signification

The indigenous Boorong people of northwestern Victoria saw this star (together with σ Sco) as wives of Djuit (Antares).
In Chinese, 心宿 (Xīn Xiù), meaning Heart, refers to an asterism consisting of τ Scorpii, σ Scorpii and Antares. Consequently, the Chinese name for τ Scorpii itself is 心宿三 (Xīn Xiù sān), "the Third Star of Heart".