Imai //, Delta Crucis (Latinised from δ Crucis, abbreviated Delta Cru, δ Cru), is a star in the southern constellation of Crux, and is the faintest of the four bright stars that form the prominent asterism known as the Southern Cross. This star has an apparent magnitude of 2.79, and its proper name was adopted by the astronomical community on 10 August 2018. Imai is a massive, hot and rapidly rotating star that is in the process of evolving into a giant, and is located at a distance of about 345 light-years (106 parsecs) from the Sun.
Epoch J2000 Equinox J2000
|Right ascension||12h 15m 08.71673s|
|Declination||–58° 44′ 56.1369″|
|Apparent magnitude (V)||2.79|
|Spectral type||B2 IV|
|U−B color index||–0.921|
|B−V color index||–0.235|
|Variable type||β Cep|
|Radial velocity (Rv)||+22.2 km/s|
|Proper motion (μ)|| RA: -35.81 mas/yr |
Dec.: -10.36 mas/yr
|Parallax (π)||9.45 ± 0.15 mas|
|Distance||345 ± 5 ly |
(106 ± 2 pc)
|Absolute magnitude (MV)||–3.2|
|Mass||8.9 ± 0.1 M☉|
|Surface gravity (log g)||3.88 cgs|
|Temperature||22,570 ± 1,840 K|
|Rotational velocity (v sin i)||210 km/s|
|Age||18.1 ± 3.2 Myr|
Imai is the name for the star designated Delta Crucis by the Mursi people of modern-day Ethiopia. The star Imai has some significance as when it "ceases to appear in the evening sky at dusk (around the end of August), it is said that the Omo [river] rises high enough to flatten the imai grass that grows along its banks, and then subsides." The Mursi use a series of southern stars to mark their calendar to track seasonal flooding of the Omo river. 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 Imai for this star on 10 August 2018 and it is now so included in the List of IAU-approved Star Names.
This star has a stellar classification of B2 IV, making it a subgiant star that is in the process of evolving away from the main sequence It is now developing into a red giant and will in the distant future end as a white dwarf. Presently it is radiating around 10,000 times the luminosity of the Sun from its outer atmosphere at an effective temperature of 22,570 K, causing it to glow with a blue-white hue. Delta Crucis is a strong candidate Beta Cephei variable. Its rotation is very fast, with a projected rotational velocity of 210 km s−1.
Delta Crucis is a member of the Lower Centaurus Crux (LCC) component of the Scorpius-Centaurus Association, which is an OB association of massive stars that share a common origin and motion through space. This is the nearest OB association to the Sun, with the LCC component having an age in the range of 16–20 million years.
In Chinese, 十字架 (Shí Zì Jià), meaning Cross, refers to an asterism consisting of δ Crucis, γ Crucis, α Crucis and β Crucis. Consequently, δ Crucis itself is known as 十字架四 (Shí Zì Jià sì, English: the Fourth Star of Cross.).
The Aranda and Luritja people around Hermannsburg, Central Australia named Iritjinga, "The Eagle-hawk", a quadrangular arrangement comprising this star, γ Cru (Gacrux), γ Cen (Muhilfain) and δ Cen (Ma Wei).
δ Cru is represented in the flags of Australia, New Zealand and Papua New Guinea as one of the stars comprising the Southern Cross. It is also featured in the flag of Brazil, along with 26 other stars, each of which represents a state. δ Cru represents the State of Minas Gerais.
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