Delta Crucis
Faintest of the Southern Cross stars, a hot subgiant variable.
Imai, also known as Delta Crucis, is the dimmest of the four bright stars that make up the Southern Cross asterism in the constellation Crux. With an apparent visual magnitude of 2.8, it is visible without a telescope. This star is massive, hot, and spins rapidly, and it is currently transitioning into a giant. It lies roughly 345 light-years from the Sun. The name Imai comes from the Mursi people of modern-day Ethiopia and was officially adopted by the International Astronomical Union on 10 August 2018. For the Mursi, Imai marks a seasonal calendar: when it stops appearing in the evening sky around the end of August, it signals that the Omo River has risen enough to flatten the imai grass along its banks before receding. Imai is used alongside the stars Thaadoi, Waar, and Sholbi to track the river’s flooding. In Portuguese, it is sometimes called Pálida, meaning "Pale one." In Chinese, it is part of an asterism called the Cross, which also includes Gamma, Alpha, and Beta Crucis. The Aranda and Luritja people of central Australia include Imai in a quadrangular arrangement named Iritjinga, "The Eagle-hawk," along with Gacrux, Muhlifain, and Ma Wei. Classified as a B2 IV subgiant, Imai is evolving away from the main sequence toward becoming a red giant. It shines with a blue-white hue, radiating about 8,100 times the Sun's luminosity from an outer atmosphere at 20,400 K. Its rotation is very fast, and it is a strong candidate for being a Beta Cephei variable, though this is debated. Imai belongs to the Lower Centaurus Crux subgroup of the Scorpius–Centaurus association, the nearest OB association to the Sun, which is 16–20 million years old. On Earth, Delta Crucis appears on the flags of Australia, New Zealand, Samoa, and Papua New Guinea as part of the Southern Cross. It also features on the flag of Brazil, where it represents the state of Minas Gerais.
- Bayer designation
- δ Crucis
- Proper name
- Imai
- Apparent visual magnitude
- 2.8
- Distance
- about 345 ly
- Stellar classification
- B2 IV
- Luminosity
- about 8,100 times the Sun
- Effective temperature
- 20,400 K
Lore & Background
The name Imai was adopted by the International Astronomical Union on 10 August 2018, originating from the Mursi people of modern-day Ethiopia. In their tradition, when Imai '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 Imai along with β Crucis (Thaadoi), β Centauri (Waar), and α Centauri (Sholbi) to mark their calendar for tracking seasonal flooding of the Omo River. It is sometimes called Pálida (Pale [one]) in Portuguese. In Chinese, the asterism Cross includes δ Crucis, γ Crucis, α Crucis and β Crucis; δ Crucis itself is known accordingly. 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 (Muhlifain) and δ Cen (Ma Wei).
Reader's Guide
Delta Crucis is a strong candidate Beta Cephei variable, though this is disputed. Its rotation is very fast, with a projected rotational velocity given in the notes. It is a member of the Lower Centaurus Crux (LCC) component of the Scorpius–Centaurus association, the nearest OB association to the Sun, with the LCC component having an age in the range of 16–20 million years. In culture, δ Cru is represented in the flags of Australia, New Zealand, Samoa and Papua New Guinea as one of the stars comprising the Southern Cross. It also appears in the flag of Brazil, along with 26 other stars each representing a state; δ Cru represents the state of Minas Gerais. The star's Bayer designation is Delta Crucis, abbreviated Delta Cru or δ Cru. Its stellar classification of B2 IV suggests it is a subgiant star evolving away from the main sequence toward becoming a red giant. It radiates around 8,100 times the luminosity of the Sun from its outer atmosphere at an effective temperature of 20,400 K, giving it a blue-white hue.
Did You Know?
- Imai is the name given by the Mursi people of modern-day Ethiopia.
- It is the faintest of the four bright stars forming the Southern Cross.
- The star represents the state of Minas Gerais on the flag of Brazil.
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