Acrux
What is Acrux?
Stars· Distance 320 ly · size ~8× Sun (primary). The brightest star of the Southern Cross, at its foot. A multiple system of hot blue stars that a small telescope splits in two, it appears on the flags of several southern nations and never rises from Korea.
📍 Visible from where you are right now — computed in your browser
bearing · altitude · rise time · best time tonight
Where Acrux is in your sky right now: https://nowsky.org/solar/sky?sel=acrux — location and time set automatically, no login.
Distance and spectral type
Acrux is about 322 light-years away, a blue-white subgiant (spectral type B0.5IV), in the constellation Cetus. Its apparent magnitude is 0.8, visible to the naked eye.
Stellar figures
| Distance | 320 ly |
|---|---|
| Magnitude | 0.77 combined — thirteenth brightest |
| Members | multiple system of three or more blue B stars |
| Constellation | Crux |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Behind the name
A modern blend of Alpha + Crux. The foot of the Southern Cross, drawn on the flags of Australia, Brazil and more.
Frequently asked
- How far is Acrux from Earth?
- 320 ly.
- How bright is Acrux (apparent magnitude)?
- 0.77 combined — thirteenth brightest.
- Which constellation is Acrux in?
- Cetus.
- Where is Acrux in the sky right now?
- Open the sky view for your location and time and it shows the position in the sky right now.
When the Southern Cross rose over the Mediterranean
Acrux cannot be seen north of latitude 27° today, yet the ancient Greeks knew it. Because of precession, around 1000 BC the Southern Cross rose above the southern horizon of the Mediterranean, and Ptolemy recorded its stars as part of Centaurus. It vanished from Europe after about AD 400 and was "rediscovered" in the southern seas in the age of exploration, becoming a constellation of its own.
Earth's axis swings round once every 26,000 years, so which stars are visible from which latitude slowly changes. Acrux is still drifting farther south, so for the next few thousand years it recedes from the northern hemisphere before climbing north again in ten thousand years. Set the sky view to Jerusalem in May of 1000 BC and the Southern Cross reappears above the southern horizon.
The star in depth
Acrux is Alpha Crucis — the name simply fuses Alpha and Crux. At a combined magnitude of 0.77 it is the thirteenth-brightest star in the night sky, 320 light-years away. To the eye it is one star, but a small telescope shows two blue stars of magnitude 1.3 and 1.8 (α1 and α2) four arcseconds apart, and α1 is itself a spectroscopic binary with a 76-day period, so the system is at least triple. All the members are hot B0–B1 stars with surfaces above 25,000 K.
Crux is the smallest constellation in the sky but the most important in southern cultures, and Acrux marks the southern end of its long axis; extend that axis 4.5 times and you reach the vicinity of the south celestial pole, which is how the Cross is used to find south. It appears on the flags of Australia, New Zealand, Brazil, Papua New Guinea and Samoa — on Brazil's flag Acrux stands for the state of São Paulo. With masses above ten Suns, its stars are future supernova candidates.
At declination −63° it is invisible from Korea and anywhere north of latitude 27°. In the southern hemisphere it stands high in the south on March to June evenings, in the direction indicated by the Pointers Hadar and Alpha Centauri. In the Sky view, switch the location to Sydney and select Crux to see Acrux blazing at the bottom of the cross.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.