Alpha Arae (Choo)
Alpha Arae (Choo) is a star in the constellation Ara, magnitude 3.0.
The Altar’s spinning top: a blue-white star whirling so fast it is close to flying apart.
Alpha Arae spins almost as fast as a star can. Its equator races round at about 470 kilometres a second, close to the speed at which it would begin to break apart. The spin squeezes the star into a pronounced bulge at its equator and throws off gas, so this blue-white star is wrapped in a disc of its own material: a Be star, with bright emission lines in its spectrum.
In 2003 and 2005 astronomers examined it with the Very Large Telescope Interferometer. They found a dense disc circling its equator, its gas orbiting the way planets circle the Sun — faster close in, slower farther out — while a wind of about 1,000 kilometres a second blows from its poles. There are also hints of a close companion.
The star has about 9.6 times the Sun’s mass and shines 5,800 times as brightly, from a surface of some 18,000 kelvins. Its light wanders between magnitudes 2.76 and 2.90, with quick, regular flickers and slower, irregular outbursts.
Alpha Arae (Choo) is 270 light-years away: the light we see from it set off around 1755, 15 years before Charles Messier published his famous catalogue.
Did you know?
- The Pestle. In Chinese astronomy Alpha, Beta and Sigma Arae form Chǔ, the Pestle. From this, R. H. Allen and later Patrick Moore gave Alpha Arae the name Choo.
- A chance neighbour. An 11th-magnitude star appears about 50 arcseconds away, but the two are unrelated: their closeness in the sky is a coincidence of line of sight.
A blue-white 3rd-magnitude star, the second brightest in Ara, the Altar, about 13 degrees due south of Shaula in the Scorpion’s tail. Compare it with Beta Arae, the Altar’s brightest star, some six degrees to the south-south-west.
When and where to see it
It is best placed in the evening sky around July, when it crosses the meridian at about 10 pm local time (around midnight in June). It can be seen from everywhere south of 40°N and never sets south of 40°S.
At its highest it stands below the horizon from 50°N, 5° up from 35°N, 40° up from the equator, 75° up from 35°S.
Facts
- Type
- Star
- Constellation
- Ara
- Designations
- Choo, α Ara, HR 6510, HD 158427, HIP 85792
- Magnitude
- 3.0
- Position
- RA 17h 31.8m, Dec −49° 53′ (J2000)
- Spectral type
- B2Vne
- Colour
- blue-white (B−V -0.17)
- Distance
- 270 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Alpha Arae (Choo) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.