Star Stars · In Ara
Choo
Choo, α Arae, is a blue-white main-sequence star in Ara: the 144th brightest star in the night sky.
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- Constellation
- Ara
- Designation
- α Arae
- Apparent magnitude
- 2.84 · 144th brightest star in the night sky
- Spectral class
- B2Vne
- Distance
- 267 light-years
- Highest at midnight
- mid June
- Position (J2000)
- RA 17h 32m · Dec −49° 53′
A blue-white main-sequence star, around 26,000 K, like Rigel and Spica. About 29× brighter than the faintest star the eye can see. Gives off about 410 times the Sun's visible light. The light you are seeing left around 1759.
When to see it
On the night of Sat 12 Jun 2027, when it is highest around midnight, on each city’s own clock:
| From | Rises | Highest | Sets |
|---|---|---|---|
| London GMT+1 | Never rises | ||
| New York EDT | Never rises | ||
| Singapore GMT+8 | 19:22 | 01:15 · 39° | 07:08 |
| Sydney GMT+10 | 14:36 | 00:06 · 74° | 09:36 |
Common questions
- Can I see it from 50° north?
- No. From there it never rises: you would have to travel south of 40° north before it cleared the horizon at all.
- Can I see it from 35° south?
- Yes. It rises and sets, and at its highest it stands 75° up in the south.
- When is the best time of year to see it?
- Around mid June, when it stands opposite the Sun and is highest at midnight. It comes round about two hours earlier each month, so by mid September it is highest in the early evening, and in mid March not until the hours before dawn.
- How far away is it, and how bright?
- It is 267 light-years away, and shines at magnitude 2.84 in our sky. From that far, the Sun would be a star of magnitude 9.4: too faint for the eye, a target for binoculars.
- Naked eye, binoculars or telescope?
- The naked eye, even from a city: at magnitude 2.84 it is brighter than the faintest stars a city sky still shows, about magnitude 4.5.
Look for
Start from Sargas: Choo is 6.9° south of it, three fingers at arm’s length.
Look for
From 50° north it never rises; from 35° north it barely clears the horizon, at 5°; from the equator it climbs to 40°; from 35° south it climbs to 75°.
Science
Choo’s surface is about 26,000 K, some 20,200 K hotter than the Sun’s 5,772 K, so its light is strongest at about 110 nm, in the ultraviolet, past the violet end of what the eye can see, where the Sun’s is strongest at 500 nm. Moved to 33 light-years, the distance astronomers compare stars at, it would shine at magnitude −1.7, brighter than Sirius, the brightest star in our sky.
Did you know
Against Sargas, the brighter star the hop starts from: Sargas looks 2.5 times as bright as Choo in our sky; Sargas is 300 light-years away and Choo 267 light-years, so side by side Sargas would give off 3.1 times as much light as Choo.
Science
Choo is the 97th nearest to us of the 156 stars on these pages with a measured distance. Its nearest neighbour among them in space is Sargas, about 48 light-years from it; on our sky the two are 6.9° apart.
Look for
The Sun passes 27° north of Choo around mid December, when it is highest at noon, in daylight: the weeks either side are its worst time of year. From 35° north it is above the horizon for about 4 hours a day and from 35° south about 19. It is 9° from the plane of the Milky Way, inside the band a dark sky shows.