Rasalhague (Alpha Ophiuchi)
Rasalhague (Alpha Ophiuchi) is a star in the constellation Ophiuchus, magnitude 2.1.
The serpent-bearer’s head — a white star spinning so fast it is close to flying apart.
Rasalhague’s name means “the head of the serpent collector” in Arabic: it marks the head of Ophiuchus, the giant who grips a snake. Just five degrees away shines Rasalgethi, the head of Hercules, so two mythical heads almost touch.
The main star is white, about twice the Sun’s mass, and it spins at least 240 km per second at its equator — close to 90 per cent of the speed that would tear it apart. The spin squashes it, so its equator bulges about 20 per cent wider than its poles. The poles are hotter too, near 9,400 kelvin against 7,600 at the equator.
Rasalhague is also a binary. A smaller orange dwarf of about 0.8 solar masses circles it every 8.6 years. The orbit was only pinned down in 2011 using adaptive optics, and the two made their closest approach in April 2012.
Rasalhague (Alpha Ophiuchi) is 49 light-years away: the light we see from it set off in 1977, 8 years after Apollo 11 landed on the Moon.
Did you know?
- The emperor’s assistant. Chinese astronomers named it Hòu, a senior assistant to the emperor, sometimes said to be an astrologer.
- Dimmer from the side. Because it is flattened and hottest at the poles, Rasalhague would look brighter seen pole-on. From our tilted view it appears almost a fifth dimmer than its true output.
A white 2nd-magnitude star marking the head of Ophiuchus, with Rasalgethi, the head of Hercules, five degrees to its west-north-west. Look at the pair together: Rasalhague’s white beside Rasalgethi’s orange-red.
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 north of 77°S and never sets north of 77°N.
At its highest it stands 53° up from 50°N, 68° up from 35°N, 77° up from the equator, 42° up from 35°S.
Facts
- Type
- Star
- Constellation
- Ophiuchus
- Designations
- Alpha Ophiuchi, α Oph, 55 Oph, HR 6556, HD 159561, HIP 86032
- Magnitude
- 2.1
- Position
- RA 17h 34.9m, Dec +12° 34′ (J2000)
- Spectral type
- A5IVnn
- Colour
- yellow-white (B−V 0.15)
- Distance
- 49 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Rasalhague (Alpha Ophiuchi) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.