Gacrux (Gamma Crucis)
Gacrux (Gamma Crucis) is a star in the constellation Crux, magnitude 1.6.
The ruby head of the Southern Cross — the nearest red giant of its kind to the Sun.
Gacrux is the nearest M-type red giant to the Sun. Only half again as massive as the Sun, it has swollen to about 73 times its width and shines with some 830 times its light. Its surface has cooled to about 3,700 K, giving the reddish-orange glow that sets it apart from the blue-white stars of the Cross. Like many giants it swells and fades a little, a semi-regular variable star.
Its atmosphere holds a puzzle: it is rich in barium. That usually means a companion once dumped processed material on it before fading into a white dwarf — yet no such companion has been found.
Precession let the ancient Greeks and Romans see it, and Ptolemy counted it as part of Centaurus. Yet it had no Western name until navigators, by the 1940s, shortened Gamma Crucis to Gacrux. In Brazil it is also called Rubídea, ‘ruby-like’.
Gacrux (Gamma Crucis) is 89 light-years away: the light we see from it set off in 1937, 7 years after Pluto was discovered.
Did you know?
- A flag with a moment in it. On Brazil’s flag Gacrux stands for the state of Bahia. The line through Gacrux and Acrux matches the meridian of the sky over Rio de Janeiro at 8:30 am on 15 November 1889, when the new republic was formally ratified.
- The Eagle-hawk. The Aranda and Luritja people of Central Australia see Gacrux as part of Iritjinga, the Eagle-hawk, a four-sided pattern with nearby stars of the Cross and Centaurus.
- Four flags more. Gacrux is one of the Southern Cross stars on the flags of Australia, New Zealand, Samoa and Papua New Guinea.
The orange-red star at the top of the Southern Cross, a vivid contrast with the blue-white stars of the rest of the pattern. A line from Gacrux through Acrux points towards the south celestial pole. Binoculars show a sixth-magnitude white star nearby — only a chance alignment.
When and where to see it
It is best placed in the evening sky around May, when it crosses the meridian at about 10 pm local time (around midnight in March). It can be seen from everywhere south of 33°N and never sets south of 33°S.
At its highest it stands below the horizon from 50°N, below the horizon from 35°N, 33° up from the equator, 68° up from 35°S.
Facts
- Type
- Star
- Constellation
- Crux
- Designations
- Gamma Crucis, γ Cru, HR 4763, HD 108903, HIP 61084
- Magnitude
- 1.6
- Position
- RA 12h 31.2m, Dec −57° 7′ (J2000)
- Spectral type
- M3.5III
- Colour
- orange-red (B−V 1.59)
- Distance
- 89 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Gacrux (Gamma Crucis) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.