Alhena (Gamma Geminorum)

Alhena (Gamma Geminorum) is a star in the constellation Gemini, magnitude 1.9.

The white star at the Twins’ feet — with a hidden partner on a 12-year orbit.

Alhena is a white star beginning to grow old. It is running low on hydrogen in its core and has started to swell into a subgiant — already about six times the Sun’s width, with 3.4 times its mass and 200 times its light, from a surface at about 9,200 K.

It hides a partner. Shifts in its spectrum reveal a companion about 1.4 times the Sun’s mass, probably a yellow star like the Sun but heavier, swinging round it every 12.6 years on a very stretched orbit.

Its name comes from the Arabic al-han‘ah, ‘the brand’ on a camel’s neck; another old name, Almeisan, means ‘the shining one’. In China it is the third star of Jǐng, the Well.

Alhena (Gamma Geminorum) is 110 light-years away: the light we see from it set off around 1915, 8 years before Edwin Hubble showed that other galaxies exist.

Did you know?

• A rescue ship. A Dutch ship named Alhena rescued many people from the stricken Italian liner Principessa Mafalda in October 1927.
• Venus is coming. Alhena lies too far south of the ecliptic for the Moon to cover it, and planets usually pass several degrees north. But from August 2143 Venus will make a series of close passes, every eight years.

A bright white star marking the feet of the Twins, at the far end of Gemini from Castor and Pollux. Bright enough to pick out even from a light-polluted city.

What is a star?

When and where to see it

It is best placed in the evening sky around January, when it crosses the meridian at about 10 pm local time (around midnight in December). It can be seen from everywhere north of 74°S and never sets north of 74°N.

At its highest it stands 56° up from 50°N, 71° up from 35°N, 74° up from the equator, 39° up from 35°S.

Facts

Type

Star

Constellation

Gemini

Designations

Gamma Geminorum, γ Gem, 24 Gem, HR 2421, HD 47105, HIP 31681

Magnitude

1.9

Position

RA 6h 37.7m, Dec +16° 24′ (J2000)

Spectral type

A1.5IV+

Colour

white (B−V 0.00)

Distance

110 light-years

Discovered

Known since antiquity

Open AstroPilot to see when Alhena (Gamma Geminorum) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.