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Alhena — Star in Gemini

HIP 31681; Gamma Geminorum; 24 Geminorum

Magnitude 1.9m Star Gemini (Gem) Visible
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About Alhena

Description

Alhena is a white subgiant of spectral type A0IV at magnitude 1.93 in Gemini. Located about 109 light-years from Earth, it has a luminosity of roughly 123 times solar. Alhena marks the foot of the Twin Pollux and is a spectroscopic binary with an orbital period of about 12.6 years.

Observing Tips

Alhena lies at the southwestern foot of Gemini, well south of the twin heads Castor and Pollux. It is a bright white star that serves as a gateway to the winter Milky Way. The open cluster M35 lies about 2.5 degrees to the northwest. Best observed December through April.

History

The name Alhena comes from the Arabic 'al-han'ah,' meaning 'the brand mark' on the neck of a camel. In the Gemini constellation figure, it marks the foot of the mortal twin Pollux. Alhena has been used as a spectroscopic standard star due to its well-characterized spectrum.

Fun Facts

Alhena is one of the brightest A0 stars in the sky and its proximity to the rich Milky Way star fields of Gemini makes it a useful guide star for finding nearby deep-sky treasures like M35 and the Jellyfish Nebula (IC 443).

Observe

1Physical Properties

Magnitude 1.93
Spectral Type A1.5IV subgiant
Star Color White (B-V 0.00)
Distance 109 ly

2Position & Identifiers

RA 06h 37m 42.7s
Dec +16° 23' 57.0"
Constellation Gemini (Gem)
HR 2421
HIP 31681
HD 47105
SAO 95912
Bayer Gamma
Flamsteed 24 Gem
SIMBAD Wikipedia

3How easy to spot?

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Equipment Bortle 3 Bortle 4 Bortle 5
Naked eye Easy Easy Easy
50mm finder Easy Easy Easy
150mm scope Easy Easy Easy
Easy Medium Hard Very hard Impossible

Bortle 3 = rural · 4 = outer suburbs · 5 = suburbs

4Visibility

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Best season Nov – Jan (peak: Dec)

5Survey Image

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Size Comparison

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Spectral Classification

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Hertzsprung-Russell Diagram

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Read: The Hertzsprung-Russell Diagram — Reading Stars Like a Map
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Stellar Lifecycle

12

Blackbody Spectrum

13

Stellar Absorption Spectrum

Simulated absorption spectrum based on spectral type. Hover over lines to identify elements.

14

Stellar Fusion

Discover

15Stellar Notes

A* 1.93 A0IV, 12.9y, a = 0.038".
12.4y. Unresolved by speckle interferometry. RV also var. 0.13d.
Color excess E(B-V) = +0.03.
ALHENA.
Diam. = 0.00132 - 0.00139".
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Light Travel Time Machine

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Relativistic Travel

Nearby in the Sky

Other targets within a few degrees — pan your scope a little and keep exploring.

Visibility scores assume a 150 mm Newton at Bortle 4.

BL Ori
Medium
Semi-regular variable, range 5.9–6.6
Variable Star mag 6.24 3.4° Orion
NGC 2304
Easy
Open cluster
Open Cluster mag 10.0 4.5° Gemini
38 Gem
Easy
Easy double, sep 7.4″
Double Star mag 4.65 5.2° Gemini
NGC 2247
Medium+
Reflection nebula around a bright star
Reflection Nebula mag 8.50 6.2° Monoceros
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