Two of the brightest stars in the evening sky are found in the constellation Gemini: Pollux, visible as a bright orange star and Castor, a smaller, more distant cluster of stars. Although Castor is labeled alpha Geminorum (α gem) and Pollux labeled beta Geminorum (β gem), it is actually Pollux, the red giant, that is the brightest of the two with a magnitude of 1.16. Castor’s magnitude is 1.58.
A star’s brightness can be measured in several ways. Magnitude is one way to measure brightness. This system was devised by the ancient Greeks, who arranged the stars into six groups based on their visible brightness. The system was later refined in the 19th century rather than abandoned and includes groups ranging from negative numbers to higher positive numbers, inclusive of zero. A lower magnitude indicates a brighter star. For example, the sun’s magnitude is -26.74. Vega’s magnitude is 0.03
The groups are referred to as the 1st magnitude, 2nd magnitude, 3rd magnitude and so on. Each magnitude is approximately 2.512 times greater than the next. A star of first magnitude is 2.512 times greater than a star of 2nd magnitude. A star of first magnitude is (2.512-squared) times greater than a star of 3rd magnitude. A star of first magnitude is (2.512-cubed) times greater than a star of 4th magnitude and so on. Note that the difference between magnitudes, (2-1=1, 3-1=2, 4-1=3 and so on) is the power to which you raise 2.512.
Referred to as the Twins in Greek mythology, their mother was Leda and they had different fathers. Castor was mortal, unlike Pollux, and when Castor died, the two were united in the sky at Pollux’s request to his father, the god Zeus. Castor’s father, King Tyndarus of Sparta, was mortal. According to myth, they also had a sister, Helena, whose capture later in life led to the Trojan wars. Although the twins were born out of misery and deceit, their love for one another was strong and pure.
* Of note, one of the most prominent meteor showers, the geminids, radiates from this constellation and can be viewed December through January.
Finding the constellation Gemini in the evening sky is easy to do, at least finding the two brightest stars is relatively easy. However, it can be difficult to locate the other stars in the constellation without a telescope and very clear conditions. Gemini is located between Cancer, Auriga and Canis Minor. Refer to the Solar System Scope to find the constellation. (See also blog post dated March 9, 2012).