When learning about the night sky, much of what you are doing is learning names. Just like with plants or birds, the agreed-upon names help you communicate to others what you are looking at.
There are several hundred billion stars in our Milky Way, but only a few hundred of the brighter stars have common names. Most of the common names are quite old, and many of those are derived from Arabic. When you “learn the stars,” learning these names is what you typically have in mind.
However, there are more stars than names. With the total number of stars that can be seen with binoculars or telescopes, it is helpful to have other naming systems available.
In the 17th century, the German astronomer Johann Bayer made one of the first star catalogs with a uniform naming convention. In the Bayer system, a star is designated by a lower case Greek letter followed by a Latin form of the constellation name. The alpha star is the brightest in the constellation, beta is second brightest and so on through the Greek alphabet.
But the ordering does not always agree with modern magnitude measurements. For example, Betelgeuse is alpha Orionis (alpha Ori), but is slightly dimmer than Rigel (beta Ori).
Jupiter is very prominent in the evening sky this month – the brightest thing beside the sun and the moon. Jupiter is in Gemini, so is a convenient marker for that constellation. If you hope to see other planets, Mars rises around midnight, and you will need to stay up until 3 a.m. to see Saturn.
If you have a small telescope, there are things to see in Gemini beside Jupiter. The open cluster Messier 35 (M35) is near the star eta Gem, the foot of the twins. The much dimmer cluster NGC 2158 is near M35 and might be in the same field of view of your telescope. Look on a sky chart for other open clusters in Gemini.
The twin stars of Castor (alpha Gem) and Pollux (beta Gem) represent the mythological twin sons of Zeus and Leda. Castor is alpha, but Pollux is brighter.
In a small telescope, Castor is a nice visual binary, meaning you can see two stars separated by an angle of 6 arc seconds. It turns out that each of the two visible components is a spectroscopic binary.
In a spectroscopic binary, the stars are too close to each other to be seen from Earth. But if you use a spectrometer, you can see the spectrum changing periodically as the stars orbit each other. This is how we know the point of light you see is actually two stars.
In addition to the brighter visible components, there is a much dimmer companion separated by just more than an arc minute that is also a binary. That makes six stars in this one system we call Castor.
email@example.com. Charles Hakes is an assistant professor in the physics and engineering department at Fort Lewis College and is director of the Fort Lewis Observatory.