Gemini constellation is winter constellation and it is in best position for evening observing in March when it is high on the southern sky close to the zenith.
In January in the evening it is climbing on the eastern sky, in February it is high on the southeastern sky, in March it is close to the zenith and in best position for evening observing, in April it is high on the southwestern sky and in May it is low on the west sky.
We will observe it in April when it is high on the southwestern sky.
Double and multiple stars
Delta Geminorum - Its name is Wasat. It is about 55 light years away from the Earth. Its components have visual magnitudes of 3.6 and 8.2 and are separated about 6 arc seconds on the sky. The main component is subgiant star with 1.6 times more mass than our Sun. Its surface temperature is about 7,000 K and it is 1.5 billion years old.
The companion is main sequence star with smaller mass than our Sun and with lower surface temperature, thus it should appear orange in a telescope. Its orbital period is about 1,200 years.
Close to Wasat the dwarf planet Pluto was discovered in 1930.
Alpha Geminorum - Its name is Castor. Its combined visual magnitude is 1.6. This star in fact is star system composed of 6 stars located at a distance of about 50 light years away from the Earth. Two components have visual magnitudes 1.9 and 3.0 and their orbital period is about 450 years. These two components are separated 4 arc seconds on the sky. They are main sequence stars, with about 3 times bigger mass than our Sun and higher temperatures, about 10,000 K and 9,000 K. In reality each of these two components is spectroscopic binary star system with period of several days. This makes total of 4 stars.
Spectrocsopic binary star system means that with a telescope visually we can see one star, but if we analyze the spectrum of that star we will determine that there are two stars. They are so close to each other that no visual telescope can resolve them. The spectrum shows that instead of one, in fact two stars exsist. Such type of binary star we call it spectroscopic binary, because we can detect it with spectrometer.
Beside these two components, in a telescope visually we can notice another component, which is separated about 70 arc seconds from the main pair. It is red dwarf star with visual magnitude 9.3-9.8. It changes the magnitude from 9.3 to 9.8 because it consists of two stars - both of them red dwarfs, with orbital period of about 20 hours, and they eclipse each other when orbiting, which causes drop in the brightness of this component. These stars are also too close to be resolved individually with a telescope. For such binary star system we use the term eclipsing binary star. They eclipse each other because the plane of their orbital movement lies in the plane of our sight of view. This components are most likely physically connected with the main pair of stars and needs several thousand years for one orbit around them. That means that Castor is true multiple star system with total of 6 stars. Each red dwarf has around half the mass of the Sun and half the diameter of the Sun and is cooler with surface temperature about 3800 K.
Deep sky objects
(can be observed only from dark location outside of the city)
NGC 2392 - It is a planetary nebula. Its name is Eskimo Nebula. Its visual magnitude on the sky is 9.2 and it is about 3 or 4,000 light years away from the Earth. The central star (white dwarf) has visual magnitude 10.1. Its diameter is about 0.7 light years.
NGC 2158 - It is an open cluster of stars. Its visual magnitude on the sky is 8.6 and it is about 12,000 light years away from the Earth. It is about 2 billion years old. It contains a lot of stars and is nice object to see in a telescope. On the sky is apparently close to M 35 cluster, they are about 30 arc minutes apart (or half degree). In reality in space they are not connected at all, they just lie in the space in our line of sight.
M 35 - It is an open cluster of stars. Its visual magnitude on the sky is 5.0. It is visible even with naked eyes. It is about 3,000 light years away from the Earth. Its diameter is about 25 light years.
NGC 2420 - It is an open cluster of stars. Its visual magnitude on the sky is 8.3. It is about 7,000 light years away from the Earth. Its diameter in space is about 30 light years. It is about 1.7 billion years old.