Gemini is the 30th biggest of the 88 recognized constellations and the 8th biggest of the zodiacal constellations, taking up an area of 514 square degrees of the northern celestial hemisphere. The brightest star in Gemini is Pollux (Beta Geminorum), which is situated 34 light years away and has an apparent visual magnitude of +1.14, also making it the 17th brightest star in the entire night sky.
Gemini can be seen from latitudes of between +90° and -60°, and from northern locations is visible during the winter and springtime, although best seen during the months of January and February at about 9 PM Local Time, when it is highest in the sky. However, for observers from mid-northern latitudes, the constellation remains visible in the west around 3 AM in early March, 1 AM (2 AM daylight saving time) in early April and 11 PM (Midnight Daylight Saving Time) in early May.
The constellation contains one Messier object, and a number of other interesting deep sky objects, the most notable of which are explored in this list.
Messier 35 (NGC 2168)
This large open cluster shown at the top of this page is situated 2,800 light years away, and contains several hundred young stars, many of which are B3 type blue-white stars less than 110 million years old. It spans 24 light years of space, with the width of its central mass about half that distance, and weighing an estimated 1600 to 3200 solar masses. Situated 3.5 degrees to the northwest of the star Mu Geminorum, M35 shines with an apparent magnitude of 5.30, and appears about as big as the full Moon in telescopic views, meaning that only low magnification is needed to appreciate the sight of this beautiful cluster.
NGC 2158 and IC 2157
Located west of M35 are two other interesting but unrelated star clusters which offer a wonderful study in contrast, as can be seen in the above image. NGC 2158 appears fainter than M35, but at around 16,000 light years from Earth is actually around five times further away. Lying less than 1.5 degrees westwards from M35 and NGC 2158 is the sparse open cluster IC 2157, which is around 6,650 light-years distant, and is similar to NGC 2158 in terms of both magnitude (+8.4) and size (6 arc-mins). NGC 2158 is about 2 billion years old, and dominated by older, yellow stars, while IC 2157 is relatively young at just 63 million years old, and contains a number off hot young stars.
Eskimo Nebula (NGC 2392, Caldwell 39)
Located about 2,870 light years away, the nebula has an apparent visual magnitude of 10.1, and is an easy target for large binoculars and small telescopes.
Jellyfish Nebula (IC 443, Sharpless 248)
Essentially, the Jellyfish Nebula consists of two unequal halves, with the bigger northeastern half (shown as the violet-colored arc at centre) consisting of sheet-like filaments that emit light from iron, neon, silicon, oxygen, and dust particles that were heated by the supernova explosion. In contrast, the smaller shell (shown as the bright blue arc) consists of clumps of relatively dense knots, and other similar structures that emit light primarily from heated dust and hydrogen. To date, no credible explanation for the unequal distribution of mass in the nebula have been found.
Note that while the Medusa Nebula is only about 1,500 light years away and has an apparent magnitude of 7.68, it has a very low surface brightness of between 15.99 and 25, which means that to see it requires a telescope with an 8 inch or larger aperture that is fitted with an OIII filter.