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.
This young cluster is estimated to be less than 10 million years old, and is located about 7,200 light years away, in the Local spiral arm of the Milky Way. The two bright spots at center are the overexposed images of the two B-type stars, called HD 250289 and HD 250290, that dominate the cluster’s core. The stars share a common proper motion, which suggests that they form a close binary system. At its distance from Earth, the cluster’s apparent dimensions translate into a true diameter of about 10.4 light years, with the cluster shining with an apparent magnitude of 6.7 as seen from earth.
By way of contrast, NGC 2355 is among the oldest known open clusters with an estimated age of about 1 billion years. Located about 5,400 light years away, and more than 1,000 light years above the plane of the Milky Way, the apparent dimensions of the cluster translate into a true diameter of more than 46 light years. The part of the cluster shown in the image opposite represents the core of NGC 2355, which is about 4.6 light years in diameter, but with its central component radius around 11 light years wide, and containing more than 30 loosely scattered stars. NGC 2355 has an apparent magnitude of 9.7.
Eskimo Nebula (NGC 2392, Caldwell 39)
Named the “Eskimo Nebula” due to its resemblance to a human face covered by a parka hood, this bi-polar planetary nebula displays a double shell of material ejected by the progenitor star, which is suspected to have been a G-type, Sun-like star. Its discoverer, William Herschel, described NGC 2392 as “… [A] star [of] 9th magnitude with a pretty bright middle, nebulosity equally dispersed all around.”
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)
Located about 5,000 light years away, this strange, 70-light year wide supernova remnant is thought to be between 3,000 and 30,000 years old. However, while it is known that the nebula contains a neutron star, which is the collapsed remains of the progenitor star, the weird structure and emissions that surround the nebula remains a mystery.
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.
Only discovered in 1955 by George O. Abell, this four-light year wide nebula located close to the Gemini-Canis Minor border was first thought to be a relic from a supernova explosion. However, a study performed by Russian investigators in the 1970’s found that the nebula is actually a planetary nebula that formed when a red giant blew off its outer layers during its transition into a white dwarf.
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.
Since this pretty planetary nebula appears to consist of two distinctly separate components, it has two NGC designations, NGC 2371 and NGC 2372. Nonetheless, it is included in the Royal Astronomical Society of Canada’s (RASC) Finest NGC Objects list, and makes an easy target for small to medium amateur telescopes. NGC 2371-2 has an apparent visual magnitude of 13, and is located about 4,400 light years away- look for it a short distance towards the southwest of the bright star Castor.