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The Messier Catalogue is a famous catalogue in Astronomy. It consists of 110 deep sky objects, including open and globular star clusters, galaxies, nebulae, an asterism, a double star and even a supernova remnant. It was compiled in the 18th century by Charles Messier.
Messier was a comet hunter who was born in Badonviller, France on June 26, 1730. When searching for comets he was frustrated by fixed objects that looked like comets in the night sky but actually weren't. These fuzzy "comet" like objects hindered his searches so he catalogued them in order to avoid them in future.
The first version of his catalogue contained 45 objects and was published in 1774 in the journal of the French Academy of Sciences in Paris. Over the next 7 years he updated the catalogue and published the final version in 1781, containing 103 objects. Many of these objects he actually discovered himself.
On several different occasions between 1921 and 1966, astronomers and historians discovered evidence of another 7 deep-sky objects that were observed either by Messier or his friend and assistant, Pierre Mechain, shortly after the final version was published. These objects, numbered M104 to M110, are now accepted by astronomers as "official" Messier objects.
M101 is a large face-on spiral galaxy located 22 million light-years away in the constellation of Ursa Major. At magnitude +7.9, it can be glimpsed in binoculars or small telescopes from dark sites. However, this galaxy suffers from low surface brightness and in bad seeing conditions or light polluted areas is sometimes difficult to spot even with 200mm (8-inch) scopes. M101 is best seen from the Northern Hemisphere during the months of March, April and May.
M101 is also known as the Pinwheel galaxy and was discovered by Pierre Méchain on March 27, 1781. He described it as "nebula without star, very obscure and pretty large, 6' to 7' in diameter, between the left hand of Boötes and the tail of the great Bear." He communicated this to Charles Messier, who verified its position and then included it in his catalogue as one of the final entries.
Locating the part of sky where M101 is positioned is easy, since it's close to the handle of the bowl that forms the Plough or Big Dipper asterism of Ursa Major. The Pinwheel galaxy is located at one corner of an equatorial triangle formed with second magnitude stars Mizar (ζ UMa - mag. +2.2) and Alkaid (η UMa - mag. +1.8). M101 is 5.5 degrees east of Mizar (the celebrated naked eye double star) and 5.5 degrees northeast of Alkaid.
M31, the famous Andromeda Galaxy, is the largest member of the Local Group that also includes the Milky Way, the Triangulum Galaxy (M33) and about 50 other smaller galaxies. With an apparent mag. of +3.4, it's one of the brightest Messier objects and easily visible to the naked eye even from areas with a certain amount of light pollution. M31 is usually regarded as the most distant object that can be easily seen without optical aid.
The Andromeda Galaxy has been known for a long time. It was first recorded over 1000 years ago by Isfahan based Persian astronomer Abd-al-Rahman Al-Sufi. In 964 AD, he described it as the little cloud in his Book of Fixed Stars. This object was almost certainly known - for a number of years before this date - to other Persian astronomers. The first person to telescopically observe and describe M31, was German astronomer Simon Marius on December 15, 1612. Unaware of both Al Sufi's and Marius' discovery, Giovanni Batista Hodierna independently rediscovered the object sometime before 1654. Then on August 3, 1764, Charles Messier added the great spiral to his catalogue. Incidentally, Messier incorrectly credited Marius as the discoverer, apparently unaware of the earlier work of Al-Sufi.
For a long time, the true nature of M31 was unknown. It was simply regarded as a large nebula located within our galaxy. Up until the 20th century, it was referred to as either the Andromeda Nebula or the Great Andromeda Nebula. In the 18th century, William Herschel incorrectly believed M31 was one of the nearest nebulae, located at a distance of not more than 17,000 light-years. However, he was correct in viewing the galaxy as an island universe like the Milky Way, although at such a small distance it would be much smaller in size than our own galaxy.
On a clear night, from a dark site with exceptional seeing there are about 2,500 stars visible to the naked eye at any one time and in total there are about 6,000 stars, maybe a few more, visible to the naked eye from Earth. However this represents a very tiny fraction of the number of stars in our Milky Way galaxy. Conservative estimates put the total number at 100 billion with probably many more.
With at least 100 billion galaxies in the Universe the total number of stars in the Universe is enormous, almost unimaginable. In this section we list some of the more famous, unusual and historical significant stars / star groupings of interest to amateur astronomers.
Alpha Centauri is the third brightest star in the night sky. To the naked eye it shines at apparent magnitude -0.27, which is fainter than Canopus (mag. -0.72) but brighter than Arcturus (mag. -0.04). However, Alpha Centauri is not a single star; it's a triple consisting of two bright components and a feeble red dwarf. For most of their orbit, the main stars are easily split with small telescopes. This is also the nearest star system to the Solar System.
There are at least 6,000 stars bright enough to be visible with the naked eye. Under dark skies, up to about 3,000 can be seen at any one time (since one half of the Earth is in daylight). However, none can rival the brilliance of Sirius, the brightest night time star.
Located in the constellation of Canis Major, Sirius shines with an apparent magnitude of -1.46. It's noticeably brighter than its nearest rival Canopus (α Car mag. -0.72) and four times more brilliant than Arcturus, the brightest star in the northern section of sky. Sirius is also known as the "Dog Star". When close to the horizon it twinkles. Of course, the colour variations are only a result of the Earth's unsteady atmosphere. In practice, all stars do twinkle and to a lesser extent the planets as well.
M43 is a HII region located in the constellation of Orion that was discovered by Jean-Jacques Dortous de Mairan sometime before 1731. As part of the famous Orion Nebula (M42), it's positioned just north of the main nebula and separated from it by a narrow dust lane. With an apparent mag. of +9.0, M43 is about 100 times fainter than M42, but still bright enough to be seen with binoculars.
Occasionally, ninth mag. nebulae like M43 can be difficult to find - especially if located in barren parts of the sky - but not this one. Firstly, it's located in majestic Orion, perhaps the most recognizable of all constellations, secondly it's part of the Orion Nebula and therefore positioned right next to the great showpiece object and finally it has a relatively high surface brightness. Of course, finding M42 is easy, it's positioned 5 degrees south of the three bright stars that form Orion's belt (Mintaka, Alnilam and Alnitak). M43 is located just 8 arc minutes north of M42 surrounding a 7th magnitude star. M43 (and M42) are best seen during the months of December, January and February.
M51, also known as the Whirlpool Galaxy, is a grand design spiral galaxy located in the constellation of Canes Venatici. It's one of the most famous galaxies in the sky, appearing face-on when viewed from Earth. At magnitude +8.4, it's relatively bright and visible in binoculars, especially from dark sites. M51 has a much smaller dwarf companion, known as NGC 5195, and together they form the finest and most studied example of an interacting galaxy pair in the sky.
M51 was one of Charles Messier original discoveries on October 13, 1773. His friend Pierre Méchain discovered NGC 5195 on March 20, 1781. Messier described M51, as a faint nebula without stars, that was difficult to see. In his catalogue of 1781, he put M51 and NGC 5195 in the same note and therefore some confusion exists. Was he referring to M51 as the larger galaxy, or did he mean the pair itself. If it was the pair, then the main galaxy should really be referred to as M51A, with NGC 5195 separately known as M51B.
Canes Venatici is a small northern constellation of faint stars that was created by Johannes Hevelius in the 17th century. Apart from its brightest member, Cor Caroli (α CVn - mag. +2.9), it contains no stars better than 4th magnitude. However, finding M51 isn't difficult as it positioned only a few degrees from the handle of the seven stars, that form the famous Plough or Big Dipper asterism of Ursa Major. To pinpoint M51, first identify Alkaid (η UMa - mag +1.9) the end star of the handle of the Plough. Located 3 degrees directly west of Alkaid is 24 UMa (mag. +4.5). Positioned a degree northeast of 24 UMa is a mag. +6.5 star. Now imagine a line connecting this star with 24 UMa and continue it southwards for a further two degrees. This leads to a triangle of stars of magnitudes +7.1, +7.1 and +7.5. All three are easily visible in binoculars, with M51 located just west of the southernmost star.
The New General Catalogue of Nebulae and Clusters of Stars (NGC) is an extensive catalogue of astronomical deep sky objects that was compiled by John Louis Emil Dreyer in 1888. Danish born Dreyer spent much of his life in Ireland where he compiled the catalogue, which was based on Sir William Herschel's Catalogue of Nebulae and Clusters of Stars.
Herschel first published his catalogue containing 1,000 entries in 1786 with assistance from his sister Caroline. He then added another 1,000 entries in 1789 and a final 500 in 1802, bringing the total number of entries to 2,500. In 1864, Sir John Herschel the son of William then expanded the catalogue into the General Catalogue of Nebulae and Clusters and Clusters of Stars (GC), which contained 5,079 entries.
Dreyer build on the early work of the Herschel's to produce the renowned NGC, which is still used by astronomers all over the World today. In total he listed 7,840 objects that are referred to as NGC objects. The catalogue contains all types of deep sky objects including galaxies, nebulae, globular clusters, open clusters, supernova remnants and planetary nebulae. Compiling the catalogue was a massive task for Dreyer, he had to deal with many observation reports from a host of different scopes that contained numerous amounts of contradictory information. The sheer volume of information and number of objects meant Dreyer couldn't validate all of them himself and as a result, he had to accept some data as recorded. Although Dreyer himself was very accurate in his transcripts, it's perhaps not surprising that the catalogue contains several, mostly position and description errors.
In addition to the NGC, Dreyer also published two supplements, known as the Index Catalogues (IC). The first containing 1,520 objects was published in 1895. The second published in 1908 listed an extra 3,866 objects. In total, the two IC's catalogues contain 5,386 objects.
Various attempts have been made to correct the original NGC errors. These include the Revised New General Catalogue by Jack Sulentic and William Tifft in 1973 and NGC 2000.0 by Roger Sinnott in 1989 using J2000.0 coordinates. More recently, the Revised New General Catalogue and Index Catalogue (Revised NGC/IC) published by Wolfgang Steinicke in 1996.