Explanation of terms:

Messier

Charles Messier was born in France in 1730 and became interested in stargazing very early in life. When he was 14 years old a great 6-tailed comet appeared which stimulated his deeper interest in all things astronomical. In 1751 he went to Paris and was employed by the Navy astronomer Joseph Deslisle because of his neat handwriting. Then in 1757 be began looking for the return of comet Halley and in doing so began to record fuzzy objects to avoid them being confused with comets. His first object to be avoided would later be cataloged as M32. Comet Halley was predicted to return in 1758 and his boss Deleslie had produced a rough plan plotting the expected position in the sky where Halley would appear. Messier produced a fine star chart and plotted the predicted path of Halley, but unfortunately Deslisle had made a mistake in his calculations which Messier duplicated on his star chart. This lead to him looking in the wrong part of the sky for Halley and he therefore missed it. However he wasn't put off and began a life long passion for searching for comets. He became very excited on 28th August 1758 when he thought he had discovered a new comet in Taurus. He was disappointed when the fuzzy object turned out to be something other than a comet, so he cataloged it to prevent it causing future confusion. That first cataloged fuzzy object was recorded as M1 and the rest, as they say, is history. Click here for more ~ click here for the original messier catalogue ~ click here for messier's comets

Open Cluster

An open cluster is a group of up to a few thousand stars that were formed from the same molecular cloud and are approximately the same age. Gravity loosly binds them together and are easily disturbed by the close proximity of other clusters and clouds of gas as they orbit the galactic center, resulting in a migration to the main body of the galaxy, as well as a loss of cluster members through internal close encounters.

Open clusters might survive for a few hundred million years. In contrast, the more massive Globular Clusters of stars exert a stronger gravitational attraction on their members, and can survive for many billions of years. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.

Young open clusters may still be contained within the molecular cloud from which they formed, illuminating it to create an H II region. In time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away.

Open clusters are key objects in the study of stellar evolution. Because the cluster members are of similar age and chemical composition, the effects of other stellar properties are more easily determined than they are for isolated stars. A number of open clusters are visible with the naked eye.

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Globular Cluster

A Globular Cluster is a spherical collection of stars that orbit a galactic core as a satellite.
Globular Clusters are tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers.

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Supernova Remnant

A Supernova Remnant is the structure resulting from an exploding star in a supernova. The supernova remnant is bounded by an expanding shock wave and consists of ejected material expanding from the explosion.

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Nebula

A Nebula, or Nebulae (plural) is an interstella cloud of dust and ironised gasses such as Hydrogen and Helium. Throughout history, the term Nebula has been used to describe any fuzzy object in the night sky. However since Edwin Hubble\'s discovery of Galaxies we only use the term to describe fuzzy objects that are within our own galaxy.

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H II Region

H II is the term used by astronomers to describe a large cloud of ironised gas in which bright young stars have recently formed. Hydrogen is the most common material in the Universe, and can be found in virtually every environment investigated by astronomers, from the regions surrounding black holes, in stars and galaxies, as an important component in clusters of galaxies, and spread on large scales through the universe itself. H II regions exist most commonly in the disk of a spiral galaxy. For an H II region to exist, there must be a source to proved the ionizing heat required to strip the electron, so H II regions are common near very hot stars

H II regions are so called because of the large amount of ionised atomic hydrogen they contain. H I is neutral atomic hydrogen and H2 is molecular hydrogen.

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Planetary Nebula

Planetary Nebular are created at the end of the life of certain types of stars. During the Red Giant phase the outer layers of the star are blown off in energetic pulsations and strong stellar winds.

Planetary Nebula have nothing to do with planets. The name originated when they were first discovered in the 18th century due to their similarity in appearance to giant planets when viewed through the telescopes of the day.

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Spiral Galaxy

Spiral galaxies are characterised by a flat, rotating disk containing stars, gas and dust. They have a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which form globular clusters. Spiral galaxies usually have two arms in their structure that extend from the cental mass. They were originally described as Spiral Galaxies by Edwin Hubble in 1936 when he published The Realm of the Nebulae.

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Barred Spiral Galaxy

A Barred Spiral Galaxy is like a spiral galaxy but with a central bar-shaped structure composed of stars. gas and dust. The spiral arms usually extend from each end of the central bar.

Edwin Hubble classified these types of spiral galaxies as \"SB\" (Spiral, Barred) and arranged them into three sub-categories based on how open the arms of the spiral are:

• SBa types feature tightly bound arms.
• SBc types are at the other extreme and have loosely bound arms.
• SBb type galaxies lie in between a and c.
• A fourth type, SBm, was created to describe irregular barred spirals, such as the Magellanic Cloud galaxies, which were previously classified as Irregular Galaxies.

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Eliptical Galaxy

An elliptical galaxy, as the name implies, has an eliptical shape and a smooth, nearly featureless bright profile. They range in shape from spherical to virtually flat and range in size from hundreds of millions to over a trillion stars. It is thought that they are the result of two galaxies colliding.

They are one of the three main classes of galaxy described by Edwin Hubble in 1936 when he published The Realm of the Nebulae.

Most elliptical galaxies are composed of older, low-mass stars, with little interstellar gas and dust and therefore, little new star formation activity.

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Defuse Nebula or Nebulae

Diffuse nebula (or nebulae) consist of clouds of interstellar dust and gas. Large or massive amounts of dust and gas give rise to new star formation and produce large clusters of stars. Some of these stars are often very massive and so hot that their high energy radiation can excite the surrounding hydrogen gas of the nebula and cause it to shine, we call these Emission Nebula.

Stars that are not so massive, or so hot, simply cause their light to be reflected in the gas and dust, we call these Reflection Nebula. However, Emission Nebula will usually contain a lot of dust, which will only reflect the light, so they also have Reflection Nebula characteristics.

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Lenticular Galaxies

Lenticular Galaxies are disk galaxies without any obvious structure in their disks. This is probably because they have either used up most of their interstellar matter, so that they consist of old stars only and have therefore settled into a smooth and even distribution in the disk by the time, or because the galaxy has not closely encountered any neighbour for some hundred million years, which would have interacted with it and caused disturbances.

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Asterism

An Asterism is a star pattern that is not a constellation. Asterisms may be contained within a single constellation or consist of stars belonging to different constellations. The Plough for instance is an asterism within the constellations of Ursus Major and Ursa Minor. The Summer Triangle is also an asterism for the same reason in that it contains stars from several constellations.

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NGC Numbers

NGC stands for New General Catalogue which contains all of the Messier objects plus thousands of others which Messier missed or simply couldn't observe from his location. The catalogue was compiled in 1888 by Danish astronomer Johan Ludvig Emil Dreyer, who based his work on earlier lists made by the Herschel family of British astronomers. Dreyer included some 8,000 celestial objects which was increased to around 13,000 by his first and second Index Catalogues published in 1895 and 1908, respectively. With these supplements the NGC covers the entire sky, although many objects visible with modern instruments are not listed.

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Magnitude

The term Magnitude, is used to provide us observers with an idea of how bright, or dim, a star , nebula or galaxy will appear to be. This is a great help because we know what to expect when looking for an object in the night sky. The Scale of Magnitude is based on human measurements and start with the brightest object in the sky, the Sun at -26, through the star Vega at 0 to the faintest naked eye object at around Magnitude 7.

So if a star chart says the object is Magnitude +6 you know you are going to struggle with the naked eye but binoculars or a small telescope will pick it up.

Following is a simple table to help. I know I am going to receive 100's of emails from people disagreeing with my assessments but I intend it purely as a rough guide:

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Astronomical Distances

Astronomical Distances are so large that us humans have difficulty imagining them. Such terms as Light Yrs are used to reduce the numbers but we all struggle to comprehend.

For a bit of fun, and to provide a real example, I have created the following table which shows how long it would take to travel to distant objects using Miles Per Hour:

1 Light Year = 5,865,696,000,000 Miles