The night sky is populated by thousands of objects of various shapes and sizes. It is not always easy to find your way around. Here are a few elements to help you understand what kind of objects you are dealing with and how to classify them:

Object category

These various objects can be grouped into several major categories, each containing similar types of targets:

Solar systemEverything within our immediate space environment: the planets, the Sun, the Moon, comets, and associated phenomena such as eclipses or auroras (polar lights).
Stellar objectsIn relatively “nearby” space, the term stellar encompasses everything related to stars on their main sequence—meaning during their “adult” life, powered by the nuclear fusion of hydrogen. This includes individual stars (variable, binary, or colored stars) as well as stellar associations, such as open clusters or globular clusters.
NébulaeObjects with a “diffuse” appearance within a galaxy, which are primarily gas clouds, most often subjected to electromagnetic radiation. They can be:
1. Molecular or cold: These are dark nebulae, which are found everywhere in the universe.
2. Hot or ionized, emitting or reflecting light: These are generally associated with star formation or stellar youth. This is notably the case for H II regions within galaxies.
3. Associated with the end of a star’s life: Such as planetary nebulae, Wolf-Rayet bubbles, or supernova remnants (SNRs). These can be referred to as “stellar ejection nebulae.”
GalaxiesEverything that lies beyond our own Milky Way. This includes individual galaxies, both nearby and distant, as well as all galaxy groupings—whether they are compact groups or spread out into clusters and superclusters—where interactions between galaxies take shape. Gravity is the dominant force here, manifesting through tidal forces or gravitational lensing.

Catalogues

Différents catalogues sont utilisés par les astronomes amateurs ou professionnels, pour référencer les objets du ciel :

MessierThe “catalog of diffuse deep-sky objects” is highly popular among amateur astronomers in Europe. It was compiled in 1774 by French astronomer Charles Messier to help comet hunters avoid confusing these permanent, fuzzy structures with comets, as their true nature was completely unknown at the time. Today, it features the most “classic” deep-sky targets accessible from the Northern Hemisphere (the full list is available on Wikipedia)
CaldwellThis catalog acts as a supplement to the Messier catalog for “classic” targets accessible to amateur astronomers, particularly by including prominent objects from the Southern Hemisphere. It was published in 1995 in Sky & Telescope by Patrick Moore Caldwell. This catalog is more widely popular among Anglo-Saxon amateurs (the full list is available onWikipedia)
NGCThe “New General Catalogue” originated from the “General Catalogue” initiated by William and later John Herschel. It is the most widely used catalog among amateur astronomers. It features 7,840 deep-sky objects, including many galaxies, compiled by John Dreyer up until 1888—the year the catalog was first published in the Memoirs of the Royal Astronomical Society. Its defining feature is that the objects are sorted by right ascension and declination (a list of prominent objects is available on Wikipedia)
ICThe “Index Catalogue”—also referred to as the “Index Catalogue of Nebulae” or “Index Catalogue of Nebulae and Clusters of Stars”, IC I or IC II—serves as a supplement to the New General Catalogue. John Dreyer thus compiled over 5,386 objects up until 1908, bringing the combined NGC + IC total to more than 13,000 objects (a list of prominent objects is available on Wikipedia)
AbellThe Abell catalog of rich clusters of galaxies lists 4,073 rich galaxy clusters whose redshift values (z) are, by definition, between 0.02 and 0.2. Originally published by George Ogden Abell in 1958, it was expanded and completed with the help of Harold Corwin and Ronald Olowin up until 1989. The main elements are available on Wikipedia
ArpL’Atlas of Peculiar Galaxies, aussi appelé communément atlas Arp, est un catalogue astronomique produit par l’astronome Halton Arp et recensant 338 galaxies particulières. L’atlas a été publié pour la première fois par le California Institute of Technology en 1966 (Liste sur Wikipedia)
HicksonA Hickson Compact Group (or HCG) is a grouping of galaxies selected and published by Paul Hickson in 1982. These are most frequently galaxies undergoing gravitational interaction, which triggers distortions and starbursts (the full list is available on Wikipedia)