The Milky Way is our own galaxy. It consists of about 100 billion stars, and our Sun is one of them. All stars that you can see in the sky belong to the milky way, but only about 10.000 of them can be seen as individual stars with the naked eye from a very dark spot on earth. The remaining billions of stars can be seen as a faint light band across the sky. You have to go outside a city in order to see it, and when you go to a remote area, it will be magnificent.

A light year corresponds to the distance that light covers within one year in space. Hence, it is a distance measure, not a time measure. Very approximately, light travels the distance of some 300.000 km (or 185.000 miles) in one second. A light year therefore corresponds to the distance of 365 days times 24 hours times 60 minutes times 60 seconds times 185.000 miles, resulting in about 5,9 trillion miles. A light year is the most commonly used distance unit when measuring distances in outer space.

Charles Messier (1730 - 1817) was a french astronomer who compiled a catalogue of deep-sky objects in the second haf of the 18th century, know as the Messier objects. It contains 110 objects that are numbered from Messier object M1 to Messier object M110. They are among the most beautiful objects in the night sky, and most of them are either visible with the naked eye or with a pair of good binoculars.
 
Today, other more comprehensive catalogues are in use, such as the New General Catalogue (NGC) by J. L. E. Dreyer, comprising several thousand objects. Still, the Messier Catalogue is probably the most popular catalogue among amateur astronomers all over the world.

Magnitude measures the brightness of stars. Smaller numbers mean brighter stars. It is a logarithmic scale, and a star of first magnitude (1 mag) appears 100 times brighter than a star of sixth magnitude (6 mag). The brightest star in the night sky is Sirius with a magnitude of -1.46mag. The faintest stars that can be seen with the naked eye are around 6mag.

A deep-sky object is an astronomical object that is outside our solar system. Typical deep-sky objects are gas nebulas and star clusters, that typically belong to our Milky Way. Other deep-sky objects such as other galaxies do not belong to our Milky Way and are therefore even farther away.

The area of the sky is divided into 89 areas, which constitute the 88 modern constellations. They were agreed upon be the International Astronomical Union (IAU) in 1922 and have been used by astronomers world-wide ever since.