A nebula is a giant cloud of dust and gas in space. Some nebulae (more than one nebula) come from the gas and dust thrown out by the explosion of a dying star, such as a supernova. Other nebulae are regions where new stars are beginning to form. For this reason, some nebulae are called "star nurseries." (source: NASA - Space Place).

Here you can find pictures of star clusters. We distinguish two main kind of clusters: Globular clusters, that are spherical groups of thousands to millions of stars gravitationally bound and often old or very old; globular clusters are often satellites of main galaxies as Milky Way or Andromeda Galaxy (M31). Open clusters are group of relatively young stars born from cosmic gas concentration; often the stars are dispersing before they die.

Galaxìas kyklos (milky circle) is the name borrowed from ancient Greek for the milky way; it appears like a milky band in the night sky. Today we know galaxies are gravitationally bound systems of stars (dead or alive) and other interstellar materials. Milky Way is a medium range galaxy; it's estimated to have a diameter 30-60.000 parsec diameter (approximately 100-200.000 light-years) and contain 100-400 billion of stars.