Eclipses

An eclipse (from the Greek ekleipô, “to vanish”) is an astronomical event that occurs when one celestial object moves into the shadow of another. The term is most often used to describe either a solar eclipse, when the Moon’s shadow crosses Earth’s surface, or a lunar eclipse, when the Moon moves into the shadow of Earth. However, it can also refer to such events beyond the Earth-Moon system: for example, a planet moving into the shadow cast by one of its moons, a moon passing into the shadow cast by its parent planet, or a moon passing into the shadow of another moon. An eclipse is a type of syzygy (a line up of astronomical bodies), as are transits and occultations. Eclipses are impossible on Mercury and Venus, which have no moons.

Clusters

Star clusters are groups of stars that are gravitationally bound. Two distinct types of star cluster can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars, while open clusters generally contain less than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group.

Nebulae

The term “Nebula” (plural nebulae) has a varied use in the history of astronomy. In pre-telescopic times it was used to distinguish fuzzy objects that looked different to the point like stars. Most nebulae known at that time have been shown to be open star clusters. The term nebula was a catch-all term used for what we now call deep sky objects. If you have an old astronomy book, the Andromeda Galaxy was once called the Andromeda Nebula.

In early telescopic times, the nature of these objects was still widely unknown. With open clusters resolved, still all other deep sky objects were summarised as “Nebulae”. Only the use of large telescopes, the discovery of spectroscopy and the invention of photography in the second half of the 19th century made it possible to distinguish “real” nebulae – i.e., gas and dust clouds – with certainty from objects made up of stars (globular clusters and galaxies).