vast thin volumes of gas swirling in space. The gases in nebulae are excited by the
radiation from nearby stars. The gas re-radiates the energy at different
characteristic blue, green, yellow or red wavelengths. The color depends upon the
composition of the gas in the nebula and the type and strength of the radiation it is
receiving from its nearby stars. For example the red light seen in many nebulas is
caused by glowing Hydrogen.
are groups of stars numbering from loose aggregations of a few dozen in loose clusters all
the way up to Globular clusters which are huge concentrations of many thousands, or even a
million stars. Some globular clusters rival dwarf galaxies for size.
are home to many billions of stars swirling together about a bright central and mysterious
core. Galaxies contain many nebulae, clusters and probably even planets. At
the centre of every galaxy there is probably a huge black hole.