Star Clusters and Doubles
Most stars do not exist alone, they are often multiple or double star systems. They drift apart over time from their original batches, formed from the same gas cloud. Optical doubles are stars that are not true doubles (they do not revolve around each other and are not in a system); they are instead stars that are too far apart to be doubles yet appear close because they are almost in the same line of vision.
This particular type of cluster comprise of young and hot stars. They are arranged in a spread-out manner and are not close together. A common example is M45, Pleiades, the brightest open cluster in the northern sky, in the constellation Taurus.
This type of star cluster is exactly the opposite of the open cluster; it is arranged in a spherical shape and is especially concentrated in the centre. They are old and most of the stars are red giants.
The Globular Cluster Omega Centauri, NGC 5139 is the relatively bright orange spot at eleven o' clock to the centre of this image.
These stars have variable brightness due to their rotation, eclipsing a brighter/dimmer star (Demon star Algol in the constellation Perseus) or even fluctuations in mass, it being a red giant and thus unstable and eruptive (Mira in the constellation Cetus).
Cataclysmic Variables include novas and supernovas which undergo sudden nuclear explosions.
Eruptive Variables are stars which brighten or fade with no regular pattern, throwing off gas and dust in the process which lowers the amount of light emitted. Eta Carinae in the constellation Carina constantly throws out thick clouds of dust which causes its brightness to fluctuate.
Pulsating Variables are often stars which are nearing death. As they pulsate and spin, their brightness, size and temperature also vary over time. Mira stars (named after the variable star of this category in the constellation Cetus) are red giants that pulsate over 1000 days.
Rotating Variables vary in brightness because of their surface being unequally covered in sunspots. As they rotate, different groups of dimmer sunspots come into view and the magnitude varies slightly.
When there is a pair of binary stars of different magnitude. When the dimmer one eclipses the brightness one by passing in front of it, the light from the star would be reduced and the star would appear to dim.
Life Cycles of Stars | Diffuse Nebula | Main-Sequence Stars | Red Giants after Main-Sequence | Death of a Low Mass Star | Death of a High-Mass Star | Star Families | Magnitude Scale | Measuring Stellar Distances | Classification of stars | Wien's Law and Stefan-Boltzmannn Law for a Blackbody | Stellar Spectra