Variable Stars

The first variable star was discovered in 1595th, when amateur astronomer David Fabricius noticed that star Omicron Ceti changes its brightness so that it can sometimes be seen with the naked eye, and then disappears from the area of visibility. That is why in the 17th century named her Mira (miraculous). Today we know that it belongs to variable stars and to a special long-periodic type. These stars are relatively cool red giants whose maximum magnitude may reach hundreds of times higher brightness than the minimum. Like all red giants, mira variables are located in the upper left corner of the H-R diagram(Hertzsprung-Russell diagram) and their luminosity varies between 10 and 10000 luminosity of the Sun.
Other type of variable stars is very well known and well researched. It is the Cepheids. First Cepheid, Delta Cephei, was discovered in 1784th and today serves as a prototype. Classical Cepheid can be identified by the specific shape of the curve shine. After a short time she was at maximum brightness, and then its glow gradually weakening, until it reaches a minimum. This mechanism is now well-known to scientists, because Cepheids are well studied. Cepheids pulsate because of the periodic warming and cooling star gases. When the star is squeezed, the internal pressure and high temperature pressing the upper layers and the star begins to expand. With expansion pressure drops, and temperature with him. So the heat is lost, and when sufficiently cooled, the material again falls on the star. Cepheid is the brightest when its radius is the smallest.
It is assumed that the Cepheids appear late in the evolution of stars of relatively large mass, while they pass through the so-called area of instability in the H-R diagram. There are two types of Cepheids: Cepheids type I and Cepheids type II, and the marks relating to the population of stars. Population II stars are poor with elements heavier than helium, while the population I stars, other than hydrogen and helium, consist of specific quantities of heavier elements (metals), which affects the transparency of their atmospheres.
There are other types of variable stars, but they are not as well known or explored as Cepheids and RR Lyrae. Some of them have irregular periods and some of them are flowing. Variable stars should not be confused with a close or binary stars because it makes their glow variable, which is the result of two ordinary stars in the interaction.