A variable star is a star whose Luminosité varies.
Whereas the majority of stars are of almost constant luminosity, as our Sun which practically does not have measurable variation (approximately 0,1% on a 11 years cycle), the luminosity of certain stars varies in a perceptible way over periods of time much shorter.
This variation of luminosity was discovered at the 16th century during the appearance of the supernova of 1572 by Tycho Brahé and observation of the increase and regular reduction in the glare of the star Mira (O Ceti) in 1596. One discovered more and more variable stars progressively of the improvement of the instruments of observation; currently, the catalogs, of which most important is the General Catalog off Variable Stars , contain more: 40,000 stars variable or suspectées to be it.
In the beginning, the luminosity of stars was visually given by comparing a star with its neighbors. Later, the development of the Photographie made it possible to compare these luminosities on a photographic plate. Currently, they are measured precisely using a photoelectric detector or camera CCC.
These luminosities are traced on a graph named curve of light which represents the magnitude according to the Temps. This graph makes it possible to determine the amplitude of the variations and them period. The recording of these curves of light is one of the only fields of astronomy where the amateurs can really help the professionals, to even carry out true research task.
Strictly speaking, all the stars are variable because their structure and their luminosity change with their evolution, but in general these changes are very slow. However, for certain evolutionary phases, the variations can be extremely fast or be periodic, like the pulsation of the external layer of certain stars. Other small variations of luminosity can be caused by cold or hot spots on the surface of star which even appear and disappear with rotation from star on it. For this reason, our Sun is a star very slightly variable because of the sunspots and it is extremely probable which the majority of stars have of the similar spots.
The variable stars are classified in two great groups, themselves subdivided in a multitude of sub-groups generally bearing the name of a star which characterizes them:
Intrinsic variable stars
These are stars whose variations of luminosity are caused by changes of the structure even of star. An intrinsic variable star perhaps attached to various types according to its behavior:
The pulsating stars contain most of the variables. These stars present a periodic variation of their volume, which results in a modification of their luminosity:
Variables by rotation
The variable stars by rotation see their luminosity varying by the presence of dark or clear spots on their surface. Thus, when the star rotates, more or less of light arrives to us.
Variable eruptive (in the past called variable irregular )
An eruptive variable star knows an ongoing activity in its Chromosphère or its crown which causes variations of luminosity impossible to envisage and which can be accompanied by a fort stellar Vent or matter ejections. The principal types of eruptive variables are:
Extrinsic variable stars
The variation of luminosity of extrinsic variable stars, as observed by a terrestrial observer, is due to an external cause with star and not to a modification of its properties.
Optical variable (or with eclipses)
The extrinsic main cause of variability is the presence of another star around principal star, forming together a star doubles. Sight under a certain angle, one of these two stars can with regular intervals eclipse R the other, thus causing a reduction in the total luminosity.
Cataclysmic variables (in the past called variable eruptive )
A cataclysmic variable star sees its luminosity evolving/moving abruptly, generally on several magnitudes, thereafter of physical phenomena extremely violent one.
In some binary systems, the two stars are so close one to the other that the force of Gravitation of the most massive star tears off part of the matter of his/her partner. In many cases, this mass forms a Disque of accretion. These systems are called binary system in interaction . The distance in-on this side which this situation can arrive corresponds to the “Lobe of Rock” of star, according to Edouard Roche, the astronomer having created the theory of this kind of system.
On the most massive star, the arrival of this additional mass and different composition can, by the release of nuclear reactions, to cause various phenomena, sometimes cataclysmic. The Nova E traditional, known as also recurring , are one of the most spectacular forms of this phenomenon which appears by intense variations of luminosity. The dwarf novae are another category of cataclysmic variables whose variations of luminosity, less spectacular, would be caused by a variation of accretion rate in the disc.
The variations of luminosity can also occur in other partie of the electromagnetic Specter that the visible one, in particular in the fields of the x-rays. In the named systems binary X which would consist of a star normal or at the end of the lifetime, called secondary star and of a star compacts, such as a white Naine, a neutron star, even a Black hole, called primary star ; the interaction of the matter coming from secondary star and the intense gravitational field of primary star produces an enormous quantity of energy of which a part reaches us in the form of x-rays.
- Variable céphéide
French Association of the variable star observers
- The American Association off Variable Star Observers
- very complete Site gathering and describing all the kinds of variable stars
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