Introduction
My scientific interests lie among the category of variable stars.
Eclipsing binaries
The Eclipsing Binaries are geometrical variable stars that consist of two stars-members orbiting their common center of mass. The angle between their orbital plane and the line of sight allows us to observe successive eclipses between the components of the system. They are one of the most important categories of stellar systems due to the high importance of the information provide. The absolute parameters of the stars-members (i.e. masses, radii, temperatures, luminosities) can be directly calculated with the combination of spectroscopic and photometric observations, a fact which is very important since there is no other way deriving these parameters in single stars. According to their light curve shape, which is related to their evolutionary status and some times indicates the system's configuration, they can be divided in three main categories: a) Algol type (EA), b) β Lyrae type (EB) and c) W UMa type (EW). Moreover, a new sub-category of EA stars, namely oEA (oscillating EA) stars, includes semidetached systems in which at least one of the components shows pulsational behaviour.
Pulsating Stars
Pulsating stars are physical variables and they usually present asymmetric light curves. There are many categories of these stars (e.g. β Cep, δ Sct, classical Cepheids etc.) and also too many criteria for categorize them. The light curve variations are caused due to the oscillations of their internal layers or of their atmosphere. The pulsations could be radial or non-radial, and they can be divided into f-mode (i.e. fundamental-radial), p-mode (i.e. pressure mode-non-radial) and in the cases where the star rotates rapidly, or is a member of a binary system, into g-mode (i.e. gravitational mode-non-radial). The radial pulsations are created due to the pause of the hydrostatic equilibrium in the internal layers of the star, while the non-radial ones (i.e. p-modes) occur on the star's surface due to the large difference in pressure and temperature into various regions of the surface. This kind of stars is believed that, they are in the middle of two different stages of evolution, namely from main-sequence dwarfs to giants, and their pulsational behaviour is a temporary stage of their lifetime. The last few years the field of "Asteroseismology" examines case by case all the candidate pulsating stars, in order to extract conclusion about the physical mechanisms which form this kind of behaviour.
Novae
This kind of binary systems are consisted usually of at least one evolved star (e.g. white dwarf), and a companion which cannot have been evolved further than the giant or supergiant stage. The more massive star, usually a white dwarf, gains mass from its companion and an accretion disk around it is formed. The companion looses mass with a steady and rapid rate and its evolution differs a lot from that of a single star. The matter that reaches the surface of the white dwarf causes local eruptions and sudden increase of the system's brightness. Of course, these eruptions are usually non-periodic and additionally they are presented as extremely asymmetric. The later of the system is extremely violent, since after the stop of the mass flow from the companion to the white dwarf, the companion might be evolved more quickly into a white dwarf too and the system will be transformed into a X-ray binary.
Eclipsing binaries
The Eclipsing Binaries are geometrical variable stars that consist of two stars-members orbiting their common center of mass. The angle between their orbital plane and the line of sight allows us to observe successive eclipses between the components of the system. They are one of the most important categories of stellar systems due to the high importance of the information provide. The absolute parameters of the stars-members (i.e. masses, radii, temperatures, luminosities) can be directly calculated with the combination of spectroscopic and photometric observations, a fact which is very important since there is no other way deriving these parameters in single stars. According to their light curve shape, which is related to their evolutionary status and some times indicates the system's configuration, they can be divided in three main categories: a) Algol type (EA), b) β Lyrae type (EB) and c) W UMa type (EW). Moreover, a new sub-category of EA stars, namely oEA (oscillating EA) stars, includes semidetached systems in which at least one of the components shows pulsational behaviour.
Pulsating Stars
Pulsating stars are physical variables and they usually present asymmetric light curves. There are many categories of these stars (e.g. β Cep, δ Sct, classical Cepheids etc.) and also too many criteria for categorize them. The light curve variations are caused due to the oscillations of their internal layers or of their atmosphere. The pulsations could be radial or non-radial, and they can be divided into f-mode (i.e. fundamental-radial), p-mode (i.e. pressure mode-non-radial) and in the cases where the star rotates rapidly, or is a member of a binary system, into g-mode (i.e. gravitational mode-non-radial). The radial pulsations are created due to the pause of the hydrostatic equilibrium in the internal layers of the star, while the non-radial ones (i.e. p-modes) occur on the star's surface due to the large difference in pressure and temperature into various regions of the surface. This kind of stars is believed that, they are in the middle of two different stages of evolution, namely from main-sequence dwarfs to giants, and their pulsational behaviour is a temporary stage of their lifetime. The last few years the field of "Asteroseismology" examines case by case all the candidate pulsating stars, in order to extract conclusion about the physical mechanisms which form this kind of behaviour.
Novae
This kind of binary systems are consisted usually of at least one evolved star (e.g. white dwarf), and a companion which cannot have been evolved further than the giant or supergiant stage. The more massive star, usually a white dwarf, gains mass from its companion and an accretion disk around it is formed. The companion looses mass with a steady and rapid rate and its evolution differs a lot from that of a single star. The matter that reaches the surface of the white dwarf causes local eruptions and sudden increase of the system's brightness. Of course, these eruptions are usually non-periodic and additionally they are presented as extremely asymmetric. The later of the system is extremely violent, since after the stop of the mass flow from the companion to the white dwarf, the companion might be evolved more quickly into a white dwarf too and the system will be transformed into a X-ray binary.