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# Variable Stars

When we are watch the night sky we could notice the twinkling stars of which some are bright while others are dim depending on its own luminosity. But there are stars whose brightness varies in brightness even in some second or may take years or centuries. These are called Variable stars and their variation might be as small as a part in a million, or may be a factor of a thousand or more. Lets see about them here.

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## What are Variable stars?

Variable stars are the stars whose brightness varies or fluctuates each time as seen from earth.They are often used for astronomical study in discovery, measuring analyzing the stars that are variable one as they give information about the universe in general. There are many possibilities for these variations:
1) The variations might be due to the rotation of a spotted star, or to an eclipse of a star by a companion star, or even by an unseen planet.

2) It might be due to the vibrations of a star. If they are complex enough the Ct scan provide their internal structure.

3) The variations may be due to eruptions on a star (flares), or an accretion disc (dwarf novae) or major explosions on a star (novae), or to the total disruption of a star in a supernova.

## Characteristics and Composition of Variable Stars

Characteristics: The variable are so, due to many reasons. It might be the change in the luminosity of the star or the mass of the star or would be any obstruction between the earth and that star which hinders the amount of light reaching the earth. Well the variable stars are of irregular class because of the complete lack of periodicity in their light variations. Among these we find a large number that are associated with nebulosity and are very significant. These are nebular variables which, because of their connection with either dark or bright nebulae, important for the study. Their complex light fluctuations and spectral characteristics with those of surrounding nebulosity, has given the astronomers a vast study of data gives the pre-main sequence evolution of such stars.

Composition:
Basically we can categorize the variable stars in to intrinsic variables and extrinsic variables. The intrinsic variables are the one whose luminosity varies due to the pulsations, eruptions or the stars undergoing swell and shrinks. Extrinsic variables are the stars that changes the brightness because of eclipse happening due to the stellar rotation or might be another star or may be the planet.

## Types of Variable Stars

In the early 1880, the astronomer Edward pickering classified the variable stars in to five classes namely new stars, long period variables, irregular variables, short-period variables and eclipsing variables. As the time eventually revealed, the system had flaws and as the number of variable stars increased and hence in twentieth century Henry Norris Russell classified them as novae, long period variables, irregular variables, Cepheids and eclipsing stars.

Types of Variable stars

But by tradition the variable stars are classified into two families: Intrinsic variables are the stars in which there would be variation in brightness as there would be some changes in the star itself like those changes in pulsating stars like Cepheids while Extrinsic variables are the one in which there would be some changes in the brightness due to the affect external factors of the star or might be the star rotation. The intrinsic variables are again classified into two classes namely pulsating and eruptive. The pulsating stars are Cepheids, RR Lyrae, RV Tauri and long period variables. The eruptive ones are supernovae, novae, Recurrent novae, dwarf novae, symbiotic stars and R Coronae Borealis. The extrinsic variables are just classified as Eclipsing binaries and rotating variables as shown in fig.

## Intrinsic and Extrinsic Variables

The actual classes of variable stars do not have very clear cut boundaries. The distinction between them is found within characteristic of making classification which can be empirically measured but on clear cut basis like some spectral class. Variable stars are generally classified into two types:

1) Intrinsic Variables
2) Extrinsic Variables

Intrinsic variables and Extrinsic variables. Intrinsic variable stars vary due to  to the physical processes happening within the star itself and extrinsic variable stars vary due to the processes external to the star, such as rotation. Extrinsic variables are eclipsing binaries and rotational variables. Intrinsic variables are pulsating variables, eruptive and explosive  variables.

## Cepheid Variable Stars

These are intrinsic variables named after the prototype $\Delta$ Cepheid which pulsates radiating and hence varying in both temperature as well as diameter with changing brightness but having stable amplitude and period. It gives a definite relationship between its intrinsic luminosity and its period. Its period is 5.4 days and its apparent magnitude varies from 3.6 to 4.3 mag. They have periods from 1 to 100 days and almost 1000 identified in our galaxy. Another familiar Cepheid is Polaris that changes by only 0.1 mag, from 2.5 to 2.6. Since it has such a slight change in magnitude for long time it was not known that Polaris is variable and hence they were used as a reference for measuring the magnitude of other stars. Cepheids have masses of approx 6 $M_\odot$, and radii of about 25 $R_\odot$. They are extremely luminous and hence can be observed even though it is at distant apart.

## Pulsating Variable Stars

Pulsating stars are most frequent type of variable stars, which are found throughout the Hertzsprung-Russell diagram. They are intrinsic variable stars whose brightness varies due to the change in the physical state of the star. Here there is actually expansion and contraction in star's surface layers and hence the star size actually increases and decreases periodically. These stars actually reveal many properties of the stellar interiors. There is also a group of white dwarf that are pulsationally unstable and show a very rich and complicated pattern of pulsation, consisting of the superposition of many modes. They helpout in understanding the internal structure and chemical composition of these stellar corpses.

## Cataclysmic Stars

 Name Constellation 1 R And Andromeda 2 S And Andromeda 3 U Ant Antlia 4 $\theta$ Aps Antlia 5 $\eta$ Aql aquila 6 V Aql Aquila 7 R Aqr Aquarius