Variable star

Variable stars, variable stars, or shortly variable, are stars seen from Earth brightness fluctuations have less than a century, in a period of time. These brightness variations are not to be confused with the scintillation, the twinkling of the stars, which is caused by air turbulence. While earlier variable stars were regarded as something special, it is believed today that all the stars show in the course of its development temporarily brightness fluctuations. There are two different types of variability:

• Intrinsic variability, thereby changing the luminosity of the star
• Extrinsic variability, with the brightness does not change in all directions, but in the direction of the earth. An example is Bedeckungsveränderlichkeit in which a star is covered by a companion.

In recent decades, monitoring and development in measurement technology have expanded the knowledge of variable stars. The number of stars, where you can find brightness variations, has thereby increased many times over. By increasing the measurement accuracy, it has become more difficult to find a universal definition to delineate variable stars of the immutable:

• A brightness change takes place in time periods that are compared to the general stellar evolution to be considered as short ( light change within decades to centuries ).
• The light change is observable in the optical, near ultraviolet or near infrared region.
• The photometrically measurable amplitudes have in the last 100 years, from about 0.05 to 0.0001 might like refined in satellite measurements, which puts a dividing line between " fixed " stars.

• 4.1 eclipsing
• 4.2 Rotation Variable
• 4.3 Pulsationsveränderliche
• 4.4 cataclysmic variables
• 4.5 Eruptive variables

• 8.1 videos

History

Antiquity

The first descriptions of variables can be found in Chinese chronicles. The new stars were either novae or supernovae. However, it may have also acted to comets or planetary configurations. After the Aristotelian world view of heaven was eternal and all changes phenomena of the atmosphere. There is therefore no reports of Variable Stars from antiquity. Only with the beginning of the Renaissance, the variable stars were perceived.

Renaissance

The first observed variable was Mira, the Wonderful, which was first described in 1596 by David Fabricius. The cycles of light intensity of the temporarily visible to the naked eye Mira star with a period of 11 months and an amplitude of 8 likes was first described in 1639 by Johann Holwarda. This was the first known variable in addition to the guest stars ( nova or supernova ). Already in 1572 Tycho Brahe was demonstrated by the immeasurably small parallax of the supernova of the year that the guest stars are not phenomena of the atmosphere. However, Nova and Super Nova are not counted until the beginning of the 20th century to the variables.

The visual era

Mira has long been considered unique until the discovery of the variability of Algol by Geminianus Montanari in 1669. Until 1844, only 21 variable stars were known, which had either been found by chance or were discovered in the search for asteroids. In the same year Friedrich Wilhelm August Argelander published his " invitation to the Friends of Astronomy", which can be regarded as an impetus for the systematic discovery and observation of variable stars well.

Through the Bonn survey, there were in the second half of the 19th century for the first time a star atlas for safe telescope stars; Thus stars which are not visible with the naked eye. Through the comparison of the night sky through the telescope with the Bonner Durchmusterung numerous variables of large amplitude have been discovered. The brightness determination was achieved by estimating the variables to constant comparison stars. This method achieves an accuracy of at best 0.3 mag and is subject to subjective influences. Amateur astronomers have observed to date with this method and their combined long-term light curves over a range of more than 100 years in the research of great value.

Introduction of photographic methods

When, after 1880, the sensitivity of the photographic plates allowed the initiation of stars, it ushered in a new era in the study of variable stars. A photographic plate stores the brightness of thousands of stars for subsequent studies and facilitates the discovery. Two plates of the same region of the sky are flashed. The recordings are arranged so that overlap the stars and with the help of a shutter each one or the other plate is shown alternate. Variable stars show through a flashing. In this way, most of the variables have been found up to approximately 1990. It can be variable with amplitudes of less than 0.3 may be discovered, which also corresponds to the accuracy of the brightness measurements. The important for astronomical distance measurement period-luminosity relation of Cepheids in 1912 by Henrietta Swan Leavitt first described in a study of the variables in the Magellanic Clouds. However, it had not been possible to calibrate this relationship.

The 20th century

New and improved monitoring techniques have made it together with the advancement of theoretical physics allows the causes of the changes in brightness of variable stars in the context of astrophysics to understand.

• The star spectroscopy has made the measurement of the radial velocity changes, temperature, surface accelerations stellar magnetic fields and the chemical composition of possible stars. So could the conjecture in 1784 that Algol is an eclipsing binary, only be proved by measuring the radial velocity curve.
• The photoelectric photometry began shortly after the discovery of the photoelectric effect and thus could the accuracy of the measured brightness at first 0.01 may be increased. After the Second World War was able to lower noise amplifier, larger telescopes and more stable power supply, the measurement accuracy even improved to some millimag. At the same time the integration time could be reduced to fractions of a second, which has led to the discovery of very fast phenomena such as flickering at Akkretionprozessen.
• The observation of stars outside the visual range has contributed much to the understanding of variable stars, and only awakened the interest in new classes of stars. These include, in the satellite-based measurements in the field of gamma radiation, X-rays, ultraviolet radiation, infrared radiation and microwave radiation. Only the near-infrared and radio waves as well as the visual field can be observed from the Earth's surface.
• Using the simulation of stars in computers stellar models and their development could be tested and constantly improved.

The 21st century

In this century so far continue the trends that have already indicated in the last decades of the last century.

• CCD sensors have replaced the photoelectric photometry and photographic technique with a few exceptions. Since hundreds of already available with the help of CCDs, the brightness of thousands of stars to as digital data, the discovery and classification of variable stars is automatic. So alone the OGLE project has discovered more than 80,000 new variable in or towards the Magellanic Clouds.
• Increasing computer performance makes the transition from 2D to 3D simulations. Many dynamic processes such as supernova bursts, pulsations and stellar magnetic fields resulting red giant in 3D simulations different results than in the calculations of 2D cutouts.
• The satellite-based observations have the accuracy of the brightness measurements in the range of a few 0.0001 may increase. This has led to the discovery of extrasolar planetary transits and allows by means of asteroseismology deeper insight into the structure of stars. Furthermore, for example, has the Hubble Space Telescope brought a significant increase in sensitivity and angular resolution. So the first time a jet are imaged by a T Tauri star next to the causative accretion disk.
• The enlargement of the observation technique for the detection of variable stars no longer relies solely on electromagnetic radiation. Nowadays, to improve the detection sensitivity can be worked in the field of neutrino astronomy and high-energy, emitted directly from the stars particles.
• The digitization of record collections, for example, within the DASCH project at the Harvard-Smithsonian Center for Astrophysics leads to the discovery of slow and rare changes in brightness.

Designation

The naming of variable stars in the general galactic field is a combination of an identifier and the constellation. After the letters have already been to Q used in the catalog of Johann Bayer was the first variable the identifier of R. An example is the first variable in the constellation shield that has the name of R Scuti. After you've arrived at Z followed RR, RS and SS ... RZ, ST -SZ etc to ZZ. When was this namespace exhausted AA to AZ, etc was up to QZ ( J has been omitted to avoid confusion with I). After that was started per constellation with the number V335 and counted.

Variable stars of the Milky Way are listed in the General Catalogue of Variable Stars and these are approximately 46,000 with the state in late 2010. Besides variables in the GCVS are still 10,000 variables in other galaxies as well as over 10,000 " suspected " section. Both of these attachments will no longer be updated. Whether the naming continues is open. It is expected that the artificial satellite Gaia will discover about 18 million new variable stars in the Milky Way.

Importance

Variable stars are interesting in many ways for Astrophysics:

• Variable stars are the basis for measuring distance within and outside the Milky Way over the period-luminosity relation for pulsating variables and by the identical maximum brightness of all supernovae of type Ia.
• The asteroseismology pulsating variables made ​​possible by the analysis of oscillations in a star an insight into the inner structure.
• For double- periodic pulsating variables to calculate the stellar mass is possible. This can otherwise be done only in binary systems. There, however, may differ from that of a single star by previous mass transfer, the structure of the stars.
• In eclipsing binary stars, a resolution of the stellar surface is achieved by analyzing the change in brightness when a star a second hidden.
• For periodic variables the smallest changes of the stellar structure can be found, since the sum of these changes and thus easier to be detected than with a direct measurement.
• No classification requires less effort than the measurement of brightness. Therefore Variable Stars for stellarstatistische tests are used when the stars are too faint for recording spectra.

Classification

There are different classifications of variable stars, which are all based primarily on the determination of the amplitude, periodicity and Forme of the light curve. The following are the groupings from the General Catalogue of Variable Stars ( GCVS ) lists be supplemented with newer subgroups that were not known when creating the GCVS in 1985:

Eclipsing

Eclipsing binary stars can be observed when the components of a binary star system from the perspective of the earth after another pass and thereby cover each other. By covering a star is no longer the brightness of both stellar disks from the earth visible and we observe a minimum. Eclipsing binaries are divided into three main groups:

• Algolsterne
• Beta Lyrae stars
• W Ursae Maiori star

Rotation Variable

Rotation Variable stars are stars that change in the course of its rotation its brightness. This is either because they are deformed as components of close binary stars ellipsoidal or because they show a non- uniform brightness distribution on the stellar surface. A non-uniform luminance distribution can be caused by sun spots or thermal or chemical inhomogeneities caused by a magnetic field, which does not coincide with the rotation axis. Rotation variables are divided into:

• Alpha2 Canum - Venaticorum - variable
• SX Arietis star
• BY Draconis star
• Rotating ellipsoidal variable
• FK- Comae Berenices - star
• Pulsars

Pulsationsveränderliche

Pulsating variables show a periodic contraction and expansion of its surface. The radial or non-radial vibration leads to a luminosity change due to the change of the radius of the star shape and / or the surface temperature. Based on their periods, masses and their development status, the following sub- types can be distinguished:

• Alpha Cygni stars
• Beta Cephei stars
• Cepheids
• Anomalous Cepheids or BL Bootis stars
• Delta Scuti stars
• SX Phoenix Star
• Gamma Doradus stars
• Slowly pulsating B stars (LPS )
• Mira stars
• Semi- regular variable stars
• Irregular slow
• PV TELESCOPI star
• RR Lyrae stars
• RV Tauri stars
• Quick pulsating hot B stars ( RPHS )
• ZZ Ceti stars

Cataclysmic variables

Cataclysmic variables are stars with brightness outbreaks, caused by an anomaly in thermonuclear reactions on the surface or inside the star. However, the outbreaks can also have their origin in an accretion disk. The most cataclysmic variables consist of a white dwarf, the matter of a accretion disk gets from a companion. This definition cataclysmic variable deviates from the otherwise in the literature (,) used. The following sub- groups:

• Novae
• Recurrent novae
• Supernovae
• Dwarf novae
• Nova Similar variable
• Symbiotic stars
• Polar
• DQ Herculis stars
• AM Canum - Venaticorum star

Eruptive variables

The brightness changes of eruptive variables based on flares, eruptions or Massenausflüssen shell in the form of stellar winds and / or interaction with the interstellar medium. They are divided into the following subgroups:

• FU Orionis stars
• Be stars
• Gamma Cassiopeiae star
• R Coronae Borealis star
• RS Canum - Venaticorum star
• Luminous Blue Variables
• Flare stars
• Wolf- Rayet stars
• Quick irregular variables including subgroups such as the T Tauri stars