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This makes the gas more opaque, and radiation temporarily becomes captured in the gas. This heats the gas further, leading it to expand once again. Thus a cycle of expansion and compression swelling and shrinking is maintained. In a given constellation, the first variable stars discovered were designated with letters R through Z, e. This system of nomenclature was developed by Friedrich W. Argelander , who gave the first previously unnamed variable in a constellation the letter R, the first letter not used by Bayer.
Once those combinations are exhausted, variables are numbered in order of discovery, starting with the prefixed V onwards. These subgroups themselves are further divided into specific types of variable stars that are usually named after their prototype. For example, dwarf novae are designated U Geminorum stars after the first recognized star in the class, U Geminorum.
The pulsating stars swell and shrink, affecting their brightness and spectrum. Pulsations are generally split into: Depending on the type of pulsation and its location within the star, there is a natural or fundamental frequency which determines the period of the star. Stars may also pulsate in a harmonic or overtone which is a higher frequency, corresponding to a shorter period. Pulsating variable stars sometimes have a single well-defined period, but often they pulsate simultaneously with multiple frequencies and complex analysis is required to determine the separate interfering periods.
In some cases, the pulsations do not have a defined frequency, causing a random variation, referred to as stochastic. The study of stellar interiors using their pulsations is known as asteroseismology.
The expansion phase of a pulsation is caused by the blocking of the internal energy flow by material with a high opacity, but this must occur at a particular depth of the star to create visible pulsations.
If the expansion occurs below a convective zone then no variation will be visible at the surface. If the expansion occurs too close to the surface the restoring force will be too weak to create a pulsation. The restoring force to create the contraction phase of a pulsation can be pressure if the pulsation occurs in a non-degenerate layer deep inside a star, and this is called an acoustic or pressure mode of pulsation, abbreviated to p-mode.
In other cases, the restoring force is gravity and this is called a g-mode. Pulsating variable stars typically pulsate in only one of these modes. This group consists of several kinds of pulsating stars, all found on the instability strip , that swell and shrink very regularly caused by the star's own mass resonance , generally by the fundamental frequency. Generally the Eddington valve mechanism for pulsating variables is believed to account for cepheid-like pulsations.
Each of the subgroups on the instability strip has a fixed relationship between period and absolute magnitude, as well as a relation between period and mean density of the star. The period-luminosity relationship was first established for Delta Cepheids by Henrietta Leavitt , and makes these high luminosity Cepheids very useful for determining distances to galaxies within the Local Group and beyond.
Edwin Hubble used this method to prove that the so-called spiral nebulae are in fact distant galaxies. Note that the Cepheids are named only for Delta Cephei , while a completely separate class of variables is named after Beta Cephei.
Classical Cepheids or Delta Cephei variables are population I young, massive, and luminous yellow supergiants which undergo pulsations with very regular periods on the order of days to months.
On September 10, , Edward Pigott detected the variability of Eta Aquilae , the first known representative of the class of Cepheid variables. However, the namesake for classical Cepheids is the star Delta Cephei , discovered to be variable by John Goodricke a few months later.
The Type II have somewhat lower metallicity , much lower mass, somewhat lower luminosity, and a slightly offset period verses luminosity relationship, so it is always important to know which type of star is being observed. These stars are somewhat similar to Cepheids, but are not as luminous and have shorter periods.
Due to their common occurrence in globular clusters , they are occasionally referred to as cluster Cepheids. They also have a well established period-luminosity relationship, and so are also useful as distance indicators. These A-type stars vary by about 0. They were once known as Dwarf Cepheids. They often show many superimposed periods, which combine to form an extremely complex light curve. Their spectral type is usually between A0 and F5. They exhibit fluctuations in their brightness in the order of 0.
They have extremely rapid variations with periods of a few minutes and amplitudes of a few thousandths of a magnitude. The long period variables are cool evolved stars that pulsate with periods in the range of weeks to several years. Mira variables are AGB red giants. Over periods of many months they fade and brighten by between 2. The very large visual amplitudes are mainly due to the shifting of energy output between visual and infra-red as the temperature of the star changes.
In a few cases, Mira variables show dramatic period changes over a period of decades, thought to be related to the thermal pulsing cycle of the most advanced AGB stars. These are red giants or supergiants.
Semiregular variables may show a definite period on occasion, but more often show less well-defined variations that can sometimes be resolved into multiple periods. At least some of the semi-regular variables are very closely related to Mira variables, possibly the only difference being pulsating in a different harmonic.
These are red giants or supergiants with little or no detectable periodicity. Some are poorly studied semiregular variables, often with multiple periods, but others may simply be chaotic. Many variable red giants and supergiants show variations over several hundred to several thousand days. The brightness may change by several magnitudes although it is often much smaller, with the more rapid primary variations are superimposed.
The reasons for this type of variation are not clearly understood, being variously ascribed to pulsations, binarity, and stellar rotation. They are at their brightest during minimum contraction. Many stars of this kind exhibits multiple pulsation periods. Slowly pulsating B SPB stars are hot main-sequence stars slightly less luminous than the Beta Cephei stars, with longer periods and larger amplitudes.
The prototype of this rare class is V Hydrae , a 15th magnitude subdwarf B star. They pulsate with periods of a few minutes and may simultaneous pulsate with multiple periods. They are p-mode pulsators. Stars in this class are type Bp supergiants with a period of 0.
Their spectra are peculiar by having weak hydrogen while on the other hand carbon and helium lines are extra strong, a type of Extreme helium star. These are yellow supergiant stars actually low mass post-AGB stars at the most luminous stage of their lives which have alternating deep and shallow minima.
This double-peaked variation typically has periods of 30— days and amplitudes of 3—4 magnitudes. Superimposed on this variation, there may be long-term variations over periods of several years. Their spectra are of type F or G at maximum light and type K or M at minimum brightness.
They lie near the instability strip, cooler than type I Cepheids more luminous than type II Cepheids. Their pulsations are caused by the same basic mechanisms related to helium opacity, but they are at a very different stage of their lives.
Their periods range from several days to several weeks, and their amplitudes of variation are typically of the order of 0. The light changes, which often seem irregular, are caused by the superposition of many oscillations with close periods. Deneb , in the constellation of Cygnus is the prototype of this class.
Their periods are around one day and their amplitudes typically of the order of 0. These non-radially pulsating stars have short periods of hundreds to thousands of seconds with tiny fluctuations of 0.
Known types of pulsating white dwarf or pre-white dwarf include the DAV , or ZZ Ceti , stars, with hydrogen-dominated atmospheres and the spectral type DA;  DBV , or V Her , stars, with helium-dominated atmospheres and the spectral type DB;  and GW Vir stars, with atmospheres dominated by helium, carbon, and oxygen. The Sun oscillates with very low amplitude in a large number of modes having periods around 5 minutes. The study of these oscillations is known as helioseismology.
Oscillations in the Sun are driven stochastically by convection in its outer layers. The term solar-like oscillations is used to describe oscillations in other stars that are excited in the same way and the study of these oscillations is one of the main areas of active research in the field of asteroseismology. Eruptive variable stars show irregular or semi-regular brightness variations caused by material being lost from the star, or in some cases being accreted to it.
Despite the name these are not explosive events, those are the cataclysmic variables. Protostars are young objects that have not yet completed the process of contraction from a gas nebula to a veritable star. Most protostars exhibit irregular brightness variations. Orion variables are young, hot pre—main-sequence stars usually embedded in nebulosity.
They have irregular periods with amplitudes of several magnitudes. A well-known subtype of Orion variables are the T Tauri variables. Variability of T Tauri stars is due to spots on the stellar surface and gas-dust clumps, orbiting in the circumstellar disks. These stars reside in reflection nebulae and show gradual increases in their luminosity in the order of 6 magnitudes followed by a lengthy phase of constant brightness.
They then dim by 2 magnitudes six times dimmer or so over a period of many years. V Cygni for example dimmed by 2. FU Orionis variables are of spectral type A through G and are possibly an evolutionary phase in the life of T Tauri stars. Large stars lose their matter relatively easily. For this reason variability due to eruptions and mass loss is fairly common among giants and supergiants.
Also known as the S Doradus variables, the most luminous stars known belong to this class. They have permanent high mass loss, but at intervals of years internal pulsations cause the star to exceed its Eddington limit and the mass loss increases hugely. Visual brightness increases although the overall luminosity is largely unchanged.
Giant eruptions observed in a few LBVs do increase the luminosity, so much so that they have been tagged supernova impostors , and may be a different type of event.
These massive evolved stars are unstable due to their high luminosity and position above the instability strip, and they exhibit slow but sometimes large photometric and spectroscopic changes due to high mass loss and occasional larger eruptions, combined with secular variation on an observable timescale.
The best known example is Rho Cassiopeiae. While classed as eruptive variables, these stars do not undergo periodic increases in brightness. Instead they spend most of their time at maximum brightness, but at irregular intervals they suddenly fade by 1—9 magnitudes 2. Most are classified as yellow supergiants by luminosity, although they are actually post-AGB stars, but there are both red and blue giant R CrB stars. DY Persei variables are a subclass of R CrB variables that have a periodic variability in addition to their eruptions.
Wolf—Rayet stars are massive hot stars that sometimes show variability, probably due to several different causes including binary interactions and rotating gas clumps around the star. They exhibit broad emission line spectra with helium , nitrogen , carbon and oxygen lines.
Variations in some stars appear to be stochastic while others show multiple periods. In main-sequence stars major eruptive variability is exceptional.
It is common only among the flare stars , also known as the UV Ceti variables, very faint main-sequence stars which undergo regular flares. They increase in brightness by up to two magnitudes six times brighter in just a few seconds, and then fade back to normal brightness in half an hour or less. Several nearby red dwarfs are flare stars, including Proxima Centauri and Wolf These are close binary systems with highly active chromospheres, including huge sunspots and flares, believed to be enhanced by the close companion.
Variability scales ranges from days, close to the orbital period and sometimes also with eclipses, to years as sunspot activity varies. Supernovae are the most dramatic type of cataclysmic variable, being some of the most energetic events in the universe. A supernova can briefly emit as much energy as an entire galaxy , brightening by more than 20 magnitudes over one hundred million times brighter. This collapse "bounces" and causes the star to explode and emit this enormous energy quantity.
The outer layers of these stars are blown away at speeds of many thousands of kilometers an hour. The expelled matter may form nebulae called supernova remnants. A well-known example of such a nebula is the Crab Nebula , left over from a supernova that was observed in China and North America in The core of the star or the white dwarf may either become a neutron star generally a pulsar or disintegrate completely in the explosion.
Supernovae can result from the death of an extremely massive star, many times heavier than the Sun. Sie sind bereits im Ausland und benötigen dringend Bargeld in der Landeswährung? Mit Western Union können sie sich bequem Bargeld in der Landeswährung senden lassen. Überall in Ihrer Nähe! Filiale, die Ihren gewünschten Kriterien entspricht. Wir verwenden Cookies und Analysesoftware, um unsere Website möglichst benutzerfreundlich zu gestalten.
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