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Pulsating Variable

A term referring to stars whose magnitude varies due to physical changes in the star's structure. Processes within variable stars of this kind cause the star's surface layers to expand and contract over time, and these pulsations in turn cause the star's brightness to fluctuate periodically. These stars show an imbalance between the outward pressure of radiation from the star's core and the inward pressure of gravity, and this imbalance causes the star's form the oscillate over time.

The processes behind the pulsations vary from star to star, but they are broadly divided into two categories: radial (in which the entire star swells or contracts symmetrically), and non-radial (in which the star's shape deforms as the pulsations only affect certain surface regions). Besides this basic division, pulsating variables are subdivided into many different types, of which some of the major classifications are listed below.

Delta Cephei, the prototype for the classical Type I Cepheid pulsating variables. The star's outer layers pulsate, causing its magnitude to climb to +3.48, and then rapidly fall to +4.37, over a periodic cycle of five days and nine hours. Imagery provided by Aladin sky atlas

Major Classifications

Type Description Examples
Alpha Cygni Variables Blue or white supergiant stars, typified by Alpha Cygni or Deneb in Cygnus, showing complex patterns of overlaid oscillations that form a regular cycle. The cycle takes at least several days, and sometimes weeks, to repeat. Rigel
Beta Cephei Variables Hot stars of the B-type and O-type spectral classifications, Beta Cephei variables typically swell and contract over a period of less than a day, with the star emitting most light energy as it reaches its minimum diameter. Most variables of this kind are of the radial type, but there are a few rare non-radial examples. Hadar
Cepheid or Delta Cephei Variables
Type I
Supergiant stars of which Delta Cephei is a prominent example, and the source of the name Cepheid. The magnitude of Cepheid variables typically varies over periods of dozens of days and, crucially, this period correlates closely with the luminosity of the star. This relationship means that the period of a given Cepheid variable can be used to accurately calculate its distance. Polaris
l Carinae
Beta Doradus
Delta Cephei
Cepheid or Delta Cephei Variables
Type II
These variables are related to the classical or Type I Cepheids, though typically rather less luminous and following shorter periods of pulsation. Like classical Cepheids, these stars also show a relationship between their luminosity and their period of variability. W Virginis
BL Herculis
Delta Scuti Variables Variable stars that follow a complicated pattern of radial and non-radial pulsations, so that in some cases different parts of the star may be expanding or contracting concurrently. Like Cepheids, Delta Scuti variables show a relationship between their period and their luminosity, allowing their distance to be calculated with considerable accuracy. Seginus
Epsilon Cephei
Delta Scuti
Gamma Doradus Variables A relatively rare variable type whose members are typically white or bright yellow dwarf stars, and whose variability is caused by gravity waves within the star's structure. The pulsations of Gamma Doradus variables typically follow a period a few hours, and are rarely longer than a day apart. V529 Andromedae
9 Aurigae
Gamma Doradus
Mira Variables Named for their prototype, Mira in Cetus, these are red giant stars in the last stages of their lives. Mira variables can show extreme variations in magnitude (for example Mira itself varies between a relatively bright +2 and an extremely faint +10). Variables of this kind typically have periods running into hundreds of days in length. Mira
Chi Cygni
PV Telescopii Variables A rare class of variables, typically blue supergiants with notably high levels of carbon in their structure. Stars of this kind commonly show shifts in brightness over a period of less than a day, but much longer patterns of variability can also occur. Upsilon Sagittarii
HM Librae
PV Telescopii
RR Lyrae Variables These are old, low-mass stars (typically about half the mass of the Sun) that are commonly found in the globular clusters surrounding the Milky Way Galaxy (and indeed other galaxies). Like Cepheids, their pattern of pulsation - in this case in the infrared - provides a 'standard candle' for the calculation of their distance. RR Lyrae
V675 Sagittarii
RV Tauri Variables A group of exceptionally luminous stars whose colour shifts alongside their brightness. A typical variable of this type will shine with a red or orange light at its miminum brightness, but as it undergoes radial expansion its brightness increases and its spectrum shifts towards the yellow. The cycles between peak magnitude for these stars can take several months to complete. R Scuti
AC Herculis
RV Tauri
Semiregular Variables A populous and diverse group of variable stars characterised by the fact that, while their variability shows a regular pattern of pulsation, their luminosity can also shift unpredictably within that underlying pattern (which can take years to recur). Stars belonging to this extensive group are usually giants or supergiants, and are divided into five further subcategories based on the details of their variability. Betelgeuse
Gorgonea Tertia
SX Phoenicis Variables A variant of the Delta Scuti variable type, but showing more rapid variations in brightness (over periods of minutes of hours). SX Phoenicis variables are typically older stars of the dwarf or subdwarf types, commonly found outside the main disc of the Milky Way in the surrounding galactic halo or in globular clusters. SX Phoenicis
DY Pegasi
ZZ Ceti Variables A class of variable white dwarfs that show non-radial shifts in brightness taking place over periods of seconds or minutes. Stars of this kind are occasionally observed to show sudden flares in brightness, though it is uncertain whether this is due to the underlying pulsing mechanism. ZZ Ceti
V777 Herculis


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