The unqualified term instability strip usually refers to a region of the Hertzsprung–Russell diagram largely occupied by several related classes of pulsating

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...

Delta Scuti variable A Delta Scuti variable (sometimes termed dwarf cepheid when the V-band amplitude is larger than 0.3 mag.) is a class of pulsating star, comprising several sub-classes of object with A- or F-type spectra. The variables follow a period-luminosity ...

SX Phoenicis variable An SX Phoenicis variable is a type of variable star. These stars exhibit a short period pulsation behavior that varies on time scales of 0.03–0.08 days (0.7–1.9 hours). They have spectral classifications in the range A2-F5 and vary in magnitude ...

s, and rapidly oscillating Ap stars (roAps) near the

main sequence In astronomy, the main sequence is a classification of stars which appear on plots of stellar color index, color versus absolute magnitude, brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or d ...

;

RR Lyrae variables RR Lyrae variables are periodic variable stars, commonly found in globular clusters. They are used as standard candles to measure (extra) galactic distances, assisting with the cosmic distance ladder. This class is named after the prototype an ...

where it intersects the

horizontal branch The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha proc ...

; and the

Cepheid variable A Cepheid variable () is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period (typically 1–100 days) and amplitude. Cepheids are important cosmi ...

s where it crosses the supergiants. RV Tauri variables are also often considered to lie on the instability strip, occupying the area to the right of the brighter Cepheids (at lower temperatures), since their stellar pulsations are attributed to the same mechanism.

Position on the HR diagram

The Hertzsprung–Russell diagram plots the real

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

of stars against their

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

(their

color Color (or colour in English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is the visual perception based on the electromagnetic spectrum. Though co ...

, given by the temperature of their

photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

). The instability strip intersects the

, (the prominent diagonal band that runs from the upper left to the lower right) in the region of A and F stars (1–2 solar mass ()) and extends to G and early K bright supergiants (early M if RV Tauri stars at minimum are included). Above the main sequence, the vast majority of stars in the instability strip are variable. Where the instability strip intersects the main sequence, the vast majority of stars are stable, but there are some variables, including the roAp stars and the Delta Scuti variables.

Pulsations

Stars in the instability strip pulsate due to He III (doubly

ionized Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...

helium), in a process based on the Kappa–mechanism. In normal A-F-G class stars, He in the stellar

is neutral. Deeper below the photosphere, where the temperature reaches 25,000–, begins the He II layer (first He ionization). Second ionization of helium (He III) starts at depths where the temperature is 35,000–. When the star contracts, the

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

and

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

of the He II layer increases. The increased energy is sufficient to remove the lone remaining electron in the He II, transforming it into He III (second

ionization Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged at ...

). This causes the opacity of the He layer to increase and the

energy flux Energy flux is the rate of transfer of energy through a surface. The quantity is defined in two different ways, depending on the context: # Total rate of energy transfer (not per unit area); SI units: W = J⋅s−1. # Specific rate of energy tran ...

from the interior of the star is effectively absorbed. The temperature of the star's core increases, which causes it to expand. After expansion, the He III cools and begins to recombine with free electrons to form He II and the opacity of the star decreases. This allows the trapped heat to propagate to the surface of the star. When sufficient energy has been radiated away, overlying stellar material once again causes the He II layer to contract, and the cycle starts from the beginning. This results in the observed increase and decrease in the surface temperature of the star. In some stars, the pulsations are caused by the opacity peak of metal ions at about . The phase shift between a star's

radial Radial is a geometric term of location which may refer to: Mathematics and Direction * Vector (geometric), a line * Radius, adjective form of * Radial distance (geometry), a directional coordinate in a polar coordinate system * Radial set * A ...

pulsations and

brightness Brightness is an attribute of visual perception in which a source appears to be radiating/reflecting light. In other words, brightness is the perception dictated by the luminance of a visual target. The perception is not linear to luminance, and ...

variations depends on the distance of He II zone from the stellar surface in the

stellar atmosphere The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone. Overview The stellar atmosphere is divided into several regions of distinct character: * The photosphere, whi ...

. For most Cepheids, this creates a distinctly asymmetrical observed light curve, increasing rapidly to maximum and slowly decreasing back down to minimum.

Other pulsating stars

There are several types of pulsating star not found on the instability strip and with pulsations driven by different mechanisms. At cooler temperatures are the long period variable AGB stars. At hotter temperatures are the

Beta Cephei Beta Cephei (β Cephei, abbreviated Beta Cep, β Cep) is a triple star system of the third magnitude in the constellation of Cepheus. Based on parallax measurements obtained during the Hipparcos mission, it is approximately 690 light-y ...

and PV Telescopii variables. Right at the edge of the instability strip near the main sequence are

Gamma Doradus variables Gamma Doradus variables are variable stars which display variations in luminosity due to non-radial pulsations of their surface. The stars are typically young, early F or late A spectral classification, type main sequence stars, and typical bright ...

. The band of White dwarfs has three separate regions and types of variable: DOV, DBV, and DAV (= ZZ Ceti variables) white dwarfs. Each of these types of pulsating variable has an associated instability strip created by variable opacity partial ionisation regions other than helium. Most high luminosity supergiants are somewhat variable, including the Alpha Cygni variables. In the specific region of more luminous stars above the instability strip are found the yellow hypergiants which have irregular pulsations and eruptions. The hotter luminous blue variables may be related and show similar short- and long-term spectral and brightness variations with irregular eruptions.

References

{{DEFAULTSORT:Instability Strip Hertzsprung–Russell classifications Stellar evolution