Instability strip
A region of the Hertzsprung–Russell diagram where pulsating variable stars are found, driven by the Kappa–mechanism involving doubly ionized helium.
The unqualified term instability strip usually refers to a region of the Hertzsprung–Russell diagram largely occupied by several related classes of pulsating variable stars: Delta Scuti variables, SX Phoenicis variables, and rapidly oscillating Ap stars (roAps) near the main sequence; RR Lyrae variables where it intersects the horizontal branch; and the Cepheid variables 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.
- location
- Hertzsprung–Russell diagram
- intersects_main_sequence
- A and F stars (1–2 solar mass)
- extends_to
- G and early K bright supergiants (early M if RV Tauri stars at minimum included)
- variable_star_types
- Delta Scuti, SX Phoenicis, rapidly oscillating Ap stars, RR Lyrae, Cepheids, RV Tauri
- pulsation_mechanism
- Kappa–mechanism via He III (doubly ionized helium)
- other_pulsating_stars_nearby
- Gamma Doradus variables at edge; Beta Cephei and PV Telescopii at hotter temperatures
Lore & Background
Stars in the instability strip pulsate due to He III (doubly ionized helium), in a process based on the Kappa–mechanism. In normal A-F-G class stars, He in the stellar photosphere is neutral. Deeper below the photosphere, where the temperature reaches 25,000–30,000 K, begins the He II layer (first He ionization). Second ionization of helium (He III) starts at depths where the temperature is 35,000–50,000 K. When the star contracts, the density and temperature 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). This causes the opacity of the He layer to increase and the energy flux 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 200,000 K. The phase shift between a star's radial pulsations and brightness variations depends on the distance of He II zone from the stellar surface in the stellar atmosphere. For most Cepheids, this creates a distinctly asymmetrical observed light curve, increasing rapidly to maximum and slowly decreasing back down to minimum.
Reader's Guide
The instability strip intersects the main sequence 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.
Did You Know?
- The instability strip is also home to RV Tauri variables, which lie to the right of the brighter Cepheids.
- Pulsations in the instability strip are driven by the Kappa–mechanism involving doubly ionized helium (He III).
- Most high luminosity supergiants are somewhat variable, including the Alpha Cygni variables.
- There are several types of pulsating star not found on the instability strip, such as Beta Cephei and PV Telescopii variables.
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