Instability strip

Instability strip

The Instability strip is a nearly vertical region in the Hertzsprung–Russell diagram which is occupied by pulsating variable stars (including RR Lyrae variable, Cepheid variable, W Virginis variable, ZZ Ceti variable, RV Tauri variable, Delta Scuti variable, SX Phoenicis variable and rapidly oscillating Ap stars).[1]

The instability strip intersects the main sequence in the region of A and F stars (1–2 solar mass) and extends upwards almost vertically (slightly inclined to the right) to the highest luminosities. The lower part of instability strip appears as the Hertzsprung gap on the Hertzsprung–Russell diagram.


Stars in the instability strip pulsate due to He III (doubly ionized helium). In normal A-F-G stars He is neutral in the stellar photosphere. Deeper below the photosphere, at about 25,000–30,000K, begins the He II layer (first He ionization). Second ionization (He III) starts at about 35,000–50,000K.

When the star contracts, the density and temperature of the He II layer increase. He II starts to transform to He III (second ionization). Opacity increases and the energy flux from the interior of the star is effectively absorbed. The temperature of the layer increases and it starts to expand. After expansion, density and temperature decrease and He III begins to recombine into He II. The outer layers contract and the cycle starts from the beginning.

The phase shift between a star's radial velocity pulsations and brightness variability depends on the distance of He II zone from the stellar surface in the stellar atmosphere.


  1. ^ "Cepheid instability strip". A Dictionary of Astronomy. Retrieved 2010-03-28. 

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Gaza Strip — a coastal area on the E Mediterranean: formerly in the Palestine mandate, occupied by Israel 1967 94; since 1994 under Palestinian self rule. * * * Gaza Strip Introduction Gaza Strip Background: The Israel PLO Declaration of Principles on Interim …   Universalium

  • Pulsating white dwarf — V777 Herculis star redirects here. For the star, see GD 358. A pulsating white dwarf is a white dwarf star whose luminosity varies due to non radial gravity wave pulsations within itself. Known types of pulsating white dwarfs include DAV, or ZZ… …   Wikipedia

  • Main sequence — A Hertzsprung Russell diagram plots the actual brightness (or absolute magnitude) of a star against its color index (represented as B V). The main sequence is visible as a prominent diagonal band that runs from the upper left to the lower right.… …   Wikipedia

  • Naine blanche à pulsations — Une naine blanche à pulsations est une étoile naine blanche à la luminosité variable du fait de pulsations d ondes de gravité non radiales dans sa partie interne. Les types connus de naines blanches à pulsations comprennent les étoiles DAV, ou ZZ …   Wikipédia en Français

  • Enana blanca — Saltar a navegación, búsqueda Comparación de tamaños entre la enana blanca IK P …   Wikipedia Español

  • Пульсирующий белый карлик — Пульсирующие белые карлики  один из типов пульсирующих переменных звёзд. Светимость этих белых карликов меняется из за их нерадиальных пульсаций, вызванных волнами гравитации (gravity wave) (не гравитационными волнами!)[1]. У этих звёзд… …   Википедия

  • White dwarf — For other uses, see White dwarf (disambiguation). Image of Sirius A and Sirius B taken by the Hubble Space Telescope. Sirius B, which is a white dwarf, can be seen as a faint pinprick of light to the lower left of the much brighter Sirius A …   Wikipedia

  • Stellar evolution — Life cycle of a Sun like star Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years (for the most… …   Wikipedia

  • Supernova — This article is about the astronomical event. For other uses, see Supernova (disambiguation). Multiwavelength X ray, infrared, and optical compilation image of Kepler s supernova remnant, SN 1604. A supernova is a stellar explosion that is more… …   Wikipedia

  • Metallicity — The globular cluster M80. Stars in globular clusters are mainly older metal poor members of Population II. In astronomy and physical cosmology, the metallicity (also called Z[1]) of an object is the proportion of its matter made …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.