Instability strip

The instability strip is a specific region on the Hertzsprung–Russell diagram. This diagram is a tool used by astronomers to plot a star's luminosity against its effective temperature. The effective temperature is essentially the color of the star's photosphere, or its outer layer. Stars located within this strip are often pulsating variable stars. This means they do not stay at a constant brightness. Instead, they physically grow and shrink in a regular cycle.

These pulsations are driven by a process known as the Kappa mechanism. This mechanism relies on how helium gas handles energy through ionization. In most stars, like those in the A, F, or G classes, helium in the photosphere remains neutral. However, deeper inside the star, the temperature reaches levels that change the helium. At temperatures around 25,000 Kelvin, a layer of He II, or first ionization helium, forms. At even deeper levels where temperatures reach 35,000 Kelvin, He III, or doubly ionized helium, begins to form.

The cycle begins when a star starts to contract. As the star contracts, the density and temperature of the He II layer increase. This extra energy is enough to strip the last electron from the He II ions. This process transforms the helium into He III. This change causes the opacity of the helium layer to increase. Opacity refers to how much a substance blocks light or energy. Because the layer becomes more opaque, it absorbs the energy flux coming from the star's interior.

As the energy becomes trapped, the temperature of the star's core rises. This increase in core temperature causes the star to expand outward. During this expansion, the He III begins to cool down. As it cools, the helium undergoes recombination. This is when the ions capture free electrons to turn back into He II. This process makes the helium layer less opaque. Once the opacity decreases, the trapped heat can finally propagate to the surface.

After the star radiates away enough energy, the overlying stellar material causes the He II layer to contract once more. This restarts the entire cycle from the beginning. This rhythmic expansion and contraction results in observed changes in the star's surface temperature. For many Cepheid variables, this creates a specific light curve. This curve is asymmetrical, meaning it does not look the same on both sides. In Cepheids, the brightness increases rapidly to a maximum and then decreases slowly back to a minimum.

Many different types of stars occupy the instability strip. Near the main sequence, we find Delta Scuti variables and SX Phoenicis variables. We also find rapidly oscillating Ap stars, known as roAps, in this region. RR Lyrae variables are located where the strip intersects the horizontal branch. Cepheid variables are found where the strip crosses the supergiants. RV Tauri variables are also considered part of the strip. They sit to the right of the brighter Cepheids at lower temperatures.

Not all pulsating stars are found within this specific strip. Some stars pulsate using different mechanisms or at different temperatures. For example, AGB stars are long-period variables found at cooler temperatures. Beta Cephei and PV Telescopii variables are found at much hotter temperatures. Gamma Doradus variables sit right at the edge of the strip near the main sequence. Even white dwarfs have their own separate instability strips. These include DOV, DBV, and DAV stars, which use different ionization regions to move energy.

The study of these stars helps us understand stellar evolution and the physical properties of matter. Some highly luminous supergiants, like Alpha Cygni variables, show slight variability. Even more extreme are the yellow hypergiants located above the instability strip. These stars undergo irregular pulsations and eruptions. There are also luminous blue variables that show irregular changes in their spectra and brightness. Each of these stars provides a different way to observe how energy and gravity interact in the universe.