[1] Here, the Greek letter kappa (κ) is used to indicate the radiative opacity at any particular depth of the stellar atmosphere.
[2] If a layer of a stellar atmosphere moves inward, it becomes denser and more opaque, causing heat flow to be checked.
In return, this heat increase causes a build-up of pressure that pushes the layer back out again.
In Beta Cephei variables, stellar pulsations occur at a depth where the temperature reaches approximately 200,000 K and there is an abundance of iron.
The increase in the opacity of iron at this depth is known as the Z bump, where Z is the astronomical symbol for elements other than hydrogen and helium.