The surfaces of all stars “jiggle” a bit. For many stars, the jiggles are quite small. For a few stars, though, they’re big enough to see with the eye alone.
An example is in Scutum, the shield. As night falls, the constellation is above the teapot outlined by the bright stars of Sagittarius.
Delta Scuti is a good bit bigger, brighter, and heavier than the Sun. And although it’s much younger than the Sun, it’s much farther along in its life cycle. It’s no longer fusing the hydrogen in its core to make helium. But it’s not yet ready to fuse the helium to make heavier elements.
There’s a helium-rich layer in the star’s thick envelope of gas. This layer heats up, which causes the helium to become more opaque. That traps heat from inside the star, causing its outer layers to expand. As they expand, the extra heat escapes, so the star looks brighter.
As the heat escapes, though, the outer layers cool and fall back inward. That causes the helium layer to heat up once more, restarting the process.
In the case of Delta Scuti, there are several such cycles. The main one lasts about four and a half hours, and causes the star’s brightness to vary by about 20 percent. There are other cycles, too, in which some of the surface moves outward, but some moves inward.
Delta Scuti is the prototype of an entire class of variable stars. The brightest member of the class stands well to the left or upper left of Scutum: Altair, a member of the bright Summer Triangle.