Stars like the Sun boil and seethe, because the interior energy generated in them cannot escape quickly enough, and then convection vibrations resulting in light variations happen, asteroseismologist Victoria Antoci and her colleagues wrote recently.
Asteroseismology is the study of movements and structures within stars, especially the turbulence within variable stars, and there is still much to learn in the field, because not all periodic solar oscillations happen through convection -- like on the Sun, where this has been observed extensively, according to the University of Vienna's news portal:
In stars 1.5 times or more as massive as the Sun, periodic pulsations are less and less excited in an outer convective envelope, and driven more by an interior process known as the "kappa mechanism," (or heat build up and radiation from opaque levels to regions transparent enough for it to escape) -- and there are stars that fall between, where only one percent of the radius could be convective, so fluctuating energy is mostly, or perhaps entirely, moved between core and outer regions by radiation; yet scientists have been predicting convection still excites pulsations in the outer layer of some stars in this mass range, such as the type called Delta Scuti variables (named for the group's prototype star Delta Scuti, in the constellation Scutum, the Shield).
The Vienna team announced using the NASA Kepler Observatory to observe convective pulsations occurring in the shallow convective layer of a Delta Scuti star, demonstrating stars in this group and mass range can still possess convective envelopes, confirming the long-held theory; their findings have been published in the journal Nature.
The Delta Scuti star observed by Antoci, called HD 187547, is the first of its kind definitively exhibiting both convection and radiation types of oscillations, according to.astronomer Gerald Handler of the Nicolaus Copernicus Center in Warsaw, who supervised Antoci’s research and explained, “With HD 187547 we found the ideal object to study different processes and their interaction under extreme physical conditions."
Discovering the boundary-line star mass, where the convective envelope no longer exists, remains a research goal, a difficult one, because of the distances and conditions involved, but the Vienna team's new discovery points another beam of light towards an answer.
According to the Delta Scuti Network, astronomers from around the planet have been collaborating in observing this group of variable stars since 1983.
University of Illinois astronomer Jim Kahler has profiled Delta Scuti variables in an updating online star reference.