Regular rhythms among pulsating stars

by | May 14, 2020 | Neutron Stars / Pulsars, Stars | 0 comments

IMAGE: Still image from video of pulsating star.
CREDIT: NASA’s Goddard Space Flight Center

Today’s top story looks at an area of astronomy that rarely makes headlines: pulsating variable stars. As some of you may know, I started my research career looking first at TT Tauris in radio and then moved back to optical astronomy with Cepheids and RR Lyraes. In grad school, I switched to extra-galactic astronomy because I was told that pulsating variables were essentially a solved problem, and there just wouldn’t be funding or jobs doing the research I most enjoyed. Here’s the thing: stars have long been under-appreciated and labeled solved by folks that study things that look more complicated on the outside – folks who often fail to realize just how complex stars can be on the inside. As more and more researchers have to look at stars to find planets, the messiness of stars is starting to get more attention, and I am so here for it.

Variable stars come in lots of kinds. Some flare up randomly. Some darken suddenly. Pulsating variable stars are stars that have just the right in-balance of gravity, temperature, and energy generation that as light pushes outward it sometimes gets stolen by the stars’ outer atmosphere. Unable to escape, thanks to effects like ionization and changes in opacity, the photons increase in pressure until they can push the star into becoming a bigger star… this changes the temperature and the behavior of the outer star, making it possible for photons to fly free again. … which cools the star which subsequently collapses. We see this effect as stars visibly changing in brightness and color, and for many stars, these changes can be seen without a telescope or binoculars. All you need is your eye and clear darkish skies.

One of the favorite targets of backyard astronomers is Delta Scuti stars. These young stars have chaotic looking brightness changes that reflect rapid rotation and pulsations we haven’t been able to understand….. At least until now.

Astronomers using the TESS and Kepler exoplanet observatories along with ground-based telescopes have found that there are two different classes of Delta Scuti stars that can be sorted. One set of stars behaves in insensible ways, with the entire surface of the star moving in concert. The other set is harder to imagine – different parts of the star actually pulsate out of phase with each other, with different quadrants alternatingly going in and out. This is ridiculously hard to sort when stars are rotating rapidly. The amount we see is a function of which quadrants are pulsed bright and which are facing us, and the facing us is changing.

But, one team, led by Tim Bedding, has just published a paper in the journal Nature that shows that ridiculously hard is not the same as impossible. They have sorted solutions, and just like the harmonics of a horn tell us the shape of the horn, and seismic waves through the earth tell us the structure of our planet, these asteroseismic waves tell us the shapes inside the stars.

These new findings are already allowing researchers to use these pulsating young objects to determine the age of their surroundings, including measuring the age of a stream of stars orbiting our Milky Way.

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