For at least 2,500 years, humans have perceived musical dimensions in the behavior of stars and other celestial objects, a tradition that is still going strong today. Pulsating stars, which vary in brightness, are one of the most potent examples because their beats are reminiscent of time signatures and tempos.
But while some pulsating stars beat at regular intervals, others are a little jazzier. For instance, astronomers have puzzled for years by the seemingly discordant oscillations of so-called delta Scuti stars, which are about 1.5 to 2.5 times the mass of the Sun. The light from these stars reveals oscillations that are so complex that scientists have been unable to decode them into regular patterns, known as “pulsation modes”—until now.
A team of scientists led by Tim Bedding, an astronomer at the University of Sydney, detected “remarkably regular sequences of high-frequency pulsation modes” in 60 delta Scuti stars, according to a study published on Wednesday in Nature. The discovery offers a new window into the intricate interior dynamics of these mysterious pulsators, and could also help astronomers unlock broader details about the evolution of our galaxy, the Milky Way.
The pulsations of delta Scuti stars have remained relatively inscrutable to astronomers primarily because these stars rotate very quickly. Whereas the Sun spins once every 24 days or so, delta Scuti stars complete one rotation within a day or two. This high rotational speed causes the stars to literally stretch out at the edges, unlike slower-spinning spherical objects like the Sun.
“It basically makes them quite elongated,” said Bedding in a call. “Because they are not spherical, their pulsations don’t look like a spherical object. A spherical object has quite nice pulsations. They are quite regular in frequency and well-understood.”
Delta Scuti stars, in contrast, send out messy beats that are difficult to untangle. Complicating matters further, these stars become much more cacophonous as they age. Over the course of their lives— typically a few billion years—the stars burn hydrogen in their cores and convert it to helium. As their cores become mostly helium and their outer layers remain mostly hydrogen, the stars evolve two gas mediums with different properties.
“Because you divided the star into two sections, there are two parts of the star that can both oscillate, or two different cavities,” Bedding said. “It’s sort of like having two musical instruments. We talk about them being coupled. The result of that is very complicated.”
“That's why we were so happy when we found that at least some delta Scuti stars have these regular patterns,” he added.
Even stars like the Sun start to show these shifts, which are called “bumped” modes in the study, when they get to their senior years. But delta Scuti stars begin to experience mode-bumping early in their lives, which makes it progressively harder to recognize pulsation modes in these stars as they age.
For this reason, Bedding and his colleagues identified regular patterns only in the youngest stars they observed, which were born just a few million years ago.
“One of the things we’re trying to do next is to find more regularity where it’s less obvious,” Bedding noted. “We’ve done the easy stars, the ones that are really regular, but there’s a whole bunch of stars that we looked at where they are sort of a bit regular, but not quite. Using what we’ve learned from the nice ones, we’re going to try to bootstrap our way up.”
The team was able to measure the beats of these “easy stars” thanks to NASA's Transiting Exoplanet Survey Satellite (TESS), a telescope launched in 2018 that is primarily designed to resolve fine details about planets in nearby stellar systems.
Serendipitously, Bedding realized that the telescope’s rapid 30-minute samples of the sky would enable his team to get the first detailed look at delta Scuti stars over small time intervals. TESS will upgrade to 10-minute intervals in July, which should reveal even more intimate insights about these stars.
A better understanding of delta Scuti stars will help astronomers advance at least two trippy fields of space science: Asteroseismology and galactic archaeology.
Asteroseismology, much like seismology on Earth, uses the waves and oscillations that ripple through an object to study its interior. As astronomers identify more pulsation modes in stars—even harmonically challenged ones like the delta Scuti band—they will be able to reconstruct unknown details about stellar evolution.
Galactic archaeology, as the epic name implies, is the quest to understand how galaxies form and evolve into massive entities such as the Milky Way. Delta Scuti stars are a key piece in this puzzle because these stars tend to be born in clutches with several members that eventually disperse throughout the galaxy.
The regular beats of the younger delta Scuti stars allow scientists to pinpoint the general age of these stellar youth groups, which can help constrain how the galactic gravitational field separates them and distributes them around the Milky Way.
“The ultimate aim is to better understand how the galaxy is formed,” Bedding said. “How did our galaxy, the Milky Way, get assembled from all these different bits? Our hope is that we can contribute something to that endeavor.”
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