Puzzling pulsating stars reveal secrets in breakthrough study

Chris Davies - May 13, 2020, 12:06 pm CDT
Puzzling pulsating stars reveal secrets in breakthrough study

A breakthrough study using NASA’s TESS space telescope has helped unlock the secrets of weirdly pulsing stars, paving the way to astronomers better understanding how old they are and what’s going on beneath their roiling surface. Delta Scuti stars are only one of a number of types of star out in the universe, but they’ve also become one of the most confusing.

That’s because they pulsate in strange sequences. Many stars pulsate, but it’s typically a predictable, straightforward pattern. Delta Scuti stars, however, do so chaotically, their hemispheres swelling and shrinking out of sync, and their forms expanding and contracting.

It’s enough to stymie traditional ways of gaging just what’s going on inside a star. Normally, astronomers would rely on what’s known as asteroseismology, which relies on the fact that as sound waves travel through the interior of a star, they can be affected by different densities and more. That results in fluctuations in brightness on the star’s surface which, though tiny, can nonetheless be used to ascertain things like age, temperature, composition, and internal structure.

That is, if you’re not dealing with a star that obscures its pulsation patterns in some ways. The first Delta Scuti star – and the one all subsequent discoveries have been classified after – was spotted in 1900, and is visible to the human eye. Thousands more have since been identified, with NASA’s Kepler space telescope pinpointing many of those.

There are several elements to Delta Scuti stars which make them particularly tough to assess. For a start, while they’re large – typically 1.5 to 2.5 times the mass of our own Sun – they’re also fast-spinning. Rotating once or twice a day, they’re at least a dozen times faster than the Sun. It’s enough to make the stars flatten slightly at their poles, which works to jumble up the pulsation patterns.

To spot a pattern, astronomers needed consistent measurements of the stars over a period of time. Problem was, that simply wasn’t being generated. TESS – the Transiting Exoplanet Survey Satellite – takes a full image each 30 minutes, but that’s just not frequent enough for the minute-by-minute changes a Delta Scuti star can exhibit.

The key came in TESS’ other snapshots, where a selection of stars numbering in the thousands are subject to photos every two minutes. A team led by Tim Bedding, a professor of astronomy at the University of Sydney, found that some of those were Delta Scuti stars, and demonstrated regular pulsation patterns. They’ve released a new paper on their findings.

“Once they knew what to look for, they searched for other examples in data from Kepler, which used a similar observing strategy,” NASA explains. “They also conducted follow-up observations with ground-based telescopes, including one at the W.M. Keck Observatory in Hawaii and two in the global Las Cumbres Observatory network. In total, they identified a batch of 60 Delta Scuti stars with clear patterns.”

Even with that new data, though, the strange stars defied easy categorization. The so-called “well behaved” group of Delta Scuti stars actually behave in two distinct ways. One sees the whole star expanding and contracting symmetrically, while the other has the star’s opposite hemispheres alternately expand and contract.

Beyond that, though, other Delta Scuti stars defy such regularity. That, the researchers believe, is a factor of age: younger stars exhibit patterns with a greater regularity, but as they age, their pulsations become more sporadic.

TESS is set to upgrade later in the year, and take full images every 10 minutes rather than 30 minutes. The expectation is that the images will not only help find exoplanets elsewhere in the universe, but also allow for the discovery of other strangely-behaving stars.

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