New study caps off 12 years of research into pulsating stars

(CN) — A new study, published Friday in the Journal of Astrophysics and Astronomy, has brought together more than a decade of research into a peculiar class of stars called Cepheids.

Cepheids are massive stars that seem to brighten and dim — and even grow and shrink — in rhythmic pulsations. They're not pulsars, another class of pulsating stellar bodies that are the leftover remains of a giant star's death. Instead, Cepheid shine brightly enough to be seen with the naked eye from Earth, with fluctuations so dramatic that observers first discovered them more than 200 years ago.

Cepheids are also useful for measuring distances in space. Scientists can measure the relationship between a Cepheid's rate of fluctuation and its luminosity as seen from Earth, helping them extrapolate the distance between cosmic objects.

"They vary in timescales that are visible to humans — maybe a month or so," Richard Anderson, an astrophysicist with the Swiss Federal Institute of Technology in Lausanne, told Courthouse News in an interview.

Despite the cosmological importance of Cepheids, their variability — coupled with their complex interactions with surrounding stellar phenomena — can make studying them difficult.

To solve this problem, Anderson in 2010 helped launch the VELOCE Project (short for for "VELOcities of CEpheids"), a longterm deep dive into Cepheid research. Anderson led the project in collaboration with other scientists from around the world, including those from Switzerland's University of Geneva and Belgium's KU Leuven.

"At the time I was really interested in the application of these stars for distance measurement," Anderson said — adding in a statement that there was an urgent need for high-quality measurements of Cepheids' radial velocities. That's the speed, he explained, at which Cepheids expand and contract along our line of sight.

Using observations from the Euler Telescope in Chile and the Mercator Telescope on the island of La Palma, the VELOCE Project team collected more than 18,000 radial-velocity measurements from more than 250 Cepheids between 2010 and 2022. Anderson said part of the goal was to gather as much data as possible to build a foundation for future Cepheid research and observation.

Beyond simply gathering data, the VELOCE team made some interesting discoveries of their own. They found that some Cepheids' pulsations changed their line-of-sight velocity by as much as 70 kilometers per second. They also discovered that some Cepheids exhibit a "double peak" pattern in their pulsations that scientists had not seen before.

The revelation complicates the idea that Cepheids pulsate in predictable rhythms, Anderson explained. "With the data from VELOCE we're seeing that's not true with sufficiently accurate measurements," he told Courthouse News.

The newly-discovered complexities in Cepheid pulses may have to do with how the star's internal layers interact, forming what Anderson calls "acoustic variations."

“This suggests that there are more intricate processes occurring within these stars, such as interactions between different layers of the star, or additional ... pulsation signals that may present an opportunity to determine the structure of Cepheid stars by asteroseismology,” Anderson’s postdoctoral student Henryka Netzel said in her own statement.

The finding also calls into question the accuracy of cosmic distance measurements scientists have made using Cepheid pulses. While that doesn't mean our estimations of the size of the observable universe need to be thrown out, Anderson said, it does mean scientists will have to look at the subtle variations of Cepheid pulse rhythms more carefully when taking their measurements.

The scientist predicted VELOCE's discoveries will become more relevant as more data comes in from the European Space Agency's Gaia space telescope. Gaia has also observed Cepheids. When compared with VELOCE data, they may help clarify unanswered questions about measuring cosmic distances and the universe's rate of expansion.

“This dataset will serve as an anchor to link Cepheid observations from different telescopes across time and hopefully inspire further study by the community,” Anderson said in a statement.

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