When viewed from Earth, pulsars appear as flickering stars. (Image credit: MARK GARLICK/SCIENCE PHOTO LIBRARY via Getty Images)

Astronomers at the SETI (Search for Extraterrestrial Intelligence) Institute have learned to read the subtle "twinkle" of a distant cosmic lighthouse, revealing how interstellar space distorts radio signals as they travel across the galaxy.

The research shows that gas between stars can shift the arrival time of a pulsar’s signal by mere billionths of a second.

"Pulsars are wonderful tools that can teach us much about the universe and our own stellar neighborhood," study lead author Grayce Brown of the SETI Institute said in a statement. "Results like these help not just pulsar science, but other fields of astronomy as well, including SETI."

Beginning in late February 2023, Brown and her team conducted a nearly daily observing campaign lasting 10 months using the SETI-operated Allen Telescope Array in California. The team tracked subtle changes in radio signals from the pulsar PSR J0332+5434 — the fast-spinning remnant of a neutron star located more than 3,000 light-years from Earth and the brightest pulsar visible to the telescope.

From nearly 400 observations, the team identified changes in the pulsar’s "twinkling" pattern, known as scintillation, over timescales of hundreds of days. As the radio waves blasted from the pulsar’s poles travel through space, they pass through clouds of charged gas, primarily free electrons, that bend, scatter and slightly delay the signal. This interaction produces scintillation, the radio equivalent of how stars appear to twinkle in Earth’s atmosphere, according to the study.

As Earth, the pulsar, and the intervening interstellar gas move relative to one another, bright and dim patches form across radio frequencies and evolve over time. These shifting patterns subtly alter when the pulses arrive, introducing timing delays on the order of tens of nanoseconds, the statement says.

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Such tiny discrepancies between the predicted and observed arrival times of pulsar pulses can have outsized consequences. Pulsar timing arrays search for low-frequency gravitational waves by looking for correlated deviations in pulse arrival times caused by the stretching and squeezing of spacetime. If delays introduced by interstellar gas are not properly accounted for, they can obscure — or even mimic — the faint signals researchers are trying to detect, the study notes.

Beyond helping to improve pulsar timing, scientists say the findings also provide a valuable tool for SETI researchers working to distinguish genuine cosmic signals from human-made interference. "Noticeable scintillation can help SETI scientists distinguish between human-made radio signals and signals from other star systems," the statement reads.

"We need some way to differentiate between signals coming from Earth and signals coming from beyond our Solar System," Brown told The Debrief. "Because of this research, we know how much scintillation to expect from a radio signal traveling through this pulsar’s region of interstellar space."

"If we don’t see that scintillation," she added, "then the signal is probably just interference from Earth."

The observations were part of a broader effort that monitored roughly 20 pulsars over about a year, following a pilot phase in late 2022. While the team did not identify a repeating pattern in the scintillation changes, the study notes future observing campaigns lasting longer than a year could further refine predictions and improve corrections for interstellar distortion.

The study was published on Dec. 10, 2025 in The Astrophysical Journal.

Sharmila Kuthunur is an independent space journalist based in Bengaluru, India. Her work has also appeared in Scientific American, Science, Astronomy and Live Science, among other publications. She holds a master’s degree in journalism from Northeastern University in Boston.

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