A team of researchers led by Dr John Lindner from the University of Hawaiʻi at Mānoa and the College of Wooster, Ohio, has found evidence for fractal behavior in the pulsations of the so-called ‘golden’ RR Lyrae variables.

RR Lyrae variables are at least 10 billion years old and their brightness can vary by 200 percent over half a day, which is hard to study from Earth due to our day-night cycle.

“Unlike our Sun, RR Lyrae stars shrink and swell, causing their temperatures and brightness to rhythmically change like the frequencies or notes in a song,” Dr Lindner explained.

While some of these stars pulsate with a single frequency, observations confirm that others pulsate with multiple frequencies.

Several of these stars, including the RR Lyrae variable star KIC 5520878, pulsate with two principal frequencies, which are nearly in the golden ratio. Astronomers call these stars ‘golden’ RR Lyrae variables.

“We call these stars ‘golden’ because the ratio of two of their frequency components is near the golden mean, which is an irrational number famous in art, architecture, and mathematics,” Dr Lindner said.

He teamed with colleagues from the University of Hawaiʻi at Mānoa and the Institute for Data Analysis in Neukirchen-Vluyn, Germany, to study KIC 5520878 (located in the constellation Lyra, about 16,000 light-years away) and several other golden RR Lyrae variables – KIC 4064484, KIC 8832417 and KIC 9453114.

“Just as flamboyant rock stars deliver pulsating rhythmic beats under their song melodies, so, too, do these variable stars,” said Dr Lindner, the first author of a paper published in the journal Physical Review Letters (arXiv.org preprint).

By removing the ‘backbeat,’ the scientists discovered a subtle ‘melody’ in the brightness variations of KIC 5520878.

“From prior work I suspected that a fractal structure known as a strange nonchaotic attractor might guide the dynamics,” said co-author Dr William Ditto of the University of Hawaiʻi at Mānoa.

“Strange nonchaotic attractors have been observed in laboratory experiments involving magnetoelastic ribbons, electrochemical cells, electronic circuits, and a neon glow discharge, but never before in non-experiments in nature,” the scientists wrote in the paper.

“The pulsating star KIC 5520878 may be the first strange nonchaotic dynamical system observed in the wild.”

“The observation of stellar strange nonchaotic dynamics provides a new window into variable stars that can improve their classification and refine the physical modeling of their interiors,” they concluded.

_____

John F. Lindner et al. 2015. Strange Nonchaotic Stars. Phys. Rev. Lett. 114, 054101; doi: 10.1103/PhysRevLett.114.054101