Polaris is the closest and brightest Cepheid variant. Recently, something changed.

When you look up at the night sky and find your way to the North Star, you are looking at Polaris. Not only is it the brightest star in the constellation Ursa Minor (the Little Bear), but its position relative to the north celestial pole (less than one degree) makes it useful for orientation and navigation. Since the era of modern astronomy, scientists have known that the star is a binary system consisting of an F-type yellow giant (Polaris Aa) and a smaller yellow dwarf (Polaris B). Further observations revealed that Polaris Aa is a classic Cepheid variable, a regularly pulsing star class.

For most of the 20th century, records indicate that the pulse duration was increasing while the pulse amplitude was decreasing. But this has changed recently as the pulse period began to become shorter while the amplitude of the velocity changes stopped increasing. According to a new study by Guillermo Torres, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA), these behaviors can be attributed to long-term changes related to the binary nature of the system, as the two stars move closer to each other. Other, the secondary disturbs the atmosphere of the primary.

Cepheid variables are stars that pulsate radially, causing their diameter and temperature to vary. These pulsations are directly related to changes in their brightness, making them a useful tool for measuring galactic and extragalactic distances. The variable nature of Polaris was confirmed in 1911 by Danish astronomer Einar Hertz Dabrung, after whom the Hertzsprung-Russell chart is partly named. Observations conducted throughout the twentieth century showed that Polaris had a constant pulsation period of about four days, which was steadily increasing each year.

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Polaris (Alpha Ursae Minoris) as seen by the Hubble Space Telescope. Credit: NASA/HST

As Dr. Torres explained to Universe Today via email, this has begun to change recently, leading many astronomers to wonder what causes Polaris pulsations. “For more than 150 years until about 2010, the period was lengthening by about 4 or 5 seconds each year,” he said. “Recent observations have shown that this trend has now reversed, and that the pulsation period has become shorter. This is an unexpected change, showing that there is still a lot we do not understand about Polaris and other stars like it.”

To learn more about the Polaris pulsation period, Torres consulted radial velocity (RV) measurements dating back to 1888. The technique consists of measuring spectra from a distant star and looking for redshift and blueshift, which are indicators that the star is moving back and forth. (This technology also produces accurate estimates of their speed.) The Torres sample included more than 3,600 spacecraft measurements, including nearly 1,200 spectroscopic observations made by Lick Observatory over more than 60 years.

This allowed Torres to trace the evolution of Polaris’ pulsation characteristics, which showed how often pulsations occurred and also the amplitude of these pulsations. Said Torres:

“In the early 1990s, the amplitude became so small that it was thought the pulsations were about to stop. However, Polaris decided otherwise, and by the late 1990s capacity began to increase again, which continued until approximately 2015. The latest observations indicate that capacity is no longer increasing, and may begin to decline again. In addition, spacecraft have shown that this behavior may be related to the fact that another star orbits Polaris, which approaches it every 30 years and may disturb the outer layers of Cepheid, where the pulsations occur.

One artist’s image shows Polaris A with a close companion known as Polaris Ab. Another companion star, Polaris B, can be seen as a dot in the background on the right. Credit: STScI

In short, the changes in Polaris’ pulse period may have been caused by its companion disturbing it as they passed closer together. Once this was taken into account, Torres was able to derive an improved spectral orbit for the binary system, something astronomers have been trying to solve for generations. This may also lead to more accurate estimates of the dynamical masses of each companion star, which were also subject to uncertainty. As Torres summarized:

“We now know that Polaris behaves in an erratic and unpredictable way. If this is confirmed to be related to the presence of its companion, this could shed light on the behavior of other pulsars with similar properties and help us understand the nature of the oscillations. So it is important that we keep an eye on this,” Because it may still hold surprises for us.

In-depth reading: arXiv

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