Polaris is the closest, brightest Cepheid variable. Recently, something has changed.

When you look up at the night sky and find your way to the North Star, you’re looking at Polaris. Not only is it the brightest star in the constellation Ursa Minor (the Little Dipper), but its position relative to the northern sky (less than 1° away) makes it useful for viewing and navigation. Since the era of modern astronomy, scientists have realized that the star is a binary system consisting of an F-type yellow supergiant (Polaris Aa) and a smaller yellow dwarf (Polaris B). Further observations revealed that Polaris Aa is a variable type of Cepheid, a class of stars with frequent collisions.

For most of the 20th century, records show that the pulse duration is increasing while the pulsation amplitude is decreasing. But recently, this has changed as the pulse duration starts to shorten while the amplitude of the speed variation has stopped increasing. According to a new study by Guillermo Torres, an astronomer with the Harvard & Smithsonian Center for Astrophysics (CfA), these behaviors can be linked to long-term changes related to the binary nature of the system, where two stars are approaching each other. one, and the second disturbs the primary spirit.

Cepheid variables are stars that rotate radially, causing them to vary in diameter and temperature. These pulsations correspond directly 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 the Danish astronomer Ejnar HertzsDaprung, after whom the Hertzsprung-Russell diagram is named in part. Observations made throughout the 20th century have shown that Polaris has a constant pulse period of about four days, which has steadily increased each year.

Remove all ads from Universe today

Join our Patreon for just $3!

Get an ad-free experience of a lifetime

As Dr. Torres explained to Universe Today by email, this recently began to change, leading many astronomers to wonder what drives the pulsations of Polaris. “Over the 150 years until 2010, the period was getting longer by 4 or 5 seconds per year,” he said. “Current observations have shown that this trend has now returned, and the breathing period is becoming shorter. This is an unexpected change, which shows that there is much we do not understand about Polaris and other stars like it.”

To learn more about the timing of Polaris’ pulsation, Torres consulted radial velocity (RV) measurements going back to 1888. This technique involves measuring spectra from a distant star and checking for redshift and blueshift, which indicate that the star is moving back and forth. (this technique also produces an accurate estimate of its speed). The Torres sample includes more than 3,600 RV measurements, including nearly 1,200 spectroscopic observations made by the Lick Observatory over 60 years.

This allowed Torres to track the evolution of Polaris’ breathing properties, showing how often the pulses occur and their size as well. Torres said:

“In the early 1990s the size of the hands was so small that it was thought that the heartbeat was about to stop.” However, Polaris decided otherwise, and in the late 1990s the amplitude began to increase again, which lasted until about 2015. The latest observations show that the amplitude is no longer increasing, and may start to decrease again. In addition, the RVs showed that this behavior can be related to the fact that Polaris orbits another star, approaching it every 30 years and that it can disturb the outer layers of Cepheid, where the impact occurs.

In short, changes in Polaris’ pulsation period can cause its partner to disrupt it whenever they make their closest approach to each other. Once this object was installed, Torres was able to obtain an improved spectroscopic orbit of the binary system, something astronomers have been trying to solve for generations. This can lead to more accurate estimates of the dynamical masses of each star companion, which are also subject to uncertainty. As Torres summarized:

We now know that Polaris behaves in strange and unpredictable ways. If it is confirmed that this has something to do with the existence of its companion, this can shed light on the behavior of other rotating stars with similar characteristics and help us understand the nature of the oscillations. It is therefore important that we monitor this, because it may bring us surprises.”

Further Reading: arXiv