Humanity\u2019s been fortunate to have a star situated over Earth\u2019s north pole. The star, known as Polaris, or the North Star, has guided many sailors safely to port. But Polaris is a fascinating star in its own right, not just because of its serendipitous position. <\/p>\n
<\/p>\n Polaris is also called the Pole Star, and it\u2019s actually a triple star system. The primary star is a yellow supergiant named Polaris Aa, about 448 light-years away, and it orbits with a smaller companion named Polaris Ab. The outer star is named Polaris B and may also have a dim companion. In this article, Polaris refers to the primary star, Polaris Aa. <\/p>\n Polaris hasn\u2019t always been the North Star, and it won\u2019t always be. Thuban was the North Star from the 4th to 2nd millennium BC until Earth\u2019s axial precession gave that position to Polaris. The Pole Star changes during a 26,000-year cycle, so Thuban will take over from Polaris in the year 20346.<\/p>\n But whether Polaris is the Pole Star at a particular time or not, it\u2019s an interesting object whose properties can help us understand the expansion of the Universe. <\/p>\n Polaris is a variable star that pulses and changes brightness over time. Specifically, it\u2019s a Cepheid variable. Cepheid variables expand and contract rhythmically, and their brightness changes in a predictable pattern. Because there\u2019s a direct relationship between their pulsation period and their luminosity, they\u2019re useful in measuring distances. They\u2019re called \u201cstandard candles\u201d and are part of the cosmic distance ladder. <\/p>\n Astronomers use standard candles to help measure the Hubble constant, or how rapidly the Universe is expanding. But there\u2019s some tension between our measurements of the Hubble constant. When we use local objects like Cepheid variables to measure the Hubble constant, we get a different number than when we use larger-scale things like the Cosmic Microwave Background to measure it. <\/p>\n Since Polaris is such a nearby standard candle, a team of astronomers used a telescope array to watch the star for 30 years. By more accurately observing Polaris and its smaller companion Polaris Ab, they hoped to constrain Polaris\u2019 mass and other characteristics more accurately. This, in turn, could help us understand the tension in the Hubble constant. Along the way, the researchers uncovered some surprises surrounding this long-observed star. <\/p>\n Their results are in a paper titled \u201cThe Orbit and Dynamical Mass of Polaris: Observations with the CHARA Array.\u201d It\u2019s published in The Astrophysical Journal, and the lead author is Nancy Evans. Evans is an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian. <\/p>\n In order to understand Polaris better, it\u2019s critical to get a good look at its dim companion. But that\u2019s not easy to do.<\/p>\n \u201cThe small separation and large contrast in brightness between the two stars makes it extremely challenging to resolve the binary system during their closest approach,\u201d Evans said.<\/p>\n The CHARA (Center for High Angular Resolution Astronomy) Array was built to bring clarity to objects like Polaris and its dim companion. It\u2019s an interferometer, an array of six separate telescopes, each with a one-meter-diameter primary mirror. By combining the images from each separate scope, CHARA attains the higher resolution of a telescope with a primary mirror that\u2019s 330 meters in diameter, the area covered by the individual \u2018scopes. CHARA has a special camera designed to work with it called MIRC-X (Michigan InfraRed Combiner-eXeter).<\/p>\n With these tools, the astronomers tracked Polaris and its dim companion over a 30-year period. They measured how the Cepheid variable changed size as it pulsated. They learned that it\u2019s five times as massive as the Sun and has a diameter 46 times larger than the Sun. However, the mass measurement is affected by the star\u2019s large orbital eccentricity, 0.63, so there\u2019s still some uncertainty about Polaris\u2019 mass.<\/p>\n The measured mass and luminosity also show that Polaris is more luminous than it should be for a star on its evolutionary track. \u201cPolaris is at least 0.4 mag brighter than the predicted tracks,\u201d the authors write in their paper. This is important because of the \u201cCepheid mass problem.\u201d It\u2019s a discrepancy between masses inferred from stellar evolutionary tracks and masses from pulsation calculations. <\/p>\n A Cepheid variable\u2019s mass can be determined when it\u2019s in a binary relationship. \u201cMass determination starts with a radial velocity (RV) orbit and pulsation curve for a binary containing a Cepheid,\u201d the authors explain. Very few Cepheid variables are in binary relationships like Polaris, so it\u2019s an important target for constraining and understanding their masses. These measurements are all important because they relate back to the cosmic distance ladder, standard candles, and the Hubble constant.<\/p>\n \u201cThe accuracy of inputs from any of these measurements depends on many characteristics of the star: brightness, orbital period, inclination, and the separation, distance, and mass ratio of the components. This means that each Cepheid system is unique and has to be analyzed independently,\u201d the authors explain.<\/p>\n The observations also showed variable spots on the star\u2019s surface. <\/p>\n \u201cThe CHARA images revealed large bright and dark spots on the surface of Polaris that changed over time,\u201d said Gail Schaefer, director of the CHARA Array.<\/p>\n \u201cThe identification of starspots is consistent with several properties of Polaris,\u201d the researchers write. It\u2019s different from other Cepheid variables because it has a very low pulsation amplitude. That could mean that its atmosphere is more like a nonvariable supergiant. Those atmospheres often seem to be active, much like the spots on Polaris. \u201cIt is not clear how full amplitude pulsation affects the atmosphere and magnetic field in pulsators, so Polaris is an interesting test case,\u201d they explain.<\/p>\n The spots are variable, which could explain why astronomers have struggled to identify other \u201cadditional periodicities\u201d in the star. They could also explain an observed ~120-day radial velocity variation as a rotation period.<\/p>\n The spots on Polaris\u2019 surface have added to the star\u2019s complexity, and they\u2019re begging to be understood. <\/p>\n \u201cWe plan to continue imaging Polaris in the future,\u201d said study co-author John Monnier, an astronomy professor at the University of Michigan. \u201cWe hope to better understand the mechanism that generates the spots on the surface of Polaris.\u201d<\/p>\n![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/08\/North-Up-Polaris-surface.jpg\")
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