The North Star is only 47th brightest, and it keeps misbehaving
Many people assume Polaris is the brightest star in the sky. It actually ranks around 47th. Its fame comes from position: it sits less than a degree from the north celestial pole, so while every other northern star wheels around it, Polaris barely moves, and its height above the horizon roughly gives your latitude.
That role is temporary. Earth's axis slowly wobbles, tracing a circle over some 26,000 years. Around 2750 BC the pole lay near Thuban, and in classical antiquity it was about 10 degrees from both Polaris and Kochab. Around 320 BC the Greek navigator Pytheas described the pole as empty of stars. Polaris will come nearest, about 0.45 degrees, shortly after 2100, then drift away as the pole heads toward Errai around 4000, Deneb around 10,000 and Vega around 14,500.
What looks like one point is really three stars. The main one, Polaris Aa, is a yellow supergiant. William Herschel spotted a distant companion, Polaris B, in August 1779 with a reflecting telescope he built himself. A much closer partner, Ab, was confirmed only in the early twentieth century, after W. W. Campbell noticed in 1899 that the star's velocity along our line of sight was changing. The inner pair takes roughly 29 to 30 years per orbit, and a 2024 study using the CHARA interferometer put the supergiant at about 5.13 times the Sun's mass.
Polaris Aa is also the closest Cepheid, a type of pulsating star whose rhythm reveals its true brightness, which makes it a key calibrator for measuring cosmic distances. Its behaviour, though, is odd. The pulses shrank sharply in strength after 1966 and are now reportedly growing again, a reversal seen in no other Cepheid. Its roughly four-day period lengthened by about 4.5 seconds a year for decades, but has been shortening since around 2010.
Stranger still, research published in Science suggests Polaris is 2.5 times brighter than when Ptolemy saw it. Astronomer Edward Guinan remarked that if real, such changes are 100 times larger than stellar evolution theory predicts. Age estimates add another puzzle: the supergiant appears much younger than its two companions, which is hard to square with stars supposedly born together.
Source: Polaris
