Polaris - The North Star

Known as the Northern Star, Pole Star, or Lodestar, Polaris is the star that marks the way north. It is located in the constellation of Ursa Minor, the Little Bear. In the night sky, it appears as a single star, although it is actually a triple star system. It consists of a yellow supergiant star, and two smaller main sequence stars.

The two small companions, Ab and B, are on the "main sequence," generating energy by fusing hydrogen nuclei into helium nuclei only in the core. They are not the brightest stars in the northern hemisphere, but are visible through a telescope. The larger star, Aa, is the outer pair of the asterism of Dubhe and Merak. These stars appear to follow Polaris in the sky, but their movements are not well discernible to the naked eye.

Despite being a class F (F7) yellow supergiant, the mass of Polaris is 4.5 times the mass of the Sun, and the surface temperature is approximately 6,000 Kelvin. The distance of Polaris from the Earth is 434 light years. During the classical age of antiquity, the celestial pole was closer to Kochab, which is known as the "b UMi" star. It is expected that Polaris will be replaced by Vega in the year 14000.

Polaris has been the North Star for many centuries. In the classical era, sailors used it for navigation. The celestial pole was not completely fixed, as it wobbled about 26,000 years. The axis of the earth also wobbles, and the stars that appear to trace the path of the axis seem to change.

Although Polaris has a large surface area, its mass is lower than conventional Cepheid models predict. It has experienced some mass loss, but the uncertainty of its mass is close to the mass found from luminosity and temperature. However, further data are needed to get an accurate mass.

Since 2005, a team of astronomers from the Center for Astrophysics (CfA) in San Diego has been monitoring Polaris with the Hubble Space Telescope. In addition to obtaining better masses for Polaris, the research is aimed at measuring its motion.

Polaris is now in a phase of instability, and this is causing its brightness to fluctuate. During the earliest stages of this phase, the star appears to pulsate, but it is not pulsating with its natural fundamental period. As a result, the apparent variation of the star over a three-day period is only 0.03 magnitudes. But, this variation is quite remarkable. In fact, it is about 100 times the change predicted by current theories of stellar evolution. It is not yet known whether the instability will continue or end.

The axis of the Earth precesses in a conical motion, and this movement causes the stars in the northern sky to shift. The motion of the stars causes them to move around the pole, which is so tiny that humans can not see it. The motion of the stars is observed by other stars.