The revised Hipparcos parallax gives a distance to Polaris of about 433 light-years (133 parsecs), while calculations by some other methods derive distances up to 35% closer. Polaris is a triple star system, composed of the primary star, Polaris Aa (a yellow supergiant), in orbit with a smaller companion (Polaris Ab); the pair in orbit with Polaris B (discovered in August 1779 by William Herschel). ==Stellar system== Polaris Aa is a 5.4 solar mass () F7 yellow supergiant of spectral type Ib.
William Herschel discovered the star in August 1779 using a reflecting telescope of his own, one of the best telescopes of the time.
In Shakespeare's play Julius Caesar, written around 1599, Caesar describes himself as being "as constant as the northern star", though in Caesar's time there was no constant northern star. Polaris was referenced in Nathaniel Bowditch's 1802 book, American Practical Navigator, where it is listed as one of the navigational stars.
The variability of Polaris had been suspected since 1852; this variation was confirmed by Ejnar Hertzsprung in 1911. The range of brightness of Polaris is given as 1.86–2.13, but the amplitude has changed since discovery.
The variability of Polaris had been suspected since 1852; this variation was confirmed by Ejnar Hertzsprung in 1911. The range of brightness of Polaris is given as 1.86–2.13, but the amplitude has changed since discovery.
By examining the spectrum of Polaris A, it was also discovered in 1929 that it was a very close binary, with the secondary being a dwarf (variously α UMi P, α UMi an or α UMi Ab), which had been theorized in earlier observations (Moore, J.
Prior to 1963, the amplitude was over 0.1 magnitude and was very gradually decreasing.
It has steadily increased by around 4.5 seconds per year except for a hiatus in 1963–1965.
After 1966, it very rapidly decreased until it was less than 0.05 magnitude; since then, it has erratically varied near that range.
It is also the only one with a dynamically measured mass. The Hipparcos spacecraft used stellar parallax to take measurements from 1989 and 1993 with the accuracy of 0.97 milliarcseconds (970 microarcseconds), and it obtained accurate measurements for stellar distances up to 1,000 pc away.
It is also the only one with a dynamically measured mass. The Hipparcos spacecraft used stellar parallax to take measurements from 1989 and 1993 with the accuracy of 0.97 milliarcseconds (970 microarcseconds), and it obtained accurate measurements for stellar distances up to 1,000 pc away.
The Hipparcos reduction specifically for Polaris has been re-examined and reaffirmed but there is still not widespread agreement about the distance. The next major step in high precision parallax measurements comes from Gaia, a space astrometry mission launched in 2013 and intended to measure stellar parallax to within 25 microarcseconds (μas).
When Gaia entered regular scientific operations in July 2014, it was configured to routinely process stars in the magnitude range 3 – 20.
The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Polaris for the star α Ursae Minoris Aa. In antiquity, Polaris was not yet the closest naked-eye star to the celestial pole, and the entire constellation of Ursa Minor was used for navigation rather than any single star.
All text is taken from Wikipedia. Text is available under the Creative Commons Attribution-ShareAlike License .
Page generated on 2021-08-05