![]() It is the first classical Cepheid to have a mass determined from its orbit. Polaris Aa is an evolved yellow supergiant of spectral type F7Ib with 5.4 solar masses ( M ☉). ![]() Polaris components as seen by the Hubble Space Telescope The outer pair AB were discovered in August 1779 by William Herschel, where the 'A' refers to what is now known to be the Aa/Ab pair. Calculations by other methods vary widely.Īlthough appearing to the naked eye as a single point of light, Polaris is a triple star system, composed of the primary, a yellow supergiant designated Polaris Aa, in orbit with a smaller companion, Polaris Ab the pair is in a wider orbit with Polaris B. The revised Hipparcos stellar parallax gives a distance to Polaris of about 433 light-years (133 parsecs), while the successor mission Gaia gives a distance of about 448 light-years (137 parsecs). The stable position of the star in the Northern Sky makes it useful for navigation.Īs the closest Cepheid variable its distance is used as part of the cosmic distance ladder. The position of the star lies less than 1° away from the north celestial pole, making it the current northern pole star. With an apparent magnitude that fluctuates around 1.98, it is the brightest star in the constellation and is readily visible to the naked eye at night. It is designated α Ursae Minoris ( Latinized to Alpha Ursae Minoris) and is commonly called the North Star or Pole Star. Polaris is a star in the northern circumpolar constellation of Ursa Minor. The variation of the Sun’s declination through the year.Polaris, North Star, Cynosura, Alpha UMi, α UMi, ADS 1477, CCDM J02319+8915 We now show that the choice of as a reference point is closely connected with the Sun’s yearly journey round the stellar background of the celestial sphere.įigure 1. In drawing a celestial sphere it is advisable not only to mark the observer’s meridian heavily, inserting on the equator a westwards arrow with HA (hour angle) beside it but also to mark on the equator an eastwards arrow with RA (right ascension) beside it. Like the definition of hour angle, this convention holds for observers in both northern and southern hemispheres. in the direction opposite to that in which the hour angle is measured. Right ascension is measured from 0 h to 24 h or from 0 ◦ to 360 ◦ along the equator from eastwards, i.e. The point chosen is the vernal equinox, also referred to as the First Point of Aries, and the angle between it and the intersection of the meridian through a celestial object and the equator is called the right ascension (RA) of the object. In general, then, all celestial objects may have their positions on the celestial sphere specified by their declinations and by the angles between their meridians and the meridian through. If a point,, fixed with respect to the stellar background, is chosen on the celestial equator, its angular distance from the intersection of a star’s meridian and the equator will not change, in contrast to the changing hour angle of that star. Some other meridian must be chosen which is connected directly to the celestial sphere. Because of the Earth’s rotation under the celestial sphere, the projection of the Greenwich meridian sweeps round the sphere, passing through each star’s position in turn. The Greenwich meridian cannot be used for celestial position-fixing. ![]() The longitude of a place on the Earth’s surface is defined with respect to a meridian through a particular geographical position, namely the Airy Transit Instrument at Greenwich, England, and the meridian through the place in question. In spherical astronomy, declination, referred to the celestial equator, carries out the same task in fixing the place of a celestial object. Latitude is defined with respect to the terrestrial equator. The problem is solved in a manner analogous to the way in which places on the Earth’s surface are defined uniquely in position, although the Earth is rotating on its axis. The other, the hour angle, changes steadily with the passage of time and so is unsuitable for use in a catalogue of stellar positions. We have seen that in the equatorial system, one of the coordinates of the star, namely the declination, is constant with time. ![]()
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