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In the case of the Sothic cycle the two cycles are the Egyptian civil year and the Sothic year. The Sothic year is the length of time for the star Sirius to visually return to the same position in relation to the sun. Star years measured in this way vary due to axial precession, [9] the movement of the Earth's axis in relation to the sun.
Sirius is a binary star consisting of a main-sequence star of spectral type A0 or A1, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B. The distance between the two varies between 8.2 and 31.5 astronomical units as they orbit every 50 years.
If a star is in a binary system, as is the case for Sirius B or 40 Eridani B, it is possible to estimate its mass from observations of the binary orbit. This was done for Sirius B by 1910, [31] yielding a mass estimate of 0.94 M ☉, which compares well with a more modern estimate of 1.00 M ☉. [32]
An artist's impression of Sirius A and Sirius B, a binary star system. Sirius A, an A-type main-sequence star, is the larger of the two. An A-type main-sequence star (A V) or A dwarf star is a main-sequence (hydrogen burning) star of spectral type A and luminosity class V (five). These stars have spectra defined by strong hydrogen Balmer ...
Representative lifetimes of stars as a function of their masses The change in size with time of a Sun-like star Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right Chart of stellar evolution A mass-radius plot ...
The star is a vertex of the "Winter Hexagon", an asterism that includes Aldebaran, Capella, Pollux, Procyon, and Sirius. Rigel is a prominent equatorial navigation star , being easily located and readily visible in all the world's oceans (the exception is the area north of the 82nd parallel north ).
The transition in primary energy production from one form to the other spans a range difference of less than a single solar mass. In the Sun, a one solar-mass star, only 1.5% of the energy is generated by the CNO cycle. [32] By contrast, stars with 1.8 M ☉ or above generate almost their entire energy output through the CNO cycle. [33]
Sirius is the fixed star with the greatest apparent magnitude and one which is almost non-variable. The Pleiades, a key feature of Taurus shown across Orion in the same photograph also experience an annual period of visibility ("rising and setting").