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Sirius is a star system and the brightest star in the Earth’s night sky. It is almost twice as bright as Canopus, the next brightest star.
What is Sirius?
What the naked eye perceives as a single star is a binary star system, consisting of a white main-sequence star of spectral type A1V, termed Sirius A, and a faint white dwarf companion of spectral type DA2, called Sirius B. The distance separating Sirius A from its companion varies between 8.2 and 31.5 AU.
Sirius appears bright because of its intrinsic luminosity and its proximity to Earth. At a distance of 2.6 parsecs (8.6 ly), as determined by the Hipparcos astrometry satellite, the Sirius system is one of Earth’s near neighbours. Sirius is gradually moving closer to the Solar System, so it will slightly increase in brightness over the next 60,000 years. After that time its distance will begin to increase and it will become fainter, but it will continue to be the brightest star in the Earth’s night sky for the next 210,000 years.
It is also known colloquially as the “Dog Star”, reflecting its prominence in its constellation, Canis Major (Greater Dog). The heliacal rising of Sirius marked the flooding of the Nile in Ancient Egypt and the “dog days” of summer for the ancient Greeks, while to the Polynesians in the Southern Hemisphere the star marked winter and was an important reference for their navigation around the Pacific Ocean.
It can be observed in daylight with the naked eye under the right conditions. Ideally, the sky should be very clear, with the observer at a high altitude, the star passing overhead, and the Sun low down on the horizon. These observing conditions are more easily met in the southern hemisphere, due to the southerly declination of Sirius.
The orbital motion of the Sirius binary system brings the two stars to a minimum angular separation of 3 arcseconds and a maximum of 11 arcseconds. At the closest approach, it is an observational challenge to distinguish the white dwarf from its more luminous companion, requiring a telescope with at least 300 mm (12 in) aperture and excellent seeing conditions. A periastron occurred in 1994 and the pair have since been moving apart, making them easier to separate with a telescope.
Image: NASA, ESA and G. Bacon (STScI)
Text: Wikipedia contributors. “Sirius.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 5 Aug. 2017. Web. 9 Aug. 2017.