page_title: Sirius
section_title: Sirius A
hierarchical_section_title: Sirius / Stellar system / Sirius A
caption_attribution_description: English: Comparison of size and relative surface brightness of Sirius A and the Sun
context_page_description: Sirius is the brightest star in the night sky. Its name is derived from the Greek word Σείριος Seirios "glowing" or "scorching". With a visual apparent magnitude of −1.46, Sirius is almost twice as bright as Canopus, the next brightest star. 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.
  Sirius appears bright because of its intrinsic luminosity and its proximity to the Solar System. At a distance of 2.64 parsecs, the Sirius system is one of Earth's nearest 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.
  Sirius A is about twice as massive as the Sun and has an absolute visual magnitude of +1.42.
context_section_description: Sirius A has a mass of 2 M☉. The radius of this star has been measured by an astronomical interferometer, giving an estimated angular diameter of 5.936±0.016 mas. The projected rotational velocity is a relatively low 16 km/s, which does not produce any significant flattening of its disk. This is at marked variance with the similar-sized Vega, which rotates at a much faster 274 km/s and bulges prominently around its equator. A weak magnetic field has been detected on the surface of Sirius A.
  Stellar models suggest that the star formed during the collapsing of a molecular cloud and that, after 10 million years, its internal energy generation was derived entirely from nuclear reactions. The core became convective and used the CNO cycle for energy generation. It is predicted that Sirius A will have completely exhausted the store of hydrogen at its core within a billion (10⁹) years of its formation. At this point, it will pass through a red giant stage, then settle down to become a white dwarf.
  Sirius A is classed as an Am star because the spectrum shows deep metallic absorption lines, indicating an enhancement in elements heavier than helium, such as iron. The spectral type has been reported as A0mA1 Va, which indicates that it would be classified as A1 from hydrogen and helium lines, but A0 from the metallic lines that cause it to be grouped with the Am stars. When compared to the Sun, the proportion of iron in the atmosphere of Sirius A relative to hydrogen is given by , meaning iron is 316% as abundant as in the Sun's atmosphere. The high surface content of metallic elements is unlikely to be true of the entire star; rather the iron-peak and heavy metals are radiatively levitated towards the surface.
caption: Comparison of Sirius A and the Sun, to scale and relative surface brightness
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