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The abbreviation Byr means "billion years" (109 or 1,000,000,000 years). A closely related abbreviation is bya "billion years ago".

Byr was formerly used in English-language geology and astronomy as a unit of one billion years. Today, the term gigaannum (Ga) is also used, but Gy or Gyr are still sometimes used in English-language works (at the risk of confusion with the gray). Astronomers use Gyr or Gy as an abbreviation for gigayear.[1] Specifically, one Gyr is equal to 109 years.

Because a billion means 1000 million in some countries but can mean a million million in others its use is deprecated in favour of giga- which unambiguously means 109 and annum which for precision may be qualified by (sidereal), (Julian), etc.

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  1. ^ Selsis; Kasting, J. F.; Levrard, B.; Paillet, J.; Ribas, I.; Delfosse, X. et al. (2007). "Habitable planets around the star Gl 581?". Astronomy and Astrophysics 476 (3): 1373–1387. arXiv:0710.5294. Bibcode:2007A&A...476.1373S. doi:10.1051/0004-6361:20078091.