Gyr

Gyr is an abbreviation for gigayear, a unit of time equal to one billion years. It is commonly used in geology, paleontology, astronomy, and cosmology to express long-timescale phenomena and ages of objects on cosmic timescales. The term is not an official SI unit; the year is the standard unit of time in these contexts, and "Gyr" serves as a practical multiplier of the year.

Definition: 1 Gyr = 10^9 years. In seconds, approximately 3.15576×10^16 s. For accuracy, use the year 365.25 days. In practice, scientists often use Gyr to summarize ages: the Earth is about 4.54 Gyr old; the Universe is about 13.8 Gyr old. Galaxy formation and star-formation histories are frequently discussed in terms of Gyr.

Notation: The common abbreviation is "Gyr." The capitalization matters to avoid confusion with the gray (Gy), a unit of absorbed radiation dose. Some authors may also write "Gy" for gigayears, but "Gyr" is widely accepted for time.

History: The usage arose in the 20th century as geochronology and cosmology developed long-timescale models. It provides a compact way to refer to billions of years without lengthy phrasing.

See also: year, gigayear, geological timescale, cosmology.