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A light-year, alternatively spelled light year (ly or lyr [3]), is a unit of length used to express astronomical distances and is equal to exactly 9 460 730 472 580.8 km, which is approximately 9.46 trillion km or 5.88 trillion mi.
≈ 1 hour walk, Currently defined in US as 3 Statute miles, [3] but historically varied from 2 to 9 km ≈ 4828 m: light-day: ≡ 24 light-hours ≡ 2.590 206 837 12 × 10 13 m: light-hour: ≡ 60 light-minutes ≡ 1.079 252 8488 × 10 12 m: light-minute: ≡ 60 light-seconds ≡ 1.798 754 748 × 10 10 m: light-second: ≡ Distance light ...
This number is likely much higher, due to the sheer number of stars needed to be surveyed; a star approaching the Solar System 10 million years ago, moving at a typical Sun-relative 20–200 kilometers per second, would be 600–6,000 light-years from the Sun at present day, with millions of stars closer to the Sun.
For example, for visible light, the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200 000 km/s (124 000 mi/s); the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 90 km/s (56 mi/s) slower than c.
A megaparsec is one million parsecs, or about 3,260,000 light years. [22] Sometimes, galactic distances are given in units of Mpc/ h (as in "50/ h Mpc", also written " 50 Mpc h −1 "). h is a constant (the " dimensionless Hubble constant ") in the range 0.5 < h < 0.75 reflecting the uncertainty in the value of the Hubble constant H for the ...
{{Convert}} uses unit-codes, which are similar to, but not necessarily exactly the same as, the usual written abbreviation for a given unit. These unit-codes are displayed in column 3 of the following tables. These are accepted as input by {{convert}} as the second and third unnamed parameters: {{convert|100|kg|lb}} → 100 kilograms (220 lb)
Distance of the outer limit of Oort cloud from the Sun (estimated, corresponds to 1.2 light-years) — Parsec: 206 265 — One parsec. The parsec is defined in terms of the astronomical unit, is used to measure distances beyond the scope of the Solar System and is about 3.26 light-years: 1 pc = 1 au/tan(1″) [60] [61] Proxima Centauri: 268 000 ...
Objects with size order of magnitude 1e16m: Ten light-years (94.6 Pm) radius circle with yellow Vernal Point arrow; Bubble Nebula (NGC 7635), left; Dumbbell Nebula (NGC 6853), right; one light-year shell lower right with the smaller Cat's Eye Nebula (NGC_6543) and Barnard 68 adjacent. 1e16m lengths: Ten light-years (94.6 Pm) yellow shell ...