Light Year and Parsecs

A diagram depicting 12ly. Credit: Atlas of the Universe.
A diagram depicting 12ly. Credit: Atlas of the Universe.

What is a Light Year?

A light-year, also light year or lightyear (symbol: ly), is an astronomical unit of length equal to just under 10 trillion kilometres (or about 6 trillion miles). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year. The light-year is mostly used to express distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The preferred unit in astrometry is the parsec (approximately 3.26 light-years), because it can be more easily derived from, and compared with, observational data.

Note that the light-year is a measure of distance. It is not a measure of time, for which it is sometimes mistaken.


Putting Light Years into Perspective:

The closest star to earth  (other than our Sun) is Alpha Centauri, which is 4.3 light years away (or approximately 25 trillion miles way). Because the speed of light is only 186,000 miles per second, it can only travel 6 trillion miles in one year . Therefore, it takes 4.3 years for Alpha Centauri’s light to reach earth. For human technology, however, the story is quite different. The Voyager Spacecraft, for example, is currently traveling at approximately 38,000 miles per hour. At this speed, it would take Voyager about 77,000 years to reach Alpha Centauri!


So how big is the Observable Universe?

One of the farthest known objects in the observable universe is the galaxy MACS0647-JD, which is about 13.3 billion light-years away (or 76,306,276,800,000,002,359,296 sextillion miles away). It would take Voyager more than the age of the universe to get there.

MACS0647-JD is a candidate for the farthest known galaxy from Earth. Image from NASA.
MACS0647-JD is a candidate for the farthest known galaxy from Earth. Image from NASA.

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