The plural of light-year is light-years.
When it comes to measuring astronomical distances, "light-year" is a term that is commonly used. A light-year is the distance that light travels in one year, which is roughly 5.88 trillion miles or 9.46 trillion kilometers.
It is a unit of length, not time, despite the name. Unfortunately, the name "light-year" has led to a common misconception that it is a measure of time, rather than speed.
The general rule to form the plural of hyphenated compound nouns is to change only the primary noun to its plural form.
This is because the primary noun is the most important part of the noun phrase, and changing it to its plural form creates the appropriate number agreement for the noun phrase as a whole.
For example, in the case of "daughter-in-law", the primary noun is "daughter", so we pluralize it to "daughters-in-law" to indicate that there are multiple daughters in law.
Similarly, in the case of "editor-in-chief", the primary noun is "editor", so we pluralize it to "editors-in-chief" to indicate that there are multiple editors in chief.
Now, when it comes to forming the plural of "light-year", the process is relatively straightforward. The word "year" is the primary noun in this compound noun, so we simply add an "s" to make it plural. Therefore, the plural of "light-year" is "light-years".
A light-year is a unit of astronomical distance equivalent to the distance that light travels in one year.
One light-year is equal to about 5.88 trillion miles.
Our galaxy, the Milky Way, is estimated to be 100,000 light-years in diameter.
Astronomers have discovered a black hole that is 700 million light-years away from Earth.
The light from some distant galaxies has taken billions of light-years to reach Earth.
Whilst the diameter of the Milky Way spans around 100,000 light-years, the diameter of the galaxy so-named IC 1101 is as wide as 4 million light-years!