LIGHT YEAR

 Light Year:

• Light year is an astronomical unit.
• It is used to express the astronomical distance between the celestial objects.
• A light year is the distance travelled by the light in vacuum in one year.
• It has nothing to do with the time, it is the measurement of length.
• It is represented by the symbol ‘ly’
• One light year is equal to 9.46 trillion kilometers (9.46×10¹² km) and 63241.1 AU.

Calculation of 1ly:

• The velocity of light is 
• Which means, the light travels 299,792,458 $\mathrm{<span\; class="mi"><span\; style="border:\; none\; windowtext\; 1.0pt;\; mso-border-alt:\; none\; windowtext\; 0cm;\; padding:\; 0cm;">m</span></span><span\; role="presentation"\; style="box-sizing:\; content-box;\; clip:\; rect(1px,\; 1px,\; 1px,\; 1px);\; overflow:\; hidden;\; transition:\; none\; 0s\; ease\; 0s;\; user-select:\; none;"><math\; xmlns="http://www.w3.org/1998/Math/MathML"><mi> every\; second.<span\; style="font-size:\; 14.0pt;"><o:p></o:p></span></mi></math></span>}$
• For 1 second = 299,792,458 m
• for 60 seconds = 60×299,792,458 $\mathrm{<span\; style="border:\; none\; windowtext\; 1.0pt;\; font-family:\; "Times\; New\; Roman",serif;\; font-size:\; 14.0pt;\; line-height:\; 107\%;\; mso-ansi-language:\; EN-IN;\; mso-bidi-language:\; AR-SA;\; mso-border-alt:\; none\; windowtext\; 0cm;\; mso-fareast-font-family:\; Calibri;\; mso-fareast-language:\; EN-US;\; mso-fareast-theme-font:\; minor-latin;\; padding:\; 0cm;">m</span><span\; role="presentation"\; style="box-sizing:\; content-box;\; clip:\; rect(1px,\; 1px,\; 1px,\; 1px);\; overflow:\; hidden;\; transition:\; none\; 0s\; ease\; 0s;\; user-select:\; none;"><math\; xmlns="http://www.w3.org/1998/Math/MathML"><mi></mi></math></span> }$
• for 60 minutes = $\mathrm{ }$60×60×299,792,458$\mathrm{ }$m$$
• for 24 hours = 
• for 365.25 days = 365.25×24×60×60×299,792,458 $\mathrm{}$m$$
• Therefore, 1 LY = 9.461×10^15 m.

While dealing with the galaxies and the stars the measure of their distances if calculated in kilometers would be of very large. So the values would be impractical for the astronomers to measure practically. For an example, the Crab nebula is about 4000 ly away from the Earth. If it is calculated in meters, then the value would be 3.7844×10¹⁹m.

                                 

                                      https://spaceplace.nasa.gov/light-year/en/

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