National 5PhysicsSQARevision NotesSpaceCosmologyLight-Years

Light-Years (SQA National 5 Physics): Revision Note

Exam code: X857 75

Katie M

Written by: Katie M

Reviewed by: Caroline Carroll

Updated on

Light-years

  • Astronomical distances, such as the distances between stars and galaxies, are extremely large

  • To describe these distances, astronomers use a special unit called the light year

  • One light-year is defined as:

The distance travelled by light in one year

  • The diameter of the Milky Way is approximately 100 000 light-years

  • This means that light would take 100 000 years to travel from one side of the Milky Way to the other

Converting between light-years and metres

  • To calculate one light-year in metres, use the relationship between speed, distance, and time:

d space equals space v space cross times space t

  • Where

    • v = speed of light (in a vacuum) = 3 × 108 m s-1

    • t = seconds in one year = 60 × 60 × 24 × 365

d space equals space open parentheses 3 cross times 10 to the power of 8 close parentheses space cross times space open parentheses 60 cross times 60 cross times 24 cross times 365 close parentheses

d space equals space 9.46 cross times 10 to the power of 15 space straight m

  • Therefore, one light-year is equal to 9.46 × 1015 m

Worked Example

The centre of our galaxy is 30 000 light years away from Earth.

(a) How long does it take light to reach the Earth?

(b) Calculate this distance in metres.

Answer:

(a)

  • The centre of our galaxy is 30 000 light years away

  • It takes light 30 000 years to reach the Earth from the centre of our galaxy

(b)

Step 1: Write down the known quantities:

  • The centre of our galaxy is 30 000 light years away

  • 1 light year = 9.46 × 1015 m

Step 2: Calculate the distance in km:

distance = 30 000 × (9.46 × 1015)

distance = 2.84 × 1020 m

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

    Previous:Calculating Weight across the UniverseNext:The Big Bang Theory
    Author
    Reviewer
    Katie M

    Author: Katie M

    Expertise: Physics Content Creator

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

    Caroline Carroll

    Reviewer: Caroline Carroll

    Expertise: Physics & Chemistry Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.