E.5 · IB Physics
Luminosity and apparent brightness
Set the luminosity and distance of two stars, watch their light spread over ever larger spheres, and compare how bright each appears to an observer.
b = L / (4πd²)
1 L☉ = 3.83 × 10²⁶ W
1 ly = 9.46 × 10¹⁵ m
1 L☉ = 3.83 × 10²⁶ W
1 ly = 9.46 × 10¹⁵ m
A: L = , d =
bA =
B: L = , d =
bB =
bA / bB =
Try this
- Keep both luminosities equal and double star B's distance. Check that bA/bB becomes 4.00.
- Make star B 100 times more luminous than star A, then press the match button: how many times farther away must B be to look equally bright?
- Set A to 25.0 L☉ at 8.60 ly (similar to Sirius) and B to 1.00 L☉ at 4.20 ly. Predict which looks brighter before reading the ratio.
Common error: forgetting to square the distance, or using light-years or parsecs directly in b = L/(4πd²) instead of converting to metres first.