Star Magnitude Calculator
Brightness ratio between two magnitudes, and the absolute magnitude of a star at a given distance.
Results
What this tool does
The defaults compare Sirius at −1.46 with the faintest star a good eye can see at 6, and place Sirius at its actual 2.64 parsecs. Absolute magnitude is what a star would look like from ten parsecs, which is the only fair way to compare stars at different distances.
Formula
brightness ratio = 10^(0.4 × Δm) ; M = m − 5 log₁₀(d ÷ 10)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
m1 | Apparent magnitude | — |
m2 | Apparent magnitude | — |
dp | Distance | pc |
RA | Brightness ratio | — |
AM | Absolute magnitude | — |
DM | Distance modulus | — |
LY | Light years | — |
Worked example
- Apparent magnitude-1.46
- Apparent magnitude6
- Distance2.64 pc
- Brightness ratio963.8290
- Absolute magnitude1.4320
- Distance modulus-2.8920
- Light years8.6105
Limitations
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
Frequently asked questions
Why are brighter stars given smaller numbers?
Because the scale is inherited from Hipparchus, who sorted the visible stars into six classes with the brightest called first magnitude. When it was later made precise, the ordering was kept for continuity, with each step defined as a factor of about 2.512 in brightness so that five steps make exactly one hundred. Sirius comes out at −1.46 and the faintest naked-eye stars at about 6.