✨ Star Magnitude & Distance Calculator
Enter any two of apparent magnitude, absolute magnitude and distance to solve for the third — and compare how much brighter one star is than another.
Absolute magnitude (M)
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Distance modulus (m − M)
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Brightness comparison
The brighter is 100× brighter.
Apparent magnitude (m) is how bright a star looks from Earth; absolute magnitude (M) is how bright it would look at 10 parsecs — a fair comparison of true luminosity. They relate through the distance: m − M = 5·log₁₀(d) − 5. The magnitude scale is backwards (smaller = brighter) and logarithmic: every 5 magnitudes is exactly a 100× brightness ratio, so one magnitude ≈ 2.512×. 🔒 In your browser.
How the star magnitude & distance calculator works
A star's apparent magnitude (m) is how bright it looks from Earth; its absolute magnitude (M) is how bright it would appear at a standard 10 parsecs, letting you compare true luminosities. The two are linked by the distance modulus, m − M = 5·log₁₀(d) − 5, so knowing any two gives the third. The tool also compares brightness with Pogson's scale, where every 5 magnitudes is exactly a factor of 100.
The magnitude scale is counter-intuitive: smaller numbers are brighter (the Sun is −26.7, a faint naked-eye star is +6), and it's logarithmic, so one magnitude is about 2.512× in brightness. Distances here are in parsecs (1 pc ≈ 3.26 light-years). This ignores interstellar extinction (dust dimming), which shifts real measurements.
Frequently asked questions
What is the difference between apparent and absolute magnitude?
Apparent magnitude (m) is how bright a star looks from Earth, which depends on distance. Absolute magnitude (M) is how bright it would look from a fixed 10 parsecs — a measure of true luminosity you can compare between stars.
What is the distance modulus?
The difference m − M, which depends only on distance: m − M = 5·log₁₀(d) − 5, with d in parsecs. It's a cornerstone of measuring cosmic distances — find m and M and you get the distance.
Why is a smaller magnitude brighter?
The scale is historical, running from the brightest stars (1st magnitude) to the faintest visible (6th). It was later made precise and extended, keeping the backwards direction, so brighter objects have smaller — even negative — magnitudes.
How much brighter is one magnitude?
About 2.512 times. The scale is defined so that a difference of exactly 5 magnitudes is a brightness ratio of 100, and 100^(1/5) ≈ 2.512. So a 1st-magnitude star is 100× brighter than a 6th-magnitude one.
How do I find a star's distance from its magnitudes?
Rearrange the distance modulus: d = 10^((m − M + 5) ÷ 5) parsecs. If you know both magnitudes, this gives the distance directly — the tool does it in the "find distance" mode.