StudyBrixFree worksheets, study guides and courses

Astronomy & space · University

Flux, luminosity and stellar magnitudes

Luminosity is emitted power; flux is received power per area. For isotropic emission with negligible absorption, F=L/(4πd²).

Make a worksheet

Prerequisites

Logarithms and inverse-square relationships.

Study notes

Luminosity and flux

Luminosity is emitted power; flux is received power per area. For isotropic emission with negligible absorption, F=L/(4πd²).

Magnitude

Magnitude differences use a logarithmic scale: m2−m1=−2.5 log10(F2/F1). A smaller magnitude denotes a brighter source in the same band.

Absolute magnitude

Absolute magnitude compares sources at the conventional distance of 10 pc. Bandpass and extinction assumptions must be specified.

Distance modulus

Ignoring extinction, m−M=5 log10(d/10 pc). Dust can dim and redden a source, so uncorrected brightness is not purely a distance indicator.

Worked examples

For equal luminosity and negligible absorption, one source is twice as far away. Find its flux divided by the nearer source's flux.

The inverse-square law gives 1/2².

0.25

A source becomes 100 times brighter in the same band. Find mnew−mold.

−2.5 log10(100)=−5.

-5

Check your understanding

  1. Which quantity describes emitted power rather than received power per area?

    1. Luminosity
    2. Parallax angle
    3. Apparent magnitude by itself
    4. Flux

  2. Ignoring extinction, find m−M for d=100 pc.

  3. At which conventional distance is absolute magnitude defined?

    1. 1 km
    2. 1 AU
    3. 1000 light-years
    4. 10 pc

  4. Why can two equally bright-looking stars have different luminosities?

Answers and marking guidance

Answers and marking notes
  1. Luminosity

    Luminosity is intrinsic emitted power under the model.

  2. 5

    5 log10(100/10)=5.

  3. 10 pc

    Absolute magnitude standardizes the distance at 10 parsecs.

  4. Their distances and intervening extinction can differ. The received flux alone does not uniquely determine luminosity.

    Credit distance and at least the possibility of absorption.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

AI-assisted preview; independent educational review required. Level labels describe prerequisites, not accreditation or official curriculum alignment.