Spark
Loading...

Distance Modulus and Apparent vs. Absolute Magnitude

Apparent magnitude (m) is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light caused by interstellar dust or atmosphere along the line of sight to the observer. Unless stated otherwise, the word magnitude in astronomy usually refers to a celestial object's apparent magnitude. The magnitude scale likely dates to before the ancient Roman astronomer Claudius Ptolemy, whose star catalog popularized the system by listing stars from 1st magnitude (brightest) to 6th magnitude (dimmest).

Figures (10)

Brightness with phase for diffuse reflection models. The sphere is 2/3 as bright at zero phase, while the disk can't be seen beyond 90 degrees.
Phase integrals for various values of G
A scatter plot showing how familiar objects measure in magnitude, surface luminance, and angular diameter.
Graph of relative brightness versus magnitude
The phase angle α {\displaystyle \alpha } can be calculated from the distances body-sun, observer-sun and observer-body, using the law of cosines.
Diffuse reflection on sphere and flat disk
Relationship between the slope parameter G {\displaystyle G} and the opposition surge. Larger values of G {\displaystyle G} correspond to a less pronounced opposition effect. For most asteroids, a value of G = 0.15 {\displaystyle G=0.15} is assumed, corresponding to an opposition surge of 0.3 mag {\displaystyle 0.3{\text{ mag}}} .
Asteroid 65 Cybele and two stars in the constellation Aquarius, with their magnitudes labeled
Image of 30 Doradus taken by ESO's VISTA. This nebula has a visual magnitude of 8.

Key Points

  • Apparent magnitude is technically a measure of illuminance, which can also be measured in photometric units such as lux.
  • Absolute magnitude is a related quantity which measures the luminosity that a celestial object emits, rather than its apparent brightness when observed, and is expressed on the same reverse logarithmic scale.
  • Spectroscopic parallax is an astronomical method for measuring the distances to stars.
  • Galaxies (and other extended objects) are much larger than 10 parsecs; their light is radiated over an extended patch of sky, and their overall brightness cannot be directly observed from relatively short distances, but the same convention is used.

Terms

Tap a term for a plain-language explanation.

Sources & licensing(4)