Mbol

Mbol, short for bolometric magnitude, is a measure of a star’s total energy output across all wavelengths, expressed on a logarithmic magnitude scale. Unlike magnitudes tied to a specific photometric band, the bolometric magnitude accounts for the entire electromagnetic spectrum. Mbol is an absolute magnitude: it represents the value the object would have if placed at a distance of 10 parsecs from the observer. The Sun’s bolometric magnitude is commonly adopted as Mbol,sun = 4.74.

Mbol is related to luminosity by the equation Mbol − Mbol,sun = −2.5 log10(L/Lsun), where L is the star’s luminosity and Lsun is the Sun’s luminosity. Equivalently, L/Lsun = 10^(−0.4 × (Mbol − Mbol,sun)). In practice, astronomers often use the bolometric magnitude in conjunction with a bolometric correction: Mbol = MV + BCV, where MV is the absolute magnitude in the V band and BCV is the bolometric correction for that band. BCV depends on a star’s effective temperature, metallicity, and spectral type.

Observationally, bolometric magnitudes can be estimated by integrating a star’s spectral energy distribution or, more commonly, by applying bolometric corrections to multi‑band photometry after correcting for interstellar extinction. From a distance, Mbol can also be derived from the apparent bolometric magnitude mbol via the distance modulus: mbol − Mbol = 5 log10(d/10 pc).

Bolometric magnitudes are central to determining stellar luminosities and placing stars on the Hertzsprung–Russell diagram. They provide a distance- and extinction-independent measure of intrinsic brightness, enabling comparisons across different spectral types and evolutionary stages.