Why is the bolometric correction always negative for any star?

The bolometric correction is not always negative for any star; rather, it can be both positive or negative depending on the star's temperature and luminosity. It is defined as the difference between the bolometric magnitude (which accounts for all wavelengths of light emitted) and the apparent magnitude in a specific band (like visible light). For hot, massive stars, the bolometric correction is often negative because they emit a significant amount of their energy in the ultraviolet and blue regions, which are not fully captured in visible light measurements. Conversely, cooler stars often have a positive bolometric correction as they emit more energy in the infrared.