International Ultraviolet Explorer spectra of two stars with solar-like (B − V) color. HD 1835 (bottom) is younger and has a more negative Q value than HD 76151 (top). The Mg II lines near 2800 Å are stronger for HD 1835 than HD 76151, and as Mg II indicates chromospheric activity, this figure illustrates that FUV photometry, and hence Q, also traces activity levels.