Spectrum of visible light emitted from the sub-giant star Procyon (Alpha Canis Minoris). At bottom left is the shortest wavelength visible, around 400 nanometres (nm). Wavelength increases from left to right along each strip, and from bottom to top. The longest wavelength visible, 700nm, is at top right. The dark lines are Fraunhofer absorption lines. These are due to emitted light being absorbed by elements in the star's atmosphere. Data obtained by the 100-inch (2.5 metre) reflector telescope at Mount Wilson Observatory, California, USA.