At any time the wave occupies the surface of a sphere. We can use the inverse square law to set up a scale of absolute magnitudes . The brightnesses of objects we observe are their apparent magnitudes . The quantity m-M is known as the distance modulus . Spectroscopy Generally, the light we see is composed of a mixture of wavelengths.
White light is composed of red, green, yellow, and blue photons.
We can separate light into its component wavelengths by using a dispersing element, either a prism or a grating. Once the light is dispersed , it forms a spectrum . The light is concentrated into specific colors or wavelengths. Spectra are a powerful tool for studying astronomical objects. A hot transparent gas emits a bright line spectrum, or an emission line spectrum.