Almost all continuous spectra are due to thermal radiation. Thermal radiation is radiation that only depends on temperature. Looking at the brightness or intensity of the spectra tells us the temperature of the object. The hottest stars have shorter wave lengths (cid:523)wien"s law(cid:524: radiation does not always occur in the optical range, black body curve- stephen bolton law (the area under the curve ) Think of a train horn sound as it goes by. The source of the sound is changing position which changes the wavelengths of the sound. Apparent brightness is a distance: energy per distance decreases as it spreads out over space. The bigger lens will capture more photons, you will be able to see fainter objects. Or resolving power what angle your looking at.