If we send an elctric current to the electric bulb (lamp), the filament in the bulb heats up and shines. Its luminosity increases with increasing the voltage. That is, when the lamp is in the low voltage, the current is small, and the joule heating of the filament is also small. In this case the temperature of the filament is not so high, and the filament shines, radiating reddish light. On the other hand, when the lamp is in the high voltage, the current becomes large, and the joule heating becomes large. As a result, the temperature of the filament becomes hight, and it shines, radiating bluish light.

When we resolve the radiation from the electric bulb, it is seen that the blue light becomes strong with the voltage. The component of the light emitted from the body closely relates to its temperature. The smoothly distributed radiation spectrum, like that from the electric bulb, is called continuum/continuous spectrum. Roughly speaking, the solar spectrum is continuum one, except for many fine dark lines. (By courtesy of Okayama Astrophysical Observatory)

Star Color

Stars in the Orion When we observe many stars in the night sky, we can easily find that there exist various colors for star lights. In the case of stars in the Orion, for example, Betelgeuse (alpha-Ori) is reddish, while Rigel (beta-Ori) is white. Some other stars in the Orion are bluish. This is because the surface temperature of individual stars is different. As is expected from the electric bulb, stars with low surface temperature radiate red and orange light, and look reddish. On the other hand, stars with high surface temperature radiate green, blue, and violet light, and therefore, look white or bluish. The Spectral Type of stars closely relates to the star color or the surface temperature. Star color in the Orion By courtesy of Y. Norimoto (OAO)