Color and Temperature

Have you ever looked closely at a candle flame? The hottest part of the flame near the wick is blue or white, while the cooler flame tip is orange. A propane torch flame is blue. Dying campfire embers are red. You can estimate the temperature of a flame from its color. In the same way, a star's color indicates the temperature of its surface. The hottest stars, with surface temperatures above 30,000 K, appear blue. The surfaces (photospheres) of relatively cool red stars are still a toasty 3000 K or so. Stars with surface temperatures between 5000 and 6000 K appear yellow, like the sun. As shown in Figure 10, the color differences between hot blue stars and cool red stars can be seen with the unaided eye. More precise measurements of stars' temperatures can be made by studying stars' spectra.

Brightness

When you are outside at night in a city, look for a row of street lights like the one in Figure 11. The closer lights look bright and the more distant lights look dim. However, the more distant lights are not really dimmer. They appear dim to you because, at a greater distance, their light is spread out over a greater area, so a smaller portion enters your eyes. The same is true for the light emitted by stars.

You might think that closer stars will always appear brighter than more-distant stars. Astronomers have discovered, however, that the brightness of stars can vary by a factor of more than a billion. So, stars that look bright may actually be farther away than stars that appear dim.

Although the sun appears to be the brightest star in our sky, it is really a star of only average brightness. The sun appears very bright to us because it is much closer than other stars. The brightness of a star as it appears from Earth is called its apparent brightness. The apparent brightness of a star decreases as its distance from you increases.