We can measure our body temperature with a thermometer. Thermocouples and other devices are used to measure the temperature of furnaces. But how can we measure the temperature of stars?
The surface temperature of stars is determined by various techniques. The most conventional and fairly accurate estimate can be made by colour alone. Red-coloured stars are cool while blue ones are extremely hot. On the basis of colour, stars have been classified in the table given below.
A more accurate determination of the temperature is made by the comparison of spectra of stars. Light, which comes from the sun and other stars, is made up of many different wavelengths. It can be separated into different wavelengths by a spectrograph (an instrument used to record spectrum). From the spectroscopic studies, it has been observed that stars are largely composed of hydrogen (about 75% on the average). Next in abundance is helium followed by various other metals. In the cooler stars, some compounds are present but at high temperatures, they disintegrate into atoms. In order to know the temperature, the spectra of stars are recorded. It will be different for different stars, depending upon their temperature.
Moreover, the intensity of spectral lines, bright or dark, varies with the temperature. It has been found that blue stars have O-type spectra; our sun has G-type spectra and so on. Blue stars emit 20 or more times the radiation per unit area than that of our sun does, whereas a red type may emit as little as 1/20 as much per unit area.
From these spectra, by measuring and comparing the intensity of different lines and using Wien’s Displacement Law, the temperature may be determined. Intensity of emitted light is plotted against wavelength and the curve is drawn. The temperature of the star is directly proportional to the frequency at which most of its radiation is given off, i.e. to the highest point of the curve.