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Every object in the Universe, including people, ice cubes and fire, emit radiation at all times, because charged particles in them are in constant random motion.

Image of basic black body curve, courtesy of Dr. Eric Chaisson Whenever charges change their state of motion, electromagnetic radiation is emitted. The temperature of an object is a direct measure of the amount of microscopic motion within it. The hotter the object, the faster its constituent particles move, and the more energy they radiate. Yet no natural object emits all of its radiation at just one frequency. Rather, there is a range of frequencies emitted. The intensity of the various frequencies is not random, but increases as the frequencies increase, then levels of and falls to zero, as the frequencies continue to increase.
The curve above is the radiation-distribution curve for a mathematical idealization known as a 'black body' (or 'Planck's curve)—an object that absorbs all radiation falling upon it. In a steady state, a black body must re-emit the same amount of energy as it absorbs; the black-body curve shown in the image describes the distribution of that re-emitted radiation. No real object absorbs and radiates as a perfect black body. In many cases, however, the black-body curve is a very good approximation to reality, and the properties of black bodies provide important insights into the behavior of real objects.
The black-body curve shifts toward higher frequencies (shorter wavelengths) and greater intensities as an object's temperature increases. Even so, the shape the curve remains the same.
Image of sample black body curve, courtesy of Dr. Eric Chaisson This shifting of radiation's peak frequency with temperature is familiar to us all: very hot glowing objects, such as toaster filaments or stars, emit visible light. Cooler objects, such as warm rocks or household radiators, produce invisible radiation—warm to the touch but not glowing hot to the eye. These latter objects emit most of their radiation in the lower-frequency infrared part of the electromagnetic spectrum.
As a further example, imagine a piece of metal placed in a hot furnace. At first, the metal becomes warm, although its appearance doesn't change. As it heats up, it begins to glow dull red, then orange, brilliant yellow, and finally white. How do we explain this? As illustrated in Figure 2.10, when the metal is at room temperature (300 K), it emits only invisible infrared radiation. As the metal becomes hotter, the peak of its black-body curve shifts toward higher frequencies. At 1000 K, for instance, most of the emitted radiation is still infrared, but now there is also a small amount of visible (dull red) radiation being emitted (note in Figure 2.10 that the high-frequency portion of the 1000 K curve just overlaps the visible region of the graph). As the temperature continues to rise, the peak of the metal's black-body curve moves through the visible spectrum, from red (the 4000 K curve) through yellow. The metal eventually becomes white hot because when its black-body curve peaks in the blue or violet part of the spectrum (the 7000 K curve), the low-frequency tail of the curve extends through the entire visible spectrum (to the left in Figure 2.10), meaning that substantial amounts of green, yellow, orange, and red light are also emitted. Together, all these colors combine to produce white.

From detailed studies of the precise form of the black-body curve, we obtain a very simple connection between the wavelength at which most radiation is emitted and the absolute temperature (that is, temperature measured in kelvins—see More Precisely 2-1) of the emitting object:

This relationship is known as Wien's law . Simply put, it tells us that the hotter the object, the bluer its radiation. Finally, it is also a matter of everyday experience that as the temperature of an object increases, the total amount of energy it radiates (summed over all frequencies) increases rapidly. For example, the heat given off by an electric heater increases sharply as the heater warms up and begins to emit visible light. In fact, the total amount of energy radiated per unit time is proportional to the fourth power of an object's temperature:
This relationship is called Stefan's law . It implies that the energy emitted by a body rises dramatically as the body's temperature increases. Doubling the temperature, for example, causes the total energy radiated to increase by a factor of 16. Astronomical Applications Astronomers often use black-body curves as thermometers to determine the temperatures of distant objects. For example, study of the solar spectrum makes it possible to measure the temperature of the Sun's surface. Observations of the radiation from the Sun at many frequencies yield a curve shaped somewhat like that shown in Figure 2.9. The Sun's curve peaks in the visible part of the electromagnetic spectrum; the Sun also emits a lot of infrared and a little ultraviolet radiation. Using Wien's law, we find that the temperature of the Sun's surface is approximately 6000 K. (A more precise measurement, applying Wein's law to the black-body curve that best fits the solar spectrum, yields a temperature of 5800 K.) Other cosmic objects have surfaces very much cooler or hotter than the Sun's, emitting most of their radiation in invisible parts of the spectrum (Figure 2.11). For example, the relatively cool surface of a very young star might measure 600 K and emit mostly infrared radiation. Cooler still is the interstellar gas cloud from which the star formed; at a temperature of 60 K, such a cloud would emit mainly long-wavelength radiation in the radio and infrared parts of the spectrum. The brightest stars, by contrast, have surface temperatures as high as 60,000 K and hence emit mostly ultraviolet radiation.

Below, is a comparison of black-body curves for four cosmic objects.

(a) A cool, invisible galactic gas cloud called Rho Ophiuchi. At a temperature of 60 K, it emits mostly low-frequency radio radiation.

(b) A dim, young star (shown red in the inset photograph) near the center of the Orion Nebula. The star's atmosphere, at 600 K, radiates primarily in the infrared.

(c) The Sun's surface, at approximately 6000 K, is brightest in the visible region of the electromagnetic spectrum.

(d) A cluster of very bright stars, called Omega Centauri, as observed by a telescope aboard the space shuttle. At a temperature of 60,000 K, these stars radiate strongly in the ultraviolet.

Image of four real black body curves, courtesy of Dr. Eric Chaisson

And now, we are ready to learn about Spectroscopy!