1. What is the electromagnetic spectrum?
The electromagnetic spectrum is a continuum of wavelengths, from zero to infinity. We name regions of the spectrum rather arbitrarily, but the names give us a general sense of the energy; for example, ultraviolet light has shorter wavelengths than radio light. The only region in the entire electromagnetic spectrum that our eyes are sensitive to is the visible region.
Gamma rays have the shortest wavelengths, < 0.01 nm (about the size of an atomic nucleus). This is the highest frequency and most energetic region of the electromagnetic spectrum. Gamma rays result from nuclear reactions taking place in objects such as pulsars, quasars, and black holes.
X-rays range in wavelength from 0.01 - 10 nm (about the size of an atom). They are generated by super-heated gas from exploding stars and quasars, where temperatures are near a million to ten million degrees.
Ultraviolet radiation has wavelengths of 10 - 310 nm (about the size of a virus). Young, hot stars produce a lot of ultraviolet light and bathe interstellar space with this energetic light.
Visible light covers the range of wavelengths from 400 - 700 nm (from the size of a molecule to a protozoan). Our sun emits the most of its radiation in the visible range, which our eyes perceive as the colors of the rainbow. Our eyes are sensitive only to this small portion of the electromagnetic spectrum.
Infrared wavelengths span from 710 nm - 1 mm (from the width of a pinpoint to the size of small plant seeds). At a temperature of 37 degrees C, our bodies radiate with a peak intensity near 900 nm.
Radio waves are longer than 1 mm. Since these are the longest waves, they have the lowest energy and are associated with the lowest temperatures. Radio wavelengths are found everywhere: in the background radiation of the universe, in interstellar clouds, and in the cool remnants supernova explosions, to name a few. Radio stations use radio wavelengths of electromagnetic radiation to send signals that our radios then translate into sound. These wavelengths are typically a few feet long in the FM band and up to 600 yards or more in the AM band. Radio stations transmit electromagnetic radiation, not sound. The radio station encodes a pattern on the electromagnetic radiation it transmits, and then our radios receive the electromagnetic radiation, decode the pattern and translate the pattern into sound.
New instrumentation and computer techniques of the late 20th century allow scientists to measure the universe in many regions of the electromagnetic spectrum. We build devices that are sensitive to the light that our eyes cannot see. Then, so that we can "see" these regions of the electromagnetic spectrum, computer image-processing techniques assign arbitrary color values to the light.
1. What does a light wave look like?
Light travels in the form of a wave. It is important to be familiar with the general anatomy of a wave to understand the relationship of wavelength to energy. The lowest part of the wave is called a trough. The highest part of the wave is the crest. The amplitude is the height of a wave. A wavelength is the distance from two corresponding points on subsequent waves, for example, from crest to crest or from trough to trough. The number of waves that pass through a given point in one second is called the frequency, measured in units of cycles per second called Hertz.
2. What is the relationship between frequency and wavelength?
Wavelength and frequency are related. The higher the frequency, the shorter the wavelength. In one second more short wavelengths pass by a given location than long wavelengths. The length of a wave times its frequency gives the velocity at which the wave travels. The equation that relates wavelength and frequency is: ln=v where l is the wavelength, n is the frequency and v is the velocity of the wave. For electromagnetic radiation, the speed is equal to the speed of light, c, and the equation becomes: ln=c.
3. What is the relationship between wavelength, frequency and energy?
The energy of a wave is directly proportional to its frequency, but inversely proportional to its wavelength. In other words, the greater the energy, the larger the frequency and the shorter (smaller) the wavelength. Given the relationship between wavelength and frequency described above, it follows that short wavelengths are more energetic than long wavelengths.
1. How are wavelength and temperature related?
All objects emit electromagnetic radiation, and the amount of radiation emitted at each wavelength determines the temperature of the object. Hot objects emit more light at short wavelengths, and cold objets emit more light at long wavelengths. The radiation temperature of an object is related to the wavelength at which the object gives out the most light. We call the amount of light given out at a particular wavelength, the intensity. When you plot the intensity of light from an object at each wavelength, you have a smooth curve called a blackbody curve. For any temperature, the blackbody curve shows how much energy (intensity) is radiated at each wavelength, and the wavelength where the intensity peaks is the color that the object appears to us. Therefore, you can tell the temperature of a star or galaxy by seeing what color it is, meaning, at what wavelength (color) the light peaks.
Blackbody curves, for all temperatures, have the same shape, as shown in the graphs for heating the robot up to different temperatures. However, the peak of the curve for an object heated to 30,000 K will be larger (more intense) than will the peak of the curve for a cooler object. The graphs on the heating the robot page do not show this difference-we have made the scale of the intensity axis adjust itself for each temperature change, but you should be aware of this feature of blackbody radiation. The intensity difference between the peak of curve for the robot at 30,000 K and the peak of the curve for the robot at body temperature, is a factor of 10 billion-meaning, a star at 30,000 K puts out more energy by a factor of 10 billion than does a human at body temperature. So it is not easy to show both of these blackbody curves together on the same graph without using logarithms. This aspect of blackbody radiation will be highlighted in a forthcoming addition to Star Light, Star Bright.
2. How are temperature and color related?
The amount of light produced by an object at each wavelength depends on the temperature of the object; a hot object (about 20,000 degrees C) puts out most of its light in the blue region, and cool objects put out light in the red (about 1,000 degrees C) or even infrared (37 degrees C).
1. How can light teach us information about the stars?
Electromagnetic radiation, or light, is a form of energy. Visible light is a narrow range of wavelengths of the electromagnetic spectrum. By measuring the wavelength or frequency of light coming from objects in the universe, we can learn something about their nature. Since we are not able to travel to a star or take samples from a galaxy, we must depend on electromagnetic radiation to carry information to us from distant objects in space.
The human eye is sensitive to a very small range of wavelengths called visible light. However, most objects in the universe radiate at wavelengths that our eyes cannot see. Astronomers use telescopes with detection devices that are sensitive to wavelengths other than visible light; this allows astronomers to study objects that emit this radiation, otherwise invisible to us. Computer techniques then code the light into arbitrary colors that we CAN see. The Hubble Space Telescope is able to measure wavelengths from about 0.1150 to 2 micrometers, a range that covers more than just visible light. These measurements of electromagnetic radiation enable astronomers to determine certain physical characteristics of objects, such as their temperature, composition, and velocity.
