Star Light, Star Bright
The following information is provided to give the teacher some additional knowledge on the topic of light and color. You can also choose to use this information with the students to do research on topics that you see mentioned here or use the question headings as a form of review for class discussion. This science background is organized to provide information as it relates to each of the lesson's four modules.
CATCH THE WAVES:
1. What is the electromagnetic spectrum?
The electromagnetic spectrum consists of all the different wavelengths of electromagnetic radiation, including light, radio waves and x-rays. It 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, < 0.001 nm (about the size of an atomic nucleus). This is the highest frequency and most energetic region of the electromagnetic spectrum. Gamma rays can result from nuclear reactions taking place in objects such as pulsars, quasars, and black holes.
X-rays range in wavelength from 0.001 - 10 nm (about the size of an atom). They are generated, for example, by superheated gas from exploding stars and quasars, where temperatures are near a million to ten million degrees.
Ultraviolet radiation has wavelengths of 10 - 400 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 700 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 300 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 is a light wave?
Light is a disturbance of electric and magnetic fields that travels in the form of a wave. Imagine throwing a pebble into a still pond and watching the circular ripples moving outward. Like those ripples, each light wave has a series of high points known as crests, where the electric field is highest, and a series of low points known as troughs, where the electric field is lowest. The wavelength is the distance between two wave crests, which is the same as the distance between two troughs.. 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. The speed of the wave therefore equals the frequency times the wavelength.
2. What is the relationship between frequency and wavelength?
Wavelength and frequency of light are closely related. The higher the frequency, the shorter the wavelength. Because all light waves move through a vacuum at the same speed, the number of wave crests passing by a given point in one second depends on the wavelength. That number, also known as the frequency, will be larger for a short-wavelength wave than for a long-wavelength wave. The equation that relates wavelength and frequency is:
For electromagnetic radiation, the speed is equal to the speed of light, c, and the equation becomes:
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 of their light at short wavelengths, and cold objects emit more of their 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 emitted at a particular wavelength, the intensity. When you plot the intensity of light from an object at each wavelength, you trace out 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 determines the color of that the object. The intensity peak will be at shorter (bluer) wavelengths for hotter objects, and at longer (redder) wavelengths for cooler objects. Therefore, you can tell the temperature of a star or galaxy by its color because color is closely related to the wavelength at which its light intensity peaks.
Blackbody curves, for objects of all temperatures, have a similar shape, as shown in the graphsbelow. However, the peak of the curve for a hotter object will be larger (more intense) than will the peak of the curve for a cooler object. For example, the intensity difference between the peak of the curve for an object at 30,000 K and the peak of the curve for an object at 300 K (body temperature) is a factor of 10 billion. This means that a star at 30,000 K puts out more energy by a factor of 10 billion than does a human at body temperature. Because of the large intensity difference, it would be difficult to show both of these curves on the graph below without using logarithms. To plot blackbody curves with large intensity differences on the Heating Up page of Amazing Space's "Star Light, Star Bright", we have made the scale of the intensity axis adjust itself for each temperature change.
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 producing the light. Stars hotter than the sun (over 6,000 degrees C) put out most of their light in the blue and ultraviolet regions of the spectrum. Stars cooler than the Sun (below 5.000 degrees C) put out most of their light in the red and infrared regions of the spectrum. Solid objects heated to 1.000 degrees C appear red but are putting out far more (invisible) infrared light than red light.
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.
Every day when the sun sets, the sky puts on a beautiful show of stars. It is natural to ask, how far are these stars? Do they move? What are they made of? Are they all alike? Since must of us cannot physically go to the stars to learn about them, we depend on light we receive from these distant objects to study them. In my everyday work I look at ultraviolet radiation from galaxies' centers to study the physical and kinematic characteristics of the gas that is radiating this energy.
Henbest , Nigel and Michael Marten, The New Astronomy Second Edition , Cambridge University Press, 1996.
Kuhn , Karl F. , The Quest of the Universe, West Publishing Company, 1991.
Space Based Astronomy, NASA, 1994.
Griffith, Mary, The Homeschooling Handbook, Prima Publishing, CA, 1997.
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