By Robert Hazen, Ph.D., George Mason University
Most people love astronomy, but few people realize that astronomy is the science, and the art, of collecting, analyzing, and interpreting photons from space. Virtually, all the information we have about distant stars comes from photons, that is, electromagnetic radiation. How is this data collected? What can we learn from it?
An Observational Science
Astronomy is first and foremost an observational science. Virtually all the information we have about distant stars comes from photons, that is, electromagnetic radiation that travels at 186,000 miles per second through space.
The classic way, in the 18th and 19th centuries, was to use one’s eye and write down in a notebook what was visible. Then, the Herschel family introduced photography. Photography in astronomy dates from about the middle of the 19th century and it was a wonderful way of collecting photons.
Today, most detectors are electronic. They record individual photons hitting a plate, and record the position of those photons. They are much more sensitive than film, and it allows one to do wavelength resolution as well. So, while one’s collecting photons, one is also determining the wavelength of those photons. It’s really a major advance in astronomy.
The art of astronomy, then, is to collect, analyze, and interpret these photons, these data. The astronomers measure photons; in particular, they measure four different properties of photons.
The four different properties of photons that the astronomers measure are: the wavelength of photons, the intensity of the photons arriving at Earth, the direction in space to those photons; and the variation of these three properties—wavelength, intensity, and direction—as a function of time.
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Wavelength
The wavelength is measured by spectroscopy. For example, a prism breaks all light into a spectrum of different wavelengths, the different colors of light. By the same token, one can measure all the different wavelengths of different kinds of electromagnetic radiation by different techniques.
There are radio waves; microwaves; infrared; visible light; ultraviolet; x-ray; and gamma ray. For each of the different wavelengths of electromagnetic radiation, we can measure the wavelength through spectroscopic means.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Photoelectric Effect
The intensity of photons is measured by a device very much like a light meter. It is tied to the photoelectric effect. In photoelectric effect, as a photon comes in to certain materials, it actually triggers an electron jump; an electron can leave an atom, and then flow through the material like an electric current.
One photon causes one electron, two photons cause two electrons to move, and so forth. The level of current that flows through the material is a direct measure of the number of photons that are coming to us. Thus, we can use the photoelectric effect as a way of detecting the intensity of these photons.
Measuring the Direction of Photons
The direction of photons is measured by two angles. We can measure the direction of an object in space by first taking a compass and measuring the number of degrees around from north on the horizon, and then the second angle is an angle from the horizon up to the object.
These two numbers provide an indication of the position of that object. Of course, since objects move, we also have to record the exact time that we’ve made our observation.
Wavelength, Intensity, and Position with Time
Finally, astronomers document the variation of wavelength, intensity, and position with time, because all objects in the heavens change. They may get brighter or dimmer; their wavelengths may change; and their position in the sky may change. So these are things one has to measure.
In order to do this well, one needs measurements of ever greater accuracy. The more accurately we can measure wavelength and intensity along with the position and variation, the more we can understand about the heavens.
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Understanding Our Universe
The ultimate question remains: What have we learned from these data? The answer is simple. From these data, astronomers attempt to understand the spatial distribution of objects along with their dynamic state.
Thus, the astronomical data that we are able to collect helps us closely examine the very nature of stars. It not only enhances and greatly adds to our scientific understanding of the heavenly bodies but also reveals the past as well as the future of our universe.
Common Questions about Astronomical Data and the Study of Photons
They measure four different properties of photons. They measure the wavelength of photons, the intensity of the photons arriving at Earth, the direction in space to those photons; and the variation of these three properties—wavelength, intensity, and direction—as a function of time.
In photoelectric effect, as a photon comes in to certain materials, it actually triggers an electron jump; an electron can leave an atom, and then flow through the material like an electric current.
The direction of photons is measured by two angles. We can measure the direction of an object in space by first taking a compass and measuring the number of degrees around from north on the horizon, and then the second angle is an angle from the horizon up to the object.
