Imaging, Photometry, and Spectroscopy: the Primary Tools of Astronomy

Astronomical Imaging: These are but a few examples of how astronomical pictures convey information about distant objects in the Universe. Click on any of these pictures to learn more about the object shown and what the picture tells us.

So if light is the only thing we have to work with, what can we do with it? One obvious thing is that we can take "pictures" of the things we see. We are all familiar with the many beautiful pictures of star clusters, nebulas, and galaxies that have been taken with optical light and ground-based telescopes. Several examples are shown above. In many cases, these pictures are awe-inspiring and they certainly convey information to a trained eye. For instance, in the picture on the right (above), the diffuse pink glow arises from hydrogen gas that is excited by hot stars in the gas; the diffuse blue glow, on the other hand, is a dusty region that is scattering nearby star light into our direction. Pictures alone, though, tend to lack the "quantitative" aspect that is needed for most serious scientific studies. For that, one needs to perform photometry, which is the technique that measures the relative amounts of light in different "colors" or wavelength ranges. However, the technique of light analysis that produces the most detailed information about objects in astronomy is called spectroscopy, that is, breaking the light up into a spectrum which can then be analyzed for all sorts of information.

Over the last 20 years or so, electronic light detectors have been developed that permit astronomers to essentially combine photometry with either imaging or spectroscopy. Electronic detectors, such as charge-coupled devices, or CCDs for short, have now displaced photographic materials at most astronomical observatories. These light detectors have the advantages of greater sensitivity, a more quantitative response to the light they measure, and they produce data that can be recorded digitally and entered directly into a computer for processing and analysis. Taking electronic images with certain optical filters in place essentially permits astronomers to perform imaging and photometry at the same time. CCDs are now also widely in use in digital cameras and video cameras, and even advanced amateur astronomers these days often have CCD cameras for their telescopes!

Likewise, recording a spectrum electronically (and making certain supporting observations) lets astronomers combine spectroscopy and photometry into a powerful new tool called spectrophotometry (that is, measuring how much light an object produces at various wavelengths of light).

The Hubble Space Telescope being deployed from the Space Shuttle, April 1990. Since Hubble is above the atmosphere, it can obtain a clear view of the Universe, in optical, ultraviolet, and near-infrared wavelengths. Hubble contains multiple instruments, including both cameras and spectrographs. (While you see mainly the photos from Hubble in the newspapers and magazines, much of the scientific work performed by astronomers using Hubble is done with the spectrographs!)

The other big development over the last several decades is the ability to get telescopes in orbit. Some of these satellite-telescopes take pictures or obtain spectra in portions of the electromagnetic spectrum that don't get through the atmosphere, like X-rays or ultraviolet light. Other telescopes, like the Hubble Space Telescope (see above),take advantage of their position above the atmosphere to obtain clearer and sharper views than usually obtainable from the ground. (Hubble observes in both optical and ultraviolet light, and even into the near infrared region.) The data from these telescopes show familiar objects in new and different ways.

Light conveys physical information about the source of the light, the material through which the light passes, or the material off of which the light reflects.

The fact that this is true is what makes the science of astronomy possible, and the technique of spectroscopy is what provides the bulk of the information! For instance, spectroscopy of a star using optical light and a ground-based telescope can tell astronomers such things as the temperature of the star and its chemical composition (at least for those elements with spectral lines in the optical part of the spectrum!). It can also tell us the surface gravity of the star, and hence whether the star is a giant or supergiant star, or a "normal" dwarf star like the sun. This information allows astronomers to estimate the distance to the star. By measuring the actual positions of the spectral features compared to their predicted values (as measured in a laboratory on Earth), we can tell whether the star is moving toward us or away and how fast. The list goes on and on!

Once we have measured the spectra of many stars, other investigations become possible. Let's say our first star was a bright, nearby star. Now we have gone to the telescope and observed a fainter star whose spectrum has exactly the same pattern of spectral features as the first star, but whose distribution of light through the spectrum is different (say, less "blue" light in comparison with "red" light). The fact that the pattern is the same in the two stars tells us that the temperatures and chemical composition are quite similar. Hence, the differences have to come from somewhere else. In this example, some of the second star's light has been absorbed and/or scattered away on its trip from the star to us by diffuse clouds of gas and dust in interstellar space. By carefully comparing our two stellar spectra we can learn something about the composition and density of this material out between the stars!

These are but two examples of how information can be gleaned from the tenuous beams of light sent toward us from these distant objects. Of course, we can observe the spectra of many different objects, not just stars, and we can now observe spectra all across the electromagnetic spectrum, not just optical light! This allows us to learn an incredible amount about the Universe, both nearby and far away.

This tool is used not only in astronomy, but in many areas of science.

There is nothing unique about the light coming from astronomical sources--light is light. Hence, spectral analysis is a tool that is used in many Earth-bound settings as well, throughout the biological, chemical, and physical sciences.

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Bill Blair (