A popular misconception is that astronomers spend long hours "peering intently" through their telescopes. Astronomers do spend long hours "peering intently" but most likely at computer or video monitors. People rarely look through large telescopes. The beam from a large telescope is destined to be "seen" by a host of at times exotic pieces of instrumentation that can measure and tease out the information encoded in the precious stream of photons collected by the primary mirror. In this section you will learn about a few of these instruments.
In his work Opticks Isaac Newton described experiments with a prism to illustrate the "phenomena of colours". Although crude - this was the first recorded experiment in spectroscopy - the science devoted to the analysis of light and other forms of electromagnetic waves. We more properly associate the birth of spectroscopy with Joseph von Fraunhofer, a German optician who, in the early 19th century observed the spectrum of the sun. Fraunhofer's spectrograph used a prism to disperse light emerging from a thin slit into the familiar rainbow of colours. Figure 5.17a and c show a prism splitting visible light into its spectrum as well as an antique spectrograph similar to the one used by Fraunhofer. Figure 5.18 provides a schematic of the Fraunhofer spectroscope.
Fraunhofer was one of the first to make an astronomical observation with the spectroscope. This occurred while he was testing a newly polished prism.
Fraunhofer's usual practice was to use a candle as a light source and to pass light through the prism looking for any defects in the glass. When he used the sun as a substitute light source he observed an intriguing difference in what he saw - but a difference that was due to the light and not a defect. What he saw is captured in Figure 5.19.
The solar spectrum has a multitude of thin, dark lines of various intensities. Figure 5.19 shows a small part of the solar spectrum in the green region of the spectrum, the thin dark lines are called "Fraunhofer" lines. When these lines were first discovered they were inexplicable. With the birth of modern atomic theory we now use spectral lines to "unlock" the secrets of the stars. Stellar spectra tell us many things about a star or celestial object including: temperature, density, pressure, rotation, motion and more! Spectroscopy is the fundamental tool of modern astronomy.
Example 5.6 Estimate the wavelength region (express in nm) shown in Figure 5.19.
Solution: Use the electromagnetic spectrum applet in Chapter 5.2. The green part of the visible spectrum is from about 500 nm - 550 nm.
The CCD (Charge Coupled Device)
At the heart of your digital camera is a tiny microelectronic chip that has a Canadian connection and has revolutionized astronomy! The chip is a Charge Coupled Device or CCD for short. It has replaced photographic film for both photo-enthusiast and most astronomers.
Light (or other forms of electromagnetic radiation) are the "raw materials" of astronomy. The spectrograph allows astronomers to "dissect" light into its colours. The photometer allows astronomers to "weigh" light - that is to measure its brightness and compare how much light we get from one star or another. Photometry, along with spectroscopy are the most fundamental techniques of modern astronomy. In the case of both photometry and spectroscopy the CCD has had a profound impact. Figure 5.23 shows an animation of a photometric measurement made using a CCD camera attached to a small telescope. Since a CCD creates an image by storing electrical charge in direct proportion to the amount light hitting individual pixels it can also be used as a very precise "light meter" or photometer. As well, because of the sensitivity of CCD's even small telescopes can now be used to make precise measurements of star intensities.
The variation in light from DY Pegasi amounts to about 0.7 magnitudes. If we convert this to intensity units it means that at its brightest, DY Pegasi emits 1.9 times as much light as it does at its dimmest.
Example 5.7 A CCD is used to collect data on a variable star. During the course of a night the star varies by 2 magnitudes in brightness. How much more light energy does the CCD receive when the star is its brightest compared to when it is dimmest?
Solution: Recall from Unit 1 that for every 1 magnitude difference in brightness there is an intensity factor of about 2.5 times. So, if the star varies in total by 2 magnitudes then its intensity (and therefore amount of light energy received) varies by 2.5 X 2.5 = 6.3 times. The CCD is receiving 6.3 times as much energy when the star is its brightest.