How does stellar spectroscopy work

These days, modern spectroscopy uses diffraction gratings to disperse the light, which is then projected onto CCD s Charge Coupled Devices similar to those used in digital cameras.

The 2-dimensional spectra are easily extracted from this digital format and manipulated to produce 1-dimensional spectra like the galaxy spectrum shown below. The spectrum of an S7 spiral galaxy showing both emission and absorption line features. First of all, it has taken many years of careful laboratory work on Earth to determine the precise wavelengths at which hot gases of each element have their spectral lines.

Long books and computer databases have been compiled to show the lines of each element that can be seen at each temperature. Second, stellar spectra usually have many lines from a number of elements, and we must be careful to sort them out correctly. Sometimes nature is unhelpful, and lines of different elements have identical wavelengths, thereby adding to the confusion. And third, as we saw in the chapter on Radiation and Spectra , the motion of the star can change the observed wavelength of each of the lines.

So, the observed wavelengths may not match laboratory measurements exactly. In practice, analyzing stellar spectra is a demanding, sometimes frustrating task that requires both training and skill. Studies of stellar spectra have shown that hydrogen makes up about three-quarters of the mass of most stars. Generally, but not invariably, the elements of lower atomic weight are more abundant than those of higher atomic weight. Take a careful look at the list of elements in the preceding paragraph.

Two of the most abundant are hydrogen and oxygen which make up water ; add carbon and nitrogen and you are starting to write the prescription for the chemistry of an astronomy student. We are made of elements that are common in the universe—just mixed together in a far more sophisticated form and a much cooler environment than in a star. The metallicity of the Sun, for example, is 0. The Chemical Elements lists how common each element is in the universe compared to hydrogen ; these estimates are based primarily on investigation of the Sun, which is a typical star.

Some very rare elements, however, have not been detected in the Sun. Estimates of the amounts of these elements in the universe are based on laboratory measurements of their abundance in primitive meteorites, which are considered representative of unaltered material condensed from the solar nebula see the Cosmic Samples and the Origin of the Solar System chapter.

When we measure the spectrum of a star, we determine the wavelength of each of its lines. If the star is not moving with respect to the Sun, then the wavelength corresponding to each element will be the same as those we measure in a laboratory here on Earth.

But if stars are moving toward or away from us, we must consider the Doppler effect see The Doppler Effect. We should see all the spectral lines of moving stars shifted toward the red end of the spectrum if the star is moving away from us, or toward the blue violet end if it is moving toward us Figure 2.

The greater the shift, the faster the star is moving. Such motion, along the line of sight between the star and the observer, is called radial velocity and is usually measured in kilometers per second. Figure 2: Doppler-Shifted Stars. When the spectral lines of a moving star shift toward the red end of the spectrum, we know that the star is moving away from us. If they shift toward the blue end, the star is moving toward us. William Huggins , pioneering yet again, in made the first radial velocity determination of a star.

He observed the Doppler shift in one of the hydrogen lines in the spectrum of Sirius and found that this star is moving toward the solar system. Today, radial velocity can be measured for any star bright enough for its spectrum to be observed. As we will see in The Stars: A Celestial Census , radial velocity measurements of double stars are crucial in deriving stellar masses.

There is another type of motion stars can have that cannot be detected with stellar spectra. Unlike radial motion, which is along our line of sight i. We see it as a change in the relative positions of the stars on the celestial sphere Figure 3. These changes are very slow. Even the star with the largest proper motion takes years to change its position in the sky by an amount equal to the width of the full Moon, and the motions of other stars are smaller yet.

Figure 3: Large Proper Motion. For this reason, with our naked eyes, we do not notice any change in the positions of the bright stars during the course of a human lifetime.

If we could live long enough, however, the changes would become obvious. For example, some 50, years from now, terrestrial observers will find the handle of the Big Dipper unmistakably more bent than it is now Figure 4.

Figure 4: Changes in the Big Dipper. This figure shows changes in the appearance of the Big Dipper due to proper motion of the stars over , years. That is, the measurement of proper motion tells us only by how much of an angle a star has changed its position on the celestial sphere. As an analogy, imagine you are standing at the side of a freeway. The rise of spectroscopy for astronomical use was in part due to its linkage with another emerging technology - photography.

Astronomical spectra could be recorded by photographing them on glass plates. This was a far superior approach to viewing them with through an eyepiece and trying to draw the image.

Photographic records of spectra could be stored for later analysis, copied for distribution or publication and the spectral lines could be measured relative to spectral lines from a stationary lamp producing spectral lines of known wavelength.

It was only by observing and photographing the spectra of thousands of stars that astronomers were able to classify them into spectral classes and thus start to understand the characteristics of stars. Photographic spectra were generally recorded on glass plates rather than photographic film as plates would not stretch. The image of the spectrum was normally presented as a negative so that the absorption lines show up as white lines on a dark background.

Photoelectric spectroscopy allows spectral information to be recorded electronically and digitally rather than on photographic plates. This means a CCD can convert almost 9 out of 10 incident photons into useful information compared with about 1 in for film. Using a CCD an astronomer can therefore obtain a useful spectrum much quicker than using a photographic plate and can also obtain spectra from much fainter sources.

CCDs have a more linear response over time than photographic emulsions which lose sensitivity with increased exposure. A spectra recorded on a CCD can be read directly to a computer disk for storage and analysis. The digital nature of the information allows for rapid processing and correction for atmospheric contributions to the spectrum. Modern spectra are therefore normally displayed as intensity plots of relative intensity versus wavelength as is shown below for a stellar spectrum.

The last decade has seen the growth in multifibre spectroscopy. This involves the use of optical fibres to take light from the focal plane of the telescope to a spectrograph. A key advantage of this technique is that more than one spectrum can be obtained simultaneously, dramatically improving the efficiency of observing time on a telescope. The 2dF project revolutionised the emerging field of multifibre spectroscopy by using a computerised robot to precisely position minute prisms onto a metal plate so that each prism could gather light from an object such as a galaxy or quasar.

Attached to each prism was an optical fibre that feeds into a spectrograph. The 2dF instrument sits at the top of the AAT and can take spectra from objects simultaneously over a 2 degree field of view. Whilst observing one field, the robot sets up a second set of prisms on another plate which can then be flipped over in a few minutes to begin observing a new field. This incredibly efficient system allows spectra from thousands of objects to obtained in a single night's observing run.

These surveys produced accurate data on over , galaxies and 25, quasars that have proved an immense boon for cosmologists studying the formation and large-scale structure of the Universe. Australian astronomers and engineers continue to design, develop and build new multifibre devices for the latest generation m class telescopes overseas.

It develops the techniques used in 2dF and currently allows spectra to be gathered simultaneously. Future instruments such as Echidna and AAOmega are under development at present. Spectroscopy is not just the tool of optical astronomers. It can be carried out at all wavebands, each of which provides new insights into the structure and characteristics of celestial objects.