Astronomical Targets

Observing time for most professional telescopes is booked months in advance. The flexible schedules of amateurs, however, give them a clear advantage when it comes to observing transient events such as new comets, novae, or supernovae. There is also a growing trend for professional cooperation with amateurs, especially in areas of long-term monitoring. There is no reason why, within the limits of amateur instrumentation, this should not now include amateur spectroscopic observations.

What can the amateur spectroscopist successfully observe, whether for science or satisfaction? The solar system, dominated by bright targets, is a good starting place. The atmosphere of Venus contains carbon dioxide that has been detected with amateur spectrographs. The gas giants — Jupiter, Saturn, Uranus, and Neptune — have strong methane absorption lines in their spectra, which are relatively easy to record. Saturn's satellite Titan also shows methane lines in its atmosphere. Complex molecular lines in cometary spectra are within reach of the backyard spectroscopist.

Beyond the solar system there are hundreds of stars worthy of spectroscopic attention, with variables leading the list. Mira-type long-period variables and many irregular variables are cool giant stars with spectra that vary along with their brightness. Since some of the fading is caused by a shift of light from visual to infrared wavelengths, these variables are ideal targets for the extended red sensitivity of a CCD spectrograph. This sensitivity also gives amateurs an opportunity to explore the coolest stars (spectral classifications M, N, R, and carbon stars). These spectra are easily recognized by their "string-of-beads" appearance caused by absorption bands due to gases like titanium oxide and other complex molecules.

At the opposite end of the temperature scale are the hot O and B stars and those with an e suffix added to their spectral classification, indicating they have emission-line spectra. Again some notable variable stars are included, such as Gamma Cassiopeiae and the eclipsing binary Beta Lyrae, which shows variation in its spectrum that tracks the light cycle. Many interesting targets can be culled from the spectral classification published in Sky Catalogue 2000.0 and other similar listings.

Stellar emission lines are generally easier to record than dark absorption lines when one is using a low-resolution spectrograph. Wolf-Rayet stars can show variations in their emission-line spectrum during a short time scale. These stars are extremely hot (up to 100,000° Kelvin) and have their own WR spectral classification. Some show spectacular emission lines due to highly ionized elements like carbon and nitrogen.

The outburst of a nova or bright supernova is perfect for the amateur spectroscopist (see "A Field Guide to Supernova Spectra"). The Blaze Star, T Coronae Borealis, is a recurrent nova that last flared from 10th to 3rd magnitude in 1946 and bears monitoring. Other stars worthy of amateur attention are nearby red dwarfs. Among them are flare stars like UV Ceti or Wolf 359 in Leo, which exhibit unpredictable bursts of brightness amounting to several magnitudes, accompanied by enhanced emission lines in their spectra.

One of the most powerful aspects of spectroscopy is determining the velocity an object moves toward or away from the observer. This motion is recorded as a Doppler shift of the spectral lines. Objects approaching the observer show a shift toward the blue, while those receding are redshifted. Extremely subtle spectral shifts are what professional astronomers currently use to detect planets around stars. This is currently beyond the amateur realm, but higher velocity measurements are not.

The Sun's rotation is just detectable in a powerful amateur spectrograph when one compares light from the east (approaching) and west (receding) limbs. Numerous spectral lines due to oxygen and water vapor in Earth's atmosphere are superposed on the solar spectrum and serve as static markers for determining the Doppler shift. Amateurs have even measured the subtle radial velocities of bright stars with a high-resolution spectrograph — something that would have been virtually impossible before the advent of sensitive CCD cameras.

At the opposite velocity extreme are highly redshifted quasars. Since they are remarkably bright for their distance and are receding at a sizable fraction of the speed of light itself, they are detectable with very-low-resolution equipment. They offer the amateur a tantalizing glimpse into the early history of the universe by looking back through cosmological time toward the Big Bang.

We have come a long way since Angelo Secchi looked at stars for the first time through his spectroscope more than a century ago. CCDs give amateurs an opportunity to relive the excitement he experienced in those pioneering days. The adventure can begin afresh for the inquiring amateur.