Getting the Spectrum

A 35-mm SLR film is the only type of camera that can be used effectively for acquiring stellar spectra with an objective prism. If you don't already have one, maybe you can find an inexpensive used one. It needs to be capable of 2-minute exposure times and have a cable release for the shutter. Any lens will do. The standard 50-mm lens will provide enough dispersion to at least demonstrate the differences between spectral classes for bright stars; the longer the focal length the greater the dispersion.

The camera can be mounted piggyback on the telescope, as shown in Figure 16.3, or by itself on an equatorial. A polar axis drive is not necessary. The camera has to be arranged so that the prism's dispersion is in the declination direction and the lens points approximately 60° away from the target star. The best way to do this is to line up the telescope or finder (Figure 16.3) on the target star and then rotate the camera until the spectrum is visible in the viewing screen.

Figure 16.3. The spectrograph mounted on an 80-mm f/5 refractor.

The spectrum as seen in the camera is a dispersed line without width. It becomes widened through the process of letting the star drift across the field of view in the right ascension direction. A two-minute exposure will give the spectrum sufficient width with a 135-mm lens. Short focal lengths will require longer exposures. Use black-and-white film with an ISO of 400.

Figures 16.4 and 16.5 are spectra of the star Sirius and M42, taken with the spectrograph illustrated in Figure 16.2.

The spectrum of Sirius clearly shows the hydrogen Balmer series of absorption lines characteristic of spectral class A stars, with the red end of the spectrum to the right. The hydrogen a line in the far-red section is not visible because the film sensitivity (T Max 400) cuts off sharply at that wavelength. The first visible absorption line is the hydrogen ft in the blue-green area. Although the human eye is insensitive to wave lengths much beyond the hydrogen 8, the third absorption line from the right, film sensitivity extends into the far violet.

In the Orion nebula's spectrum there are three emission lines caused by the excitation of the gases by ultraviolet light from its embedded young stars. These are bright lines in contrast to dark absorption lines characteristic of stellar spectra. The third emission line from the red end is the hydrogen ft. The other two to the right of the Hft were of controversial origin when they were first discovered because their wavelengths did not match the spectra of any then known element. It was hypothesized they were caused by a previously unknown element characteristic of interstellar gas, which they designated nebulium.

With advances in quantum theory that could predict the rate at which possible energy transitions could take place, however, it was discovered that the nebulium lines were actually produced by doubly ionized oxygen. These lines are not visible because in the best laboratory vacuums, the rate at which those particular

Violet

Red

he h.

Figure 16.4. The spectrum of Sirius.

transitions take place is greater than the time between collisions of atoms. The density of the Orion nebula is so low, however, that the interval between collisions is much longer. The oxygen atoms are essentially isolated. As a result the low probability transitions can be observed.

Outer space expands the limits of experimental physics. In that vast laboratory we can observe the behavior of matter at higher and lower pressures, higher and

"■"-•''^«BHF "

Nebula

3 Orion Belt Stars

Violet

i *m

Red

- r " ' .J. ' ;

i?

Figure 16.5. The emission spectrum of M 42.

lower densities, and higher and lower temperatures under the influence of much greater gravitational and electromagnetic fields than is possible in any facility we might construct on Earth. Thus we gather evidence necessary to support or reject theories of the fundamental properties of matter and the interactions that govern the structure of the universe.

CHAPTER SEVENTEEN

The Proper Motion of Barnard's Star

Because all the stars in our Galaxy are in orbits around its center, nearby stars demonstrate motion relative to those more distant. This motion has two components. One of these, the motion toward or away from us in the line of sight is the radial velocity. The motion perpendicular to the line of sight, measured in units of arc seconds/year, is termed the proper motion of the star. The proper motion, itself, has components in right ascension and declination.

Barnard's star, a tenth magnitude object at a distance of 5.9 light years, has a proper motion in declination of 10.3 arc seconds/year. This is easily within the observational limits of a small telescope.

Barnard's star, 17h 58m RA and Decl. +4° 42', is within two degrees of the fourth magnitude star 66Oph and 3.5° due east of ft Oph. Figure 17.1 is a chart for a 2° field made from Guide 8.0. The position of Barnard's star is indicated by the arrow.

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