I enjoy the way direct spectroscopy opens one more aspect of our fascinating universe to the amateur astronomer. I hope that this article will pique your curiosity about this interesting field upon which most modern astronomy is based.
How Spectral Lines are Created
Resources and Recommended Books
Light from a radiating body - such as a star - may be dispersed into its constituent wavelengths or colours (called a continuous spectrum or continuum) with a prism or diffraction grating.
The atmospheric temperatures, densities and compositions of the surface envelope of a star gives rise to various dark and bright lines and bands within this spectrum. The dark lines are called absorption spectra. The bright lines are called emission spectra. These lines allow us to determine basic stellar characteristics, estimate luminosities, distances, magnetic fields, rotation speeds, and mass. We classify stars according to their spectral lines.
It is possible for the amateur to explore this facet of astronomy directly.
I recently purchased a diffraction grating and a one-way lens (from Jim Badura of Rainbow Optics of California) to observe absorption spectra with an amateur telescope. (This product was reviewed in the October 1995 issue of Sky and Telescope magazine.)
The diffraction grating screws into an eyepiece thread, then the one-way lens is attached to the top of the eyepiece. On my 10 inch telescope, the diffraction grating produces a continuous spectrum and, in the case of stars of about 5th magnitude or brighter, a low dispersion pattern of absorption lines.
Due to this past year's weather, I have not had as much opportunity to become trained in the use of my spectroscope as I would have liked. I have had time, however, for some initial observations.
The spectra I viewed may be broken down into three categories - I am using part of a classification system that was originally developed by Father Angelo Secchi in the 1860s.
The first category of spectra is easy to observe on hot, white class A main sequence stars. It consists of a series of three dark, narrow lines in the red (656 nm) - it is interesting to note that you will be better able to see this line if your eye is not dark-adapted - green blue (486 nm), and blue violet (434 nm) areas of the visible continuum. These are ,respectively, the Balmer alpha, beta, and gamma lines that are produced by the hydrogen atom. They, like all of the absorption features I will be discussing, arise when electrons in the atoms and molecules in the surface layers of a star absorb photons from the stellar continuum, which arises from the interior of the star. Sirius A I V, Vega A 0 V, Castor AI V and Altair A 7 V are good examples of stars that produce these lines.
The second category of lines that can be observed is created by the absorption spectra of more complex neutral atoms. These lines appear in the cooler yellow and orange stars, the visible majority of which are class III giants. They constitute a series of lines running from orange to blue and indicate neutral atoms such as sodium (589 nm yellow), iron (572 green, 438 blue), and magnesium (517 green). Called metal lines, they are visible in late G and early K stars, of which Aldeberan K 5 III, Pollux K 0 III, Capella G 8 III and Arcturus K 2 III are good examples.
The third category constitutes the spectrum of the cool red giant star of spectral class M. Visible as a series of wide molecular bands that run the full length of the spectrum, they arise from the presence of titanium oxide molecules in the distended outer surfaces of these stars. The bands are very distinct in stars such as Betelgeuse M Ia, Arcturus M Ib, and Mu Cephi M 2 Ia. You can also see strong wide bands in the Mira stars. I observed R Leonis (M 8 IIIe) and O Ceti (M 7 IIIe), and the bands of these latter two stars wipe out large parts of the continuum.
The various types of spectra arise in the surface layers of a star's atmosphere and are related to the temperature and density at the star's surface.
On the surfaces of cool M stars, molecules that have formed are not broken down as readily as they are in warmer stars. These molecules will therefore be visible in the spectrum.
As we proceed to warmer stars, the molecules will not so easily survive the higher temperatures, but heavier atoms will-and we will see the metal absorption lines of the G and K class star.
This progression carries on into the still hotter A class star where the lines of the simpler neutral hydrogen atom predominate.
Surface densities cause some line strengths to weaken and others to increase, depending on how different molecules or atoms react to pressure and density. These are known as negative and positive luminosity functions, respectively, and this is a prime method of distinguishing between luminous giant stars and less luminous main sequence stars. In denser Class A, main sequence stars, the lines formed are much stronger than the lines of the supergiants of the same spectral class. The supergiant stars have distended surfaces and lower surface density. In a higher density medium there are more collisions among atoms. This creates an increase in line strength, a negative luminosity affect (because the more luminous the star, the weaker the line strength). Recently, I was able to make a brief observation of the fainter lines of the A 2 Ia supergiant Deneb. The hydrogen beta line was only faintly visible, and not nearly as distinct as that of a main sequence class A star.
The observations I made are preliminary, and are not complete. I expect to be able to refine and extend them considerably in the future. (For example, I want to observe carbon stars.)
To compliment the use of a spectroscope, it is a good idea to have some basic resources.
The Phillips Colour Star Atlas, and Starlist 2000
by Richard Dibon-Smith, contain information on stellar colours
With these resources, you can check observations against the spectral and luminosity information they contain. This will greatly facilitate the learning process.
For the present, I am sketching the spectra I observe. I plan to obtain a CCD camera and hope, by this method, to obtain still fainter and more detailed spectra. (The use of a CCD to obtain spectra was discussed in the fall 1995 issue of CCD Astronomy. This article is also available, at the time of this writing, at the Sky and Telescope home page.)
(from "Stars and their Spectra" by James B. Kaler, Cambridge University Press, 1989)
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