Abstract
Now for a tool that is central to the topic of astrophysics—spectroscopy, and how it used to classify stellar spectra. This is an amazing subject; from just looking at the light from an object, we can tell how hot it is, how far away it is, in which direction it is moving, if it is rotating, and (from all this data) infer its age, its mass, how long it has left to live, etc. In fact, so important is this topic that it has been given its own chapter. From this point on in the book, a star will be referred to by its spectral classification.
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Notes
- 1.
We can only easily deduce whether an object is moving away from us or toward us. To measure if it is moving laterally to us requires some complicated measurements.
- 2.
Some spectroscopes place the prism or grating in front of the telescope, and thus the light from every star in the field of view is analyzed simultaneously. This is called an objective spectroscope. The drawback is the considerable loss of detail (i.e., information about the stars), but initial measurements can be made.
- 3.
See Chap. 1 and the discussion of Wien’s Law.
- 4.
In most cases this is a very short time, a few millionths of a second.
- 5.
Do not confuse the energy of a photon with the brightness of emission lines. The energy is what determines the wavelength, or color, of the line, whereas the total number of photos emitted at that wavelength determines the brightness of the line.
- 6.
A brilliant astronomer, Cecilia Payne-Gaposchkin, discovered how the now familiar order, OBAFGKM, really works. She found that all stars are made primarily of hydrogen and helium and that a star’s surface temperature determines the strength of its spectral lines. For instance, O stars have weak hydrogen lines because, due to their high temperature, nearly all the hydrogen is ionized. Thus, without an electron to “jump” between energy levels, ionized hydrogen can neither emit nor absorb light. On the other hand, M stars are cool enough for molecules to form, resulting in strong molecular absorption lines.
- 7.
Usually only the classes O, A, B, F, G, K and M are listed, as these are stars that can be observed with amateur equipment. The other classes are used and defined as and when they are needed.
- 8.
There are, of course, many other spectroscopes available for the amateur astronomer, but I have mentioned Tom’s due to its simplicity, price, and effectiveness. It is really very good, and no, I didn’t get one free for mentioning him.
- 9.
This value is in question. The data are awaiting reassessment.
- 10.
In reality several spectra are taken in the observatory, not just that of hydrogen, in order to ensure that some spectral lines will be recognizable in the object’s spectrum.
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Inglis, M. (2015). Spectroscopy and the Spectral Sequence. In: Astrophysics Is Easy!. The Patrick Moore Practical Astronomy Series. Springer, Cham. https://doi.org/10.1007/978-3-319-11644-0_3
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DOI: https://doi.org/10.1007/978-3-319-11644-0_3
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