Gas with Atoms & Molecules


Continuum Source r


3.2 I Spectral types

When spectra were taken of stars other than the Sun, they also showed absorption spectra. Presumably, the continuous radiation produced in a star passes through an atmosphere in which the absorption lines are produced. Not all stars have absorption lines at the same wavelength.

Astronomers began to classify and catalog the spectra, even though they still did not understand the mechanism for producing the lines. This points out an important general technique in astronomy -studying large numbers of objects to look for general trends. In one very important study, over 200 000 stars were classified by Annie Jump Cannon at the Harvard College Observatory. The benefactor of that study was Henry Draper, and the catalog of stellar spectra was named after him. The stars in this catalog are still known by their HD numbers.

One set of spectral lines common to many stars was recognized as belonging to the element hydrogen. The stars were classified according to the strongest hydrogen absorption lines. In this system, A stars have the strongest hydrogen lines,

Conditions for the formation of emission and absorption lines. (a) We look at a cloud of gas with the atoms or molecules capable of producing spectral lines. Since there is no continuum radiation to absorb, we can only have emission. (b) We now look through the gas at a background continuum source.This can produce absorption lines.

B stars the next strongest, and so on. These letter designations were called spectral classes or spectral types. We now know that the different spectral types correspond to different surface temperatures. However, the sequence A, B, . . . is not a temperature-ordered sequence. For reasons we will discuss below, hydrogen lines are strongest in intermediate temperature stars.

The spectral classes we use, in order of decreasing temperature, are O, B, A, F, G, K, M. We break each of these classes into ten subclasses, identified by a number from zero to nine; for example, the sequence O7, O8, O9, B0, B1, B2, ..., B9, A0, A1, (For O stars the few hottest subclasses are not used.) For some of the hotter spectral types, we even use half subclasses, for example, B1.5. It was originally thought that stars became cooler as they evolved, so that the temperature sequence

H Samples of stellar spec-tra.These are high resolution spectra, with the visible part of the spectrum (400 to 700 nm) broken into 50 slices.Wavelength increases from left to right along each strip and from bottom to top. (a) Procyon, also known as Alpha Canis Majoris (the brightest star in Canis Major). It has spectral type F5 (see Section 3.2), making it a little warmer than the Sun. (b) Arcturus, also known as Alpha Bootes. It is spectral type Kl, being cooler than the Sun. [NOAO/AURA/NSF]

was really an evolutionary sequence. Therefore, the hotter spectral types were called early and the cooler spectral types were called late. We now know that these evolutionary ideas are not correct. However, the nomenclature still remains. We even talk about a B0 or B1 star being 'early B' and a B8 or B9 as being a 'late B'.

3.3 I The origin of spectral lines

The processes that result in atoms being able to emit or absorb radiation at certain wavelengths are tied to the nature of matter and light. In Chapter 2, we saw the beginnings of the quantum revolution with the realization that light exhibits both particle and wave properties. We now see how the ideas of quantization apply to the structure of the atom.

The modern picture of the atom begins with the experiments of Ernest Rutherford, who studied the scattering of alpha particles (helium nuclei) off gold atoms. Most of the alpha particles passed through the gold atoms without being deflected, suggesting that most of the atom is empty space! Some alpha particles were deflected through large angles, suggesting a concentration of positive charge at the center of each atom. This concentration is called the nucleus. A sufficient number of electrons orbit the nucleus to keep the atom electrically neutral.

There were still some problems with this picture. It did not explain why electron orbits were stable. Classical electricity and magnetism tells us that an accelerating charge gives off radiation. An electron going in a circular orbit is accelerating, since its direction of motion is always changing. Therefore, as the electrons orbit, they should give off radiation, lose energy and spiral into the nucleus. This is obviously not happening. The second problem concerns the origin of spectral lines. There is nothing in the Rutherford model of the atom that allows for spectral lines.

The arrangement of spectral lines in a particular element is not random. For example, in 1885, Johann Jakob Balmer, a Swiss teacher, realized that there was a regularity in the wavelengths of the spectral lines of hydrogen. They obeyed a simple relationship which became known as the Balmer formula:

The constant R is called the Rydberg constant, and its value is given by 1/R = 91.17636 nm. The quantity n is any integer greater than two. By setting n to 3, 4, ..., we obtain the wavelengths for the visible hydrogen lines (also known as the Balmer series). Of course, this was just an empirical formula, with no theoretical justification.

Example 3.1 First Balmer line Calculate the wavelength of the longest wavelength Balmer line. This line is known as the Balmer-alpha, or simply Ha.

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