Читать книгу Astrobiology - Charles S. Cockell - Страница 66

The implications of the above equations and ideas in relation to astrobiology can now be explained.

When a collection of hot gases absorbs light, the electrons in its constituent gases jump energy levels and drop back down again, emitting light at discrete wavelengths. These give rise to emission spectra (Figure 3.25b). Each individual gas has a very characteristic emission spectrum that is a fingerprint, if you will, of its atomic structure, with lines arranged across the spectrum corresponding to the different energy levels of its electronic orbitals (Figure 3.25b). This emission spectrum can be used to identify the different gases in astronomical objects. These effects are pressure dependent, and at very high pressures, hot gases tend to produce continuous spectra without characteristic emission lines.

The detection of gases in the atmospheres of distant stars, beginning in the late nineteenth century, finally confirmed that distant stars are other suns. By characterizing these gases in stars of different colors and luminosities, it became possible to systematically categorize stars into different spectral types.

By contrast, if light travels through a collection of cold gases, the gases tend to absorb the light at the particular wavelengths corresponding to the energies needed to make electrons jump energy levels, dependent on the atomic structure of the individual gases. These create a characteristic absorption spectrum, essentially places in the spectrum where the light is “missing.” Absorption spectra from stars are characterized by very distinct lines in the spectrum called Fraunhofer lines, named after German optician Joseph von Fraunhofer (1787–1826). The temperature and density within a star affect the intensity of the lines, and so they can reveal information about the characteristics of a given star.

As we shall see later, absorption spectroscopy allows us to investigate the gaseous composition not just of stars, but also of planetary atmospheres, such as those of extrasolar planets. By collecting light from a star that has traveled through a planetary atmosphere, we can determine the gaseous composition of the atmosphere and seek gases that are signatures of life (Chapter 20).

We leave the summary of this last section to German physicist Gustav Kirchhoff (1824–1887) who listed, before the structure of the atom was understood, what are sometimes called the three laws of spectroscopy:

1 A hot solid object gives off a continuous spectrum (called a blackbody – we return to this in Chapter 9).

2 A hot gas, under low pressure, produces a bright-line or emission spectrum, which depends on the energy levels of the atoms in the gas.

3 A hot solid object surrounded by a cooler gas (e.g. light from a star passing through an exoplanet atmosphere) produces light with a spectrum (an absorption spectrum) that has gaps at discrete wavelengths depending on the energy levels of the atoms in the gas.