2.2 Interstellar space is not empty
The difference between the apparent brightness of a star (as measured by its apparent magnitude), and its luminosity (represented by its absolute magnitude) is defined by the distance of the star. We can explicitly state this relationship as in Equations B and C:
where L, F and d are respectively the luminosity, flux density and distance of the star, M and m are the absolute and apparent magnitude and d the distance in parsecs.
However, in stating this relationship we are making the assumption that there is no intervening material that could alter the amount of light from the star that reaches the observer. In fact, interstellar space is not empty and some light is absorbed by gas and dust.
Let's imagine a star for which the visible flux density Fv is measured and its luminosity is derived using the method of spectroscopic parallax.
If we derive the distance of the star using Equation B rearranged, d = [Lv/(4Fv)]1/2, how would the interstellar absorption affect the result?
The absorption by the interstellar material would make the star appear fainter (FV smaller) and hence the derived distance would be too large.
Conversely, if the distance to a star is known then the luminosity of the star will be underestimated if there is interstellar absorption present that is not accounted for.
In order to take account of this absorption, Equation D is written
where A is the absorption in magnitudes. The value of A depends on the amount of material between the star and the observer and how efficiently that material absorbs the light. That efficiency depends on the composition of the material and the wavelength of light being observed.
We have used the term absorption rather loosely here. In fact, there are a range of processes which remove energy from the beam of light coming from the star in the direction of the observer (and some that add to it!).
Until as recently as the 1920s, most astronomers believed that interstellar matter was confined to a handful of isolated clouds, some glowing brightly (e.g. the Orion nebula, shown in close-up in Figure 13) and some, through their obscuration of stars, appearing dark, as in Figure 14. The truth began to emerge from long-exposure photographs, which showed that such clouds are far more common than had previously been thought. Furthermore, by 1930 it had become clear that interstellar matter is not confined to such clouds, but is widespread in the spaces between them. There were three pieces of evidence for this.
First, it had already been observed that, in many directions in space, there are absorption lines in stellar spectra that, for various reasons, could not have originated in the stellar atmospheres, but must have originated in cool gas between us and the star. For example, the lines are very narrow, suggesting that they originate in a medium far cooler and less dense than a stellar atmosphere. In a stellar atmosphere the higher random thermal speeds of atoms or ions mean that they may be moving towards or away from an observer when they are absorbing photons. This causes a blue- or red-shift (due to the Doppler effect relative to the average position of the line. In addition, the higher densities in stellar atmospheres cause broadening of lines ('pressure broadening').
Secondly, a characteristic type of attenuation of starlight had been observed in many directions in space, and it had been shown that this is caused by dust particles with sizes of the order of the wavelength of visible light, about 10−6 m. This dust attenuates starlight, partly by absorbing it and partly by scattering it. You can picture scattering as a process in which photons bounce off particles in random directions, and so some of the photons that were travelling towards us from the star do not reach us. Scattering plus absorption is called extinction.
Thirdly, not only did stars in distant clusters appear to be fainter than expected, they were also redder than expected. This change in colour is a result of the greater effectiveness of the dust grains at scattering shorter wavelengths (we will discuss this further in Section 2.4).
Today, the interstellar medium (often shortened to ISM) is studied at a great variety of wavelengths. These studies allow astronomers to determine the composition of the gas, and to infer the likely composition of the dust. Such studies also reveal the temperatures, densities, motions, and magnetic fields within the ISM.