2.4 The effect of interstellar dust
Let's now consider the dust. Photoexcitation (by absorption of photons) and collisional excitation (by atoms/molecules) occur in the atoms and molecules that constitute the surface of a dust grain. Much of this energy is shared throughout the grain, raising its temperature until thermal radiation from the grain balances the energy absorbed. An alternative fate for an incident photon is to be scattered (Figure 15), a process that is very efficient at certain wavelengths. Figure 20 illustrates what is seen by observers when a cloud of interstellar dust is in the line of sight to the star and when it is out of the line of sight. The typical size of the interstellar dust grains means that they scatter short wavelengths most efficiently. This means that relatively more blue light is removed from the star's spectrum after passing through the cloud and it therefore appears redder when viewed from behind the cloud (position b in Figure 20). This process is called interstellar reddening. If the cloud is observed from out of the line of sight to a star then the dust cloud can appear as a faint blue glow from the scattered starlight.
The combined effects of absorption and scattering (extinction) by interstellar dust are shown in Figure 21. Note how the extinction increases strongly through visible wavelengths, and on into the UV. Note also how broad the spectral features are, which makes it difficult to determine the composition of the dust from such spectral studies. Not much more about composition is revealed by the emission spectrum of the dust, which is a broad smooth thermal spectrum, depending on the dust temperature, the particle size, and only weakly on its composition. At 20 K, the dust emission lies right across the far-IR and microwave parts of the spectrum.
We have seen that absorption of starlight by interstellar dust can cause stars to appear fainter than they should and therefore cause us to overestimate their distance or underestimate their luminosity. In addition, interstellar reddening can cause stars to appear redder than they should. Since colours, as measured by the colour index, are often used to infer temperature, the temperature can also be underestimated. If plotted on the H-R diagram a star will appear in the wrong place if the effects of interstellar absorption and reddening are not accounted for.
A star like our Sun is located in a star cluster at a known (large) distance and is subject to significant interstellar extinction. If its absolute visual magnitude mV is derived from its apparent visual magnitude mv using Equation C and its temperature determined from its observed colour index, B - V, what will be the effect on its position in the H-R diagram (Figure 5)? Explain how its true position can be determined if its spectrum is observed.
Equation C does not take account of interstellar extinction, A, as in Equation D:
The derived absolute visual magnitude will therefore be too faint (M numerically too large). Since interstellar dust also causes reddening, the B - V colour will be redder and therefore the derived temperature will be too low. Examination of the axes of the H-R diagram in Figure 5 shows that the star will appear below and to the right of its correct position.
If a spectrum is observed, the temperature can be derived from its spectral type (based on the strengths of certain spectral lines) and therefore not affected by interstellar reddening. Its luminosity can also be inferred directly from its spectrum and hence its true position on the H-R diagram can be determined.