Lesson - The Interstellar Extinction Curve

One of the primary ways astronomers have for studying the make up of the interstellar medium is the average interstellar extinction curve.

Light from stars is partially obscured as it travels the distance to Earth. While we cannot know the total amount of absorbed light without knowing the distance to the star, we can determine the relative amount of absorption at different wavelengths of light by comparing to known stellar types. The standard method of displaying this is the extinction curve, which is a plot of absorption versus inverse wavelength. (Inverse wavelength is used purely for historical reasons.)

The above image (taken from Mathis, Rumpl, and Nordsieck (1977)) shows the average interstellar extinction curve as the solid line.

When we do this, we see some features which are found repeatedly in stellar observations. We see a smooth peak at ~0.2 microns for all sources, we see a linear rise in the visible for all sources, and we see a strong rise in the ultraviolet for all sources.

Exercise:

1. Try matching the shape of the interstellar extinction curve with a single grain size of a single material. Is it possible to get a close match to the extinction curve in this way?
2. What is the effect of changing the grain size?
3. What is the effect of changing the material?
4. What are the basic characteristics of the extinction curve of small graphite grains?
5. What are the basic characteristics of the extinction curve of large graphite grains?
6. What are the basic characteristics of the extinction curve of small silicate grains?
7. What are the basic characteristics of the extinction curve of large silicate grains?
8. What are the basic characteristics of the extinction curve of small amorphous carbon grains?
9. What are the basic characteristics of the extinction curve of large amorphous carbon grains?
10. How well do each of the above match the interstellar extinction curve? Can you find a mixture of the three that matches the curve?