Summary

After having studied this lecture, you should be able to

• Describe how we know the properties of interstellar dust from scattering and absorption of star light.
• Explain why we find different ionisation states of interstellar gas, depending on density, temperature, and ionising background.
• Derive the Strömgren radius for an HII region, and explain the concept.
• Explain the origin of the hydrogen 21-cm line, and explain its importance in understanding the structure of the MW.
• Explain the concept of Jeans mass, and its relation to fragmentation of clouds.