1. Lecture 20…The Formation of Stars: where and how Nature provides a hint: young star clusters The proto- typical object

2. A star cluster with O Main Sequence stars cannot be more than a few million years old. It may still retain information about the processes of formation

3. The prime example: M42, the Orion Nebula

4. Another characteristic of star formation regions, Dark Clouds Prior to late 1960’s the nature of the connection between these and star formation was not clear

5. What causes the red glow near young star clusters? Answer: photo- ionization and recombination of Hydrogen by ultraviolet starlight

6. Some physics of the hydrogen atom: “quantized” energy levels Electrons can only be in certain states, with certain energies

7. “Transitions” between states via absorption or emission of light Ionization by ultraviolet light with 91.2 nm

8. Now talk about molecules (explanation later). They have quantized energy levels, too Wavelengths of molecular transitions typically from 0.1 millimeter to several millimeters

9. Examples of simple, astrophysically-relevant molecules Water Carbon Monoxide Molecular hydrogen ammonia

10. Technology for radiotelescopes operating at millimeter wavelengths came of age in late 1960’s

11. Observations showed huge clouds of molecular gas at dark clouds

12. Orion to the eye and in molecules

13. The Milky Way is filled with these molecular clouds

14. Summary of Molecular Clouds They are to be found wherever stars are forming The molecular gas is the “raw material” from which stars are formed Roughly 2500 “giant molecular clouds/star formation regions” in the Milky Way Temperatures in these clouds as cold as 10K Remaining questions: (a) How do stars form from this gas? (b) What is the chemistry of these clouds?