Lecture 20…The Formation of Stars: where and how

Nature provides a hint: young star clusters

The proto-typical object

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

The prime example: M42, the Orion Nebula

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

What causes the red glow near young star clusters?

Answer: photo- ionization and recombination of Hydrogen by ultraviolet starlight

Some physics of the hydrogen atom: “quantized” energy levels

Electrons can only be in certain states, with certain energies

“Transitions” between states

via absorption or emission of light

Ionization by ultraviolet light with 91.2 nm

Now talk about molecules (explanation later). They have quantized energy levels, too

Wavelengths of molecular transitions typically from 0.1 millimeter to several millimeters

Examples of simple, astrophysically-relevant molecules

• Water

• Carbon Monoxide

• Molecular hydrogen

• ammonia

Technology for radiotelescopes operating at millimeter

wavelengths came of age in late 1960’s

Observations showed huge clouds of molecular gas at dark clouds

Orion to the eye and in molecules

The Milky Way is filled with these molecular clouds

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?