1. spectrum of Sun

2. stellar spectra

4. The Hertzsprung- Russel diagram a.k.a. HR diagram Vertical axis: L * Horiz. axis: T s * color of the star

5. cartoon showing current idea for how a star forms: (a) denser “cores” form in dense molecular clouds (c) outflows shoot out of poles of the star/disk system (b) the center of a “core” collapses (due to its gravity) to form “proto-star” and disk (d) After clearing: protostar and proto- planetary disk

6. stage c (almost d): a protostar with disk and jet infrared image optical image

7. stage “a”: a dense molecular cloud core