summary
Basic nature of Sun well understood
- referring to W 21 picture
- mechanical stability (pressure vs. gravity)
- Energy generation near the center
- energy flow through interior
- energy flows from hotter to colder regions
- radiation to space from photosphere
- Sun has been stable for billions of years
- oldest life on earth more than 3 billion years old
- if sun twice as luminous
- if sun were half as luminous
- oceans would freeze solid
- Some things aren’t stable