1. Solar-Stellar Connection
The Sun: Home Star
Solar-Stellar Connection

2. The Sun Key to solar-stellar connection Close-up model for other stars
Local “lab” for testing ideas about the physics of stars
Energy source for most life on earth

3. Must know distance to find these!
Sun: Properties
Luminosity
Mass
Radius
Must know distance to find these!

4. Analyze spectrum to find these; do not need to know distance!
Sun: Properties
Surface temperature
Chemical composition
Analyze spectrum to find these; do not need to know distance!

5. Distance 1 AU! About 150 million km
How? Bounce radar signals off Venus! Know distance in AU from Kepler’s laws. Know speed of light, measure travel time => distance in km, known fraction of AU; get AU in km

6. Luminosity
At earth, measure flux: Energy passing through an area per second => 1370 W/s
Imagine a sphere around the sun with radius 1 AU
Collects 4 x 1026 W, sun’s luminosity or power

7. Mass
Apply: Newton’s version of Kepler’s 3rd or Newton’s 2nd law plus law pf gravitation
Need earth’s orbital period (365 days) and distance, 150 million km (1 AU)
Result: 2 x 1030 kg, 330,000 times that of earth

8. Diameter (Radius)
Angular diameter from earth about 1/2 degree => actual diameter/distance = 1/110
Know distance (1 AU in km)
Actual diameter = (1/110) x 150 x 106 km = about 1.4 million km, or 100 times earth!

9. Chemical Composition
Absorption-line spectrum: Match lines to those of known elements
Information only about photosphere, where lines form
Mostly H (72%) and He (26%)

10. Surface Temperature
Assume: Radiates like a blackbody (Planck curve of continuous emission)
Then: Match overall curve or use peak of emission
Get: Temp about 5800 K

11. Solar Energy
Source: Fusion reactions (H => He) in core, where temperature 10 to 15 million K
How: E = mc2; thermonuclear reactions convert mass to energy (heat & light)
Energy flow: From core to surface takes light about 200,000 y (opaque!)