The Sun: Home Star
Solar-Stellar
Connection
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
Sun: Properties
• Luminosity
• Mass
• Radius
Must know distance to find
these!
Sun: Properties
• Surface temperature
• Chemical composition
Analyze spectrum to find these;
do not need to know distance!
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
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
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
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!
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%)
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
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!)