The Sun
(Chapter 16, Sections 16.1-16.3, and 16.6 and 16.7.)
[Remember, we’re skipping 16.4, 16.5 on “Solar Magnetism” and “The
Active Sun.” But I think it would be foolish not to at least look through
it to see how complex our star really is, and the sun-Earth connections.
Just don’t forget that you need to read section 16.6 and 16.7!]
◆ We can only observe light coming from the “surface” of
the sun, called the photosphere—this is not a solid surface, like
on the earth, but just the layer of the sun at which the density is
low enough to allow the photons to escape. (We can also see
light from the layers above the photosphere, called the
chromosphere and corona, but they are of interest for different
reasons—they don’t lead to any understanding of the properties
and evolution of the sun.)
◆ From observations of this light from the photosphere,
along with knowing the distance to the sun, we learn the sun’s
size (about 100 earths), mass (300,000 earths), and luminosity
(how much energy per second is emitted, its brightness, like the
wattage of a light bulb—over 1026 Watts for the sun!), and its
spectrum (from which we can get the abundances of elements
(see sec. 16.3), as well as the amazing surface activity (which
we won’t cover for the exam—see 16.4, 16.5).
◆ We’ll see later that the sun is more or less an average
star, in just about all these quantities, including its composition
(abundances of elements)
