Characteristics and Origins of the Solar System
Lecture 29
November 19, 2001
The Sun: Picture of a G2V star
It is
inconceivable that one could teach a course on the solar system, and not talk
about the Sun. That will be clear when
I start giving some of the stats below.
As will also be clear, the properties of the Sun “pull together” a lot
of our understanding of the solar system.
In
the title of this lecture I have given the astronomical name for stars like the
Sun. I think it is as important to recognize
as the term “Homo Sapiens” for us. The
term also hints that there are many others like the dear old Sun, that gives us
a great insight in and of itself.
Let’s
take a look at it.
http://sohowww.nascom.nasa.gov
The
structure of the lecture will be in the form of a number of (perhaps
irritating) questions. The idea is to
get your synapses firing here early in the morning.
- How
far away is the Sun?
- What
is its mass? I wouldn’t expect you to know the answer to this, so here it
is M=1.9891 X 1030 kilograms.
This is about 330,000 times the mass of the Earth. Now for the
question: how do we know that? Is it “socially constructed”.
- What
is the temperature at the surface of the Sun? We have talked about temperatures of other objects. What is
it for the Sun, and how could we possibly know?
- What
is the power output of the Sun?
The answer is 3.8268 X 1026 Watts. Since a Watt
is defined as 1 Joule of energy per second, the Sun radiates every second this
number of Joules of energy out to space.
For purposes of comparison, the annual energy usage of the United
States is about 94 X 1018 Joules. So, every second the Sun radiates away 4 million
times the annual energy usage of the US.
- What
is the radius of the Sun? 695,000
kilometers. That is 109 times the
radius of the Earth. By now you
should be getting the idea, loud and clear why a solar system course
without a discussion of the Sun is pretty incomplete.
- What
is the Sun made of? You can take a
look at Table 14.2 of your textbook.
Before contemplating the consequences of this, you should ask
yourself how we know this? Next,
what does its chemical composition compare with that of the planets. An
additional interesting aspect of the Sun is that we can get direct
samples of it in the solar wind. In August, a new chapter in this story began. We launched the Genesis spacecraft,
which will collect a direct sample of the solar wind. See http://www.nasa.gov
- How old
is the Sun? Throughout the course
I have described the age of astronomical objects (rocks, actually). It turns out we don’t really have a
good way of independently determining the age of the Sun, and for the most
part rely on radioisotope ages of rocks to tell us when the big guy
formed. There are some qualifiers
to this statement that I will mention in a lecture or two. Actually we can
get some idea by comparing the Sun to its other G2V peers, but these
estimates are pretty rough. In the
process, however, we learn something pretty fascinating about the history
of the solar system.
- How
does the Sun (and the other stars) power itself? Stare at Picture 14.1.
- Now
here’s a fact: the Sun rotates on its axis as well. See movies at http://sohowww.nascom.nasa.gov Interestingly enough, since the Sun
is not a solid body, it rotates at different rates at different
latitudes. At the equator, it
turns around once in just under 25 days.
The Sun rotates in the same direction as the orbital motion of the
major planets. This is a
powerful clue as to the formation of the solar system.
- How
does the Sun affect us? There are
two ways: (a) sunshine, and (b) stuff the Sun throws at us, SunBurbs, etc.
- Sunshine The Sun is a pleasant yellow
color. Actually it is hot enough
to produce a lot of yellow and blue light that it necessary to activate
biological processes (photosynthesis, suntans), but not so hot that it
produces large amounts of ultraviolet light that sterilizes.
>>>>>>>> interesting diagram on the spectrum of
the Sun and the atmospheric transmission of the Earth.
- Solar
Activity and Solar-Terrestrial relations.
Central to an understanding of how the Sun affects the Earth is an
understanding of the division of the solar atmosphere into three parts, photosphere,
chromosphere, and corona.