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. 

 

  1. How far away is the Sun?
  2. 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”.
  3. 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?
  4. 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.
  5. 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.
  6. 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
  7. 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.
  8. How does the Sun (and the other stars) power itself?  Stare at Picture 14.1.
  9. 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.
  10. How does the Sun affect us?  There are two ways: (a) sunshine, and (b) stuff the Sun throws at us, SunBurbs, etc.
  11. 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.
  12. 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.