Characteristics and Origins of the Solar System

Lecture 27, Topic 19

April 16, 2004

The Sun: Picture of a G2V star

 

Preliminaries:  Some of the summary points from the lecture last time.  Let’s pull together all the objects we have discussed in the class this far.  We can place them in different categories.  You should visually plot them up on a map of the solar system. 

  1. The major planets and their satellites.  The are spaced through the solar system from about 0.40 au (Mercury) to 30 au (Neptune).
  2. Short period comets.  These are comets with periods of a few years.  They typically have aphelia (farthest point from the Sun) out among the outer planets, but come way into the inner solar system. 
  3. Kuiper Belt objects.  This is a disk of icy objects out beyond the orbit of Neptune, from 30 to 55 au.  The largest such objects would be Pluto and Quaoar,  but there are without doubt many more nearly as large as Quoar out there waiting to be discovered. 
  4. Long Period Comets.  These are comets that have probably visited the inner solar system only once in the history of the solar system.  They come from the Oort Cloud,  which is a huge resevoir of comets at distances of 10,000 – 50000 au. 

 

Meteor Showers

 

A few lectures ago I discussed meteors (shooting stars) that produce meteorites.  These come from the asteroid belt.  There is another type of meteor that is probably better known,  and these are associated with meteor showers. 

 

Meteor showers are times of the year when a higher than normal number (sometimes a much higher than normal)  of meteors are seen,  that come from the same general region on the sky.  These are pieces of matter that flake off comets.  As comets outgas, they blow pieces of ice and carbonaceous material out into space, that follows the comet.  

 

Meteor showers correspond to comets whose orbits intersect the orbit of Earth.  They occur annually because this is the time of year when the Earth passes through the comet debris path.  When you look at the radiant of the meteor shower,  you are looking back along the path of the comet in space. 

 

Look at Table 15.1 on p343 of your book.  It gives a list of the prominent meteor showers.  Notice especially the Eta Aquarids,  which reach maximum on a few days around May 4.  These are pieces off the famous Comet Halley. 

 

Exploration Beyond the Solar System

A few lectures ago,  I discussed the fact that we now have spacecraft far beyond the most distant major planets, and beyond the Kuiper Belt.  These are the Voyager 1 and Voyager 2 spacecraft. 

 

 

It is believed that these spacecraft are close to the Heliopause,  which is the boundary between the part of space where the gas was part of the Sun,  and the interstellar medium,  which is the extended atmosphere of the Milky Way galaxy. 

 

Astronomers would dearly love to send a spacecraft out through the heliopause,  and into the interstellar medium.  That won’t be possible with the Voyagers;  they are moving too slowly and are near the end of their power.  What is planned however,  (although not yet approved for launch) is the Interstellar Medium Mission  which is a spacecraft designed to leave the solar system at high speed, reach a heliocentric distance of 200 – 500 astronomical units in 20 years,  make extensive measurements of the interstellar medium,  then head out to deep space,  never to return. 

 

>>>>>>>>  Computer movie of the Interstellar Medium Mission

 

The Sun

 

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.  This is how it appeared yesterday at about 1:30 PM. 

 

You will get an opportunity to see similarly good views for yourself through the solar telescope on the roof.  A web site with many views of the Sun is the home page of the SOHO spacecraft at:

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 right after lunch. 

 

  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  Figure 7.1 of your textbook (below). 

 

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. 

 

At the beginning of this month,  the Genesis spacecraft left its position far out in space where it had been on station for two years,  collecting direct samples of the solar wind.  It will return to Earth on September 8 of this year.  Read about it in the news notes from Sky and Telescope magazine http://skyandtelescope.com/

 

The picture bellows shows the spacecraft with the “wafers”  or collectors exposed to the solar wind.

 

 

  1. 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.  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.
  2. How does the Sun (and the other stars) power itself?  Stare at Picture 17.1.

One of the most fascinating facts about the Sun is that it is a controlled hydrogen bomb going off.  I won’t talk about that much more;  it is discussed at length in our other astronomy course,  29:50,  Stars, Galaxies, and the Universe.  It is relevant for a topic we’ll come to later,  which is how long the Sun can shine.  That will be important for our discussion of the future of the solar system.

 

  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.
  2. How does the Sun affect us?  There are two ways: (a) sunshine, and (b) stuff the Sun throws at us, SunBurbs, etc.
  3. 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. A spectrum of the Sun is shown below.  We can see the wide range of wavelengths of light it emits,  as well as absorption lines that tell us a lot about conditions on the surface of the Sun.

  1. 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.