Modern Astronomy
September 6, 2002
Lecture 6: The Sun as we see it

• Lecture for last time now does have the figures embedded. It should make a lot more sense.
• The first homework set is now on the web. Take a look at it. It won't be collected, but will be of the sort of problems that will be on the quantitative exam.

``The Sun as we see it'' the surface of the Sun. SOHO Website For the moment we deal with the surface phenomena of the Sun. Fundamental questions about the interior of the Sun and ``how it works'' will be deferred until it can be dealt with in the context of other stars.

There are two reasons for studying the Sun in a course like this.

1. The Sun is a major astronomical object in its own right, with significant influence on the Earth.
2. The Sun is the nearest star, and we can study it in much more detail, and have observations which are not possible with other stars.

. Let's collect the main properties of the Sun. Some we have dealt with already, but let's mention them for completeness.

1. Distance = 1 astronomical unit = meters
2. Radius = 696,000 kilometers = meters Note that scientific notation really does come in handy!
3. Mass = kg. This is an unimaginably large mass. It is 330000 times the mass of the Earth. The point of bringing it up here is that is ``sets the scale'' for the masses of stars.
4. Power output. For stars the power output is called the luminosity. The unit of power is physics is the Watt, familiar from light bulbs. The luminosity of the Sun is Watts. This number is also unimaginably large. The only way of remotely getting `` on the page'' is as follows. Every second the Sun radiates away an amount of energy = 4 million times the annual energy usage of the United States.
5. Density. Density is an important parameter in physics. Different substances have different densities. Density is the amount of mass per volume. I will use the Greek letter to indicate density. It has units of kilograms/m .

   Let's work it out for the Sun. The volume of a sphere is

   

   For the Sun we have .
   We then have for the mean (average) density of the Sun

   

   Thus the average density of the Sun is 1.4 times that of water!

6. Why does the Sun shine? This is an audience participation question. A real basic question. I am not asking at this point for its energy source, which we will deal with later.
7. What is the Sun made of? First you should think about how we would know. The two ways are (1) spectroscopy, meaning you analyse the light from the Sun, and (2) direct measurements of solar material. This is presently being done by the Genesis spacecraft.
   URL for Genesis web site.
   

   To begin with, let's ask what the chemical composition of the Earth is. The four leading elements by mass in the Earth are: Oxygen, 47 %; Silicon, 28 %; Aluminum, 8 %; Iron, 5 %.

   The ``recipe'' for the Sun, by contrast, is given on p122 of your textbook, Figure 7.1.
   (Transparency) List of elements in the Sun . You can see that the Sun is almost entirely hydrogen. The next most abundant element is helium. This is true throughout the universe; the bulk of the universe is hydrogen and helium.
   Demonstration: ``the color of the universe''.

8. How old is the Sun. In stellar astronomy, we have some techniques that give us the age of stars. For single stars (not in star clusters) these techniques turn out to be pretty rough. In the case of the Sun however, we can date the age of formation of rocks that formed early in the history of the solar system. These turn out to give rather precisely) an age of 4.55 billion years for the age of the solar system, including the Sun.
   Mainz, Germany, and the `` Garten der Zeit''.
   Compare 4.55 billion years with other ages in Geology and Paleontology.

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