Introduction to the Sun

"The Sun is an average, middle-aged star." These words were spoken by William Shatner, Captain of the Enterprise, and later a host to the television show, "Iron Chef," or an actor in the television show "Boston Legal." Since I lave such little time for tv because of the time we spend with our daughters, I have not seen much of Shatner these days. However, Shatner is the narrator for a video we used in class to introduce my students to Astronomy. The video, entitled "The Universe," is pretty old, and Shatner's voice a bit boring at times, or overly dramatic at others, brings most of the students to sleep. However, he is right on in his little assessment about the Sun. We look at the Sun and think, "wow, it's hot!," or "Look at the beautiful sunset," or other random thoughts, but now it is time to understand the Sun, and this is the purpose of this unit.

P.S. Click on the image of the Sun at the top of the page to see a 5 minute video collection of the Sun in Action ... that includes viceo clips of awe-inspiring solar flares, shots of flares, prominences, sunspots, coronal mass ejections and other such cool things. Sorry that the units of measurement in the video clips are English instead of metric, but the footage is just so cool, that you and I can overlook this defect.

All video footage courtesy of NASA SOHO, NASA STEREO, NASA TRACE, JAXA Hinode and BBC Motion Gallery.

To begin this unit, I would like to quote the lyrics of a song by the group, "They Might Be Giants"

"The Sun is a mass of incandescent gas; a gigantic nuclear furnace

Where hydrogen is built into helium at a temperature of millions of degrees.

Yo ho, it's hot, the Sun is not a place where we could live

But here on Earth, there'd be no life without the light it gives."

The sun is the closest star to the Earth, yet compared to other stars in our galaxy, the sun is just an average middle aged star. While most stars are found in pairs, our star is a singleton. Although the sun is over 1.3 million times larger in volume than Earth, there are stars millions of times larger than our sun with masses hundreds of times greater. Conversely, there are stars millions of times smaller than our sun and with masses of a litter more than 1% of our light-giver. The sun’s relatively long life, yellow color, average size, and relatively calm surface make it indistinguishable from other stars in the Milky Way except for these important facts: The sun is the closest star to Earth, it is the source of all energy which gives life to this planet, and it is at the perfect distance to optimize conditions for life here.

These pages were updated in the Fall of 2008 to reflect some of the new things we have learned about the Sun, and how is own "weather" affects our planet. Over the past decade, a growing number of scientists, political leaders, pastors, and activitsts have raised the alarm of "Global Warming." Dire predictions of "utter devastation," "catasstrophe," "global innundation," "mass extinctions," and "runaway green house effect" have been tossed around in scientific meetings and public forums. In the overwhelming majority of instances, abusive practices of mankind have been cited as the primary cause of global warming, and unless mankind unites in a planet-wide effort, life on Earth will be threatened ... perhaps in an irreversible manner.

It is not my places as a science teacher to influence students in a totally biased manner, but it is also my responsibility as a science teacher to be skeptical of what the "experts" are saying and writing, and to critically analyze the data in a meaningful search for the truth. In my search for the truth of global warming, I have discovered many lies that were perpetrated in Al Gore's movie "An Inconvenient Truth," as well as a manipulation of statistics to present data in a format that exaggerates the rise in carbon dioxide. I also believe that the policy-makers need to raise the specter of alarm in order to present a need for the government to intervene on their behalf. In nearly every argument presented by global warming alarmists who cite mankind and industralization as the primary cause of the global warming trend, the effects of astronomical phenomenon, and in pareticular the rhythms of the Sun, have been omitted from discussion. I hope that my readers will be armed with enough information about the Sun by the end of this unit to make a more educated assessment of the trends in Earth's weather and the connection to the idosynchratic behavior of the formerly so-called "Solar Constant."

At no time do I ever contend that mankind is not responsible for some of the changes in the Earth's environment, nor do I contend that humans can continue to frivilously live their lives in a bubble of ignorance. I believe that good stewardship of our home is a moral value, and working to make our planet more hospitable for future generations is an obligation of everyone. However, I do not believe that using frightening tactics to scare people into action will provide a lasting effect or a change in human attitudes and behaviors. I hope that in the reading of this unit about the Sun and its influence, we all can be armed with better and more complete information about the relationship between changes on the Sun's surface and changes in global weather.


Mean Equatorial Diameter

1,392,000 Km

Mean Distance from Earth

149,597,900 Km

Distance from Galactic Center

30,000 Light Years

Period of Revolution Around Galactic Center

225,000,000 Years

Mean Density

1.409 g/cm3


332,946 Earths

1.989 x 10e30 Kg


1,303,600 Earths

Surface Gravity

27.90 Earths

Mean Apparent Magnitude


Absolute Magnitude


Stellar Classification


Surface Temperature

6,000 K

Core Temperature

15,000,000 K

Energy Output

380,000 x 10e21 Watts

Rotation Period

25.38 Days at Equator

up to 33 Days at the Poles

Approximate Age

4.6 to 5.0 Billion Years

That pretty much sums up my introductory comments. We will spend the next few on-line lectures learning more about the Sun than you ever knew before, and perhaps more than you ever cared to learn, but certainly not as much as is possible due to the limitations of this course, imposed upon it by my Uncle Bill's mandate to keep things simple and interesting.

What I want you to learn about the Sun - Part I

I am hoping that all students of Astronomy will know what the 6 structural parts of the Sun are, and in particular what happens in the core, and on the photosphere. I want them to learn how the absorption spectrum of the Sun tells us what elements are inside the Sun. I want them to know where ALL of the energy of the Sun is created, what the process is that creates the energy, and what form that energy is in when it is made and when it leaves the Sun. I want students to understand what sunspots are and what the sunspot cycle is. I also want students to know why scientists study solar flares and how they can play a significant role in the creation of the Aurora Borealis.

What I want you to learn about the Sun - Part II

The single most important chart that has ever been created by astronomers is the HR Diagram. This diagram is used to tell astronomers about the lifecycle of the Sun and other stars. It is used to tell astronomers where stars are in the galaxy and beyond when more precise measurements are impossible to make. The diagram can even tell us whether a star is just forming, living, dying, or already dead. In this second part of this unit, I want students to know the history of the development of the HR Diagram and who the scientists are who made this history. I want students to be able to look at an HR Diagram and know which kinds of stars can be located in the various areas of the diagram. Lastly, I want students to know the basic stages of the life of the Sun.

Here are the subunits where you will be directed to learn these most important things. There is a fair amount of reading, but students can focus on what I am emphasizing in the preceding two paragraphs, and skim other material, or else explore that material for their own interest:

Please start here first Structure ... to learn about the 6 parts of the Sun, from the inside to the outside

Thermonuclear Fusion ... gives a basic understanding of the energetic process that causes the Sun and other stars to shine

Electromagnetic Radiation ... shows you what comes out of the Sun

HR Diagram ... gives a historical look at how scientists came to understand the properties of sunlight

Developing the background for the HR Diagram, The HR Diagram and its Usefulness

Sun Lifecycle ... follows the theorized path of birth, life, and death of the Sun

Features and Solar Events ... exposes students to things that happen on the surface of the Sun

Aurora, Aurora in Minnesota Nov 7, 2004, Photon Capture and Re-emission, SolarMax, Sunspots, The Great Storm of October 29, 2003

Sun Observation - Something that is REQUIRED, but will be done from your computer :) No need to go outside.

Sun Quiz - This quiz will also be found in the Moodle Site, but with multiple choices instead of only fill-in-the-blanks.

If you ever get lost, you can return to the Sun Introduction to refind your path, or to the Syllabus.

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Much of the information within these pages is from NASA, JPL, TRACE, and SOHO websites. All are government-sponsored projects whose images and information can be utilized for educational purposed such as this course.