Have we talked about the Sun, yet? Seems like we must have made a comment or two. Amongst all of the stars in the universe, our Sun is my favorite. Hands down. I'm Sirius (1)! The reason I like it best is that it constantly warms my favorite planet! And that ought not to be taken for granted!
"Solar Constant" refers to the constant amount of solar energy that reaches the Earth's outer atmosphere. The Solar Constant is calculated at the outer atmosphere because the amount of energy reaching the Earth's surface is not constant. Clouds reflect energy more in the tropics than elsewhere because it's cloudiest there. Polar ice reflects energy very effectively, as well. Thus, differing amounts of solar energy reach the Earth's surface at different places than others and at different times seasonally. The atmosphere is a marvelously designed blanket, perfectly fitted to protect us from harmful solar radiation and to transmit beneficial radiation.

We'll look more closely at the atmosphere next. But the constant energy that the Sun radiates our way really should be a surprise. Most stars do not radiate energy at constant rates at all! Most stars, in fact, are extremely un-constant in terms of their energy output. As atomic particles collide, nuclei (the centers of atoms) fuse in the super heated environment of a hot star on a random and unpredictable basis. These nuclear fusions release tremendous energy bursts. (Note that the word "nuclear" refers to atomic nuclei.) During times of greater nuclear activity a star will actually expand as it's awesome explosions resist gravitational force. When nuclear activity subsides, the star will actually shrink in size.

If our star, the Sun, were to expand and shrink from time to time the effect on the Earth would be catastrophic. 8 minutes after a solar expansion occurred the surge of radiant energy would hit the Earth with destructive fury. White guys like me would get a quick sun-tan! Actually, we'd all get cooked. Then, when the Sun were to shrink back, it would be frosty mugs for everyone! It would be cold over the entire Earth!

3 guesses: of all the stars so far studied in the universe, which star is the most stable? Another way to frame this question: which star varies the least in terms of expansions and contractions? You won't believe it! What a coincidence! It's my star! El Sol! If the Sun were more like most stars, however, size fluctuations would result in the extinction of most (or all) higher life forms.

Earlier I told you that the Earth's distance from the Sun is perfectly set to most effectively distribute heat to a planet with the exact distribution of land and water that the Earth (coincidently) has. Such an idea, however, assumes that the solar energy that radiates to the depths of the dark universe is constant. Such an assumption, however, should not be made glibly. If our star were like other stars, there could be no perfect distance to support life! Nuclear flare-ups on the Sun would be so disruptive as to kill off higher life forms!
Actually, our star does flare from time to time. Such flares have a marvelous effect on the Earth that we'll look at later, but those flares are extremely minimal by comparison with other stars.

The Solar Constant, by the way, is about 1370 watts per square meter. Now there are gazillions of square meters at the top of the Earth's atmosphere, so that constant amount of energy is awesome. The most remarkable thing, however, is the degree of constancy. The Solar Constant has been monitored since about 1980 (when technology achieved the capability of running comparative measurements) from the satellite Nimbus-7. Variations, called solar irradiance, have amounted to 0.1% during the 2 decades of record keeping! Thus this amazing coincidence, perfectly suited to encourage the security of higher life forms!

As an aside, what do you suppose the chances are that mankind can tap into solar energy high up in the atmosphere? We haven't really even tried yet, but that energy source is abundant beyond measure, clean and free. And it is just one of perhaps limitless potential sources of energy which are as yet uncovered.

I was once a great doomsayer... but I've repented. The more I learn about the tremendous untapped potentiality of the blue planet, the less impressed I am with down-in-the-mouth predictions of energy and raw material and agricultural depletions. We think too highly of ourselves if we imagine that we've so thoroughly understood nature as to proclaim it unable to provide for a mankind at peace.

There's the rub though, don't you think? Mankind doesn't have a very good track record for peace! All we need for material well being surrounds us, but the human race seems allergic to well being!
My revised perspective is that nature could hardly have thought farther ahead for the provision of her inhabitants. (Does nature think? Seems that somebody did!)

OK, what else can I tell you about the Sun? To share about the Sun, and it's love affair with the Earth, it may be helpful to share a bit about quantum physics, as well. Quantum physics (often called quantum mechanics) is the science of atoms and sub-atomic particles.

The Sun, like other stars, is mostly composed of hydrogen (74% or so) and helium (24% or so). In the small remainder fraction are lots of other ingredients as well. But hydrogen is predominant. The proportion of hydrogen to helium is very instructive. It gives a simple clue to the energy reserve remaining in the Sun. Hydrogen, you see, converts to helium in the nuclear fusion process. In the process light and other forms of radiation are released. The Sun is, in effect, burning out as its hydrogen is used up. Helium being the by-product. The fact that there is so much more hydrogen still in the Sun than helium tells us that the Sun is still in its prime!

You know that there are three states of matter.See my fingers?Three: solid, liquid and gas. Right? Hydrogen, helium, oxygen, nitrogen, you name it, can all exist in any of the three states of matter. The colder the element, the more likely it will be to exist in a solid state. In the middle state it will become liquid and when hot enough it will jump out of the liquid state into the gaseous state.

One definition of "heat" (Merriam-Webster) is, "excite." That's an important definition! According to World Book Encyclopedia (1999): "All things are made up of atoms or molecules, which are always moving. The motion gives every object internal energy. The level of an object's internal energy depends on how rapidly its atoms or molecules move. If they move slowly, the object has a low level of internal energy. If they move violently, it has a high level. Hot objects have higher internal energy levels than do cold objects."
Here's the way it works. I think of it as a slam dance. When I go out on the dance floor and folks start crashing into me, I want more space! And atoms feel just like me! When atoms get hot, they move around a lot. When they move around enough to cause nasty collisions with other atoms, they react by demanding more space... just like me on the dance floor! When atoms demand more space with sufficient force, they break the mold of their material state. They violate the solid condition (that state of matter which is the most cool and calm and mellow) and expand (2) into the liquid state where there's a little more elbow room!

If these agitated and irritable ato ms continue to warm up, they'll demand still more space and explode into the gas state! Wanna see my hand again, just to confirm the three states of matter?Did I say three?(I'm easily confused.)There are four states of matter!
In fact, the three states of matter that we are all familiar with on Earth can hardly be found elsewhere in the universe. The fourth state of matter is actually by far the most common.
An atom exists as nucleus and electron(s). When heat is turned up enough, atoms increase their speed of motion so greatly that (like a meteor falling through the Earth's atmosphere) it begins coming apart! In the super heated environment of stars, electrons separate from atomic nuclei and a whole new porridge is formed. Much too hot for Goldilocks!

The porridge (more often referred to as "soup") is called plasma.
In the three states of matter known on Earth, atoms remain intact. Check out the excellent World Book Encyclopedia drawing on the next page to see the simplest atom (hydrogen) beside the most complex (plutonium) as they look on Earth (not!). (3)

CONTINUE

1. Sirius is the brightest star. Seen in the winter sky, it has a bluish hue
2.Nearly all elements expand from the solid into the liquid state because, of course, when atomic structure is agitated by heat, atomic motion increases and cannot be held together in it's solid condition. But, boy oh boy, will you be surprised to learn of a defiant exception which is required for higher life forms to exist in their most perfect environment!
3. Sorry, but I lied. Atoms don't really look anything like the drawings. They really look more like an egg! Read on!