The Solar Atmosphere

The sun does not have a surface as such. What we call its surface is just the layer that emits the most light. Let’s begin our journey at the outer layers of the sun (the layers that we can actually see), and work our way in. When you look up at the sun during the day, what you are really looking at is the sun’s photosphere. The layer from which the visible photons that we see arise, the photosphere has a temperature of about 6,000 K. Lower layers are hidden behind the photosphere, and higher layers are so diffuse and faint (though very hot) that we only see them during total solar eclipses or with special satellites. Above the photosphere in the solar atmosphere are the chromosphere, the transition zone, and the corona. As we move higher in the sun’s atmosphere, the temperatures rise dramatically.

Four Trillion Trillion Light Bulbs

Next time you are screwing in a light bulb, notice its wattage. A watt is a measure of power, or how much energy is produced or consumed each second. A 100 watt bulb uses 100 joules of energy every second. For comparison, the sun produces 4 1026 watts of power. That’s a lot of light bulbs—four trillion trillion of them, to be exact. This rate of energy production is called the sun’s luminosity. Many stars have luminosities much higher than that of the sun.
The source of the sun’s power—and that of all stars, during most of their lifetimes—is the fusing together of nuclei. Stars first convert hydrogen into helium, and heavier elements come later. The only fusion reactions that we have been able to produce on the earth are uncontrolled reactions known as hydrogen bombs. The destructive force of these explosions gives insight into the enormous energies released in the core of the sun. Nuclear fusion could be used as a nearly limitless supply of energy on the earth; however, we are not yet able to create the necessary conditions on Earth for controlled fusion reactions.

A Spectacular, Mediocre Star

In terms of its size, mass and energy released, the sun is by far the most spectacular body in the solar system. With a radius of 22.8 X108 feet (6.96 X108 m), it is 100 times larger than the earth. Imagine yourself standing in a room with a golf ball. If the golf ball is the earth, the sun would touch the eight-foot ceiling. With a mass of 1.99 1030 kg, the sun is 300,000 times more massive than the earth. And with a surface temperature of 5,780 K (compared to the earth’s average 290 K surface temperature), the sun would melt or vaporize any matter we know.

What’s Sun Made Of?

The sun is mostly hydrogen (about 73 percent of the total mass) and helium (25 percent). Other elements are found in much smaller amounts, adding up to just under two percent of the sun’s mass. These include carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron. Over 50 other elements are found in trace amounts. There is nothing unique about the presence of these particular elements; they are the same ones that are distributed throughout the solar system and the universe. In particular, hydrogen atoms of the sun’s core plow into one another to create helium atoms. In the process, a little mass is converted into energy. That little bit of energy for each collision means enormous amounts of energy when we count all of the collisions that occur in the core of the sun. The fact that c is a very large number means that a tiny amount of mass results in a very large amount of energy. With this energy source, the sun is expected to last not a thousand years, or even 100 million years, but about 8 to 10 billion years, typical for a star with the sun’s mass.

The Solar Furnace

Greek philosopher Anaximenes of Miletus believed the sun, like other stars, was a great ball of fire. His was an important insight, but not entirely accurate. The sun is not so simple. In terms of human experience, the sun is an unfailing source of energy. Where does all of that energy come from? In the nineteenth century, scientists knew of two possible sources: thermal heat (like a candle burning) and gravitational energy. The problem with thermal energy is that even the sun doesn’t have enough mass to produce energy the way a candle does—at least, not for billions of years. Calculations showed that the sun “burning” chemically, would last only a few thousand years.
While a sun that was a few thousand years old might have pleased some theologians at the time, there was a variety of evidence showing that the earth was much older.
So scientists turned their attention to gravitational energy, that is, the conversion of gravitational energy into heat. The theory went this way: As the sun condensed out of the solar nebula, its atoms fell inward and collided more frequently as they got more crowded. These higher velocities and collisions converted gravitational energy into heat. Gravitational energy could power the sun’s output at its current rate for about 100 million years.
But when it started to become clear that the earth was much older (geological evidence showed that it was at least 3.5 billion years old), scientists went back to the drawing board. The nineteenth century ended without an understanding of the source of energy in the sun.

Understanding Sun

An evening spent looking up under dark skies will convince you that stars can be breathtaking in their loveliness. We can appreciate why, for thousands of years, human beings thought that the stars were embedded in a perfect sphere, spinning and changeless. Yet, because of their great distance, theirs is a remote beauty. Many amateur astronomers are disappointed to discover that stars (other than the sun) look pretty much the same through even the best telescope. Our common sense sees little similarity between the distant, featureless points of light against a sable sky and the great yellow disk of daytime, whose brilliance overwhelms our vision and warms our world. Yet, of course, our sun is a star—and, as stars go, not a particularly remarkable one. We now turn our attention to the very center of our solar system, the parent of the terrestrial and jovian planets and their rings and moons. We have spent the last three chapters discussing the planets and their moons. But taken together, these objects represent only 0.1 percent of the mass of the solar system. The other 99.9 percent of the mass is found in the sun. Peoples of many times and cultures have worshipped the sun as the source of all life, and in some sense, they were right. The sun is our furnace and our light bulb: the ultimate source of most energy and light here on the earth. And because it contains almost all of the mass, it is the gravitational anchor of the solar system. Indeed, its very matter is ours. The early sun was the hot center of a swirling disk of gas and dust from which the solar system formed some 4.6 billion years ago. If the sun were a cake, the earth and the rest of the planets would be some flour left on the counter. But the sun is only one star in a galaxy containing hundreds of billions of stars. Astronomers feel fortunate that the sun is so nondescript a citizen of the galaxy. It is, of course, the star closest to us and its very averageness lets us generalize about the many stars that lie far beyond our reach. In this chapter we examine our own star, and begin to explore how the sun (and stars in general) generate the enormous energies that they do.

Where Did Pluto Come From?

Except that it doesn’t orbit another planet—and, indeed, has a moon of its own—Pluto looks more like a jovian moon than a planet. It fits into neither the terrestrial nor jovian mold. Some astronomers believe that Pluto is really a renegade moon, escaped from Neptune’s gravitational influence due to a collision or interaction involving Triton, Pluto, Charon, and Nereid. Others regard it as a kind of spare part, something left over from the creation of the solar system, and perhaps only one of a number of such objects in the outer reaches of the solar system, the Kuiper Belt.