The Outer & Inner Parts of the Sun

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Even if you have no special interest in astronomy – yet – you've no doubt wondered just what's going on in that massive bright ball in the sky that is both dangerously hot and literally life-giving at the same time. You probably know that the sun is a star, much like the countless points of light that take the sun's place overhead at night when darkness sets in, only closer. You may know it has its own fuel supply and that this supply, while not infinite, is so vast as to be incalculable. You probably realize that it wouldn't be a great idea to get a whole lot closer to the sun even if you had the ability to do it – but that it would be almost as bad an idea to stray much farther from it than you already are, a distance of about 93 million miles.

In your pondering, however, you may not have considered the idea that the sun is not a uniform orb of light and heat, but instead has layers in its own right, just like the Earth and the other seven planets in the solar system do. What are these layers – and how in the world are human scientists even able to know about them from such a great distance anyway?

The Sun and the Solar System

The sun lies at the center of the solar system (hence the name!) and accounts for 99.8 percent of the solar system's mass. Owing to the effects of gravity, everything in the solar system – the eight planets, the five (for now) dwarf planets, the moons of those planets and dwarf planets, the asteroids and other minor elements such as comets – revolves around the sun. The planet Mercury takes a little less than 88 Earth days to complete one trip around the sun, whereas Neptune takes almost 165 Earth years.

The sun is a fairly nondescript star as stars go, earning the classification of "yellow dwarf." With an age of about 4.5 billion years, the sun sits about 26,000 light-years from the center of the galaxy it inhabits, the Milky Way Galaxy. For reference, a light-year is the distance light travels in one year, about 6 trillion miles. As vast as the solar system itself is, Neptune, the farthest planet from the sun at a distance of nearly 2.8 billion miles, is barely 1/2000 of a light-year from the sun.

The sun, in addition to functioning as a gigantic furnace, also has a strong internal electric current. Electric currents generate magnetic fields, and the sun has a vast magnetic field that propagates through the solar system as solar wind – electrically charged gas that flies outward from the sun in every direction.

Is the Sun a Star?

The sun is, as noted, a yellow dwarf, but it is more formally classified as a G2 spectral-class star. Stars are classified in order from hottest to coolest as type O, B, A, F, G, K or M stars. The hottest have a surface temperature of about 30,000 to 60,000 Kelvin (K), whereas the sun's surface temperature is a comparatively tepid 5,780 K. (For reference, Kelvin degrees are the same "size" as Celsius degrees, but the scale starts 273 degrees lower. That is, 0 K, or "absolute zero," equals −273 C, 1,273 K equals 1,000 C and so on. Also, the degree symbol is omitted from Kelvin units.) The density of the sun, which is neither a solid, a liquid nor a gas and is best classified as plasma (i.e., electrically charged gas), is about 1.4 times that of water.

Other vital solar stats: The sun has a mass of 1.989 × 1030 kg and a radius of about 6.96 × 108 m. (Since the speed of light is 3 × 108 m/s, light from one side of the sun would take a little over two seconds to pass all the way through the middle to the other side.) If the sun were as tall as, say, a typical door, the Earth would be about as tall as a U.S. nickel standing on edge. Yet stars 1,000 times the diameter of the sun exist, as do dwarf stars less than a hundredth as wide.

The sun also puts out 3.85 × 1026 watts of power, about 1340 watts per square meter of which reaches the Earth. This translates to a luminosity of 4 × 1033 ergs. These numbers probably don't mean a lot in isolation, but for reference, an exponent of "only" 9 implies billions, while an exponent of 12 translates to trillions. These are enormous figures! Yet some stars are as many as a million times more luminous than the sun is, meaning that their power output is a million times greater. At the same time, some stars are a thousand or so times less luminous.

It's interesting to note that even though the sun is classified as a modest star at best in the overall scheme, it is still more massive than 95 percent of the known stars in existence. The implication of this is that most stars are well past their prime and have shrunk considerably since their lifetime peak billions of years earlier, and are now carrying on in their old age in relative anonymity.

What Are the Four Regions of the Sun?

The sun can be divided into four spatial regions, consisting of the core, radiative zone, convective zone and photosphere. The latter sits below two additional layers, which will be explored in the next section. A sun diagram consisting of a cross-section, like a view of the inside of a ball that has been cut exactly in half, would thus include a circle in the center representing the core, and then successive rings around it from inside to out denoting the radiative zone, convective zone and photosphere.

The core of the sun is where everything that observers on Earth can measure as light and heat originates. This region extends outward to about a quarter of the way from the center of the sun. The temperature at the very center of the sun is estimated to be about, 15.5 million K to 15.7 million K, equal to about 28 million degrees Fahrenheit. This makes the surface temperature of about 5,780 K seem positively chilly. The heat inside the core is generated by a constant barrage of nuclear-fusion reactions, in which two molecules of hydrogen combine with sufficient force to cause them to join together into helium (in other words, the hydrogen molecules fuse.)

The radiative zone of the sun is so named because it is in this spherical shell – a region starting about one-fourth of the way from the center of the sun, where the core ends, and extending outward about three-quarters of the way to the sun's surface where it meets the convective zone – that the energy released from the fusion inside the core travels outward in all directions, or radiates. Surprisingly, it takes a very long time for radiating energy to travel across the thickness of the radiative region – in fact, several hundred thousand years! As unlikely as this probably sounds, in solar time, this is not very long at all, given that the sun is already 4.5 billion years in age and still going strong.

The convective zone takes up most of the outermost one-fourth of the sun's volume. At the beginning of this zone (that is, on the inside) the temperature is about 2,000,000 K and dropping. As a result, the plasma-like material forming the sun's interior is, believe it or not, too cool and opaque to allow heat and light to continue to travel toward the solar surface in the form of radiation. Instead, this energy is transmitted via convection, which is essentially the use of physical media to shuttle energy along instead of allowing it to ride solo. (Bubbles rising from the bottom of a pot of boiling water to the surface and releasing heat as they pop represent an example of convection.) In contrast to the long period of time it takes for energy to navigate the radiative zone, energy moves through the convection zone comparatively quickly.

The photosphere consists of a zone in which the sun's layers change from being completely opaque, thus blocking radiation, to being transparent. This means that light as well as heat can pass through unimpeded. The photosphere is therefore is the layer of the sun from which light visible to the unaided human eye is emitted. This layer is only 500 km thick, meaning that if the entire sun is likened to an onion, the photosphere represents the onion's skin. The temperature at the bottom of this region is hotter than it is at the sun's surface, though not dramatically so – about 7,500 K, a difference of less than 2,000 K.

What Are the Layers of the Sun?

As noted, the sun's core, radiative zone, convective zone and photosphere are considered regions, but each can also be classified as one of the layers of the sun, of which there are six in number. External to the photosphere is the sun's atmosphere, which includes two layers: the chromosphere and the corona.

The chromosphere extends about 2,000 to 10,000 km above the sun's surface (that is, the outermost part of the photosphere), depending on what source you consult. Curiously enough, the temperature somewhat predictably drops with increasing distance from the photosphere at first, but then begins to rise again, possibly owing to the effects of the sun's magnetic field.

The corona (Latin for "crown") extends above the chromosphere to a distance of several times the sun's radius and reaches temperatures as high as 2,000,000 K, similar to the interior of the convection zone. This solar layer is very tenuous, containing only about 10 atoms per cm3, and it is heavily criss-crossed by magnetic field lines. "Streamers" and plumes of gas form along these magnetic field lines and are blown outward by the solar wind, giving the sun its characteristic appearance of having tendrils of light when the main part of the sun is obscured.

What Are the Outer Parts of the Sun?

As noted, the outermost parts of the sun are the photosphere, which is part of the sun proper, and the chromosphere and the corona, which are part of the sun's atmosphere. Thus the sun may be pictured as having three inner parts (the core, the radiative zone and the convective zone) and three outer parts (the photosphere, the chromosphere and the corona).

A number of interesting events unfold at or just above the surface of the sun. One of these is sunspots, which form in the photosphere in relatively cool (4,000 K) areas. Another is solar flares, which are explosive events on the surface marked by very intense brightening of regions of the solar atmosphere in the form of x-rays, ultraviolet and visible light. These unfold over periods lasting for a few minutes, and then fade over a somewhat longer time frame of an hour or thereabouts.

References

About the Author

Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.

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