Structure of The Sun

Astrophysicists classify the Sun as a star of average size, temperature, and brightness—a typical dwarf star just past middle age. It has a power output of about 10^26 watts and is expected to continue producing energy at that rate for another 5 billion years. The Sun is said to have a diameter of 1.4 million kilometers, about 109 times the diameter of Earth, but this is a slightly misleading statement because the Sun has no true “surface.” There is nothing hard, or definite, about the solar disk that we see; in fact, the matter that makes up the apparent surface is so rarified that we would consider it to be a vacuum here on Earth. It is more accurate to think of the Sun’s boundary as extending far out into the solar system, well beyond Earth. In studying the structure of the Sun, solar physicists divide it into four domains: the interior, the surface atmospheres, the inner corona, and the outer corona.


The Interior
The Sun’s interior domain includes the core, the radiative layer, and the convective layer. The core is the source of the Sun’s energy, the site of thermonuclear fusion. At a temperature of about 15,000,000 K, matter is in the state known as a plasma: atomic nuclei (principally protons) and electrons moving at very high speeds. Under these conditions two protons can collide, overcome their electrical repulsion, and become cemented together by the strong nuclear force. This process is known as nuclear fusion, and it results in the formation of heavier elements as well as the release of energy in the form of gamma ray photons. The energy output of the Sun’s core is so large that it would shine about 10^13 times brighter than the solar surface if we could “see” it.

Structure of The Sun

Thermonuclear Fusion
The nuclear fusion, now occurring in the core of the Sun, turns hydrogen nuclei into helium nuclei. In fact, that is how the elements heavier than hydrogen are made; the thermonuclear fusion at the core of stars can produce the first 26 elements, up to iron. The Sun, because of its relatively small mass, will go through only the first two stages of fusion, the hydrogen-helium stage and the helium-carbon stage.


Hydrogen-helium fusion can occur in more than one way, but in any case the temperature must be in the vicinity of 15 million K so that two positively charged particles will be moving fast enough to overcome their electrical repulsion when they collide. The density must be large, and the immense solar gravity compresses the gas so that it is ten times as dense as gold at the center of the Sun. If the two particles can get close enough together, the very short-range strong nuclear force will take effect and fuse them together. The most common fusion reaction in the Sun is shown in.

The proton-proton fusion reaction which occurs in the core of the sun at a temperature of about

15,000,000 K

The Surface Atmospheres
The solar surface atmospheres are composed of the photosphere and the chromosphere. The photosphere is the part of the Sun that we see with our eyes—it produces most of the visible (white) light. Bubbles of hotter material well up from within the Sun, dividing the surface of the photosphere into bright granules that expand and fade in several minutes, only to be replaced by the next upwelling. The photosphere is one of the coolest layers of the Sun; its temperature is only to be replaced by the next upwelling. The photosphere is one of the coolest layers of the Sun; its temperature isonly to be replaced by the next upwelling. The photosphere is one of the coolest layers of the Sun; its temperature is only to be replaced by the next upwelling. The photosphere is one of the coolest layers of the Sun; its temperature is about 6,000 K

These periods can easily be determined by watching sunspots over several days

Photos

of the Sun on four consecutive

days taken in

H light. Features can

be seen to move as the

Sun rotates.

The Inner Corona
The inner corona is the wispy halo, extending more than a million kilometers out into space, that can be seen when the brilliant disk of the Sun is blocked by the Moon during a total eclipse. The cause of the high temperature of the corona, about 2,000,000 K, is not well understood. The corona is a large source of x-rays which do not penetrate Earth’s atmosphere. With instruments on satellites we can look at the corona in x-ray wavelengths and see many details that do not appear in visible light. From this vantage point it is clear that magnetic arches dominate the structure of the corona. Large and small magnetic active regions glow brightly at x-ray wavelengths, while open magnetic field* structures appear as gaping coronal holes. The coronal material is generally confined by closed magnetic field structures, anchored at both ends, but the open field structure of coronal holes allows the corona to escape freely to form fast, low density streams in the solar wind. This material travels outward and causes disturbances in Earth’s magnetic field. Because of their effects on Earth, we would like to be able to predict when and where coronal holes will form, but as yet we cannot do this.

Total solar eclipse of

July 11, 1991 as seen from Baja

California

The Outer Corona
The outer corona extends to Earth and beyond. Its existence is not immediately obvious, since it cannot be seen directly; astrophysicists did not become aware of it until the 1950’s. Watching the behavior of comets, Ludwig Biermann realized in the early 1950’s that the solar corona must be expanding outward. By 1958, Eugene Parker concluded from theoretical models that particles streaming off the Sun were necessary to maintain the dynamic equilibrium of the corona. Parker’s mathematical prediction that particles streamed from the Sun at speeds of several hundred kilometers per second was verified in the early 1960s when satellites detected coronal outflow. This outflow came to be called the solar wind and its speed was accurately measured in 1962 by the Mariner 2 spacecraft bound for Venus. As Parker had predicted, this speed averaged about 400 km/s.