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Basic facts:

From the inside out, the structural parts of the Sun are:

Often, you might hear radiative zone and convection zone when referring to the solar interior. The outer 30 percent of the Sun transfers heat convectively (the hot gas moves up and the cool gas moves down), but in stars with different masses, often the core might be convective and the outer parts radiative, so in general it is better to use the bulleted terminology above.

The core of the Sun is where it all begins. The temperature and density are high enough that protons can fuse and form helium nuclei, losing mass and releasing energy in the process. All of the energy emitted by the Sun was generated in the core. The Sun uses the proton-proton chain reactions for energy generation, but more massive stars with higher central temperatures and densities can use the CNO cycle as well. The temperature at the very center the Sun is about 15 million Kelvin and the density is more than one hundred grams per cubic centimeter. The temperature and density are high enough to efficiently fuse hydrogen only in the innermost few hundred thousand kilometers of the Sun, which defines the boundary of the core. The current chemical composition (by mass) of the core is approximately 37.3 percent hydrogen, 60.8 percent helium, and 1.9 percent everything else. The core of the Sun is radiative rather than convective (see above). This means that once the hydrogen in the core is exhausted, it cannot bring in a fresh supply from the outer layers of the star via convective mixing.

The envelope is the region between the core and the surface of the Sun. In the Sun, the envelope is partly radiative and partly convective, with the inner boundary of the convection zone located at a radius of about five hundred thousand kilometers, or a temperature of about two million Kelvin. The composition of the envelope maintains nearly the same chemical abundance as it had when it formed, with roughly 70 percent hydrogen, 28 percent helium, and two percent everything else. This changes slightly over time, because helium and heavier elements tend to diffuse downwards because they are heavier.

Much of the remainder of the solar anatomy has been noded by others, so I will only discuss them briefly, with the suggestion that you read the existing nodes.

The photosphere is the "surface" of the Sun, in the sense that eventually the opacity of the gas becomes so great that we cannot see any deeper into it. The photosphere is where much of the visible activity on the Sun occurs: sunspots, faculae and plages, solar flares and solar prominences originate in the photosphere, though they may extend to higher levels.

The chromosphere forms a transition region between the cooler photosphere and the much, much hotter corona. The density of the solar atmosphere continues to decrease with height, but the temperature increases. This increase in temperature is probably caused by acoustic waves from the photosphere propagating outwards, eventually turning into shock waves which dissipate their energy in the chromosphere.

The corona is the outermost layer of the Sun and it consists of gas at millions of degrees Kelvin. Unlike the interior of the Sun at similar temperatures, the density is very low, so that nuclear reactions do not occur. When a solar eclipse occurs, the corona is the wispy white cloud that surrounds the Sun up to four or five solar diameters away. In visible light, the coronal light is actually light emitted by the photosphere, which is then scattered by free electrons in the corona. But the corona also emits light of its own in the extreme ultraviolet and X-ray bands of the spectrum, the latter usually emitted by highly-ionized metal atoms. The reason for the corona's high-temperature is not fully known, but it may be due to magnetic activity.

The heliosphere is the large bubble blown up by the Sun's solar wind. The boundary between the heliosphere and the Galactic interstellar medium is the heliopause.

Additionally, some information on solar activity can be found in the following nodes:

Sources: Allen's Astrophysical Quantities ed. A.N. Cox, and Solar Physics by P. Foukal.

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