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Image of the Sun, obtained from this wikipedia page here on 4 May 2007.

The Sun is the star closest to us, and which, for obvious reasons, is most relevant to us. Our Earth is just one of eight planets in orbit round the Sun, together with numerous smaller bodies, e.g. minor planets, dwarf planets, comets, etc. The Earth takes a year to go round the Sun, and the Earth's rotation axis is tilted so the equator is inclined about 23 and a half degrees from the plane of the Earth's orbit. This gives rise to the seasons.

The Sun is essentially a huge ball of gas, held together by its own gravity. Its mean diameter (diameter of the photosphere, which is where most of the visible light emanates), is about 1.392 x 10⁶ Km, which is rather more than 100 times the diameter of the Earth. The gas is mostly hydrogen, with a certain amount of helium, and much smaller amounts of other elements. The surface gravity (gravity at the surface of the photosphere) is almost 28g. The photospheric surface temperature is about 5800 K, but the temperature at the core is believed to be between 10 and 20 million K. The Sun rotates on its axis at various rates depending on latitude (as do the giant planets), the equator rotating in about 25 days, the period increasing to over 30 days at high latitudes. Above the photosphere the Sun has an extensive atmosphere which is largely transparent, and extends far into space. Part of this atmosphere, the Corona, is visible at times of total solar eclipse.

The Sun's energy is produced by thermonuclear fusion reactions in the hot core, which convert hydrogen to helium, with consequent release of energy. This energy is in the form of X-ray and gamma-ray photons. Most of these photons are absorbed after travelling through the plasma for a few millimeters. The plamsa which absorbs these photons will in turn emit photons generally of a different (most often longer) wavelength, in an arbitary direction - this is sometimes referred to as "random walk". Because the direction of re-emission of photons is arbitary, rather than always directly upwards toward the surface of the Sun, it takes a very long time for energy in the form of radiation, produced at the core, effectively to work its way up to the surface and radiate out into space. Estimates for the time taken vary from thousands of years, to tens of millions of years. By the time the energy has reached the surface of the Sun, the spectrum of the radiation produced is in the form of much longer wavelenths than when it originated in the core, and the surface of the Sun may emit millions of visible-light photons for every high-energy gamma-ray photon generated in the core. (The solar reactions also produce neutrinos, which as they seldom interact with matter, progress straight from the core to the surface, and out of the sun, in a matter of seconds.)

The principal method of propogation of energy outward from the core of the Sun is by radiation and re-emission, from the centre of the Sun, to about 70 percent of the solar radius. The outer shell of the Sun, extending from the surface to about 30 percent of the way in, is convective, which means that hot gas at the bottom of the convective layer rises up, and the gas at the top, which will have cooled, sinks down again. (The third possible mode of energy transfer, conduction, is insignificant here, in view of the very low conductivity of gas and plasma.)

The convection of plasma in the outer layer of the Sun causes movements on the surface of the photosphere, which are visible using appropriate equipment, as granulation. This active movement is also responsible for a whole range of solar phenoma, including the generation of magnetic fields, which are largely responsible for sunspots, and associated faculae and flares. The extent of sunspot generation and other activity on the Sun's surface varies, generally over a cycle averaging about 11 years.

According to current theories of star formation, stars originate as a cloud of gas collapses under its own gravity, heating up in the process. Eventually, hydrogen burning starts in the core, by thermonuclear fusion reactions. At some stage after this starts, the star will eventually stabilise and join the what is known as the Main Sequence. It then stays on the Main Sequence, burning hydrogen in the core, to form helium, until core hydrogen starts to run low, then leaves the Main Sequence and undergoes dramatic changes. These changes include structural reorganisation, and changes in stellar diameter and surface temperature. The Main-Sequence lifetime of a star varies, depending on its initial mass. A high mass star will burn hydrogen at a much quicker rate than a low-mass star, and will thus be very-much more luminous, and will exhaust its core hydrogen supply after a shorter time. Thus the more massive and more luminous a star, the shorter the time spent on the Main Sequence.

Massive stars have a higher surface temperature than lower mass ones. The spectral classes are signified by a letter, and subdivided by a number from 0 to 9. For historical reasons, the letters occur in a curious order - O being the hottest, then B, A, F, G, K, M. This list is frequently extended with classes R, N, and S - and sometimes P and Q¹. Before stars were understood as well as they are now, it was decided to classify the stars by strength of the hydrogen lines in their spectrum - A-type stars often tend to have stronger Balmer H lines than either hotter or cooler stars. The Sun is a star of type G2. In the late 19^th and early 20^th centuries, much of the "basic" work at Harvard College Observatory was done by women. One of these women, Miss Henrietta Leavitt, a graduate at Radcliffe College, is famous for discovering the relationship between the period of a Cepheid variable and its instrinsic luminosity.

Actually, there are two methods of hydrogen burning, both of which convert 4 protons into a helium nucleus. Energy production in the Sun and other comparatively low-mass stars is dominated by the proton-proton reaction. In the cores of massive stars, a different reaction called the C-N, or Carbon-Nitrogen cycle kicks in, and plays a greater role. In this reaction, the conversion of 4 protons to a helium nucleus follows a different route, and utilises the presence of carbon atoms, which act as a catalyst. The C-N reaction is more temperature dependant than the proton-proton reaction, and can only play a significant role in the hot cores of massive stars.

The stellar structure, and other properties, vary substantially with the star's mass and temperature. With stars which are less massive than the Sun, the convective envelope extends proportionately deeper into the stellar interior - indeed, a star of less than around 0.3 to 0.4 solar masses may be convective throughout, the convective envelope effectively extending all the way to the star's centre.

As the mass of a star increases above that of the Sun, the outer convective layer becomes proportionately more shallow. Probably somewhere in the region of 1.2 to 1.5 solar masses, the convective envelope essentially disappears, and the radiative part of the star then extends all the way to the surface.

Massive stars have a convective zone in the central region, because of the steep temperature gradient brought about by the high rate of energy production due to the Carbon-Nitrogen cycle.

The exact configuration of a star varies somewhat with age, and will also vary with the concentration of elements heavier than hydrogen.

The Schwarzschild criterion, discovered by Karl Schwarzschild, determines whether convection will be driven in any particular zone within a star. Essentially convection is more likely to arise where there is a steep temperature gradient, high opacity (which makes it more difficult for energy to escape by radiation), various ionisation states, and phase changes.

Depletion of hydrogen in the star's core will eventually cause the star to leave the Main Sequence, and dramatically restructure itself. Convection in the central region of a star, e.g. massive stars with convective cores, or very-low-mass stars which are convective throughout, can result in mixing of helium "ashes" with fresh hydrogen from outside the combustion zone, with possible extension of the star's Main-Sequence lifetime beyond what it might otherwise be. The red dwarf stars at the bottom of the mass range probably have a Main-Sequence lifetime longer than the current age of the Universe anyway.

Since sun-spots and solar activity, and equivalent activity on some other stars, is driven by movement of material in the convective envelope, and also by the star's rotation, it follows that stellar activity of this kind is likely to be most significant in stars with significant convective envelopes. Low-mass red dwarf stars, whose convective envelopes extend deep into the star's interior or even all the way to the centre, often experience spots and flares which cover a major part of the star's surface, and significantly alter the star's total luminosity on an irregular basis. Stars much more massive than the Sun, whose convective envelopes are either shallow or non-existant, are believed to be much quieter in this respect - thus A-type stars such as Vega and Sirius are probably not all that "spotty". Hot, massive O-type stars, however, experience extremely prolific stellar winds, and may lose a significant portion of their mass through the stellar wind.

The strong magnetic fields associated by activity in the outer parts of sun-like stars, may cause the star to lose angular momentum, thus slowing down the star's rotation. It has been noted that whereas the Sun takes 25 to 35 days to rotate on its axix (depending on latitude), some A-type stars rotate much faster. Vega, for example, rotates once in around 12.5 hours.

Fast rotation can result in significant polar flattening, just as it does in some of the giant planets in the solar system, e.g. Saturn. Stars "distorted" in this fashion may be significantly hotter at the poles than in the low lattitudes.

Sensitive methods such as interferometry, which can measure a star's apparent angular diameter, have found that the A-type star Altair, which is believed to rotate once in less than 12 hours, does have a diameter which varies with direction, indicative of polar flattening. Polar flattening on Vega has not been directly observed, and it is believed we are looking at Vega almost pole-on (to within about 5 degrees). If we were looking at Vega at the same distance, from above its equator, then the star would appear noticably dimmer.

In summary, then:

The Sun is a huge ball of gas or plasma (mostly hydrogen) held together by its gravity, and powered by thermonuclear fusion reactions in the core central zone, which convert hydrogen to helium, with release of energy in the form of gamma-ray or X-ray photons.

These photons typically travel a few millimetres before being absorbed by plasma, which then re-emits photons (usually of slightly less energy). By this process energy is gradually transfered outward for about 70 percent of the Sun's radius. This lower 70 percent of the Sun's radius is known as the radiative zone.

The outer 30 percent of the Sun's radius consists of gas which is transfering energy upwards by convection. This is known as the convection zone.

At the visible surface of the Sun the effective temperature is about 5800 K, and the Sun radiates energy into space largely as visible light, with significant amounts of infrared and ultraviolet light.

The Sun is said to be a Main Sequence star, since it is still being powered by hydrogen burning in the core central region.

Massive stars are much more luminous than low-mass stars, and burn their fuel at a faster rate; therefore massive stars have a shorter life span, and leave the Main Sequence much earlier than low-mass stars.

Stars of solar mass or less generally have convective outer envelopes, surrounding an inner radiative zone. Stars significantly more massive than the Sun have a convective inner central zone, surrounded by a radiative outer envelope. (Extremely-low-mass star may be convective throughout.)

Movement of gas in the Sun's outer convection zone, together with the Sun's rotation, brings about solar activity, including sun-spots, magnetic fields, and flares.

Stellar activity in stars of solar mass and less, will often have slowed down the rotation of the star due to reaction between the stellar magnetic fields and material outside the star.

More massive stars, with no convective envelope, will have little or no surface activity of this kind, and the rotation is often much faster, since it has not been subjected to "magnetic braking".

IMPORTANT NOTE: Looking at the Sun can be VERY DANGEROUS, unless adequate precautions are taken. Serious eye damage could result. Glancing at the Sun through a telescope or binoculars, even for a moment, can cause instant damage, or even permanent blindness.

Looking at the Sun through certain types of filter might cause injury, even if the Sun appears dim through it - the material might not adequately filter out the invisible infra-red or ultra-violet rays emitted by the Sun.

Probably the safest way to observe the Sun is by means of projection - project an image of the sun onto a card or similar object (be careful not to set fire to anything). I have observed the Sun this way, and have easily been able to see Sun-spots.

¹ Some English-speaking astronomers thought up the following phrase as a way of remembering the order - "Oh, Be A Fine Girl, Kiss Me Right Now. Smack!"