The Sun

The Sun is a massive atomic furnace that works by converting hydrogen into helium. Hydrogen is the lightest and most abundant element in the universe. It has one proton in its nucleus. Temperatures and densities in the centre of the Sun are so great, 1.5 million°C and around 200 billion atmospheres, that colliding hydrogen nuclei sometimes fuse into helium nuclei. The creation of each helium nucleus requires four hydrogen nuclei.

One helium nucleus has 99.3% of the weight of four hydrogen nuclei. This excess 0.7% of hydrogen mass compared with helium mass is converted into energy. In perspective, the Sun converts 600 million tons of hydrogen into 596 million tons of helium every second. The extra 4 million tons is converted into energy - in this case radiation in the form of gamma rays.

You can imagine the enormity of the energy generated when you realise that, given Albert Einstein's famous equation E=MC², the 4 million ton differential is multiplied by the speed of light, squared. This energy is so great that the Sun gives off 6200 watts of light from every square centimetre of its surface. Compare this to a 60-100 Watt domestic light globe. As far as we know, the Sun has been giving off this light steadily for the last 4.5 billion years, and will continue to do so for several billion more. Only half a billionth of this energy reaches the Earth. The rest is lost in space, so to speak.

The average distance from the Earth to the Sun is 150 million kilometres, which takes sunlight around 8.5 minutes to travel. The diameter of the Sun is about 1.4 million km, 109 times that of the Earth. Its volume is big enough to hold over 1 million Earths.

Composition and Structure

The Sun is a giant ball of hot gases. By weight, it is 70% hydrogen, 28% helium, 0.5% carbon, 0.5% nitrogen, 0.5% oxygen, and 0.5% other elements. Table 1 gives a breakdown of the major elements present within the Sun by volume.

At the heart of the Sun lies its core, a mass of superheated hydrogen and helium, at 1.5 million°C and at a pressure of around 200 billion atmospheres. This is where fusion reactions take place. Around the core, fusion reactions fade away as they enter what is known as the radiative zone. Though not as dense as the core, it is still so dense that photons take around 170,000 years to pass though this layer, bouncing around and colliding, being absorbed and re-emitted millions of times during the process.

Outside this layer is the convection zone, occupying the outer third of the Sun's body. This is a layer of less density that constantly churns and swells, driven by the enormous heat being generated below. This forms a plasma which surges upwards, cools slightly and then sinks back beneath the hotter gasses that continue to surge up from below. This volatile plasma layer actually spins independently of the layers below, faster at the equator and slower at the poles. Photons that have finally passed though the radiative zone are quickly carried through the convection layer zone by the swells and surges, taking just less than a month.

Between this moving gas and the more stable radiative zone is a small transition layer called the tachocline. It is thought that this small layer of turbulent charged gas may be the source of the Sun's magnetic field, however, there is still much conjecture on this.

Above the convection zone is the photosphere, or surface of the Sun. This is simply the top of the convection zone, where the plasma becomes thin enough for the majority of photons to finally escape instead of continually colliding with other particles. At this point the plumes of surging plasma can be seen, much like erupting bubbles on the surface of a heated mud pool. At this point, the temperature of gases is around 6,000°C.

The outermost layer of the Sun is the corona, a layer of thin gas that gradually dissipates with distance from the photosphere. This is the area within which solar flares and other forms of mass ejection and prominences occur. Why this layer is so incredibly hot, over 2 million°C compared with only 6,000°C immediately below, is as yet unknown. For a more in-depth look at the Sun's layers, check out the From Core to Corona web site.

Radiant Emissions

Given the temperatures at the core of the Sun, the radiation produced during fusion takes the form of gamma rays, at the highest end of the radiation spectrum. However, from outside, the Sun gives off an enormous amount of radiation over a wide range of frequencies, from cosmic rays right down to infra-red, not just gamma radiation.

As the gamma radiation passes through the radiative and convection zones, individual photons lose energy as they fight their way out constantly colliding with and being temporarily absorbed by other particles. When they finally emerge, some may have lost almost all their energy (forming the infra-red component) whilst other may have been almost unaffected (becoming cosmic rays). For more detailed information, see the electromagnetic spectrum and solar radiation topics.

Basic Characteristics

Characteristic	Value
Mass:	1.989E30 kilograms
Volume:	1.4122E18 cubic kilometres
Radius:	695,500 kilometres
Average Density:	1,409 kilograms per cubic meters
Surface Gravity:	274 meters per second squared
Escape Velocity:	617.7 kilometres per second
Orbital Period:	(of our galaxy) About 220 million years
Equator to Ecliptic Inclination:	7 degrees, 15 minutes
Rotational Period:	26.8 to 35 days (equator to poles)

Useful References

Our Tortured Star:

Irion, Robert., New Scientist, No 2184, May 1999, pp44-48.
http://www.newscientist.com/

A Look Inside Our Nearest Star!:

http://solar.physics.montana.edu/YPOP/Spotlight/SunInfo/Structure.html

From Core to Corona - Fusion Energy Educational Web Site:

http://fusedweb.pppl.gov/CPEP/Chart_Pages/5.Plasmas/SunLayers.html

Stanford Solar Center:

http://solar-center.stanford.edu/

Living Reviews in Solar Physics:

http://solarphysics.livingreviews.org/

Earth & Sun Systems Lab - High Altitude Observatory:

http://www.essl.ucar.edu/research/research.php
http://www.hao.ucar.edu/research/

Nine Planets Website - The Sun:

http://seds.lpl.arizona.edu/nineplanets/nineplanets/sol.html

Solar and Heliospheric Observatory (SOHO):

http://sohowww.nascom.nasa.gov/

NASA - Solar System Information:

http://liftoff.msfc.nasa.gov/toc.asp