The Sun

Introduction

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, with just a single proton in its nucleus. The extreme temperatures and densities at the centre of the Sun (1.5 million°C and around 200 billion atmospheres), mean that hydrogen nuclei are moving very fast and are forced very close together. This increases the incidence of collisions between them that will occasionally result in a complex chain of reactions that essentially converts four hydrogen nuclei into a single helium nucleus. Read about the proton–proton chain reaction for more details on the actual chemical processes involved.

One helium nucleus has 99.3% of the mass of four hydrogen nuclei. This excess 0.7% of hydrogen mass compared with helium mass is converted into some Neutrinos and a photon of electromagnetic energy, in this case a gamma ray. In total, the Sun converts around 600 million tons of hydrogen into 596 million tons of helium every second. The extra 4 million tons of matter is converted into radiative energy.

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 around 63.3 million W/m² of electromagnetic radiation from its apparent surface. 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 actually reaches the outer atmosphere of the Earth. The rest is lost to space.

The diameter of the Sun is about 1.4 million kilometres, 109 times that of the Earth. Its volume is big enough to hold over 1 million Earths. The average distance from the Earth to the Sun is 149.6 million kilometres (92.96 million miles), which takes solar radiation around 8 minutes and 20 seconds to travel. The average amount of solar radiation that actually hits the outer atmosphere of the Earth is around 1.367 kW/m².

Composition and Structure

The Sun is a giant ball of mostly ionized gasses. By carefully recording the radiative emissions of the Sun, it is possible to observe characteristic spectral lines within the radiated spectrum that can be used to identify the elements both generating the radiation and present in the outer gasses surrounding the Sun absorbing some of the radiation. As described in more detail below, we can only observe the Sun to a certain depth, after which it becomes effectively opaque. However, from observations of the visible outer layers, the relative composition of major elements making up the Sun is given in Table 1 below.

Core and Radiative Zone

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 typically take around 170,000 years to pass though this layer, bouncing around, colliding and being absorbed and re-emitted millions of times during the process.

Convection Zone

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 conjecture on this.

Photosphere

Above the convection zone is the photosphere. It is called this because it marks the point where the Sun appears to become opaque and creates what looks like a surface. This is actually just 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. The temperature of gases within the photosphere is around 6,000°C.

Chromosphere

Above the photosphere lies the chromosphere which is about 2000km thick and comprised mostly of very hot but low density hydrogen gas. Because of the extreme relative brightness of the photosphere, the chromosphere is usually only visible during a solar eclipse. Temperatures within the chromosphere increase from around 6000°C near the photosphere to more that 20,000°C

Corona

The outermost layer of the Sun is called the corona, a layer of very low density gas that gradually dissipates with distance from the chromosphere. This is the area within which solar flares and other forms of mass ejection and prominences occur. Why this layer is so incredibly hot, rising to over 2 million°C from only 20,000°C in the chromosphere immediately below, is not yet fully explained. 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 nuclear fusion takes the form of gamma rays, at the highest end of the radiation spectrum. However from outside, the Sun gives off electromagnetic radiation over a wide range of frequencies, from cosmic rays right down to infra-red - not just gamma radiation.

This is because, 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 others may have been almost unaffected (staying as gamma rays). For more detailed information, see the electromagnetic spectrum and solar radiation topics.

The average amount of solar radiation that hits the outer atmosphere of the Earth is called the Solar Constant and equates to 1.365 - 1.369 kW/m² aggregated across the entire spectrum, though the majority of this energy is in the visible light spectrum. The Earth’s magnetic field and gasses within the atmosphere deflect or absorb virtually all of the very high energy ionizing radiation, such as x-rays and gamma-rays, whilst allowing through most of the infra-red, visible and ultra-violet radiation.

The amount of solar radiation that is actually received on the surface of the Earth is dependent on the amount of atmosphere it had to pass through, which is based on the relative altitude of the Sun at any given date, time and location. However, if the Sun is directly overhead under a clear sky, the average incident solar radiation is about 1 kW/m².

Basic Solar Characteristics

Useful References

• 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
  https://fusedweb.llnl.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/

• Nine Planets Website - The Sun
  http://nineplanets.org/sol.html

• Solar and Heliospheric Observatory (SOHO)
  http://sohowww.nascom.nasa.gov/

• NASA - The Solar Interior
  http://solarscience.msfc.nasa.gov/interior.shtml