THE SUN
The Sun is the centre of the solar system. Its powerful gravity holds everything in the solar system in orbit around it.
The Sun is a star, no different from the many stars we see at night in the sky. It looks different simply because it is closer. It is 150 million Km from the Earth and has a diameter of 1,392,000 Km. Over 1,300,000 Earths would fit inside. It has a surface gravity 28 times that of the Earth. The Sun is made up of gas and doesn't have a solid surface.
Do not attempt to look at the Sun with the naked eye or through a telescope without the correct eye protection. Sun glasses will not protect you. The Sun is very powerful and if you attempt to look at it through a telescope without the correct protection you could be blinded in seconds.
-Sun
with sunspots - Credit SOHO
At the centre is the core where the sun's energy is generated. Hydrogen atoms under enormous pressure are forced together to form helium - a nuclear reaction called the Proton-Proton reaction that produces energy. A hydrogen bomb operates on the same principle, in effect every star is a huge continuously exploding hydrogen bomb. Such is the density of the sun's material that the heat energy produced at the core takes millions of years to reach the surface. More about the Proton-Proton reaction here >>>
Next is the radiation zone where the energy travels upward as heat and light radiation. Then is the convection zone where energy is carried up by the movement of the solar material, just like water boiling in a kettle. Finally, the surface at a temperature of 6000 degrees. The surface and the atmosphere above it has many features like sunspots, flares, prominences etc. most of which are the result of the sun's magnetic field.
Because the Sun is made up of gas it doesn't have a solid surface. The part of the Sun that emits light and heat is called the photosphere, it is this that we see as the surface. The photosphere is at a temperature of 6000 degrees C and at those high temperatures the electrons around the nucleus of the atoms are stripped away from the atoms leaving the nucleus with a positive charge, the negatively charged electrons wander freely between the nuclei. This state of matter is called a plasma and interacts strongly with the Sun's magnetic field. It is this interaction that produces most of the phenomena that we observe on the surface of the Sun.
Another result of the gaseous nature of the Sun is differential rotation where different latitudes of the Sun rotate at different rates. At its equator the Sun rotates once every 25 days but near the poles it can take as long as 36 days. So different latitudes of the Sun are moving at different speeds. Because the surface of the Sun is a plasma and interacts strongly with the Sun's magnetic field, the Sun's magnetic field is locked into the Sun's surface. As the surface is not rotating as a solid body the magnetic field becomes stretched and distorted as the Sun rotates. This creates kinks in the field. These kinks will protrude out of the Sun's surface and extend into the solar atmosphere. The magnetic field that penetrates the surface has the effect of slowing down the movement of the atoms in that part of the surface. This cools that part of the surface by about 2000 degrees and creates a dark area we call a sunspot at a temperature of about 4000 degrees. This is still very hot but cooler than the surrounding surface.
The number of sunspots varies with a maximum occurring every 11 years. At the start of a cycle there may be no sunspots for some months then one or two will appear at high latitudes in the northern and southern hemispheres. As the cycle progresses the number of sunspots increases and moves closer to the equator.
A
large prominence hangs above the Sun
. Credit- SOHO (ESA & NASA)
Above the photosphere is the first layer of the Sun's atmosphere known as the chromosphere about 2500 km thick. For reasons not fully understood the temperature of the chromosphere is higher than photosphere, the layer below it, reaching some 10,000 degrees. Faculae are bright clouds of hot Hydrogen that form in the chromosphere above sunspots. More spectacular are solar flares which can release enormous energy in a short time. They are caused by the magnetic field. As the field becomes stretched and distorted by differential rotation, it reaches breaking point and like an over-stretched elastic band it will snap. The energy released creates a flare.
Above the chromosphere is the mysterious corona. It is extremely hot, in fact a lot hotter than it should be, reaching temperatures of over one million degrees. As with the chromosphere the process by which the corona reaches such high temperatures is not yet understood but almost certainly involves the magnetic field. It is not normally visible but during a solar eclipse can be seen as a faint pearly glow surrounding the Sun.
Prominences are large features that form above the photosphere and often extend into the upper parts of the corona. They take the form of large loops of hot gas traveling at speeds in excess of 1000 km per second and reaching over a million km above the surface. They follow the magnetic field lines that arch up from the sunspot zones produced by the distortion of the Sun's magnetic field resulting from the differential rotation of the Sun. They can hang in the Sun's atmosphere for days.
The corona is not even, there are large areas called coronal holes through which matter to flows out from the Sun, this is called the solar wind. It consists of sub-atomic particles such as electrons and protons with some heavier charged atoms called ions which travel away from the Sun with speeds varying between 450 and 750 kilometres per second. The solar wind fills the solar system reaching far beyond the orbit of Pluto. It is the solar wind that allows the Sun to influence the Earth.
© Matthew Wallace 2006