The Sun is our closest star. It is classified as G2, which means that it is very ordinary. It has a surface temperature of about 6000K and an apparent magnitude of Ė26.7, but its absolute magnitude is only 4.83. The core of the Sun is at a temperature of about 15.6 million K, and is at a density of around 148000 kg/m3.
The Sun displays differential rotation, which means that it rotates faster at the equator than near the poles. The rotation period at the equator is about 25.4 days, while near the poles it is about 36 days. This is possible because the Sun is made of gas and has no solid surface.
The Sun is made up of 71% hydrogen, 27% helium and the other 2% is heavier elements. This is changing all the time as hydrogen is converted into helium in the nuclear reactions in the core. It has an age of 4.6 billion years and is approximately half way through its Main Sequence lifetime.
Surrounding the core is a radiative layer, which transports the energy away from the core. This then becomes convective at about 2/3 of the solar radius, and carries the energy to the surface. The outer layer of the Sun is called the photosphere. This is where the photons undergo their last interaction with the Sun, so this is the layer we see. Most of the radiation from the Sun is in the visible band.
Surrounding the photosphere is a very tenuous gas called the corona. This is only visible during a solar eclipse, but is at a temperature of around 1 million K. The reason it is so hot is not really understood, but is believed to be due to the Sunís very strong magnetic field.
The magnetic fields are also seen in other ways. Sunspots are dark patches on the surface of the photosphere, and are slightly cooler regions with high magnetic fields. The number of sunspots on the surface of the Sun varies on a 22 year cycle. We are currently just past solar maximum, when the number of sunspots is greatest. There may also be other, long-term cycles taking place: between 1645 and 1715 there were hardly any sunspots seen on the surface of the Sun. This coincided with a period of very cold weather on Earth, known as a mini ice age. The period was called the Maunder Minimum.
A further effect of the magnetic field is the solar prominences, which are big loops of hot gas coming from the Sunís surface. They seem to exit and re-enter the surface at sunspots, indicating that the gas follows a magnetic field line.
As well as a large amount of radiation, the Sun also gives out a very weak flow of particles, mostly protons and electrons, called the Solar Wind. This interacts with the Earthís magnetic field to produce the Aurora Borealis (Northern Lights) and Aurora Australis (Southern Lights).
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Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001