The Sun

The Sun is a fairly typical star, called a yellow dwarf. It has a diameter of 1,400,000 km, 109 times the diameter of the Earth. It fills a sphere big enough to hold 1,300,000 Earths and it has the same mass as 332,946 Earths. It is made of very hot gas, mostly hydrogen with a little helium. The gas is so hot that it forms a special state of matter called a plasma. In its centre, hydrogen atoms collide with each other and fuse to form helium atoms, releasing huge amounts of energy as they do this. This process, called nuclear fusion, has kept the Sun hot for 4,566,000,000 years. Physicists would like to recreate nuclear fusion on Earth to make power stations that could provide almost limitless, cheap energy for everyone.

The temperature at the centre of the Sun is 15,600,000°C. This drops to 5,507°C at the photosphere, the visible surface of the Sun. There is a diffuse region of gas spreading into space around the Sun, called the corona. You can see the corona in pictures showing total eclipses of the Sun. One of the big mysteries of solar science is how the corona can have a temperature of more than 2,000,000°C. Scientists would like to know where the extra heat comes from to raise the temperature so much.

The Sun rotates on its own axis but not at the same speed all over. Because it is made of gas, it can rotate faster at the equator (one rotation every 26.8 days) than nearer the poles (30.8 days for one rotation 60° from the equator). Its surface is covered by darker spots called sunspots. These are actually cooler regions in the photosphere and their motion across the Sun allows us to measure its rotation. The number of sunspots recorded increases and decreases in a cycle lasting approximately 11 years. When the sunspot number is near maximum, other activity on the Sun increases too. In particular the number an size of solar flares increases. These are huge explosions just above the photosphere (in the chromosphere) that release X-rays and gamma-rays. A typical flare is equivalent to detonating 2,500,000 nuclear bombs.

At solar maximum, there is also an increase in the number and strength of coronal mass ejections, or CMEs. In a CME, material is blown off from the Sun and sometimes it makes its way to the Earth. We see the effect of this material when we see the aurora borealis, in the northern hemisphere, and the aurora australis, in the southern hemisphere. These Northern and Southern Lights are caused by charged particles from the Sun, funnelling in along the Earth's magnetic field to gather at the north and south poles where they interact with the atmosphere and cause it to give off light. Big flares and CMEs can damage satellites and endanger astronauts. They can also cause big currents in the wiring systems that provide electricity to our towns and cities, burning them out or destroying power stations and switching stations. This makes the study of space weather very important.

Observation Warning