Airbus Defence and Space

Our sun - the giver of life

The star at the centre of our planetary system

With a diameter of 1.4 million kilometres, the sun is around 110 times larger than the Earth. Its mass amounts to approximately two times 10 to the power of 27 (2x1027) tons or about 330,000 times Earth’s mass. Both of these measurements show the sun to be an average star.

The sun is mainly composed of the two light elements hydrogen (about 78%) and helium (about 20%). The sun’s surface is 11 880 times that of the Earth. Its surface temperature reaches some 6,000° Celsius. The nuclear fusion processes that take place inside the sun have the effect of converting 564 million tons of hydrogen into 560 million tons of helium every second. The radiation created, which thus reaches the Earth, is a process which is essential to life. Estimates indicate that, since its formation, the sun has consumed 3% of its hydrogen reserves.

Stars are bright balls of gas with hydrogen at their centre, the most basic and most common chemical element in the cosmos, being converted into helium. Inside the sun almost 600 million tons of hydrogen undergo fusion into helium every second, generating 4 million tons of energy. Despite these enormous quantities, the sun has lost just 3% of its original mass since it was formed.

Dark spots on the sun’s surface

The sun was long seen as a flawless celestial body. It was not until the 17th century, shortly after the discovery of the telescope, that various astronomers observed dark spots here and there on the sun’s surface. The Jesuit priest Christoph Scheiner, from Augsburg, used these sunspots to calculate that the duration of the sun’s rotation is almost four weeks.

In the 19th century, the chemist and amateur astronomer Samuel Heinrich Schwabe discovered that these spots manifest themselves over a cycle of approximately 11 years.

Magnetic arcs

Sunspots are only the visible part of far more substantial solar activity caused by disruptions in the sun’s magnetic field. Magnetic field lines escape the sun’s surface in the zone of the spots and trace what are known as magnetic arcs, which curve through the sun’s atmosphere. Due to the magnetic ‘rings’ beneath the sun’s surface, blocking the inner heat source, the spots themselves are some 1,500 degrees cooler than the ‘undisturbed’ solar surface, the temperature of which is around 5,500° C.

Solar flares

Further signs of the sun’s activity are solar flares, which occur when the magnetic arcs short-circuit with the sun’s atmosphere. These ‘energy strikes’ release, among other things, large quantities of x-ray radiation. Furthermore, ejection of matter from the sun’s surface intensifies during a sunspot maximum.

The corona

Before the discovery of the telescope, people could already occasionally observe the sun’s outer atmosphere, the corona. Its paler glow would normally be outshone by the glare of the sun, but when the moon blocks the sun’s brightness in a total solar eclipse, the corona appears as a brighter ring around a black sun. Astronomers have since established that the corona’s heat can in places reach up to 2 million degrees.

Solar wind

Finally, space probes on the way to Venus and Mars in the 1960s discovered the solar wind, a stream of electrically-charged particles which continuously flow from the sun outwards in all directions. The particles normally move at speeds of 350–400 kilometres per second, but when they are affected by eruptions on the sun’s surface and also occasional solar wind gusts, they can be sent hurtling through the solar system at speeds of over 700 kilometres per second.

Under the influence of this solar wind, the side of the Earth’s magnetic field facing the sun is heavily compressed, while on the opposite side a long geomagnetic trail is drawn out. When fast electrically-charged particles reach the Earth they can substantially alter the terrestrial magnetic field.

Protective shield

This magnetic field normally works like a shield against the sun’s high-energy particles, but during such disruptions the particles can become more powerful and penetrate this shield, travelling along the magnetic field lines and entering the upper atmosphere above the Earth’s polar regions. There, they light up the atoms and molecules and we observe the northern lights (aurora borealis).

Less friendly consequences

Within the polar light zone these light phenomena occur more or less regularly, but in times of increased solar activity they may occasionally also be observed in more central geographic latitudes. During this kind of increased solar activity, the geomagnetic storms which are released can also have quite different effects. They can pose a threat to the on-board electronics of satellites; they can discharge strong electrical currents into overhead power lines in the upper northerly latitudes, which subsequently short-circuit the transformer stations and cut power from dependent land areas; and electronically-controlled systems, such as railway signals can be compromised by such events. It is therefore important to have early warning of such solar disturbances affecting the Earth.

ScienceSolar systemSolar-Terrestrial