The Sun in our Solar System

The Sun in our Solar System – One that we take for granted.

The sun is the centre of our solar system and is possibly the body from which Earth and all the other planets originated. It is the only important source of light and heat for the Earth, and it controls all forms of life. It is nevertheless, a comparatively unimportant star situated in the Milky Way and about 27,000 light years distant from the centre of the Galaxy (1 light year is 5.88 million million miles). The Sun consists of a sphere of gas which has a visible diameter of 864,000 miles, heated by thermonuclear reactions in its interior. It is supposed that the temperature near the centre of the Sun may be 14,000,000 degrees K, and on the surface the temperature is about 6,000 degrees k. (In astronomical work temperature is measured on the Kelvin Scale, which begins at absolute zero. A temperature in degrees Kelvin is equal to the temperature in Centigrade plus 273 degrees.)

The Structure of the Sun’s atmosphere

The main body of the Sun is opaque and has a well defined visible surface or ‘skin’, known as the photosphere. This is the source of practically all the light and heat radiated by the body. Extending outwards from the photosphere for a distance of about 6,000 miles is the chromosphere. This is a layer of transparent gases which are in continuous movement; it looks rather like a prairie fire. Above the chromosphere and extending to enormous distances is another transparent gaseous zone called the corona. The Earth itself lies in the solar corona. Neither the chromosphere nor the corona is normally visible. They can be seen with the aid of certain kinds of optical equipment attached to astronomical telescopes. The only time that they can be seen without these aids is during a total solar eclipse, when the body of the moon covers the photosphere and therefore obscures the brighter light radiated from it. By comparison the light radiated from the outer parts is equivalent to the brightness of moonlight.

The photosphere, chromosphere, and corona constitute the solar atmosphere and are the only parts of the Sun which can be observed. They all have a similar chemical composition and it seems that the composition of the Sun’s interior differs little from that of its atmosphere. Although the three zones are permanent features, the boundaries between them are never clearly defined, and they are always changing internally. The most important changes are collectively called solar activity and include sunspots (dark spots) and faculae (bright spots) in the photosphere, flares, flocculi, and spicules in the chromosphere, and the prominences in the corona.

The Chemical Composition of the Sun

It is estimated that the Sun is made up of 80 per cent hydrogen, 18 per cent helium, 2 percent other elements. The thermonuclear reaction of hydrogen to form helium causes the intense heat of the Sun.

One of the most important methods of chemical analysis is known as spectroscopy. This involves the examination of the light emitted by an incandescent (glowing) substance. This is done by an instrument called the spectroscope which is capable of splitting this light according to its different wavelengths. Since the Sun is composed of gases of very high temperatures, its composition can be studies in this way, and indeed the science of spectroscopy was largely founded upon early attempts to determine the nature of the Sun. If sunlight is allowed to shine through a narrow slit and prism and then through a lens system, this ‘white’ light is separated into a series of colours which range, in the visible spectrum, from red to violet, which after extensively studied and analysed gives us the conclusion that The solar spectrum of the Sun shows that about 80 percent is composed of hydrogen and 18 percent is helium. Of the other 92 natural elements, 64 have been identified as present in the Sun. The remainder are undoubtedly there also, but have not been identified as yet. However apart from hydrogen and helium, all remaining elements make up only about 2 percent of the Sun.

The Size of the Sun

From the surface of the Earth, the disc of the Sun measures approximately 32 minutes (60 mins equals 1 degree) of the arc across its diameter. The angle varies slightly according to the distance between the Sun and Earth because it is greater at perihelion than at aphelion. It is also possible to determine the angle which the Earth’s disc would make at the centre of the Sun (8.8 seconds of arc) and, knowing the diameter of the Earth, it is possible to calculate the distance between the Sun and Earth. This average distance is about 92,948,000 miles, so that a ray of light leaving the Sun takes 498 seconds to reach the Earth. Knowing the distance between the Earth and Sun and also the average measurement of the Sun’s disc, it is possible to calculate the diameter of the Sun as about 864,000 miles, or 109 times the diameter of the Earth.

Some Numerical Facts about the Sun:

Diameter: about 864,000 miles (109 times that of the Earth)

Mass: 333,430 times the mass of Earth

Mean Density: 1-41

Gravitational Force: 28 times that of terrestrial gravity

Temperature of the Interior of the Sun: about 14,000,000 degrees Kelvin

Temperature of the photosphere: about 6,000 degrees Kelvin

Temperature of the Sunspots: about 4,000 degrees Kelvin