Our Star

Our Star: SUN 

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Because the Sun appears so glorious in our sky, some people are disinclined to believe that it's only a star; indeed, astronomers relegate it to the status of a Yellow Dwarf! Its closeness to us means it's the sole star which we will examine intimately. Its diameter is 1,392,000 kilometres (865,000 miles), and it could engulf over 1,000,000 globes the quantity of the Earth, but it's considerably less dense, because it's made from incandescent gas.

 At the core, where the energy is being produced, the temperature could also be as high as 15,000,000 degrees C; even the brilliant surface which we can see – the photosphere – is at a temperature of 5500 degrees C. it's here that we see the familiar sunspots and the bright regions referred to as faculae. Above the photosphere comes the chromosphere, a layer of far more rarefied gas, and eventually the corona, which can be considered the Sun’s outer atmosphere. 

The Sun is nowhere near the centre of the Galaxy; it's around 25,000 light-years from the nucleus. it's sharing in the general rotation of the Galaxy, moving at 220 kilometres (140 miles) per second, and taking 225 million years to finish one circuit – a period often called the cosmic year; one cosmic year ago, even the dinosaurs lay in the future!

The Sun is rotating on its axis, but it doesn't spin in the way that a solid body would do. The rotation period at the equator is 25.4 days, but near the poles it's about 34 days. this is often easy to watch by the drift of the sunspots across the disk; it takes a few fortnight for a group to cross the disk from one limb to the opposite .

 The greatest care must be taken when observing the Sun. Looking directly at it with any telescope, or even binoculars, means focusing all the sunshine and (worse) the heat on to the observer’s eye, and total and permanent blindness will result.

 Even employing a dark filter is unsafe; filters are apt to shatter all of sudden , and in any case cannot give full protection. the sole sensible method is to use the telescope as a projector, and observe the Sun’s disk on a screen held or fastened behind the telescope eyepiece.

 We know that the world is approximately 4600 million  years old, and therefore the Sun is certainly older than this. A Sun made up entirely of coal, and burning furiously enough to emit the maximum amount energy because the real Sun actually does, would be reduced to ashes in just 5000 years. 

In fact, the Sun’s energy is drawn from nuclear transformations near its core, where the temperatures and pressures are colossal. Not surprisingly, the Sun consists largely of hydrogen (over 70 per cent), and near the core the nuclei of hydrogen atoms are combining to make nuclei of subsequent lightest element, helium. 

It takes four hydrogen nuclei to form one helium nucleus; whenever this happens, a touch energy is released and a touch mass is lost. it's this energy which keeps the Sun shining, and therefore the mass-loss amounts to 4 million tonnes per second. 

Fortunately there's no cause for immediate alarm; the Sun won't change dramatically for a minimum of thousand million years yet. The photosphere extends right down to about 300 kilometres (190 miles), and below this comes the convection zone, which features a depth of about 200,000 kilometres (125,000 miles); 

here, energy is carried upwards from below by moving streams and much of gas. Next comes the radiative zone, and eventually the energy-producing core, which seems to possess a diameter of around 450,000 kilometres (280,000 miles). 

The theoretical models seem satisfactory enough, and a serious problem has recently been solved. The Sun sends out vast numbers of strange particles called neutrinos, which are difficult to detect because they need no electrical charge.

 The Sun appeared to emit far fewer neutrinos than predicted but the recent discovery of their small but non-zero mass has resolved the discrepancy. 

If a neutrino scores an immediate find an atom of chlorine, the chlorine could also be become a sort of radioactive argon. Deep in Homestake Gold Mine in South Dakota , Ray Davis and his colleagues filled an outsized tank with over 450,000 litres of cleaning fluid, which is rich in chlorine; 

every few weeks they flushed out the tank to ascertain how much argon had been produced by neutrino hits. In fact the numbers were strikingly but they ought to have been, and similar experiments elsewhere confirmed this.

 (It was essential to put in the tank deep below the ground; otherwise the results would be suffering from cosmic ray particles which, unlike neutrinos, cannot penetrate far below the Earth’s surface.) Japan’s Super-Kamiokande detector, 1000 metres down within the Mozumi mine, uses 50,000 plenty of pure water; it too detects fewer neutrinos than had been expected.

Like all other stars, the Sun began its career by condensing out of interstellar material, and initially it had been not hot enough to shine, because it shrank, under the influence of gravity, it heated , and when the core temperature had risen to 10 million degrees nuclear reactions were triggered off; hydrogen was converted into helium, and the Sun began an extended period of steady emission of energy. 

As we have seen, it had been not initially as luminous because it is now, and the increase in power may have had disastrous results for any life which can have appeared on Venus. 

But at the moment the Sun changes very little; the fluctuations thanks to its 11-year cycle are insignificant.However, this may not last for ever. the important crisis will come when the availability of obtainable hydrogen begins to become exhausted. 

The core will shrink and warmth up as differing types of reactions begin; the outer layers will expand and cool. The Sun will become a red giant star star, and can be at least 100 times as luminous because it is at the present , in order that the Earth and therefore the other inner planets are bound to be destroyed. Subsequently the Sun will throw off its outer layers, and therefore the core will collapse, in order that the Sun becomes a very small, incredibly dense star of the sort referred to as white dwarf. 

Eventually all its light and warmth will leave it, and it'll become a chilly , dead globe – a black dwarf. This may sound depressing, but the crisis lies thus far ahead that we'd like not concern ourselves with it. In our own time, at least, there's no danger from the Sun.

IMPORTANT READ

▲ The Sun within the Galaxy. The Sun lies well faraway from the centre of the Galaxy; the distance from the centre is less than 30,000 light-years, and the Sun lies near the edge of one among the spiral arms. This picture shows the Milky Way in infra-red, as imaged by the COBE satellite.

Source: Atlas of the Universe - Sir Partick Moore