Philips Atlas of the Universe

Our Star: the Sun

ATLAS OF THE UNIVERSE

B

ecause the Sun appears so glorious in our sky, some

people are disinclined to believe that it is only a star;

indeed, astronomers relegate it to the status of a Yellow

Dwarf! Its closeness to us means that it is the only star

which we can examine in detail.

Its diameter is 1,392,000 kilometres (865,000 miles),

and it could engulf over a million globes the volume of

the Earth, but it is very much less dense, because it is

made up of incandescent gas. At the core, where the ener-

gy is being produced, the temperature may be as high as

15,000,000 degrees C; even the bright surface which we

can see – the photosphere – is at a temperature of 5500

degrees C. It is here that we see the familiar sunspots and

the bright regions known as faculae. Above the photo-

sphere comes the chromosphere, a layer of much more

rarefied gas, and finally the corona, which may be regard-

ed as the Sun’s outer atmosphere.

The Sun is nowhere near the centre of the Galaxy; it is

around 25,000 light-years from the nucleus. It is sharing

in the general rotation of the Galaxy, moving at 220 kilo-

metres (140 miles) per second, and taking 225 million

years to complete 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 does not 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 is about

34 days. This is easy to observe by the drift of the

sunspots across the disk; it takes about a fortnight for a

group to cross the disk from one limb to the other.

The greatest care must be taken when observing the

Sun. Looking directly at it with any telescope, or even

binoculars, means focusing all the light and (worse) the

heat on to the observer’s eye, and total and permanent

blindness will result. Even using a dark filter is unsafe; fil-

ters are apt to shatter without warning, and in any case

cannot give full protection. The only 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 Earth is approximately 4600 million

years old, and the Sun is certainly older than this. A Sun

made up entirely of coal, and burning furiously enough to

emit as much energy as the real Sun actually does, would

be reduced to ashes in only 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 hydro-

gen atoms are combining to form nuclei of the next light-

est element, helium. It takes four hydrogen nuclei to make

one helium nucleus; each time this happens, a little energy

is released and a little mass is lost. It is this energy which

keeps the Sun shining, and the mass-loss amounts to

4 million tonnes per second. Fortunately there is no cause

for immediate alarm; the Sun will not change dramatically

for at least a thousand million years yet.

The photosphere extends down to about 300 kilo-

metres (190 miles), and below this comes the convection

zone, which has a depth of about 200,000 kilometres

(125,000 miles); here, energy is carried upwards from

below by moving streams and masses of gas. Next comes

the radiative zone, and finally the energy-producing core,

which seems to have a diameter of around 450,000 kilo-

metres (280,000 miles). The theoretical models seem

satisfactory enough, and a major problem has recently

been solved. The Sun sends out vast numbers of strange

SOLAR DATA

Distance from Earth 149,597,893 km (92.970,000

mile or 1 astronomical unit)

Mean distance from centre 25,000 light-years

of Galaxy

Velocity round centre 220 km/s (140 miles/s)

of Galaxy

Revolution period 225,000,000 years

round centre of Galaxy

Apparent diameter max. 32’ 35”,

mean 32’ 01”, min. 31’ 31”

Density, water = 1 1.409

Mass, Earth = 1 332,946

Mass 2 x 10^27 tonnes

Volume, Earth = 1 1,303,600

Surface gravity, Earth = 1 27.9

Escape velocity 617.5 km/s (384 miles/s)

Mean apparent magnitude 
26.8 (600,000 Full Moons)

Absolute magnitude 4.83

Spectrum G2

Surface temperature 5500°C

Core temperature about 15,000,000°C

Rotation period (equatorial) 25.4 days.

Diameter (equatorial) 1,392,000 km (865,000 miles)

▼ Cross-section of the Sun

showing the core, radiative

zone, convective zone,

photosphere, chromosphere

and corona.

Earth

Jupiter

Core

Radiative

layer

Convective

layer

Photosphere

Chromosphere

Corona

E152- 191 UNIVERSE UK 2003mb 7/4/03 5:40 pm Page 154