HOW MUCH DO WE KNOW?

Auckland Star, Volume LXXI, Issue 17, 20 January 1940, Page 4 (Supplement)

HOW MUCH DO WE KNOW?

By - - Sir Frank Dyson, F.R.S.

T'HE first great astronomical disi covery was that the earth was a sphere of 8000 miles diameter. The next was that it turned round its axis in a day and, accompanied by the moon, went round the sun in a year.

Then followed the discovery of the Solar System with large planets, including the earth, all controlled and moving round the

The stars were incidental to these discoveries, because of their fixity in the sky relatively to one another. Orion and the Great Bear look exactly as they did thousands of years ago. One great argument used against the Copernican system was that the relative positions of the stars should change as the earth moves round the sun. The only reply was that the distances are so immense that no differences can be de-, tected from viewpoints millions of miles apart. This proved, to be correct.

Gradually the view prevailed that the stars were self-luminous bodies like the sun. Xewton, by considering the small amount of light received, on Saturn from the sun, and reflected to us, and comparing this light with that given by the brightest stars, showed that these stars must be at least 100,000 times as far away as the sun.

Next Hallcy showed that three of the brightest stars had slightly moved relatively to other stars since tho time when their positions in the sky had been accurately determined by Greek astronomers. He concluded that, as these were some of the brightest stars in the sky, their movement was an additional reason for believing them to be comparatively near. Immense Distances Efforts were made by many astronomers to determine the distance of several of the brightest stars. They said: "If the earth is at one time of the year 93,000,000 miles from tho sun, then six months later it will be 15G,000,000 miles from its first position, and any star which is much nearer than the surrounding stars will have changed its, position relative to the background." If tou hold your finger up at arm's length and look at it first with one eye and then the other, you will see how it appears to shift with reference to the objects on tho wall of the room. It is just a hundred years ago since Besael measured, tho distance of 61 Cygni. This star is not bright, but was chosen because of its rapid motion across the sky, Afteor a series of very accurate measures li© found it to be 700,000 times as distant as the sun. The next year another star, Centauri, was found by Henderson to bo 300,000 times the distance of the sun. is actually the nearest star to us. During the last century the distances of 80 stars were found. In the present century, thanks to large photographic telescopes, the distances are so large that they are conveniently expressed in "light years," or the number of years tho light takes to reach us. The nearest star is 4.3 "light years" distant, and if you remember that light travels 11,000,000 miles a minute, you can calculate its distance in miles. You will bo no wiser, for such large figures convey little meaning, and it is best to start a new scale remembering that one "light year" is 70,000 times the distance of the sun. It is possible, though difficult, to compare the light of tho sun with the light of a star. The sun is found to give us 20,000 million times as much light as Sirius, the brightest star in the sky. But if Sirius, which is distant nine '•'light years," were as near as the sun, it would give 27 times as much light. Colours of the Stars When we know the distances we arc thus able to compare the intrinsic luminosities of the stars with the sun. They vary greatly. Some are ten thousand times as bright, and some only one-thousandth. The sun is above the average in luminosity.

Viewed in a largo telescope, many stars are found to be double. When observed, year after year, they are found to circulate round each other by reason of their mutual attraction. Some take only a few years, but others may take hundreds. If the distance of the

(Late Astronomer-Royal.) pair of stars from us is known, it is easy to calculate how far apart they are. These are usually comparable with the distance of the earth from the sun, often between ten and twenty times.

When the complete period of revolution is also known it becomes possible to calculate the sum of the masses of the two stars. It is seldom more than ten times or le3s than a half that of the sun.

Wo conclude, therefore, that though thero is a wide range in luminosity there is a comparatively small range in the masses of the stars.

The stars arc of different colours. Some are yellow, like the sun; others red; others white. The colours indicate the temperature of the star near its surface. The red stars show a temperature of 3500 degrees centigrade; yellow stars of about GOOO degrees centigrade; white ones of about 12,000 degrees centigrade, and some are even hotter. Like, the sun, the temperatures increase to an enormous extent towards the centre to millions of degrees. This high temperature is necessary to give the gas sufficient elasticity to support the enormous pressure. We do not know exactly how this temperature is maintained, but radio-activity has shown that there is a great store of energy inside the atoms of matter. These are gradually depleted and the stars are thus able to maintain for millions of years a constant outflow of energy in the form of heat and light, accompanied by a gradual small diminution of mass. The stars, then, are great globes of gas at high temperatures. Their order of evolution is thought to be of this nature. We see them first as giant red stars of enormous bulk—of, say, 100 times the radius of the sun and therefore a million times its volume —in a state of such great tenuity that the total mass is not more than ten times that of the sun. A red star gradually contracts and its surface temperatures increase till it reaches a maximum in a white or blue star. The contraction continues, but now the temperature falls, going through corresponding stages of yellow and red. The mass has diminished through the outpouring of energy, so that when the star, reaches the dwarf, rei stage ite mass is considerably less than that of the sun. A great number of facts support this theory. For example, the immense diameter of the giant red stars has been verified by observation. How the giant red 6tars came into existence we do not know. Presumably the dwarfs will diminish till they are no longer visible. The stars are all in rapid motion in all manner of directions as seen from the sun. This was first discovered by their angular movement across .the face of the sky. It has been confirmed and extended by the determination of their movements towards or away from us.

Sir William Hersclicl noticcd that these movements were not altogether at random, but showed a tendency to move away from the constellation Hercules. He accounted for tliis by saying that the sun, carrying us with it, was moving towards the constellation Hercules. This has been amply confirmed and the velocity determined as 12 miles a second. Herschel with his great telescopes made gauges of the number of stars in different directions and found that the number increased greatly as the plane of the Jlilky Way was approached. He concluded that the stars around us extended much farther in the plane of the Milky Way than in the perpendicular direction. The system of fitars to which we belong is shaped like a.watch. From modern observations we conclude that the diameter of the Galaxy is 90,000 light j'ears, and its thickness about a quarter of that figure. In recent years a great deal of attention has been given by astronomers to the construction of the Galaxy. The question is complicated, but the results are conclusive. The sun is situated about 30.000 light years from the centre and 15,000 light years from the extreme edge. It is rotating about the centre of the Galaxy and takes about 200 million years to complete the circuit, although it is moving with the immense

velocity of 175 miles a second. The stars nearer the centre are moving faster and those further away more slowly. The total mass of all the stars is of the order 150,000 million Suns. Attention should be drawn to two other features of the Galaxy—nebulae and clusters. The nebulae are of two kinds—some named planetary are more or less spherical in shape, but others are diffused over large areas, of tho sky. They consist mainly of oxygen, nitrogen and helium in a state of the utmost

tenuity. The clusters are of globular form and consist of thousands of stars. They are limited in number, being not more than 100, and are extremely distant. Their unsymmetrical distribution gave the first clue to the position of the s-un, away from the centre of the Galaxy. So much for the Milky Way. But this ?s not tho end of the story. There are millions of "island universes" similar to it, and the largest telescopes can see no end to theni.