Astronomy of To-Day.*

Progress, Volume II, Issue 3, 2 January 1907, Page 98

Astronomy of To -Day.*

PART I. Do we realise when we look up to the sky how few stars we can see with our unaided vision ? It is said that we can only see between 7000 and 8000, even if we examine the sky in both hemispheres. And yet the stars are innumerable. The more powerful we make our telescopes, the more stars we discover, and the most powerful telescope has found no place in the heavens devoid of stars. We may consider the heavens m two or three aspects. Let us for a little see if we can realise (1) the immense distances of the universe, (2) the sizes of many of the stars, (3) the beauties of the heavens, and, lastly, (4) some theories of the beginning and end of the universe. (1.) If we are in the Wanganui Observatory, we would perhaps ask to see the Southern Cross. Who has not seen that brilliant constellation ? It is situated in that portion of the sky which is the richest in stars, and as we watch it from month to month in its varying altitude to us, we are always struck with its beauty. We will notice the pointers of the cross ; they belong to a constellation called Centaur. You are aware perhaps that the stars in each constellation are named by the Greek alphabet, or by numbers, and named according to their apparent brightness. For example, the brightest star in a constellation will be called Alpha, the next brightest Beta, the next Gamma, and so on. The brightest pointer is called Alpha-Centauri When you come to examine it through a telescope you will find that it is not a single star, but it is a " double star " — there are two stais. It is a beautiful object. The reason, however, why I wish to direct your attention to it is, that so far as we know, it is the nearest star to this earth. It is from considering its distance that we can get some idea of the vastness of the distances in the heavens. The distance of the sun from the earth is according to the latest calculation 92,874,000 miles. This is the calculation of Sir David Gill, who I see has just been appointed President of the British Association , he is an eminent astronomer, and during his residence at the Cape of Good Hope has done good service for astronomical science. One way of testing distances is to consider how long it takes for light to travel from the sun to the earth. Light travels at the rate of 186,300 miles a second, i c seven tunes round the earth in a second ; this would show that light will travel from the sun in about nine minutes. Now, light takes nearly 4\ years to travel from Alpha-Centaui to our earth, so when we are looking at AlphaCentaur we are looking at Ihe star as it existed about 4| years ago — it may have perished in the meantime. This shows that the distance of AlphaCentaur from our earth is about — I am giving you round numbers only — 25 billions of miles. Let me show you the distance m another way You have no doubt seen an orrery constructed, that is, you have round globes or balls showing the size of "the sun and the different planets and their relative distances. Well, let us suppose that at this table there is the sun, and we place the earth, we shall say, a yard off from the sun ; where will AlphaCentaur be ? Alpha-Centaur would have to be placed in our orrery 150 miles off. That will give you some idea of the distance of Alpha-Centaur, the nearest star to us. I may illustrate the relative distance and size by quoting to you what Sir John Herschel said about our solar system He said — Let us suppose we have a globe 2 ft. in diameter ; this would stand for the sun. If Mercury is placed 82 ft. away it will be relatively to the sun the size of a mustard seed ; Venus will be 142 ft. away, and the size of a pea ; the earth 215 ft. away, or 3 chains and 17 ft., and the size of a pea a little larger ; Mars will be 327 ft. away, and the size of a large shot ; the Asteroids — small planets — will be 600 ft. away, and of the size of grains of sand ; Jupiter will be \ mile off, and as large as an ordinary orange ; Saturn will be four- tenths of a mile off, and of the size of a small orange ; Uranus will be } mile away, and like a large cherry , and, lastly,

*The lecture was illustrated by slides, and they were explained, and further information not m these notes given

Neptune like a large plum 1£ mile away. But the nearest star Alpha-Centaun would be a globe about 3 or 4 ft across, and 10,000 miles distant. Sometimes during the year we may cast our eyes to what is the brightest star m the southern skyit is the star Sinus Well, Sinus is double the distance of Alpha-Centaur from us. Let me try and give another illustration of the distance : We will suppose that we take a tram — and it will be an express train going far faster than any express tram runs in New Zealand — let us suppose that it travels at the rate of 60 miles an hour ; travelling at that rate it would take us 130 years to reach the sun, but to reach the nearest star, namely, to reach Alpha-Centaun, it would take us about 35 millions of years. There is another star which was at one time thought to be the nearest star — it is in the constellation of the Swan, and is named 61 Cygni. To reach it would take about 40 millions of years travelling at the same rate, and, remember, these are the nearest stars to us If we take some of the more distant stars, say for example Sinus, we could not reach it, going a mile a minute, under 72 millions of years Let me give you yet another illustration, so that we may try if we can dimly realise the vast distances in the heavens. There burst out in 1901 a brilliant star m the constellation of Perseus. A star had been known m about the same position, but it was not of great magnitude or brilliancy. On the night of February 21st/22nd Dr. Anderson of Edinburgh noticed a bright star which increased so rapidly in brightness as to become about the third brightest stai in the sky. Afterwards jt began to fade away slowly. Now, this star was so distant from us that what Dr Anderson was seeing took place in the reign of Henry VIII. The star was trillions of miles from us. Many more illustrations might be given, but perhaps those used may impress us with the vast distances of the stars in the sky It is computed that the diameter of the known Sidereal universe is so great that light would take 90 millions of years to travel across it. This would make the diameter 374 trillions of miles, 374,340326,400 000,000 000, or 374 with eighteen figures following it Can we comprehend it ? (2 ) I have spoken about distances let us consider the size of the stars we see in the sky. I assume that we are still looking through the telescope at Mpha-Centaur but Alpha-Centaui consists of two stars about equal m size • they are really two vast suns, and each, compared with our sun, is moie than seven times its si?e. They take 81 years to travel round their orbit The one revolves lound thp other There is another star that we sometimes see It is called Arcturus or a Bootes, and was at one time thought to be the quickest-moving star in the sky. We see it in April in the northeast, in June in the north at 8 p.m. I refer to it now because it may give us an illustration of the vast size of some of these suns called stars. To try and realise the size of Arcturus, let us look at the size of the sun. The size of our earth is about 8000 miles in diameter, that is, if we bored a hole from pole to pole that would be about the diameter of the carth — I am giving you round numbers. The diameter of the sun is 865,000 miles, but the diameter of Arcturus is 86 millions of miles — about one hundred times the diameter of the sun. Let us not forget that the distance of the sun from us is nearly 93 millions of miles. If the star Arcturus came between us and the sun it would almost fill the space. It is m volume a million times the size of the sun, and there are no doubt many thousands of stars, perhaps hundreds of thousands of stars, as large as Arcturus. The fact is that our earth, relatively to the size of some of the heavenly bodies, is as a gram of sand is to the size of our earth. Sinus, also, is a large star, about 3J times larger than our sun, and about 48 times as bnght. There are no doubt stars smaller than our sun, but we may say that there are hundreds of thousands of stars larger than it We speak of the stars as " fixed " stars. Compared with the planets they appear immovable. The fact is, however, that some of them are exceedingly rapid in their movements. If we consider their velocity we will be further impressed with the vastness of the universe. There is a difference of opinion amongst astronomers as to what star, the movements of which can be approxi-

mately ascertained, is the most rapid. In 1893 it was said that Arcturus was the most rapid, and Prof. Newcomb in his work on " The Stars " assumes that this is correct. The immense distances of the stars make accurate observation very difficult. It has been computed that Arcturus travels at the rate of 376 miles per second or 32,486,400 miles per day. A star called 1830 Groombridge — because that is the number of the star in Groombridge's catalogue of stars — situated m the northern contellation of the Great Bear, travels, it is said, at the rate of 231 miles per second, or 19,958,400 miles per day. And yet these stars, because of their immense distance from us, seem immovable or fixed. The distance of Arcturus is so vast that light travelling at the rate of 186,300 miles per second would take 181 years to reach our earth from Arcturus It is about 41 times more distant from us than Alpha-Centaun. From 1830 Groombndge light would take 36 years to reach us. And now let me say something about the number of the stars Through the aid of photography, which has been of as great service to astronomy as the telescope, some of the most recent astronomers have come to the conclusion that there are perhaps known now a hundred millions of suns, and if you consider that each of these suns may have, like our sun, planets revolving round it, which, however, are never seen by us, and that these planets have other planets around them, such as our moon around our earth", you may get some idea of the number of heavenly bodies that exist m the universe. (3 ) Let us pass to another stage — the beauties of the heavens. The first thing of beauty that you would likely have your attention directed to is in the Southern Cross. There are what are termed m the sky clusters — magnificent clusters of stars. To the naked eye, if you have a keen vision, the cluster may appear a single clouded star, but when you look through the telescope you behold a most beautiful spectacle. You see thousands of stars sparkling such as no diamonds or other precious stones ever sparkled in the finest piece of jewellery. Sometimes the stars in a cluster are of varied colours, white, yellow, red, violet. You will often stand in wonder at the beauty of many clusters. The numbers in the clusters will surprise you. What appears as a single star may, with the aid of the telescope and the photographic lens, become thousands. You know the Pleiades, often called the Seven Stars. You see them on a clear bummer night in the north-east heavens You could see them about 11 pm. on the Ist October. One with ordinary vision can count six stars. If one has extraordinary vision, twelve may be seen. When you look through a powerful telescope you may count about 2000, and in one photographic plate that had been exposed for some time, 2326 have been counted. Around one of the stars in the eross — the star Kappa — there is a brilliant cluster The stars are not so numerous as in some clusters, but they are large and brilliant and of different colours. Another very fine cluster is round a star m the constellation of Toucan This constellation is at present high up in the sky, lying between the constellations Hydrus and Grus (the Crane), to the west of the bright star Acheron The cluster is round the star 47 loucani It is a large circular cluster, with the stars apparently closely compacted together, as seen through the telescope in Wanganui. The cluster that is said to be the brightest m the sky is Omega-Centauri, the constellation in which the Southern Cross pointers are. It is just within the border of the Milky Way. To the unaided eye, and on a clear night, it appears a hazy star of the fourth magnitude. Prof. Newcomb says over 6,000 stars have been counted m one photograph of this cluster, and the whole number is greater — some say perhaps 15,000 stars. It is a most beautiful cluster, and, if the sky is clear, is always visible to us in New Zealand. There is a fine cluster between Alpha and Beta Scorpionis. It is near Antares, that beautiful red star otherwise called Alpha-Scorpionis. Sir John Herschel considered it the richest and most condensed mass of stars in the firmament. A strange thing happened in this cluster in 1860 : one bright star appeared, almost blotting out the cluster by its brightness. The beauties of these and other clusters will grow on you, and I doubt not but that, whenever you visit an observatory, you will agam and again ask to see them. There are plenty to show you. 226 clusters have been observed in the milky way, and 38 outside. (To be continued).