IN STARRY SKIES

Evening Post, Volume CXIII, Issue 30, 5 February 1932, Page 14

IN STARRY SKIES

THE STARS WHICH NEVER SET

(By "Omega Centauri.")

To-day we show a portion of the sky which has a very special interest for us. It contains all the stars which, viewed from the latitude of Wellington, never set. An excellent way to study them is to begin by using, a camera. Turn it due south and tilt it upwards at an angle of about 41 degrees. Then expose in it a rapid plate for an hour or so. If the place of observation is free from disturbing lights, each bright star will make its characteristic impression on the plate. But the images will not be star-like points. The developed picture will suggest whirling fireworks rather than steady stars. The earth, spinning on its axis, makes the stars appear to circle round tho point in _ the sky towards which the axis points. Each star is represented therefore by an arc of a circle. The arcs are all truly concentric, but they differ enormously in size. Near the centre of the plate they are very short, but then steadily increase in length as we approach the margin. An easy way to make an accurate map of this part of the sky 13 to make a print from the negative, and then with a pin or needle prick a hole either at tho beginning or end of each arc. All tho holes, of course, must be made at tho same end.

Holding this print up facing the light you will have a good representation of the southern sky, especially if you have enlarged the holes at tlio ends of the brighter lines. This experiment shows that the heavens seem to circle round a definite point in the sky. If -you time your exposure carefully and meafire the angle subtended at the centre by any arc, you will find that it would take about a day to complete the circle. The southern stars thus form a stellar clock. This is a more accurate timepiece than any made by the hand of man, but it keeps sidereal time, not solar. It gains on the town clock exactly one day per year, or nearly four minutes cacli day. In whatever part of the country we may be, this stellar clock acts also as a compass. Its centre is always due south of us. Unfortunately the centre of the clock is in rather a barren region of the sky. There is not a single really bright star in it. But fortunately there are some about thirty degrees away, and they are spaced fairly eveiijy round the Pole co jthat they can be used as the hands of the clock. The best known group consists of the Southern Cross and the Pointers. On the opposite side of the Pole, and nearly at the same distance, is Achernar, or Alpha Eridaui. If we take a line from Achernar to Beta Centauri and bisect it, the middle point will be near the South Polo of the heavens. If you have field glasses you may adopt n. more accurate' way of finding the Pole. Start with tho smaller Magellan Cloud. Beside it we see the wonderful globular cluster, 47 Toucani. Almost at right angles with this from the centre of the cloud is the star Beta. Hydri, brighter than the third magnitude.' Go on from. Beta five degrees further from the Cloud, and yon come to a group of three stars called Gamma Hydri. Follow .on almost in the same direction and you find three stars in a line spaced out about two degrees apart. The third of these is Sigma Octantis, the Southern Pole star. It is less than one degree from th,e South Pole. You may be sure you" have got it by noticing two other faint stars coming back from it like the barb of an arrow formed, by the other three. Having fonnd the centre of our clock we can now use the line from Beta Centauri to Achernar as a giant hand turning once round in the sky every twenty-four hours. But, as we have already saidthis hand marks true sidereal rime, which, gains nearly four minutes each day on mean solar time. Beta

Ceutauri is near the horizon below the Pole at 8 o'clock in tho evening -during the week beforo Christmas. Six months later at tho same timo in the evening it is liigh in tho sky straight, above the Polo. Six hours ahead of the Cross and a little further from tho Polo is the brilliant star Canopus. Six hours after Alpha Centauri with almost the same declination is the second magnitude star Alpha Pavouis. Achernar, Canopus, Gamma Crncis, and Alpha Pavonis are very nearly on the same circle, with the Pole as centre and a radius of 34 degrees. In estimating distances in the sky it is very convenient to have a few standard ones to refer to. The length of the cross from Alpha to Gamma is six degrees. The Pointers are five degrees apart, the belt of Orion is nearly three degrees in length, and the diameter of the moon is little more than half a degree. If we want louger distauces wo may take it as 12i degrees from Alpha to Beta Pegasi, and twenty degrees from the head to the knee of Orion, that is from Lambda to Kappa. From Rigel to Betelgeuse is not much less.

Many objects of interest are shown in this map. , Alpha Centauri is a pair of suns very like our sun. They revolve round one another in 81 years. The light from them takes about four years and four months to come to us, and they are nearer to us than any other knowa stars except a neighbouring faint one called Proxima Centauri. This appears as a star between the tenth and eleventh magnitude, and is only one-tenth of a light year nearer to us than the great pair. Alpha Crucis a,lso is a wonderful multiple system,

shining with 400 times the intensity of the sun, but it is so far off that the whole system appears to us as a first magnitude star. It is, indeed, the best example of what is meant by the first magnitude. Aldebaran is almost equally bright, and these two are outshone by only a dozen stars, Alpha Centauri being two and a half and Sirius eleven times as bright. Gamma Crucis, magnitude 1.5, is another interesting system when viewed through a small telescope. Near Beta Crucis is one of the reddest stars in the sky, whilst not far off is Kappa, the magnificent cluster of coloured suns which Herscliel called the Jewel Casket.

Canopus is one of the most brilliant of the known stars. It shines with about 100,000 times the brilliancy of the sun. It is known to be exceeded by Zeta' Orionis, Eta Ursae Mnjoris, and Gamma Cygni only. The first of these is believed to be at least twice as bright as Canopus, but owing to its distance of oveT 3000 light years it appears but little brighter than a standard second magnitude star. It is rather remarkable that in this portion of the sky, which is never hidden from us, we have both the Magellan Clouds. These look like detached parts of the Milky Way, and are found to be composed of the same elements, stars, star clusters, and gaseous nebulae. One of the latter in the greater cloud is larger than any gaseous nebula in the galaxy. These clouds, which appear about 7 and les3 than 4 degrees iv diameter, .respectively, are believed to be over 100,000 light years frani us, and are receding with speeds of about 175 and 10S miles per second. Near the smaller Magellan Cloud we can see .with the naked eye as a hazy third- magnitude star, the glorio s •globular star cluster known as 47 Toucani. TJiis is a maguifleent swarm of some ten thousand suns, at a distance of 22,000 light years. With fiold-glassesfcit-"is 'easy to distinguish, this cluster from an ordinary star, and even FiH} smalL telescopes it is a most beautiful sight.

Just outside tho boundary of this map, between the Southern Cross and the Pointers, is another magnificent globular cluster, Omega Centauri. It is about the same brightness as 47 foucani, and is also at the same distance and consists of about the same number or stars. In it 132 stars have already been found to be variable. The cluster is oust as,beautiful as 47 Toucani but less condensed towards the centre