ASTRONOMICAL NOTES

Evening Star, Issue 19521, 31 March 1927, Page 14

ASTRONOMICAL NOTES

APRIL 1927 [Written by A. E. C. Cues?, M.Sc., Member of. New Zealand Astronomical Society.] As is generally the caso this year, the Moon is near the Sun at the beginning of tho month. A conjunction with Uranus occurs on the Ist at 7.20 p.m., and. New, Moon on iho 2nd, at 3.54 p.m. Tho Moon passes Venus on the 4tli at 7.30, p.m., Aldobarau, tho red Bull’s Eye in Taurus, on the 6th at 1.44 p.m., and tho planet .Mars on tho 7Lli at 3.30 p.m. On tho oth, at 11.51 a.m., the Moon is at the first quarter, and at 0.06 tlio same evening is in conjunction with tho star Bollux. On tho 12th tho Moon passes the planet Neptune at 8.30 a.m., and the star ilcgulus at 4.38 p.m. On tho 17th, at 2.5(3 a.m., Spica ’ Virginia is in conjunction with tho Moon, whic'h is full at 3.05 p.m. the same day. Tho Moon, passes Saturn on the 20th at 11.30 p.m., An tares on tho 21st at 1.20 a.m., reaches the last quarter on the 2oth at 9.51 a.m., and passes Jupiter on the 28th at 6.30 p.m,, Uranus on the 29th at 7.30 a.m., and Mercury on the 30th at 00.30 p.m. Mercury is. in the morning sky, and is in conjunction with Jupiter on the 6th at 4.30 p.m It should ho well placed for early morning observation, its crescent phases terminating with the greatest elongation west, which occurs on tho 10th at 3.30 p.m. On the ]Bth, at 1.30 a.m., Mercury,will bo m conjunction with Uranus.

The planet Venus continues to improve her position in tho evening sky, setting about an hour or more after the Sun. Sho has been seen in Wellington several times during March, hut not through the telescopes at the Observatory. She is not very prominent yet, owing to her great distance and her increasing north declination. Shq will show afgibbous phase during April. While Venus is gaming in tho eternal race of the planets Mars is being left behind. Still ho is plodding aloaig bravely, and still sets about three hours after the Sun; Ho is in Taurus, and has passed Aldebaran during March, but will bo in Gemini in April.' Jupiter is still a morning planet dose to the Sun.'

, Saturn is steadily improving as an object for evening observation, which . might well be Ijegun this month. He is hot difficult to find, being directly below Antares, in the heart, and Beta Scorpii, in the head of the Scorpion. Uranus, like Jupiter, is in the morning sky close to the Sun. I Neptune is slowly retrograding on the ecliptic west of Regains, in the '“Sickle” of the Lion. Ho is about I the seventh or eighth magnitude, too 1 faint for naked eye observation, but a telescopic search a few degrees west of Regnlus will easily show him. On the 15th at Dunedin the Sun rises at 6.42 a.m. and sets at 5.44 p.m The i planets visible are Venus, Mars, Nep- | tune, and Saturn. Venus about I 7 o’clock, Mars about 9 p.m., Neptune ■ is ou the meridian at 8.16 p.m., and ; Saturn rises about 7.30 p.m. It will i bo seen that Venus and Saturn are ! nearly opposite each other. j The star Castor is ou the meridian I at G p.ra., Procyon at C. 5 p.m., and 1 Pollux at 6.11 p.m. These three stars I are so placed that a line from Procyon j passing down between the twins Cas- : tor and Pollux serves to indicate the direction of the north celestial pole, which is, of course, invisible, being ' 4Cdeg below tho horizon. | Antares rises about 7 p.m. With the possible exception of Betelguesc, in I Orion, this red star is the largest of i tho first magnitude, although far from j coming first in apparent brightness. I The latest spectroscopic measurements 'of the distance of this great star were 1 Victoria, British Columbia, and at Mount Wilson, California, and are in close agreement. Tho respective parallaxes arc given as 0.012 and 0.011 seconds of arc respectively. Hence tho distance is between 83 and 91 parsecs, or about 280 light years. Yet this star is ono of the brightest in our skies, and from its apparent magnitude of 1.2 j and tho above distance wo find that it , gives 2,500 times as much light as the Sim. Its red surface is so much duller than that of our bright luminary, however, that its diameter cannot he less than 300 times that of the Sun. Squaring and cubing this figure, wo obtain the vas'- area of 90,000 and a volume ■ 27,000,000 times that of the Sun. 1 Antares is thus so large that, if placed in the centre of our solar system, i would contain all tho orbits of the inner planets, including Mars and several or tho minor planets. To add to the interest of this bright red star, a grant of the-first order, it has a tiny, rauit irrcen companion, which shines like a tiny emerald in the field of the tele- ! scope. This little star is of the seventh ' magnitude, and, using the parallax ' given above, we find that it is about the same size as the Sun, though about thirteen times as bright, on account ot its high temperature. Arcturus rises, and Aldebarau sets, ! about 8 o’clock, and Regnlus passes - tho meridian at 8.33 p.m. Castor, Botelguese, Rigel, and Pollux set about ! 10 p.m., while Alpha Cruris is on tne meridian at 10.52 p.m, nearly overhead, followed by Beta Crucis twenty minutes later. Procyon sets about 11.30 p.m., while Spica, in the Virgin, crosses tho meridian at 11.50. Four minutes after midnight Acbernnr is on the meridian below the South Pole. A very attractive group of constellations adorns tho early evening skies in i April. While Orion and his neighbors .are still to bo seen in tho north-west and Argo is practically overhead, the richlyI jewelled Cross and Pointers are high in the south-east, with’ tho fine constellations of Centaurus, Lupus, and Scorpio below. Although tho atlases show Centaurus as a centaur galloping over the Cross and driving his spear into Lupus, the Wolf, tho constellation as it appears in our autumn skies reminds ono irresistibly of a gallant star-studded horse charging up The heavens with curving mane and lashing tail. This figure is so well marked that it can he called tho Southern Horse at least as legitimately as Northerners speak of tho “ Dippers ” of the Great and Little Bear, or tho “Frying Pan” of Orion. The forepart of this horse belongs to Centaurus, the tail and hind-quarters to Lupus. It might serve to remind us of the horses that helped in the Palestine campaign, or those other southern horses that the liberators of South .America rode to victory over the tyranny of Spain. Wo are now directly under the arch of the Milky Way, which spans tho heavens from north-west to south-east in April, passing through the above-men-' tinned constellations.

In the west Eridamis is sinking,-while Columbia, Lepus, and a few other small groups are descending lower in the sky. Orion and Canis Major are west of the meridian, while due north is the head of Hydra, with dim Cancer below. The constellation of Leo, more like a bicycle than a lion to us in the Southern Hemisphere, is east of the meridian, while further round . are Virgo and Libra, with Crater and Coitus above. In the south-east, south of the tail of Scorpio, is the small constellation Ara, the Altar, which never wholly sets in Dunedin. * Three stars in a curve above and four below make a very presentable altar, the two close together in the base forming a very pretty pair of contrasting colors—-Beta Arao orange, and Gamma Arae green. Tncana is now below the Pole, with Pavo east, and Achernar, in Eridamis, west of the south meridian.

STAR DISTANCES

No. 11. The discovery of “ spectroscopic parallaxes” is an achievement of the present century. For many years tho science of astrophysics progressed in tho direction of analysing the stars by determining the chemical elements pre • sented by their spectra, but latterly it has been realised that the size ofi a star has much to do with the appearance or absence of certain hues in the rainbow bands produced by tho prism of, tho spectroscope, and by a careful styicly of tho spectra a young astronomer named Adams, at Mount Wilson, discovered that tho real brightness, or “ absolute magnitude,” of a star could be deduced without; any knowledge" of its distance being required. Once the absolute magnitude was obtained, however, tho apparent magnitude was combined with it in a simple formula, giving tho distance in parsecs or the parallax as desired. The great advantage of this method was that, although any parallax, spectroscopically determined, was liable to an error of not more than 20 per cent., this percentage error remained practically constant for any distance, being dependent only on errors in the absolute magnitude. Thus a spectroscopic parallax of Alpha Centauri niignt be 0.91 (Adams made it about 0.79), but the same error in some remote star of parallax 0.0070 would only change tho value to 0.0091, whereas a trigonometrical determination would be easily liable to an error greater than the parallax itself, All parallaxes must be taken with a ‘‘grain of salt,” but the quantity of salt is a percentage in the case of the spectroscopic parallaxes. It may be pointed out that . in the case of very hot stars, such as Sirius Alpha Crucus, etc., spectroscopic parallaxes are not vet quite so reliable as in the cooler types; but in the hot stars, tho group parallaxes are particularly reliable as a rule.

Tho last methods of determination deal with tlo most remote stars, in which clustering is so prominent a feature. No. method is suitable which does not show a percentage error, but in the depths of remote space trigonometrical parallaxes are quite out of the question, while spectroscopic ones are at length ruled out by the extreme faintness of the stars. Trigonometrical parallaxes are becoming unreliable at fifty parsecs, spectroscopic ones reach out to giant stars at 1,000 parsecs, and two or three methods are left for tho rest, methods that can take us almost out to the last visible star.

Scattered over the heavens, hut found chiefly near the Milky Way, are a great number of stars winch undergo regular changes in the intensity of their light, the periods ranging from a few hours to about a month. Such stars are termed “short period” or “ Cepheid ” variables, so named after one of their number in_ the constellation Cepheus. Tho variation of their light is caused by “pulsations” or alternate expansions and contractions of their gaseous bodies, not unlike the action of a gas engine, but tho same gas is used over and over again, and no external work is clone except radiation. In the case of the Milky Way Cepheids a simple relation has been found between tho absolute magnitudes and the variation periods, but research on clusters of distant stars has shown that Cepheid variables ate present wherever stars gather in their thousands, and so tho distances of cluster after cluster are now dicing unravelled, the measurements being based on,the periods and apparent-brightness of tho cluster Cepheids. An example of these measurements is provided by the great southern cluster Omega Centauri, distance 6,000 parsecs .or 20,000 light years.

In the star clouds of the Milky Way distances arc frequently estimated by making counts of the stars within certain typical, equal areas, the more distant stars being the more closely packed. The measurements of globular , clusters such as Omega Centauri show that tbo stars in them are white giant stars of enormous brilliancy, and rather uniform in- size. The sizo of these white giants may be then used to determine the distances of remoter clusters. These two methods are, however, rather of an. approximate nature.

During tho year 1926 Dr Edwin P. Hubble, of Mount Wilson, California, succeeded, by using the groat lOpin reflector, in resolving the largo spiral nebula in Andromeda into myriads of stars, including many Cepheid_ variables, as usual. Using the periods of the Cepheids and their apparent brightness, no came to tho surprising conclusion that the Andromeda nebula is distant nearly one million light years from tho solar system, thus proving that the nearest spiral nebula is another mighty universe like our own Milky Way system. The scale of tho universe may be illustrated by a comparison with familiar objects. A man at a small dining table at some hotel in Dunedin draws out a box of matches to light his cigarette. Let a wax vesta lin long represent tho distance from the earth to the sun, then a wooden safety match will stretch from the sun to Mara, a cigarette into the zone of asteroids, and a pipe to Jupiter. A vase of flowers IQin high stretches to Saturn, tho furthest planet known to the ancients, hut the little dining table might have difficulty in accommodating tho orbit of Uranus, and to take in that of Neptune it would have to_ be 511 across. The sizo of tho sun itself would be represented by a grain of sand, and most of tho planets by motes of dust, the earth being quite invisible. The man being in tho centre of Dunedin, lie would have to go as far as Anderson’s Bay, four miles, to reach two grains of'sand representing Alpha Centauri, or to Port Chalmers to travel the distance representing that of Sirius. Nearly all the tiny dwarf stars known W'onld be placed about the suburbs and environs of tho city, say, from Evansdalo to Mosgiel. A railway journev to Invercargill and on to Bluff would be about the scale value for the distance to Alpha Crucis, and all stars with reliable trigonometrical parallaxes would bo scattered about Otago, Southland, and South Canterbury, j.The Stars in Orion nr Scorpio would have to he placed as far away as Palmerston North or Wanganui, the nearest Cepheid variables and white giants like Bigel, being also placed in Wellington Province, the remoter Cepheids and white giants being beyond Auckland. A weather-beaten sailor walks into the hotel. He has made forty voyages round the world, and has covered more than the. distance we require to represent that of the Andromeda nebula.