ASTRONOMICAL NOTES

Evening Star, Issue 22326, 30 April 1936, Page 14

ASTRONOMICAL NOTES

THE SKIES IN MAY [Written by A. G. C. Ckust, M.Sc., for the ‘ Evening Star.’] POSITIONS DF THE STARS Local Sidereal Time, 12hrs; Lattitudo 48deg S. The following star positions hold good locally four minutes earlier each evening, as at 9.30 p.m. on May 3, 8.30 p.m. on May 18, and 7.30 p.m, on June 2:— Achernar is S. by W., locleg, Canopus S.W., 38deg, Sirius W., 19deg, and Procyon W.N.W. at an _ altitude of 13deg. Hegulus is now N,W. by W., 26deg, Silica N.E. b.y N., 51deg, and Arcturus, in the same direction, 18deg. Antares, in the Scorpion, is IC., 34deg, Alpha Centauri S.E., 64deg, and Alpha Crucis. now the highest bright star, S. by E., 73deg. The Moon, at 9.30 p.m. on May 3, will be placed N. by E., Sldeg; on the forenoon of May 7 she will pass 3deg south of Spica, and Full Moon will take place early on the 7th. At 9.10 p.m. on the Bth she will be E., 34deg, and will pass 3deg north of Antares about 11 p.m. Early on the morning of the 10th she will pass 2deg above the bright planet Jupiter, last quarter falls on the evening of the 14th, and a conjunction with Saturn on the 16th. New Moon occurs on the forenoon of May 21, a conjunction with Hegulus early on the 28th, first quarter on tho afternoon of that day, while at 7.50 p.m. the Moon will appear M.N.W., 37deg. The planet Mercury will he at greatest elongation east, 21deg from the Sun, on May 7, stationary on the 20th, in conjunction with Mars on May 30, and in inferior conjunction with the Sun -on the 31st. The planet Venus will pass close to the remote planet Uranus on the evening of May 10. The planet Jupiter will be placed E. by S., 20 to 23deg, at 12hrs S.T. during the month. In the following list of times of superior conjunctions of his satellites, tho times are counted up to 24hrs, e.g., May 2:21:40 means 9.40 p.m. on May 2. The superior conjunctions of Jupiter’s satellites occur as follow:—lo, first, May 2:21:40, seventeenth, May 31:04:39; Europa, first. May 4:08:58, eighth, Mav 29:05:00; Ganymede, first. May 5:14:13, fourth. May 27:00:19; Callisto, Mav 1:03:24 and May 17:18:24. Ganymede will be at west elongation on Sunday evenings to Monday mornings as the month goes on, and at each elongation on Thursdays. Callisto will be at west elongation on May 13 and 30, at east elongation on May 5 and 21. From the Queen’s Gardens, Dunedin, at 12hrs S.T., Achernar appears lowover Vogel street, Hegulus rather low over Burlington street, Antares moderately low over the Early Settlers’ Hall, and Jun'ter rather low over the seaward end of Rattray street. At 9.10 p.m. on May 8 the Moon will be close to Antares. ECLIPSES AND TRANSITS AMONG THE STARS Among the most impressive astronomical phenomena are those of eclipses. Because of the great apparent size of the Sun and Moon, eclipses of these bodies are most spectacular events, even to the naked-eye observer. In an eclipse of the Sun the Moon comes between the Sun and the Earth, turning day into deep twil’ght in a total eclipse. Oil the oth4r hand, the eclipse of the Moon occurs when the shadow of the Earth falls on that body. In the case of the scars, we have to deal with bodies so distant that, even when magnified, they are only points of light. In many instance the spectroscope has proved to us that, where the telescope shows us only a single luminous point, there are really two suns revolving in ■a very- small orbit about their common centre of gravity. Hi such cashes the distances are small indeed, often not greatly exceeding the dimensions o f the stars concerned. There may be cases in which pairs of stars revolve in the same plane as the background of the sky. These do not approach or recede from us alternately-, hence the spectra give us no indication of their existence, unless they are of the composite type, due to two stars of widely different colours, but rather well-matched photographically. Indeed it is difficult to see how such pairs can be detected at all, unless with the aid of such an instrument as the interferometer at Mount Wilson. If the orbit is more or less inclined to the background of the sky, however, the star is recognised as a “ spectroscopic binary,” either with single lines shifting alternately towards the red and violet ends of the spectrum, or with lines which become double twice in every revolution. This phenomenon of the shifting of the lines is called the Doppler Effect. The waves of light from a receding body each start from a point more distant than the last, so they are lengthened, and therefore become redder; but an advancing body emits each wave from a point nearer than the last, so they are shortened, and shifted towards the violet. Similarly -with sound waves, an aeroplane approaches with a relatively shrill sound, the pitch of which falls perceptibly as the machine passes overhead and recedes from us.

Now, if the orbit is inclined to the background of the sky at a high angle, perhaps 70deg or more, the stars, if relatively large, may partially eclipse each other, as do the pair which form Spica, and if the inclination is 90deg a total ech'xise of the smaller one of the pair is inevitable, as is also a transit of the smaller one across the disc of the larger.

Many ages ago there was a second magnitude star in the northern hemisphere which acquired a supremely evil reputation among astrologers. The Arabs called it Algol, the Demon Star. According to ancient Greek mythology the hero Perseus slew the Gorgon Medusa, a terrible monster whose very appearance was so dreadful that it turned all living creatures who beheld it into stone! Its terror, strangely enough, could not bo reflected, so Perseus conquered it with the aid of a mirror, and carried off its head in triumph. No hero has had showered upon him such celestial honours as has Perseus. With the principal characters of his story he was transferred to the starry skies, and as the eye of his grisly trophy there still shines the Demon Star! Perhaps it was known to the science, of a forgotten past that the eye of the Gorgon indulged in a grave and sinister wink about once in every three days. Unfortunately Algol is not visible as far south as Dunedin, and at Wellington, though visible in a bad season for observation, it is too low for effective study. It is noted as the first star in which the effect of an eclipse was detected. Variations were noted by Montanari in 1669 and

Maraldi in 1694, but the changes in brightness were fully studied by Goodricke in 1782. They are characteristic changes, and give to eclipsing variables the alternative name of “ Algolids.” Algol is normally of mag. 2.1, but periodically it declines for 4h 23min, till it reaches mag. 3.2 After Ih 14min at this magnitude it brightens once more for 4h 23min and remains at mag. 2.1 for about 2d lOh. The whole period is 2d 20h 48niin. Variable stars of this type are most interesting from many points of view. Among the brighter examples may be mentioned Beta Lyrac, not far from Vega, Lambda Tauri near the llyades, Delta Librae, and V Pnppis. From the relative durations of the change from maximum to minimum, and the minimum itself, the relative dimensions of the stars can be computed, and Algol itself was the first star whoso diameter was determined in this or any other way. Also the relative surface brilliancies of the two stars can be measured from the magnitudes attained. There is always a “ secondary minimum,” as in the case of Beta Lyrac, and eclipsing variables with a pronounced secondary minimum arc called “ Lyrids,” after that typical example. This secondary minimum is not as deep as the primary one, and is caused by the transit of the smaller star across the disc of the larger one.

The changes in magnitude are not, however, the only effects of interstellar eclipses, nor are they necessarily the most striking ones. During the changes of magnitude the spectra sometimes show intense absorption in certain lines, and the changes of colour, when properly observed, may be considerable. As the eclipse itself occupies a relatively brief interval in a long period of constant light, Algolid variables are difficult to detect, and it is very likely that a large number await discovery. Also, the eclipse may be of trifling extent in magnitude. For instance, Eorneforos, or Beta Herculis, declines only from 2.81 to 2.96. The discovery of this eclipsing variable was due, as our readers know, to a striking periodic change in colour. Recent observations at Wellington show that the pair of stars concerned in the eclipse are extremely peculiar. The large one is 12 times as wide as tho Sun but yields only one-hundredth part of the Sun’s light. It would be as red as the carbon star 19 Piscium if observable alone, but its small mass classes it as a dwarf star, probably the first carbon dwarf to be discovered. Its mass is about 100,000 times that of the Earth, but only one-third of that of the Sun. The small star of the pair is almost violet in colour, and only 0.21 of the width of the Sun, though twice as heavy and 15 times as bright. It is 220 times as dense as the Sun, 310 times as dense as w-ater, or about 15 times as heavy, bulk for bulk, as the heaviest metals. The orbit is eccentric, and at poriastron the blue star is only four solar diameters away from the surface of the red one. which reflects enough of the blue light to cause a striking colour effect. The colour curve of Beta Herculis is itself a powerful vindication of our method of observation. Out pf 24 measurements only four are capriciously out of place. The remaining 20 form a curve of the type described in our notes for March, 1929, except for a minor change due to the reflection mentioned above. For 2.C days the colour remains steady at 8.5 uniform scale (white with a blue tinge), then falls rapidly to 8.2 (bluish white). After 1.1 days at this colour tho star returns to 8.5, and remains thus for 2.7 days. Then comes a gradual rise to 8.8 (white) lasting one day, and this value is constant for 1.5 days. Then comes a rapid rise, in about one hour, to 10.45 (a strong butter-yellow colour), which is retained for 2.2 days, and then, the colour returns to 8.5 by steps identical with those described in the rise of colour. The slow change from 8.8 to 8.5 is due to the appearance of the area of the red star which reflects the light of the blue one, and the value of 8.2 is due to the transit of the blue star across the disc of the red one. More observations of this part of the curve are needed, but it appears as if the blue reflected light is partly absorbed and partly intensified in colour by an extensive atmosphere surrounding the blue star. The rapid change from white to yellow is, of course, due to the total eclipse of the blue star by the red one. The whole period is 12.59 flays, and the mean separation of the pair one-seventh of the distance from tho Earth to the Sun, or 13 million miles. The difficulty of observation, especially of magnitude, is duo to the overpowering light of the giant KO star, which is of 113 sunpower and distant 185 million miles from the eclipsing pair, which revolves around a common centre of gravity with it in the period of 411 days. This great star is four times as massive as the Sun and 36 times as wide. If it could be removed we should see an intensely blue star of the fifth magnitude, changing periodically to a faint red star of the thirteenth magnitude, visible only with a fairly large telescope. The orbital velocity of tho great yellow star is calculated to be 12 miles a second, while its radial component, according to Plummer. is eight miles a second. The difference is clue to tho inclination of the orbit, which precludes the giant star itself from taking part in any eclipse. The light from Beta Herculis takes 140 years to travel to our eyes.

Tho discovery of the variation of Beta Herculis is only the beginning of a rapid series of discoveries. Most remarkable is that of Psi Velorum, a fourth magnitude star which proved unsatisfactory in the visual (naked eye) comparison of b and c Valorum. People who think that nothing of importance in astronomy can he done without a huge telescope should take note of this. Naked-eye comparisons in the past two months show that this star declines from mag. 3.5 to 3.8 once every 22 days, but there is a secondary decline to 3.7 midway between these primary minima. The star has a visual companion with a period of 35 years, hut as a spectroscopic binary it is not even suspected! At present colour and magnitude observations of Alpha Gnncri. a fourth magnitude star some distance west of Beguius. arc being taken at Wellington. Its period is about four days, and tho variations at each minimum last only one hour, the range being from 4.4 to 5.1, while tho colour varies distinctly.

The era of discovery now dawning is, for the writer himself, the reward of six years of steady observation. In those six years ho has made considerable efforts to interest his countrymen in the observation of star colours, for it has been his dseire to make tho section a national institution rather than a Wellington one. Incidentally, Dunedin is well represented among our still small membership in Wellington. Also the opportunities of discovery were seen to be considerable, and the desire to share thorn was often uppermost in the writer’s mind. Now that Korneforos has given such encouraging results the work of discovery itself must take pride of place, and for astronomers New Zealand must soon become also a “ newsv laud.”