BRIGHTNESS OF STARS

Timaru Herald, Volume CXXV, Issue 18618, 14 July 1930, Page 2

BRIGHTNESS OF STARS

SCALE OF MAGNITUDE On any clear night one cannot help being struck with the marked differ- j ence in the brightness of the stars ; visible to the naked eye, the planet j Venus, the brightest of them all, often | being sufficiently brilliant to be seen with ease in broad daylight, while others are so faint that they cannot be seen on a moonlight night (writes "Regulus,” in the Melbourne ‘Age’). With field glasses, fainter stars can be observed, with more powerful telescopes still fainter stars are rendered visible, and the range of brightness is con, tinually increased as the telescope becomes larger. Earliest Estimates of Brightness The first astronomer who formed a star catalogue and classified the stars in order of brightness was Hipparchus (127 8.C.) His results appear in the Almagest of Ptolmy completed in 138 A.D.,in which is given a list of 1.028 stars with their positions on the celestial sphere, and also an indication of their brightness. For the brightness, the stars were divided into six groups; those in the first group, consisting of the brightest stars, were said to be of the first magnitude, the next group were called second magnitude stars, and so on, the faint stars barely visible to the unaided eye on a clear, moonless night being of the sixth magnitude. So then, in this connection, magnitude simply refers to the apparent brightness of the stars. Ptolemy did not subdivide his classes, but he did recognise that there was some difference in brightness between stars in the same group, for occasionally he attached the words "greater” or "less” to the stars to indicate that they were brighter or fainter respectively than the average star of the group. It is interesting to note here that the magnitudes assigned by Ptolemy have been compared with careful measurements of brightness made in recent years, and the results show that what Ptolemy has marked “less tl*an 2” is on the average of the same brightness as what he calls "greater than 3,” “less than 3” is on the average the same as "greater than 4,” and so on. William Herschel’s W’ork Ptolemy’s magnitudes were adopted without revision by his successors for many hundreds of years, except by the Persian astronomer A 1 Sufi, in the tenth century, who revised them with much care. William Herschel introduced an important new method in a paper published in 1796, which included "a catalogue of comparative brightness, for ascertaining the permanency of the lustre of stars.” He points out how confusing was the system then in vogue for assigning magnitudes to stars, since reference was made to an imaginary standard which was the average brightness , of a class, but no comparision of the stars one with another was carried out. Actually he found many discrepancies in the accepted magnitudes, so that for instance, some so-called fourth magnitude stars were fainter than some which were designated fifth. Either the stars had changed in brightness or many of the assigned magni- j tudes werej wrong. , Herschel did not assign definite magnitudes to the stars, but drew up a number of sequences, in which neighbouring stars were arranged in order of brightness. In addition, he gave arbitrary symbols to represent degrees of difference' between pairs of stars, thus c meant "the least perceptible difference less bright”; a dot, equality; a comma, "the least perceptible j difference more bright”; a dash, “a very small difference more bright,” and so on. In all Herschel’s sequences furnished observations of nearly three thousand stars. Argelander The next step was due to Argelander of Bonn, who with his two assistants, Schoenfeld and Krueger, began in 1852 a great catalogue of stars in the northern hemisphere, in which the magnitude as well as the position was given. Originally they estimated to half magnitudes, using the symbol 2.3 to denote a star lying between second and third magnitude. With practice they found that they could estimate magnitudes more closely, and so they introduced two subdivisions between the half magnitudes, thus giving a scale, such as 7, 7 bare ,7.8 full, 7.8, 7.8 bare. 8 full, 8. Still later, in 1857, they introduced a decimal subdivision j between the magnitudes, and this procedure is in use to the present day. hut extended to the second, and even the third, decimal place. Pogson’s Scale. Up to this point the assignment of magnitude had been arbitrary, each observer made his own scale. The magnitude of those stars which were visible to the naked eye were fairly well standardised, but for telescopic stars the magnitudes assigned differed very greatly. For instance. John Herschel would class as twentieth magnitude a star which Struve would class as twelfth. Towards the middle of the nineteenth century the need for a definite scale of magnitude became very great, and several instruments were designed for measuring the relative brightness of stars. Various determinations were made for stars to which magnitudes j had been assigned, and it was found that the amount of light received from a star was about two and a-half times that from a star one magnitude fainter. In 1850 Pogson proposed that this ratio be definitely fixed at 2.512, a convenient number for the purpose. Incidentally J. Herschel had found that a first magnitude star on the conventional scale gave about 100 times as much light as a sixth magnitude star, and as 2.512 is the fifth root { of 100, Pogson’s suggestion agreed with the scale that had been handed down from Hipparchus. This ratio is now universally accepted in the determination of stellar magnitude. It is necessary, too. to fix one point on the scale, and for this the scale is chosen so as to secure as good an agreement as possible with the magnitudes in Argelander’s catalogue at the sixth magnitude. The Range of Apparent Brightness, Having a definite scale of magnitudes fixed, and suitable instruments having been designed by which the relative brightness, the magnitude of | various stars can be measured, and the ' amount of light received from them found. The brightest of all the fixed stars is Sirus, of which the magnitude is minus 1.58. We receive somewhata more than ten times as much light from Sirius as from Alpha Crucis, which is a standard first magnitude star. The Range of Luminosity. The apparent brightness of a star depends on two factors, the amount of light that the star sends out, and its distance away from us. How do the stars compare with the sun in the amount of light they give out? The nearest star is 250,000 times as far away as the sun, so if the sun were moved as far away as that, its apparent brightness would be reduced in the ratio 250,000 times 250,000 to 1. This corresponds to 27 magnitudes, so that the sun would appear as a star of magnitude 0.3. The distances of a good many stars have now been measured, the apparent brightness also has been determined, and thus the luminosity can be found. The results show a very wide

range, one faint star discovered by van Maanen, at Mount Wilson Observatory giving out only one fifty-thousandth part of the light given out by the sun, /hile Canopus has at least ten thousand times the sun's luminosity. So then it is seen that the stars are oi different apparent brightness partly because of their different distances and i partly becaues of their different lumI ircf-tties. Van Maanen's star, although j comparatively close, appears faint because it gives out but little light, while Sirius owes its great apparent brightness chiefly to its- being one of the stars closest to the earth, its luminosity being but 27 times that of the sun.