A TURNING POINT IN ASTRONOMY

Up to the beginning of the 20^th century astronomers were able to measure the distances of stars only as far as 300 light years. The first measurement of a stellar distance was accomplished by Friedrich W Bessel (1784 - 1846) who, in 1838 succeeded in measuring the distance of the star 61 Cygni, a 5th magnitude star. He chose this star because it has a large proper motion, moving 5,2 seconds of arc per annum against the background of the stars. Bessel found the parallax of 61 Cygni to be 0,31 arcseconds, which meant that its distance was 1 ÷ 0,31 = 3,2 parsecs, i.e. 3,2 x 3,25 = 10,4 lightyears.

E Henderson at Cape Town had measured the parallax of Alpha Centauri in 1831 - 33 but the calculations were only completed in 1839; so he just missed being the first.

By this trigonometrical method the parallax that theoretically could be measured was 0,01 seconds of arc, namely a distance of 1 ÷ 0,01 = 100 parsecs = 100 x 3,26 = 326 light years. At distances of 300 light years the probable error in the measurements was as great as the measurements themselves, the accuracy thus being ± 50%. In all directions around the Earth, astronomers could therefore measure stellar distances encompassing a sphere of diameter 600 light years. We know today that the diameter of the disc of the Milky Way is 100 000 light years. 600 is only one one-hundred and sixty-sixth of the diameter of the Milky Way. Most of the stars in our Galaxy are crowded together near the centre of the Galaxy and we occupy a position near one of the outermost spiral arms and we had succeeded in measuring the distances of one to two thousand close-by stars.

During the first twenty years of the 20th century a great dispute arose among astronomers as to whether the nebulae seen in the sky were within the bounds of the Milky Way or whether they were outside the limits of the Milky Way. Things came to a head when Harlow Shapley and Heber Curtis faced each other in open debate at the National Academy of Sciences, USA. Shapley maintained that those nebulae which showed spiral structures were masses of gas within the Milky Way and not outside it. He ascribed their continuous spectra as due to the scattering of light from nearby stars. Curtis maintained that these nebulae were stellar systems, like the Milky Way but at distances so great that the individual stars could not be resolved.

Shapley based his arguments on Van Maanen's estimate of the distance of the spiral "nebula" M101, based on its speed of rotation which could be accurately determined by the Doppler effect. Van Mann had found a period of rotation of M101 of 850 000 years implying a speed of rotation in excess of the speed of light if M101 were located outside the Milky Way.

Shapley held that the surface brightness of the Milky Way as determined by F Seares was much greater than would be the case if the nebulae were outside the Milky Way. Shapley said that the nova S Andromedae which shone brightly at 6^th magnitude in the nucleus of the Andromeda "nebula" in 1885 was far brighter than the brightnesses of all known novae, if Andromeda lay outside the Milky Way. Shapley also asked why no spiral nebulae were to be seen in the plane of the Milky Way. He did not agree that it was due to the obscuring effects of gas and dust in the plane of the Milky Way. Shapley pointed to the great velocities of recession of the nebulae found by V M Slipher and said they could not be correct if the nebulae were external to the Milky Way.

Little did Shapley know that nova S Andromedae (1885) was indeed a most exceptional nova, being at least 6 magnitudes brighter than any nova -- it was the first Supernova.

On the other hand Curtis argued that Shapley's estimate of 20 000 light years for the distances of nearby spirals must be far too low and in any case some spirals had angular sizes that would place them 20 million light years away, proving that they were outside the Milky Way.

He argued that if the novae in the Andromeda "nebula" were of the same average brightness as novae in the Milky Way, Andromeda must be outside the Milky Way. Curtis asked how it could be that the speeds of recession of the "nebulae" could be so inordinately large.

As always in Astronomy, observation would be the deciding factor.

From 1905 to 1912 Henrietta Swan Leavitt measured the brightnesses and periods of variation of Cepheid variable stars in the Magellanic Clouds. In 1912 she published her findings. She found that the brightnesses were proportional to the logarithms of the periods of variation of these Cepheids -- a remarkable discovery! This formed the basis of the Period-Luminosity Law. Although no Cepheid could be found, near enough to check its distance, the law gave a method of determining the absolute magnitudes of these stars. By comparing the absolute magnitudes so calculated, with the observed apparent magnitudes, the distances could be calculated. The distances determined by E P Hubble, M L Humason and Shapley, showed that the Magellanic Clouds must be more than 100 000 light years away! They were thus stellar systems (galaxies) outside the Milky Way. In one fell swoop astronomers' abilities to deter-mine stellar distances had leapt from 300 light years to more than 100 000 light years! These distances showed that the diameter of the Milky Way is at least 100 000 light years and that the Andromeda Galaxy (now, no longer a nebula) was at least one million light years away, far beyond the limits of the Milky Way. (Today's known distance of the Andromeda Galaxy is 2 200 000 light years.

Furthermore it was found that the galaxies (now no longer nebulae) are all receding from each other and that the fainter, and thus further, galaxies are receding faster and faster. This was the basis of Hubble's Law: V = H_o D, and it showed that the universe is expanding. Cosmology took on a new complexion and the theory of relativity came into its own. The Period-Luminosity Law was certainly the single most revolutionary advance in Astronomy.

Due to the work of W Baade the Period-Luminosity Law became divided into three categories:

1. the very short period pulsating variables, the R R Lyrae variables of periods less than two days;

2. the classical Cepheids with periods of two to 20 days; and

3. the long period variables.

It was now possible to draw up a correct scale of distances in the universe to as far as the edge of the observable universe, 10 to 15 milliard light years away (10 to 15 thousand million light years) a universe 10 to 15 milliard years old!

Jan Eben van Zyl