HOW DID HENRIETTA DO IT? - ( part II )

With no Cepheids near enough to calculate their absolute magnitudes, based on the trigonometrical determination of their distances, indirect methods had to be used to determine the absolute magnitudes of the Cepheid variables. This was done by comparing the spectral types and classes of the Cepheids with those of stars of known distances and thus of known absolute magnitudes.

Another method was to compare the Cepheids with the members of clusters of known distances.

Harlow Shapley made the assumption that the parallaxes of nearby Cepheids could be determined on the basis that their average transverse velocities must be equal to their average radial velocities, with regard to the Sun as centre. The radial velocities could be readily measured. From the statistical values of the parallaxes the mean values of (m - M), the distance moduli, could be found and thus the distances D in parsecs could be calculated from the formula 5log D = (m - M) + 5. From the measured values of the apparent magnitudes m, the absolute magnitudes M could be calculated. These values of M were then ascribed to the Cepheids in the Small Magellanic Cloud, having the same periods as more nearby Cepheids. Shapley was assisted by M L Humason, E P Hubble and V M Slipher the foremost astronomers of the time, the 1920's, and eventually a scale of absolute magnitudes of the Cepheids, corresponding to the observed apparent magnitudes, was decided upon.

Using this scale, it was found that the Large Magellanic had a distance of about 170 000 light years and the Small Cloud, about 200 000 light years. This corroborated Henrietta Leavitt's supposition that the two Clouds were very far away.

Astronomers then had a mighty weapon for determining stellar distances greater than 100 parsecs (326 light years). In one leap measurable distances reached beyond 100 000 light years.

By measuring the periods and apparent magnitudes of Cepheid variables all over the Milky Way it became apparent that the Milky Way had a diameter of at least 50 000 light years, later found to be at least 100 000 light years. At more than 150 000 light years the Magellanic Clouds must therefore reside far beyond the fringes of the Milky Way Galaxy. Hence they had to be separate galaxies or clusters. This was the first indication that the Universe does not consist only of the Milky Way, but that the Milky Way is only one of the building blocks of the Universe. Besides the Magellanic Clouds, some of the nebulae seen in the sky could be stellar systems outside the Milky Way.

The 100-inch telescope on Mount Wilson succeeded in resolving individual stars in these nebulae and especially in M31, the Great Nebula in Andromeda, which thus had to be looked upon as being a galaxy rather than a nebula. By means of the Cepheid variables discovered in the Andromeda Galaxy by E P Hubble, it was found that the distance of M31 had to be at least 900 000 light years and that it had a distinct spiral pattern.

When the 200-inch (5 metre) telescope on Mount Palomar came into use (1948), W Baade could find no trace of the very short period Cepheids, the RR Lyrae variables but all other classes of stars were found there. The RR Lyrae variables all have the same absolute magnitude of 0,8 and at a distance of 900 000 light years (= 900 000 ÷ 3,26 parsecs), their apparent magnitudes would be 23:

(

= -4,2 + 5 x 5,44102

= -4,2 + 27,2

= 23 ).

The five-metre telescope could easily see to magnitude 24, more than 2 times dimmer but it could find no RR Lyrae variables. The Andromeda Galaxy thus had to be further than 900 000 light years. Besides, a distance of 900 000 light years does not agree with a distance of 2 million light years which had been indicated by the Supernova S Andromedae which had appeared in M31 in 1885. It was unthinkable that a huge galaxy such as M31 would contain all sorts of stars, but no RR Lyrae variables.

Baade concluded that there must be something wrong with the Cepheid scale of magnitudes and distances. He pointed out that the classical Cepheids with periods from 2 to 30 days are F and G giants, whereas the very short period RR Lyrae variables were all of Type A and the Long Period variables are of spectral types M and K. These stars that were so very different thus had to have different absolute magnitude and distance scales.

For classical Cepheids of periods from 2 to 14 days, selected by A R Sandage and G A Tammann of Mount Palomar Observatory and having absolute magnitudes ranging from -2,5 to -4,7, the relationship between absolute magnitude M and periods P (in days) works out at

M = -1,48 - 2,7 log P ± 0,32.

Using this relationship the absolute magnitude of Delta Cephei, the prototype of the Cepheids, works out to M = -3,45 and its distance to 339 parsec or 1105 light years. In the catalogues Delta Cephei was classified as having a distance of 630 light years. In 1993 C Gatewood and his assistants at the Allegheny Observatory, using the Multichannel Astrometric Photometer in which two separate telescopes form separate images of the object and thus give a greater resolution, found that the distance of Delta Cephei is indeed 1100 light years.

Today the most accurate distances are 166 000 to 170 000 light years for the Large Magellanic Cloud; 200 000 to 205 000 light years for the Small Magellanic Cloud and 2 200 000 light years for the Great Galaxy in Andromeda.

Thus the work begun by Henrietta Swan Leavitt led Astronomers far, far into the depths of space. The brightest Cepheids have enabled astronomers to measure distances up to 13 million light years. By means of the redshift and Hubble's Law distances can be determined as far as 15 milliard light years (15 thousand million light years) - as far as the observable edge of the universe. At this distance the galaxies and quasars are receding with velocities approaching the speed of light so that they become invisible even in the most powerful telescopes.

The Period-Luminosity Law, first broached by Henrietta Leavitt was the spur which enabled astronomers to form an idea of the structure of the universe.

Jan Eben van Zyl