Walter Baade : Baade was a German-born American, whose work gave new estimates for the age and size of the universe. During the wartime, blackouts aided his observatons and allowed him to indentify and classify stars in a new and useful way, and led him to increase and improve Hubble’s values for the size and age of the universe (to the great relief of geologists. ) He also worked on supernovae and radiostars.
Milton Humason : Humason was a colleague of Edwin Hubble’s at Mt. Wilson and Palomar Mtn. ho was instrumental in measuring faint galaxy spectra providing evidence for the expansion of the universe. Jan Oort : In 1927, this Dutch astronomer proved by observation (in the Leiden observatory) that our galaxy is rotating, and calculated the sirance of the sun from the centre of the galaxy and the period of its orbit. In 1950 he sugested the exsistence of a sphere of incipent cometary material surrounding the solar system, which is now called the ‘Oort cloud. ‘ He proposed that comets detached themsleves from this ‘Oort-cloud’ and went into orbit around the sun.
From 1940 onwards he carried out notable work in radio astronomy. Harlow Shapley : Shapley deduced that the Sun lies near the central plane of the Galaxy some 30,000 light-years away from the centre. In 1911 Shapley, working with results given by Henry N. Russell, began finding the dimensions of stars in a number of binary systems from measurements of their light variation when they eclipse one another. These methods remained the standard procedure for more than 30 years.
Shapley also showed that Cepheid variables cannot be star pairs that eclipse each other. He was the first to propose that they are pulsating stars. In the Mount Wilson Observatory, Pasadena Calif. , in 1914, he made a study of the distribution of the globular clusters in the Galaxy; these clusters are immense, densely packed groups of stars, some containing as many as 1,000,000 members. He found that of the 100 clusters known at the time, one-third lay within the boundary of the constellation Sagittarius.
Utilizing the newly developed concept that variable stars accurately reveal their distance by their period of variation and apparent brightness, he found that the clusters were distributed roughly in a sphere whose centre lay in Sagittarius. Since the clusters assumed a spherical arrangement, it was logical to conclude that they would cluster around the centre of the Galaxy; from this conclusion and his other distance data Shapley deduced that the Sun lies at a distance of 50,000 light-years from the centre of the Galaxy; the number was later corrected to 30,000 light-years.
Before Shapley, the Sun was believed to lie ne! ar the centre of the Galaxy. His work, which led to the first realistic estimate for the actual size of the Galaxy, thus was a milestone in galactic astronomy. Allan Sandage : Sandage (U. S) discovered the first quasi-stellar radio source (quasar), a starlike object that is a strong emitter of radio waves. He made the discovery in collaboration with the U. S. radio astronomer Thomas A. Matthews.
Sandage became a member of the staff of the Hale Observatories (now the Mount Wilson and Palomar Observatories), in California, in 1952 and carried out most of his investigations there. Pursuing the theoretical work of several astronomers on the evolution of stars, Sandage, with Harold L. Johnson, demonstrated in the early 1950s that the observed characteristics of the light and colour of the brightest stars in various globular clusters indicate that the clusters can be arranged in order according to their age. This information provided insight into stellar evolution and galactic structure.
Later, Sandage became a leader in the study of quasi-stellar radio sources, comparing accurate positions of radio sources with photographic sky maps and then using a large optical telescope to find a visual starlike source at the point where the strong radio waves are being emitted. Sandage and Matthews identified the first of many such objects Sandage later discovered that some of the remote, starlike objects with similar characteristics are not radio sources. He also found that the light from a number of the sources varies rapidly and irregularly in intensity.