The Theory of Concentric Spheres

Greek philosophers began a new approach that reached beyond astrology, attempting to explain in a rational manner why the planets move as they do. The sphere and circular motion was their preferred ideal for celestial motion (remaining so for two millennia). As geometric forms, the sphere and the circle were much investigated in Greek mathematics. Also, perfect circular motion, always returning back its original point, seemed to be suitable for celestial bodies held to be divine beings or at...

Nuclear Force

When a beryllium target is bombarded by fast alpha rays, beryllium starts to radiate unknown, very penetrating radiation. Chadwick found out first that it does not consist of electromagnetic radiation, but of particles. Then, he reasoned that the particle cannot have electric charge since it penetrates matter much better than protons. Finally, his collision experiments produced the mass of the particle, approximately equal to the proton mass. Chadwick called the particle the neutron because of...

Life After the Main Sequence

Most of the life of a star is spent at the main sequence stage where the star turns more and more of its hydrogen into helium. Examining Table 19.1 for the interior of the Sun, we see that the photosphere of the Sun has its original 73 mass abundance of hydrogen. However, to construct an equilibrium model the core must have only 36 . This is consistent with the theoretical picture that hydrogen in the core converts to helium during the life of the Sun, but conditions near the photosphere are...

Shapleys Second Copernican Revolution

The American astronomer Harlow Shapley (1885-1972) moved the Sun from the central position in the Milky Way where star counts had put it. Shapley's path to science was not straightforward. In his memoirs Shapley tells that he went to University of Missouri to study journalism, but the beginning of the course had been shifted till the following year. He decided to study something in the mean time and thumbed through the university syllabus. The first subject in the syllabus the name of which he...

The Race Toward Stellar Distances

During his short life, Joseph von Fraunhofer made important advances to telescopes. He constructed a stand on which the telescope could rotate equatorially, with the axis of rotation pointing at the North Pole. It had a clock mechanism keeping the correct rate of rotation so that the desired star remained in the field of view and its position could be carefully measured by an astronomer. He also manufactured a special kind of refracting telescope, so-called heliometer, which was very suitable...

Atoms and Nuclei

We now understand light as vibrations of electric and magnetic fields which somehow propagate through space. Obviously, we need to discuss the nature of light further, but before we do so, we should first ask, What is matter The Greek philosopher Empedocles (Chap. 2) had many interesting ideas on the workings of nature. For example, he envisioned that light travels at a finite (very large) speed, which gained acceptance only much later. He also proposed the idea that matter is made of four...

Great Star Catalogs and Kapteyns Universe

What is needed to chart the distribution of stars in space Clearly the directions to the stars, but one also needs to know their distances. Then one can define the outline of the Milky Way. But astronomers have to be satisfied with much less. As Herschel found, one can never see all the stars some are definitely too dim even for modern telescopes. Moreover, it is impossible to measure the distances to all stars. There were very few parallax (that is, distance) measurements in the nineteenth...