[History of Astronomy]
[Astro History: Asian and Greek]
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A Brief History of Astronomy
Einstein to Hubble
(1915 to 1929)
He brings Planck's ideas of quanta into prominence by using them to explain a previously mysterious effect when light shines on metals (the Photoelectric Effect).
He explains a strange movement of small particles in a liquid (called Brownian Motion) by proving mathematically that it must be due to the random motions of atoms and molecules. This is the first direct proof of atomic theory and allows the size of these small particles to be determined.
His Special Theory of Relativity explains why the Michelson and Morley experiment had apparently failed. Absolute motion cannot be measured: all motion is relative. This leads to the idea that the velocity of light is the maximum speed that any material body can have. No information can travel faster than light. When we look into distant space we are looking at the past! A further development leads to the famous equation
which shows that matter is a concentrated form of energy. This would allow future scientists to explain the source of the energy of the stars. Time and space turn out to be changeable and dependent on the position and motion of the observer. This defies common sense but would be found to be in accord with observation.
Einstein's General Theory of Relativity changes the way humans look at gravity. Newton had envisaged gravity as a force between all matter. Einstein sees matter as distorting the very fabric of space, causing it to curve. This curvature of space causes matter to move in non-linear paths. Under most conditions, the differences between the two theories of gravity are minimal. However, Einstein's theory explains the anomalies in the orbit of Mercury found by Leverrier sixty years earlier.
General Relativity also predicts that light would be bent by a gravitational field. This would be proved during a total eclipse of the sun a few years later. Another prediction is that a strong gravitational field would give a spectral red shift separate from that produced by the Doppler shift. This is proved when the spectrum of a very dense White Dwarf star is examined. The star is a companion of Sirius so the Doppler effect could be accounted for since the two stars move together.
The General Theory of Relativity gives an overall view of the entire Universe indicating that it is not static. Einstein thinks that the Universe is static and disregards this part of his equations. He would soon be proved wrong.
If this idea is correct, the Sun's planetary system could be unique since stellar encounters are very rare. Stars are too far apart to interact with others very frequently.
He assumes that the centre of these clusters is the centre of the Galaxy. If so, then that centre is 50,000 Light Years away from the Solar System. Not only is the Earth not the centre of the Solar System; the Solar System is nowhere near the centre of the Galaxy. Shapley points out that the Milky Way looks symmetrical from the Earth because of the existence of dark nebulae (interstellar clouds) blocking out distant stars.
Shapley's measurements to the centre of the Galaxy turn out to be an over-estimate, however. This is the first time that the size of the Universe is over-estimated. The currently accepted figure is 30,000 Light Years. In 1930 Robert Trumpler would show that interstellar dust dims the Globular Clusters making them look further than they actually were.
He estimates the interior temperature of the Sun to be in the millions of degrees. This is so hot that Jeans' idea of planetary formation would not work.
Eddington discoveres the Mass-Luminosity Law for stars. More massive stars are more luminous. His studies allow him to explain how Cepheid stars vary in brightness by pulsating.
The centre of the Galaxy is confirmed to be 30,000 Light Years from the Sun's position. The Sun requires 200 million years to orbit the Galactic centre. The Galaxy has enough matter to make 100 thousand million stars like the Sun.
The Andromeda spiral is in fact a galaxy outside our own and is now called the Andromeda Galaxy.
Our Galaxy, with its thousands of millions of stars, is not unique.
More galaxies are quickly found; there are billions now known. The Universe is far, far larger than previously thought. Hubble finds that there are three types of galaxies: spiral, elliptical and irregular. From its overall properties our Galaxy appeared to be a spiral.
Vesto Slipher had previously measured the velocities of many nebulae by taking photographs of their spectra.
Hubble analyses the velocities of the ones now recognised as galaxies. He finds that the overwhelming majority of galaxies are moving away from us. Their spectra show a Red Shift. He shows that there is a simple mathematical relationship between the distance of the galaxy and its velocity away from us. This relationship is now called Hubble's Law.
Hubble's Law provides another yardstick with which to measure distance. The Red Shift of a galaxy can be measured from its spectrum. This gives its velocity of recession from us. Hubble's Law provides the distance.
The simplest way to explain these observations is to assume that the Universe is expanding. Einstein's General Theory of Relativity had already predicted that the Universe would not stable if it was static. Hubble's work shows that the Universe is, indeed, not static.
Modern Cosmology (the study of the overall structure of the Universe) can be said to have begun with Hubble's work.
History of Science A large collection of resources looking at the history of astronomy, physics, chemistry and mathematics.