A Brief History of Astronomy

Einstein to Hubble

(1915 to 1929)


1915

Physics and astronomy are revolutionised by Albert Einstein.

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

E = mc2

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.

1917

An idea to explain the formation of the Solar System is postulated by James Jeans. He suggests that a passing star had drawn material from the Sun. This material had condensed to form the planets (including the Earth).

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.

1918

Harlow Shapley applies Leavitt's Cepheid yardstick to Globular Clusters. These are large spherical groups of stars. Most types of object are distributed randomly in the sky. Globular Clusters, however, are bunched up together. 70% of them occupy a 2% region of the sky. Shapley finds that these clusters are arranged in a sphere centred on a point a long way from the Sun.

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.

1924

Arthur Eddington uses gas theory to study the interiors of stars. He shows that stars are stable because there is a balance between two opposing tendencies. The energy and gas pressure coming from the hot centre push the star outwards, tending to expand it. Gravity pulls the star inwards, tending to contract it.

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.

1926

Edwin Schrödinger discovers a wave equation that puts Quantum Mechanics on a firm mathematical footing. This would lead to advances in the understanding of atomic and molecular spectra that would increase knowledge in astronomical objects.

1927

Jan Oort studies the star streaming discovered by Kapteyn. He shows that these stellar movements are due to the stars in the Galaxy revolving about the centre. The stars closer to the Galactic centre travel faster than the stars further away. Oort uses the stellar motions to find the location of the Galactic centre: its position agrees with Shapley's centre of Globular Clusters.

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.

1929

Edwin Hubble studies the spiral nebulous object in the constellation of Andromeda (first noted by Al-Sufi and Marius). Using the world's largest telescope, he manages to see stars in the object. Some of the stars are Cepheids. This allows him to determine their distance and hence the distance of the spiral. The distance of 800,000 Light Years he finds is far outside the domain of our Galaxy even though it is an under-estimate.

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.


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KryssTal Related Pages

An easy-to-understand scaling of the Universe in space. Distances in space are represented by the time light takes to travel there.

An easy-to-understand scaling of the Universe in time. The chronology of the Universe is compared to a real year.

A listing of the 20 brightest stars as well as explanations of the terms used.

Information about the planets and satellites of the Solar System with explanations of the terms used.

A historical account of the discovery of the electromagnetic spectrum and its uses in Astronomy. Radio waves, infra-red, visible light, ultra violet, X-rays and gamma rays are explained.

An account of how various properties of stars can be measured by studying starlight. Includes brightness, distance, luminosity, temperature, mass, radius, density and an introduction to the H-R Diagram.

An account of how stars evolve and change the chemistry of the Universe.

The force that moves apples and planets. A short introduction to the ideas of Kepler and Newton that culminated with the theory of Universal Gravitation.

This looks at the history of inventions and the various civilisations of the world.

Selected biographies of people from around the world including scientists and astronomers.


External Links

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History of Science A large collection of resources looking at the history of astronomy, physics, chemistry and mathematics.