1838
The first stellar distances are finally measured.
Friedrich Bessel measures the parallax of a faint star called 61 Cygni. It had been chosen because it has a large Proper Motion and is therefore assumed to be nearby.
Even the nearest star is over 270,000 times further away than the Sun!
The stars are so far away that they must be Sun-like in luminosity to be visible from the Earth. The Sun is thus shown to be an ordinary star seen from close up. Copernicus had been correct when he stated that the stars were too distant for a parallax to be easily visible.
1846
The French mathematician, Urbain Leverrier (France) studies anomalies in the motion of Uranus and predicts the existence of a new planet using
the laws of gravity. This planet (Neptune) is quickly detected and is considered another triumph for Newton.
The orbit of the planet Mercury is found to have an anomaly which Newton's laws of gravity cannot explain. This has to wait for Einstein sixty years later.
1848
Armand Fizeau shows that lines in a spectrum change position when the light source is moving to or from the observer. When the source is moving away the lines are shifted towards the red end of the spectrum (a Red Shift); when the source is moving closer the lines are shifted towards the blue end of the spectrum (a Blue Shift). This is called the Doppler Effect.
1851
Jean Foucault uses a large pendulum in a church to prove that the Earth is rotating on its axis. This is now called a Foucault Pendulum and is the first direct proof of the Earth's rotation postulated by Heracleides 2000 years previously.
The exact way the pendulum moves depends on the latitude. This can only happen if the Earth is spherical.
Foucault also measures the speed of light in the laboratory to a high level of accuracy.
1854
Gustav Kirchhoff studies the spectrum of glowing substances. He discovers that each type of atom gives a different set of lines in the spectrum. This gives a method of identifying the atoms present in glowing objects without needing a sample in the laboratory.
1863
William Huggins applies the technique of spectroscopy to astronomy. He studies the composition of the Sun, stars and planets. The same elements that exist on Earth are found in space.
He finds that the Sun and stars are mainly made of Hydrogen. He measures the Doppler Effect of the star, Sirius and finds that it is moving away from us. Comets are shown to contain glowing carbon compounds. Many Nebulae produce spectra that show that they are glowing gases rather than stars. He uses photography to obtain spectra of very faint objects.
Aristotle's 2100 year old idea that the heavens are made of a different element (the Aether) is finally proved to be wrong.
1864
Pietro Secchi photographs the spectra of over 4000 stars. He finds differences which would eventually lead to ideas of stellar evolution.
1868
Joseph Lockyer discovers a new element in the Sun's spectrum. It is named after the Greek word for Sun, Helium. 40 years later, Helium would be found on the Earth.
1882
In the USA, Albert Michelson and Edward Morley measure the speed of light to a very high level of accuracy. They attempt to measure the absolute motion of the Earth around the Sun by finding a difference in the speed of light in different directions. No difference is found.
Michelson and Morley consider that the experiment has failed because it could not be reconciled with the physics of the day. It turns out to be the most glorious failed experiment in the history of science, eventually laying the groundwork for Einstein's
Theory of Relativity.
1893
Wilhelm Wien studies radiation of energy and light from hot objects. He shows that the colour of a glowing body is related to its temperature in a definite mathematical way. This is called Wien's Law and can be applied to the surfaces of stars.
Red stars are coolest. Orange stars are hotter; then come yellow stars; hotter still are white stars. Blue stars are the hottest. Very cool stars give out their energy in the infra-red. The very hottest stars shine mainly in the ultra-violet.
The Sun's surface temperature is shown to be around 6,000°C. Some stars are hotter than the Sun.
Theoretically, Wien's energy pattern could not be explained by the physics of the day. The explanation would have to await the development of
Quantum Theory.
1895
Maximillian Wolf and Edward Barnard discover that dark areas in the Milky Way are dark nebulae made up of gas and dust.
1900
The German physicist, Max Planck, develops the idea that energy exists in lumps (called quanta) rather than continuous emissions. This idea explains Wien's work on radiation.
1906
Jacobus Kapteyn repeats William Herschel's statistical analysis of the stars. He discovers order in the motions of the stars. The stars are not moving at random in the Galaxy. This is the phenomenon of star streaming. His measurements of the size of the Galaxy increase its size but are still less than 60% of the correct figure. The presence of dark nebulae hinder accurate measurements.
1912
Henrietta Leavitt studies thousands of variable stars. She finds that a particular type (called Cepheids) have regular periods and are easily distinguished by the way their brightness changes (the Light Curve) and their spectra. She observes examples of these stars in star clusters. Stars in these clusters are all at the same distance from the earth. This allows her to discover a link between the period and the luminosity. This is called the Period-Luminosity Law. The longer the star's period, the more luminous the star.
These stars provide a yardstick for measuring distant objects in the Universe. The period gives the luminosity; the luminosity can be compared with the apparent brightness of the star as seen from the Earth; this gives the distance to the star. If the star is part of a group, cluster or nebula, the distance to that object is known.
1913
Walter Adams works out how to deduce a star's luminosity from its spectrum. Once the luminosity is known, the distance can be calculated from the apparent brightness.
This new tool allows Ejnar Hertzsprung and Henry Russell to measure the distance to nearby Cepheid variables thus providing the scale to Leavitt's cosmic yardstick.
Hertzsprung and Russell go on to find a relationship between the colour and luminosity of stars. Blue (hot) stars tend to be luminous, yellow (medium) stars tend to be less luminous, red (cool) stars tend to be faint. More than 90% of stars fit this classification and are called Main Sequence stars. Some red stars are too luminous for their colour. These are called Red Giants because they are very large. Some white stars are too dim for their colour. These are small and very dense stars called White Dwarfs.
A graph of these results is known as the Hertzsprung-Russell (or H-R) diagram. Special stars (like Cepheid variables) occupy distinct zones in the H-R diagram. The diagram is very important in the study of stellar structure and provides a foundation for ideas about stellar evolution.
Russell studies the spectrum of the Sun to determine its chemical composition. The Sun is 90% Hydrogen, 9% Helium and 1% everything else. Most stars have a similar composition.
Niels Bohr applies Planck's quantum ideas to atoms helping to explain why and how atomic spectra form.
[Astronomy History: Solar System]
[Astronomy History: Einstein to Hubble]
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.
