Coming of Age in the Milky Way (67 page)

Time
: 1802

Noteworthy Events
: William Wollaston discovers spectral lines in the spectrum of the sun.

Time
: 1814

Noteworthy Events
: Joseph Fraunhofer, using the first grating spectroscope, rediscovers solar spectral lines and charts them, laying the basis for astrophysical spectroscopy.

Time
: 1820

Noteworthy Events
: Hans Christian Örsted discovers that electric current produces a magnetic field, ushering in the study of electromagnetic force.

Time
: 1823

Noteworthy Events
: John Herschel proposes that Fraunhofer lines may indicate the presence of metals in the sun.

Time
: 1830

Noteworthy Events
: Charles Lyell publishes the first volume of his
Principles of Geology
, presenting evidence for the uniformitarian theory that the geological record can be explained in terms of the slow action, over aeons of time, of processes that continue in the world today.

Time
: 1831

Noteworthy Events
: Charles Darwin, a copy of Lyell’s book in hand, departs aboard the
Beagle
on a five-year voyage around the world.

Time
:
1832

Noteworthy Events
: Darwin adduces the essential elements of his theory of evolution by natural selection, but does not publish the theory for another twenty-two years.

Time
: 1833

Noteworthy Events
: First precise measurement, by means of parallax, of the distance to a star.

Time
: 1842

Noteworthy Events
: Christian Johann Doppler points out that the wavelength of sound or other emissions from a moving source will appear to a stationary observer to be higher in frequency if the object is approaching, lower if it is receding—the “Doppler shift.”

Time
: 1847

Noteworthy Events
: Hermann von Helmholtz proposes the law of conservation of energy.

Time
: 1849

Noteworthy Events
: Jean-Léon Foucault detects spectral emission lines.

Time
: 1850

Noteworthy Events
: First astronomical photograph—a daguerreotype of the moon—is made, by W. C. Bond at Harvard.

Time
: 1855–1863

Noteworthy Events
: Robert Bunsen and Gustav Kirchhoff workout the basics of spectral analysis, by which the spectra of laboratory materials can be compared with those of the sun and stars.

Time
: 1859

Noteworthy Events
: Darwin’s
Origin of Species
published.

Time
: 1862

Noteworthy Events
: Foucault refines estimates of the velocity of light.

Time
: 1864

Noteworthy Events
: William Huggins obtains the first spectrum of a nebula, finds that it is composed of gas.

Noteworthy Events
: James Clerk Maxwell publishes a unified theory of electricity and magnetism, portraying both as aspects of electromagnetic force.

Time
: 1865

Noteworthy Events
: Gregor Mendel announces results of his research in genetics, revealing key to persistence of
unchanging
traits in living things, a critical missing element in Darwinism.

Time
: 1874, 1882

Noteworthy Events
: Transits of Venus observed with new, more precise instruments, improving estimates of the astronomical unit.

Time
: 1877

Noteworthy Events
: David Gill measures parallax of Mars during its opposition, deduces distance to the sun of ninety-three million miles.

Time
: 1879

Noteworthy Events
: Albert Michelson, employing Foucault’s principle, determines velocity of light.

Time
: 1883

Noteworthy Events
: Henry Rowland’s diffraction grating greatly improves the resolution of spectrographs.

Time
: 1884

Noteworthy Events
: Johann Balmer determines harmonic sequence of hydrogen
lines, initiating line of inquiry that will lead to investigation of the electron shells of atoms.

Time
: 1887

Noteworthy Events
: Albert Michelson and Edward Morley perform the final and most precise in a series of experiments showing that space cannot be filled with the aether that had been thought to be responsible for transmitting light. Their work clears the ground for the ascent of the Lorentz contractions.

Time
: 1892

Noteworthy Events
: Hendrik Lorentz and George FitzGerald independently propose that contraction of length of measuring rods with velocity explains the Michelson-Morley experimental results, a concept essential to the special theory of relativity.

Time
: 1895

Noteworthy Events
: E. E. Barnard photographs the Milky Way, notes that dark patches are too numerous to be empty space but must represent dark clouds of interstellar matter.

Time
: 1897

Noteworthy Events
: J. J. Thomson discovers the electron.

Time
: 1898

Noteworthy Events
: Marie and Pierre Curie isolate the radioactive elements radium and polonium.

Time
: 1900

Noteworthy Events
: Max Planck proposes the quantum theory of radiation, the basis of quantum physics.

Time
: 1904

Noteworthy Events
: Ernest Rutherford suggests that the amount of helium produced by the radioactive decay of minerals in rocks could be employed to measure the age of the earth.

Time
: 1905

Noteworthy Events
: Albert Einstein publishes special theory of relativity, indicating that measurements of space and time are distorted at high velocity and implying that mass and energy are equivalent; in another paper he shows that light is composed of quanta.

Noteworthy Events
: Jacobus Kapteyn, studying the proper motions of twenty-four hundred stars, finds evidence of what he calls “star streaming”—that stars in our neighborhood move in a preferred direction—an early clue to the rotation of our galaxy.

Time
: 1911

Noteworthy Events
: Ernest Rutherford determines that most of the mass of atoms is contained in their tiny nuclei.

Time
: 1912

Noteworthy Events
: Henrietta Swan Leavitt discovers a correlation between the absolute magnitude and the period of variability of Cepheid variable stars, opening the door to their use as intergalactic distance indicators.

Time
: 1913

Noteworthy Events
: Niels Bohr develops theory of atomic structure, in which
electrons are said to orbit the nucleus in a manner somewhat akin to that of planets orbiting the sun.

Noteworthy Events
: Henry Norris Russell presents a plot of the luminosities and colors of stars, extending work done in 1911 by Ejnar Hertzsprung. The resulting Hertzsprung-Russell diagram will be fundamental to the understanding of the evolution of stars.

Time
: 1914

Noteworthy Events
: Walter Adams and Arnold Kohlschutter determine the absolute luminosity of stars from their spectra alone, making it possible to estimate the distances of millions of distant stars.

Time
: 1915

Noteworthy Events
: Annie Jump Cannon classifies stars into categories according to their spectral type, a major step in discerning order underlying the diversity of the stars.

Time
: Arnold Sommerfeld refines Bohr model of the atom.

Time
: 1916

Noteworthy Events
: Albert Einstein publishes the general theory of relativity, portraying gravitation as an effect of curved space and delivering cosmology from the ancient dilemma of a finite versus an infinite universe.

Time
: 1916–1917

Noteworthy Events
: Arthur Stanley Eddington demonstrates theoretically that stars are gaseous spheres; his work lays the foundation for his later assertion that gravitational contraction cannot be the mechanism that powers the stars.

Time
: 1917

Noteworthy Events
: Heber Curtis and George Ritchey announce that they have found novae (stars that have suddenly increased tremendously in brightness) in the Andromeda spiral. Opinions differ on whether this means Andromeda is a galaxy of stars, or a gaseous nebula from which new stars are condensing.

Noteworthy Events
: Vesto Slipher measures large Doppler shifts in the spectra of spirals, later found to be due to the motion of the spiral galaxies in the expanding universe.

Time
: 1918

Noteworthy Events
: Harlow Shapley determines, by studying the distances of globular clusters, that the sun lies toward one edge of a galaxy of stars.

Noteworthy Events
: The 100-inch telescope at Mount Wilson, then the world’s largest, begins operation.

Time
: 1919

Noteworthy Events
: English expedition to observe a solar eclipse confirms Einstein’s prediction that space, in a gravitational field, is strongly curved.

Time
: 1920

Noteworthy Events
: The controversy over whether spiral nebulae are gaseous
clouds or “island universes”—i.e., galaxies—comes to a head in a debate between Heber Curtis and Harlow Shapley.

Time
: 1922

Noteworthy Events
: Ernst Öpik deduces, from rotation velocities and the mass to luminosity ratio of the Andromeda spiral, that it is a galaxy in its own right.

Noteworthy Events
: Aleksandr Friedmann shows that general relativity is consistent with an expanding-universe cosmology.

Time
: 1923

Noteworthy Events
: Cecilia Payne demonstrates, from solar spectra, that the relative abundance of elements in the sun approximates that in the crust of the earth.

Time
: 1924

Noteworthy Events
: Louis de Broglie develops wave theory of matter.

Time
: 1925

Noteworthy Events
: Max Born, Pascual Jordan, and Werner Heisenberg develop quantum mechanics.

Noteworthy Events
: Wolfgang Pauli announces the exclusion principle, essential to understanding spectral lines of stars and nebulae.

Noteworthy Events
: Bertil Lindblad demonstrates that the motion of stars called “star streaming” by Kapteyn in 1905 can be explained as being due to the rotation of the Milky Way galaxy.

Noteworthy Events
: Edwin Hubble announces that he has identified Cepheid variable stars in the Andromeda galaxy, confirming that it is a galaxy of stars rather than a gaseous nebula and making it possible to measure its distance.

Time
: 1926

Noteworthy Events
: Erwin Schrödinger proposes wave-mechanical theory of the atom.

Noteworthy Events
: Lindblad produces theory of rotation of the Milky Way galaxy.

Time
: 1927

Noteworthy Events
: Jan Oort detects evidence of the rotation of the Milky Way galaxy, by examining the radial velocities of stars.

Noteworthy Events
: Georges Lemaître publishes an expanding-universe cosmology.

Noteworthy Events
: Werner Heisenberg discovers the quantum indeterminacy principle.

Time
: 1927–1929

Noteworthy Events
: Relativistic quantum electrodynamics theory developed.

Time
: 1928

Noteworthy Events
: George Gamow applies the uncertainty principle to the problem of how protons combine to build nuclei in stellar interiors, a signal step in establishing that nuclear fusion provides the energy that powers stars.

Noteworthy Events
:
Ira Bowen determines that the spectra of nebulae are produced by doubly ionized oxygen and not by an unknown element called “nebulium,” as had been thought. This strengthens the hopes of astrophysicists that the rest of the universe is made of the same elements and obeys the same natural laws as here on Earth.

Noteworthy Events
: Dirac publishes the “Dirac equation,” a relativistic quantum theory of electromagnetism.

Time
: 1929

Noteworthy Events
: Edwin Hubble announces a relationship between the redshift in the spectra of galaxies and their distances, indicating that the universe is expanding.

Time
: 1930

Noteworthy Events
: Robert Trumpler’s studies of open star clusters enable him to measure the extent to which interstellar clouds dim and redden starlight, greatly improving estimates of the distances of stars.

Time
: 1931

Noteworthy Events
: Dirac predicts the existence of the positron, the antimatter equivalent of the electron.

Noteworthy Events
: Wolfgang Pauli, studying beta decay, predicts the existence of the neutrino.