Fjordman: A History of Astrophysics — Part 2

The second part of Fjordman’s series on the history of astrophysics has been published at Tundra Tabloids. Some excerpts are below:

Photography made it possible to preserve images of the spectra of stars. The Catholic priest and astrophysicist Pietro Angelo Secchi (1818-1878), born in the city of Reggio Emilia in northern Italy, is considered the discoverer of the principle of stellar classification. He visited England and the USA and became professor of astronomy in Rome in 1849. After the discovery of spectrum analysis by Kirchhoff and Bunsen, Secchi was among the first to investigate the spectra of Uranus and Neptune.

On an expedition to Spain to observe the total solar eclipse of 1860 he definitively established by photographic records that the corona and the prominences rising from the chromosphere (i.e. the red protuberances around the edge of the eclipsed disc of the sun) were real features of the sun itself, not optical illusions or illuminated mountains on the Moon. In the 1860s he began collecting the spectra of stars and classified them according to spectral characteristics, although his particular system didn’t last.

The Harvard system based on the star’s surface temperature was developed from the 1880s onward. Several of its creators were women. The US astronomer Edward Pickering (1846-1919) at the Harvard College Observatory hired female assistants, among them the Scottish-born Williamina Fleming (1857-1911) and especially Annie Jump Cannon (1863-1941) and Antonia Maury (1866-1952) from the USA, to classify the prism spectra of hundreds of thousands of stars. Cannon developed a classification system based on temperature where stars, from hot to cool, were of ten spectral types O, B, A, F, G, K, M, R, N, S that astronomers accepted for world-wide use in 1922. Maury developed a different system.

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Edward Pickering and the German astronomer Hermann Karl Vogel (1841-1907) independently discovered spectroscopic binaries double-stars that are too close to be detected through direct observation but which through the analysis of their light have been found to be two stars revolving around one another. Vogel was born in Leipzig in what was then the Kingdom of Saxony, and died in Potsdam in the unified German Empire. He studied astronomy at the Universities of Leipzig and Jena, joined the staff of the Potsdam Astrophysical Observatory and served as its director from 1882 to 1907. Vogel made detailed tables of the solar spectrum, attempted spectral classification of stars and also made photographic measurement of Doppler shifts to determine the radial velocities of stars.

Another system was worked out in the 1940s by the American astronomers William Wilson Morgan (1906-1994) and Philip Keenan (1908-2000), aided by Edith Kellman. They introduced stellar luminosity classes. For the first time, astronomers could determine the luminosity of stars directly by analyzing their spectra, their stellar fingerprints. This is known as the MK (after Morgan and Keenan) or Yerkes spectral classification system after Yerkes Observatory, the astronomical research center of the University of Chicago.

Morgan’s observational work helped demonstrate the existence of spiral arms in our Milky Way Galaxy. Maury’s classifications were not preferred by Pickering, but the Danish astronomer Ejnar Hertzsprung (1873-1967) realized their value. As stated in his Bruce Medal profile, Hertzsprung studied chemical engineering in Copenhagen, worked as a chemist in St. Petersburg, and studied photochemistry in Leipzig before returning to Denmark in 1901 to become an independent astronomer. In 1909 he was invited to Göttingen to work with Karl Schwarzschild, whom he accompanied to the Potsdam Astrophysical Observatory later that year.

From 1919-44 he worked at the Leiden Observatory in the Netherlands, the last nine years as director. He then retired to Denmark but continued measuring plates into his nineties. He is best known for his discovery that the variations in the widths of stellar lines discovered by Antonia Maury reveal that some stars (giants) are of much lower density than others (main sequence or dwarfs ) and for publishing the first color-magnitude diagrams.

The American astronomer Henry Norris Russell (1877-1957) spent six decades at Princeton University as a student, professor and observatory director. From 1921 on he made lengthy annual visits to the Mt. Wilson Observatory. He measured parallaxes in Cambridge, England, with A.R. Hinks and found a correlation between spectral types and absolute magnitudes of stars the Hertzsprung-Russell diagram. He popularized the distinction between giant stars and dwarfs while developing an early theory of stellar evolution.

With his student, Harlow Shapley, he analyzed light from eclipsing binary stars to determine stellar masses. Later he and his assistant, Charlotte E. Moore Sitterly, determined masses of thousands of binary stars using statistical methods. With Walter S. Adams Russell applied Meghnad Saha’s theory of ionization to stellar atmospheres and determined elemental abundances, confirming Cecilia Payne-Gaposchkin’s discovery that the stars are composed mostly of hydrogen. Russell applied the Bohr theory of the atom to atomic spectra and with Harvard physicist F.A. Saunders made an important contribution to atomic physics, Russell-Saunders coupling (also known as LS coupling).

Herztsprung had discovered the relationship between the brightness of a star and its color, but published his findings in a photographic journal which went largely unnoticed. Russell made essentially the same discovery, but published it in 1913 in a journal read by astronomers and presented the findings in a graph, which made them easier to understand. The Hertzsprung-Russell diagram helped give astronomers their first insight into the lifecycle of stars. It can be regarded as the Periodic Table of stars. The Indian astrophysicist Meghnad Saha (1893-1956) provided a theoretical basis for relating the spectral classes to stellar surface temperatures.

Changes in the structure of stars are reflected in changes in temperatures, sizes and luminosities. The smallest ones, red dwarfs, may contain less than 10% the mass of the Sun and emit 0.01% as much energy. They constitute by far the most numerous types of stars and have lifespans of tens of billions of years. By contrast, the rare hypergiants may be over 100 times more massive than the Sun and emit hundreds of thousands of times more energy, but they have lifetimes of just a few million years. Those that are actively fusing hydrogen into helium in their cores, which means most of them, are called main sequence stars. These are in hydrostatic equilibrium, which means that the outward radiation pressure from the fusion process is balanced by the inward gravitational force. When the hydrogen fuel runs out, the core contracts and heats up. The star then brightens and expands, becoming a red giant.

The Eddington limit, named after the English astrophysicist Arthur Eddington, is the point at which the luminosity emitted by a star is so extreme that it starts blowing off its outer layers. It is believed to be reached in stars around 120 solar masses. In the very early stages of the universe, extremely massive stars containing hundreds of solar masses may have been able to form because they contained practically no heavy elements, just hydrogen and helium. Wolf-Rayet stars are very hot, luminous and massive objects that eject significant proportions of their mass through solar wind per year. They are named after the French astronomers Charles Wolf (1827-1918) and Georges Rayet(1839-1906) who discovered their existence in 1867.

A common, medium-sized star like the Sun will remain on the main sequence for roughly 10 billion years. The Sun is currently in the middle of its lifespan, as it formed 4.57 billion years ago and in about 5 billion years it will become a red giant. Even today, the Sun daily emits about 30% more energy than it did when it was born. The so-called faint young Sun paradox, proposed by Carl Sagan and his colleague George Mullen in the United States in 1972, refers to the fact that the Earth apparently had liquid oceans, not frozen ones, for much of the first half of its existence, despite the fact that the Sun probably was only 70 percent as bright in its youth as it is now. Scientists have not yet reached an agreement on why this was the case.

Read the rest at Tundra Tabloids. Part 1 in the series may be found here.