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.

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.

The magnetic field of the Sun can be probed because in the presence of a magnetic field the energy levels of atoms and ions are split into more than one level, which causes spectral transition lines to be split as well. This is called the Zeeman Effect, after the Dutch physicist Pieter Zeeman (1865-1943). Spectroscopic studies of the Sun by the American astronomer Walter Adams (1876-1956) with Hale and others at the Mt. Wilson Solar Observatory led to the insight that sunspots are regions of lower temperatures and stronger magnetic fields than their surroundings. The spectroheliograph for studying the Sun was developed independently by George Ellery Hale in the USA and by the talented French astrophysicist Henri-Alexandre Deslandres (1853-1948), working at the Paris Observatory around 1890. Sunspots appear dark to us because they are cooler than other regions, but in reality they are red or orange in color.

Richard Carrington and Edward Sabine in Britain in the 1800s had suggested a possible link between the occurrences of solar flares and observations of aurorae and geomagnetic storms on the Earth. It takes a day or two for the charged particles of the solar wind to travel from the Sun to the Earth. This is obviously very fast, yet significantly slower than the 8 minutes and 20 seconds or so that it takes for light to travel the same distance. This indicated that something other than light travels from the Sun to us. Following work by Kristian Birkeland from Norway, the English geophysicist Sydney Chapman and the German astronomer Ludwig Biermann, Eugene Parker in the USA created a coherent model for the solar wind in 1958.

Progress in mapping the Sun’s magnetic field was made in the mid-twentieth century by an American father-and-son team. The prominent solar astronomer Harold D. Babcock (1882-1968) studied spectroscopy and the magnetic fields of stars. Horace W. Babcock (1912-2003) was his son. The two Babcocks were the first to measure the distribution of magnetic fields over the solar surface. These fields change polarity every 11-year cycle, indicating that solar activity varies with a period of around 22 years. They developed important models of sunspots and their magnetism. In the early 1950s, Horace Babcock was the first person to propose adaptive optics, a methodology that provides real-time corrections with deformable mirrors to remove the blurring of ground-based astronomical images caused by turbulence in the Earth’s atmosphere. Adaptive optics works best at longer wavelength such as infrared.

To learn more about the Sun’s interior, astronomers record its vibrations – a study called helioseismology. In principle this is related to how geophysicists use seismic waves to study the interior of the Earth. While there are no true sunquakes, the Sun does vibrate at a variety of frequencies, which can be detected. Its magnetic field is believed to be created as a result of its rotation and the resulting motion of the ionized particles found throughout its body.

As we have seen, it was possible for European astrophysicists in the late 1800s to detect the presence of elements such as hydrogen in the Sun, but they did not yet know how big a percentage of its mass consisted of hydrogen. In the 1920s, many scientists still assumed that it was rich in heavy elements. This changed with the work of the English-born astronomer Cecilia Payne, later named Payne-Gaposchkin (1900-1979) when she married a Russian astronomer. Her interest in astronomy was triggered after she heard Arthur Eddington lecture on relativity. She joined the Harvard College Observatory in the USA. By using spectroscopy, Payne worked out that hydrogen and helium are the most abundant elements in stars. Otto Struve (1897-1963), a Russian astronomer of ethnic German origins, called her thesis Stellar Atmospheres from 1925 “the most brilliant Ph.D. thesis ever written in astronomy.”

The Irish astronomer William McCrea (1904-1999) and the German astrophysicist Albrecht Unsöld (1905-1995) independently established that the prominence of hydrogen in stellar spectra indicates that the presence of hydrogen in stars is greater than that of all other elements put together. Unsöld studied under the German theoretical physicist Arnold Sommerfeld at the University of Munich and began working on stellar atmospheres in 1927.

The English mathematical physicist James Jeans (1877-1946) worked on thermodynamics, heat and aspects of radiation, publishing major works on these topics and their applications to astronomy. The English astrophysicist and mathematician Arthur Milne (1896-1950) did research in the 1920s on stellar atmospheres, much of it with his English colleague Ralph H. Fowler (1889-1944). This led to the determination of the temperatures and pressures associated with spectral classes, explaining the origin of stellar winds. The astronomer Marcel Minnaert (1893-1970) was forced to flee from his native Belgium to the Netherlands for taking part in Flemish activism. From 1937 to 1963 he was director of the Utrecht Observatory, where he and his students did quantitative analysis of the solar spectrum.

Newton had speculated on the energy source of the Sun. He assumed that it loses mass by emitting light particles and suggested that incoming comets could provide it with more mass to compensate for this. The French physicist Claude Pouillet (1791-1868) in 1837 calculated a decent estimate of the energy emitted by the Sun. However, this would require a mass almost the equivalent of the Earth’s Moon to hit the Sun every year, which was clearly not the case.

In 1854, Hermann von Helmholtz suggested that the Sun was contracting and converting potential energy into radiated energy. This Kelvin-Helmholtz mechanism of gravitational contraction, named after Lord Kelvin and Helmholtz, is relevant for planets like Jupiter, which emits approximately twice as much energy as it receives from the Sun. Its energy comes from radioactive elements in its core and from an overall contraction amounting to a few centimeters per century. Although the required rate of solar contraction was 91 meters per year, this mechanism would have implied an impossible reduction of the Sun’s diameter with 50% over 5 million years. Nineteenth-century physicists were partially right; the release of gravitational energy ignites nuclear fusion in stars by heating up their cores. Chemical combustion was rejected as it would have burnt away the entire Sun in a few thousand years.

The discovery of radioactivity in 1896 suddenly provided a new source of heat. In 1905 Albert Einstein generalized the law of conservation of energy with his famous mass-energy equivalence formula E = mc², where E stands for energy, m for mass and c for the speed of light in a vacuum. Since the speed of light is very great, the formula implies that very little mass is required to generate huge amounts of energy. The English physical chemist Francis William Aston with his mass spectrograph in 1920 made precise measurements of different atoms. He found that four individual hydrogen nuclei were more massive than a helium nucleus consisting of four nuclear particles. Arthur Eddington argued that these measurements indicated that by converting hydrogen atoms to helium and releasing about 0.7% of the hydrogen’s mass as energy in the process, the Sun could shine for billions of years.

The great English astrophysicist Arthur Stanley Eddington (1882-1944) was born to Quaker parents and earned a scholarship to Owens College, Manchester, in 1898. He turned to physics and went to Trinity College at the University of Cambridge. He spent seven years (1906 to 1913) as chief assistant at the Royal Observatory at Greenwich. He took inspiration from the Hertzsprung-Russell diagram, made important investigations of stellar dynamics and became an influential supporter of the view that the spiral nebulae are external galaxies. Eddington’s greatest contributions concerned astrophysics. He dealt with the importance of radiation pressure, the mass-luminosity relation and investigated the internal structure and evolution of stars. He wrote several books, some of them for the general reader. His The Internal Constitution of the Stars from 1926 was extremely influential to a generation of astrophysicists. He was one of the first to provide observational support for Einstein’s general theory of relativity from 1916 and explain it to a mass audience. Eddington was also among the first to suggest that processes at the subatomic level involving hydrogen and helium could explain why stars generate energy, but it was left for other scientists to work out the details.

The Sun has a mass of about 1.989×10³⁰ kg, roughly 333 thousand times more than the mass of the Earth, and a mean density of 1408 kg/m³ or 1.408 kg/dm³, a little bit more than water. Its equatorial radius (distance from its center to its surface) is 695,500 kilometers, approximately 109 times Earth’s radius. The energy per time put out by the Sun, its luminosity, is more than 3.8 x 10²⁶ Joules per second (or Watts). The amount of mass that the Sun converts into energy equals more than 4 million metric tons, or 4 billion kg, per second.

The theoretical physicist George Gamow (1904-1968) was born in the seaport city of Odessa in the Russian Empire (now the Ukraine) on the northern shore of the Black Sea. His father came from a military family and was a teacher of Russian literature in high school; his mother’s father was Archbishop of Odessa from the Orthodox Church. At the University of Leningrad he studied briefly under the Russian mathematician Alexander Friedmann, who was interested in mathematics of relativity. After completing his PhD in 1928, Gamow worked on quantum mechanics at Göttingen, Copenhagen and Cambridge. He couldn’t endure the brutal oppression under the Communist dictator Joseph Stalin, but fled the Soviet Union and moved to the United States in 1934. Gamow introduced nuclear theory into cosmology.

According to classical physics, two particles with the same electrical charge will repel each other. In 1928, Gamow derived a quantum-mechanical formula that gave a non-zero probability of two charged particles overcoming their mutual electrostatic repulsion and coming very close together. It is now known as the “Gamow factor.” The Dutch-Austrian nuclear physicist Fritz Houtermans (1903-1966) together with his British colleague Robert Atkinson (1898-1982) in 1929 predicted that the nuclei of light atoms such as hydrogen could fuse through quantum tunneling, and that the resultant atoms would have slightly less mass than the original constituents. This loss in mass would be released as vast amounts of energy.

The final major piece of the puzzle was the structure of the atom itself. When the neutron had been detected by the Englishman James Chadwick in 1932, physicists finally had sufficient information about the atomic nucleus to calculate the details of how hydrogen can fuse to become helium. This nuclear fusion process was worked out independently by two German-born physicists in the late 1930s: Hans Bethe in the USA and Carl von Weizsäcker in Berlin.

Carl Friedrich Freiherr von Weizsäcker (1912-2007) was born in Kiel, Germany to a prominent family; his father was a German diplomat, and his elder brother was later to become German President. He studied physics and astronomy in Berlin, Göttingen and Leipzig (1929-1933) and was supervised by leading nuclear physicists such as Werner Heisenberg and Niels Bohr. After the Second World War he was appointed head of a department at the Max Planck Institute for Physics in Göttingen, and from 1957 to 1969 he was Professor for Philosophy at the University of Hamburg in northern Germany.

Hans Bethe (1906-2005) studied at the Universities of Frankfurt and Munich where he earned his Ph.D. under the great German theoretical physicist Arnold Sommerfeld in 1928. He was forced to leave Germany after Adolf Hitler (1889-1945) and the Nazi Party came to power in 1933 since his mother was Jewish, even though he had been raised as a Christian by his father. Bethe was at Cornell University from 1935 to 2005 and became an American citizen in 1941. Weizsäcker was a member of the team that performed nuclear research in Germany during WW2, while Bethe became the head of the theoretical division at Los Alamos during the development of nuclear weapons in the United States. In stellar physics, both men described the proton-proton chain, which is the dominant energy source in stars such as our Sun or smaller, and the carbon-nitrogen-oxygen (CNO) cycle. Author John North writes:

“It was not until 1938, when attending a Washington conference organized by Gamow, that he was first persuaded to turn his attention to the astrophysical problem of stellar energy creation. Helped by Chandrasekhar and Strömgren, his progress was astonishingly rapid. Moving up through the periodic table, he considered how atomic nuclei would interact with protons. Like Weizsäcker, he decided that there was a break in the chain needed to explain the abundances of the elements through a theory of element-building. Both were stymied by the fact that nuclei with mass numbers 5 and 8 were not known to exist, so that the building of elements beyond helium could not take place….Like Weizsäcker, Bethe favored the proton-proton reaction chain and the CNO reaction cycle as the most promising candidates for energy production in main sequence stars, the former being dominant in less massive, cooler, stars, the latter in more massive, hotter, stars. His highly polished work was greeted with instant acclaim by almost all of the leading authorities in the field.”

Bengt Strömgren (1908-1987) was a Danish astrophysicist, the son of a Swedish astronomer. He studied in Copenhagen and stayed in touch with the latest developments in nuclear physics via Niels Bohr’s Institute. Strömgren did important research in stellar structure in the 1930s and calculated the relative abundances of the elements in the Sun and other stars.

The nineteenth century German astronomer Friedrich Bessel was the first to notice minor deviations in the motions of the bright stars Sirius and Procyon, which he assumed must be caused by the gravitational attraction of unseen companions. The existence of these bodies was later confirmed. Bessel was also first person to measure stellar parallax in 1838, an achievement which was independently made by the Baltic German astronomer Friedrich Georg Wilhelm von Struve and the Scottish astronomer Thomas Henderson.

In 1862 the American telescope maker Alvan Graham Clark (1832-1897) discovered the very faint companion Sirius B. Because the companion was about twice as far as Sirius from their common center of mass, it had to weigh about half as much (like a child twice as far from the center of a see-saw balancing an adult). The American astronomer Walter Sydney Adams (1876-1956) in 1915 identified Sirius B as a white dwarf star, a very dense object about the size of the Earth but with roughly the same mass as the Sun. Our Sun will eventually end up as a white dwarf billions of years from now, after first having gone through a red giant phase where it will expand greatly in volume and vaporize Mercury, Venus and possibly the Earth.

The astrophysicist Subrahmanyan Chandrasekhar (1910-1995) was born in Lahore into a Tamil Hindu family and got a degree at the University of Madras in then British-ruled India. After receiving a scholarship he studied at the University of Cambridge in England and came to the University of Chicago in the United States in 1937, where he remained for the rest of his life. NASA’s Chandra X-ray Observatory from 1999 was named after him. He is remembered above all for his contributions to the subject of stellar evolution. He was the nephew of the physicist Chandrasekhara Venkata Raman (1888-1970) from Madras, whose discovery of the Raman Effect in 1928, the change in wavelength of light when it is deflected by molecules, “greatly impacted future research regarding molecular structure and radiation.” Raman was knighted by the British in 1929 and won the Nobel Prize in Physics in 1930, the first non-European to win a science Nobel. Chandrasekhar won his own Nobel Prize in 1983.

In 1930, Chandrasekhar applied the new quantum ideas to the physics of stellar structure. He realized that when a star like the Sun exhausts its nuclear fuel it will collapse due to its own gravity until stopped by the Pauli exclusion principle, which prevents electrons from getting too close to one another. Stars more massive than the Chandrasekhar limit of 1.4 solar masses do not stabilize at the white dwarf stage but become neutron stars. The upper limit for a neutron star before it collapses further is called the Oppenheimer-Volkoff limit after J. Robert Oppenheimer (1904-1967) from the USA and the Russian-born Canadian physicist George Volkoff (1914-2000). The Oppenheimer-Volkoff limit is currently estimated at approximately three solar masses. Stars of greater mass than this are believed to end up as black holes.