A History of Astrophysics

The introduction of the telescope in Western Europe in the 1600s revolutionized astronomy, but it did not found it as a discipline. Astronomy had existed in some form for thousands of years prior to this. It is consequently impossible to assign a specific date to its founding. This is not the case with astrophysics. People in ancient and medieval times might speculate on the material makeup of stars and celestial bodies, but they had no way of verifying their ideas.

Anaxagoras of Clazomenae in the fifth century BC was the first of the Pre-Socratic philosophers to live in Athens. He held many controversial theories, including his claim that the stars are fiery stones. He allegedly got this idea when a meteorite fell near Aegospotami. He assumed that it came from the Sun, and since it consisted largely of iron he concluded that the Sun was made of red-hot iron. Not a bad guess for his time, yet he had no way of proving his claims. Neither did Asian or Mesoamerican observers. Some sources indicate that Anaxagoras was charged with impiety, as most Greeks still shared the divine associations with the heavenly bodies, but political considerations may have played a part in this, too.

As late as in 1835 Auguste Comte (1798-1857), the French philosopher often regarded as the founder of sociology, stated that humans would never be able to understand the composition of stars. He was soon proved wrong by two new techniques – spectroscopy and photography.

The English chemist William Hyde Wollaston (1766-1828) in 1800 formed a partnership with his English colleague Smithson Tennant (1761-1815), whom he had befriended at Cambridge. Tennant discovered the elements iridium and osmium, extracted from platinum ores, in 1803. The platinum group metals – platinum, ruthenium, rhodium, palladium, osmium and iridium – have similar chemical properties. Osmium (Os, atomic number 76) is the heaviest natural element with a density of more than 22.6 kg/dm3, twice as much as lead at 11.3 kg/dm3.

Platinum (Pt, atomic number 78) and its dense sister metals are very rare in the Earth’s crust. It had been introduced to Europe from South American mines in the 1740s by men such as the Spanish explorer Antonio de Ulloa (1716-1795). Wollaston was the first person to produce pure, malleable platinum and became wealthy from supplying Britain with the precious metal. The Wollaston Medal, granted by the Geological Society of London, is named after him.

The German chemist Martin Klaproth (1743-1817) was born in Wernigerode in Prussian Saxony and worked as an apothecary for years before continuing his career as a professor of chemistry at the newly established University of Berlin. He discovered uranium as well as zirconium (Zr, 40) in 1789. Uranium (symbol U, atomic number 92) was named for the planet Uranus, which had been found just before this. Wollaston detected the chemical elements palladium in 1803 and rhodium in 1804. He named palladium (Pd, 46) after the asteroid Pallas, which had been discovered a year earlier by the German astronomer Olbers and was initially believed to be a planet, until the full extent of the asteroid belt had been grasped.

The birth of spectroscopy, the systematic study of the interaction of light with matter, followed shortly after the creation of scientific chemistry in Europe. William Hyde Wollaston in 1802 noted some dark features in the solar spectrum, but he didn’t follow this insight up. In 1814, the German physicist Joseph von Fraunhofer (1787-1826) independently discovered these dark features (absorption lines) in the optical spectrum of the Sun, which are now known as Fraunhofer lines. He carefully studied them and noted that they exist in the spectra of Venus and the stars, too, which meant that they had to be a property of the light itself.

In the 1780s a Swiss artisan, Pierre-Louis Guinand (1748-1824), began experimenting with the manufacture of flint glass, and in 1805 managed to produce a nearly flawless material. He passed on this secret to Fraunhofer, who worked in the secularized Benedictine monastery of Benediktbeuern. Fraunhofer improved upon Guinand’s techniques and began a more systematic study of the mysterious spectral lines. To the stronger ones he assigned the letters A to Z, a system which is also used today. Yet it was left to two other German scholars to prove the full significance of these unique lines, corresponding to specific chemical elements.

Robert Bunsen (1811-1899) is often associated with the Bunsen burner, a device found in many chemistry laboratories around the word, but the truth is that he made a few alterations to it rather than inventing it. He was born in Göttingen, where his father was a professor of languages. He obtained his doctorate in chemistry at the University of Göttingen and spent years traveling through Western Europe. He eventually settled at the scenic university town of Heidelberg in south-west Germany, where he taught from 1852 until his retirement. In the late 1850s, Bunsen began a new and very fruitful collaboration there with the physicist Kirchhoff.

Gustav Kirchhoff (1824-1887), the son of a lawyer, was born and educated in Königsberg, Prussia, on the Baltic Sea, now the Russian city of Kaliningrad. He graduated from Albertus University there in 1847 and relocated to the rapidly growing city of Berlin. After 1850 he became acquainted with Bunsen, who urged him to follow him to Heidelberg. Kirchhoff in 1859 coined the term blackbody to describe a hypothetical perfect radiator that absorbs all incident light and emits all of that light when maintained at a constant temperature. His findings proved instrumental to Max Planck’s quantum theory of electromagnetic radiation from 1900. He is above all remembered for this collaboration with Bunsen around 1860.

They demonstrated in 1859 that all pure substances display a characteristic spectrum. Together, Bunsen and Kirchhoff assembled the flame, prism, lenses and viewing tubes necessary to produce the world’s first spectrometer. They identified the alkali metals cesium (chemical symbol Cs, atomic number 55) and rubidium (Rb, 37) in 1860-61, showing in each case that these new elements produced line spectra that were unique for them, a chemical “fingerprint.” The dark lines in the solar spectrum show the selective absorption of light, caused by the transition of an electron between specific energy levels in an atom, in the gases of various elements that exist above the Sun’s surface. In the first qualitative chemical analysis of a celestial body, Kirchoff in the 1860s identified 16 different elements from the Sun’s spectrum and compared these to laboratory spectra from known elements here on Earth.

The great physicist George Gabriel Stokes (1819-1903) attended school in Dublin, Ireland, but later moved to England and Cambridge University. He theorized a reasonably correct explanation of the Fraunhofer lines in the solar spectrum, but he did not publish it or develop it further. According to the Molecular Expressions website, “Throughout his career, George Stokes emphasized the importance of experimentation and problem solving, rather than focusing solely on pure mathematics. His practical approach served him well and he made important advances in several fields, most notably hydrodynamics and optics. Stokes coined the term fluorescence, discovered that fluorescence can be induced in certain substances by stimulation with ultraviolet light, and formulated Stokes Law in 1852. Sometimes referred to as Stokes shift, the law holds that the wavelength of fluorescent light is always greater than the wavelength of the exciting light. An advocate of the wave theory of light, Stokes was one of the prominent nineteenth century scientists that believed in the concept of an ether permeating space, which he supposed was necessary for light waves to travel.”

Fluorescence microscopy has become an important tool in cellular biology. The Polish physicist Alexander Jablonski (1898-1980) at the University of Warsaw was a pioneer in fluorescence spectroscopy. Stokes was a formative influence on subsequent generations of Cambridge men and was one of the great names among nineteenth century mathematical physics, with included Michael Faraday, James Joule, Siméon Poisson, Augustin Cauchy and Joseph Fourier. The English mathematician George Green (1793-1841), known for Green’s Theorem, inspired Lord Kelvin and devised an early theory of electricity and magnetism that formed some of the basis for the work of scientists like James Clerk Maxwell.

Astrophysics as a scientific discipline was born in mid-nineteenth century Europe, and only there; it could not have happened earlier as the crucial combination of chemical and optical knowledge, telescopes and photography did not exist before. In case we forget what a huge step this was, let us recall that in the sixteenth century AD in Mesoamerica, the region usually credited with having the most sophisticated American astronomical traditions, thousands of people had their hearts ripped out every year to please the gods and ensure that the Sun would keep on shining. A little over three centuries later, European scholars could empirically study the composition of the Sun and verify that it was essentially made of the same stuff as the Earth, only much hotter. Within the next few generations, European and Western scholars would proceed to explain how the Sun and the stars generate their energy and why they shine. By any yardstick, this represents one of the greatest triumphs of the human mind in history.

Photography was born in France in the 1820s with Joseph-Nicéphore Niépce, who teamed up with the painter Louis Daguerre. As Eva Weber writes in her book Pioneers of Photography, “In March 1839 Daguerre personally demonstrated his process to inventor and painter Samuel Morse (1791-1872) who enthusiastically returned to New York to open a studio with John Draper (1811-1882), a British-born professor and doctor. Draper took the first photograph of the moon in March 1840 (a feat to be repeated by Boston’s John Adams Whipple in 1852), as well as the earliest surviving portrait, of his sister Dorothy Catherine Draper.”

The American physician Henry Draper (1837-1882), son of John Draper, was a pioneer of astrophotography. In 1857 he visited Lord Rosse, or William Parsons (1800-1867), famous for his construction in Ireland in the 1840s of the most powerful reflecting telescope in the Victorian period, frequently referred to as the Leviathan. It remained the world’s largest telescope until the twentieth century. Draper became a passionate amateur astronomer, and after reading about the work on star spectra carried out by William Huggins and Joseph Lockyer he built his own spectrograph. He died at the young age of forty-five, but his widow established the Henry Draper Memorial to support photographic research in astronomy. This funded the Henry Draper Catalog, a massive photographic stellar spectrum survey.

The first successful daguerrotype photography of the Sun was made in 1845 by the French physicists Louis Fizeau and Léon Foucault, who are mainly remembered for their accurate measurements of the speed of light. Warren de la Rue (1815-1889), a British-born astronomer, astrophotographer and chemist educated in Paris, designed a special telescope dubbed the photoheliograph. On an expedition to Spain in 1860 during a total solar eclipse, his images demonstrated clearly that the corona is a phenomenon associated with the Sun.

The technique of spectral analysis caught on after the work of Robert Bunsen and Gustav Kirchhoff. One of those who quickly took it up was the great English chemist William Crookes (1832-1919), who discovered the metal thallium (Tl, atomic number 81) in 1862. The Englishman William Huggins (1824-1910) built a private observatory in South London and tried to apply this method to other stars. Through spectroscopic methods he showed that they are composed of the same elements as the Sun and the Earth. He collaborated with his friend William Allen Miller (1817-1870), a professor of chemistry at King’s College, London.

According to his Bruce Medal biography, “Huggins was one of the wealthy British ‘amateurs’ who contributed so much to 19th century science. At age 30 he sold the family business and built a private observatory at Tulse Hill, five miles outside London. After G.R. Kirchhoff and R. Bunsen’s 1859 discovery that spectral emission and absorption lines could reveal the composition of the source, Huggins took chemicals and batteries into the observatory to compare laboratory spectra with those of stars. First visually and then photographically he explored the spectra of stars, nebulae, and comets. He was the first to show that some nebulae, including the great nebula in Orion, have pure emission spectra and thus must be truly gaseous, while others, such as that in Andromeda, yield spectra characteristic of stars. He was also the first to attempt to measure the radial velocity of a star. After 1875 his observations were made jointly with his talented wife, the former Margaret Lindsay Murray.”

The Austrian mathematician and physicist Johann Christian Doppler (1803-1853) was born in Salzburg, the son of a stonemason, and studied in Vienna. In 1842 he proposed that observed frequency of light and sound waves is dependent upon how fast the source and observer are moving relative to each other, a phenomenon called the Doppler Effect. For instance, most of us have heard how the sound of a car or a train changes in frequency as it moves toward us and then away from us. A more correct explanation of the principle involved was published by the French physicist Armand-Hippolyte-Louis Fizeau in 1848. The Doppler Effect has proved to be an invaluable tool for astronomical research. Most notably, the motions of galaxies detected through this manner led to the conclusion that the universe is expanding.

In 1864, probably as a result of discussions with his countryman William Huggins, the astronomer Joseph Norman Lockyer (1836-1920), originally from the town of Rugby in the West Midlands of England, obtained a spectroscope. In 1868 he was able to confirm that bright emission lines from prominences of the Sun could be seen at times other than during total solar eclipses. The same technique had been demonstrated by the French astronomer Pierre Janssen (1824-1907). Janssen was born in Paris, where he studied mathematics and physics, and took part in a long series of solar eclipse-expeditions around the world. Lockyer and Janssen are credited with independently discovering helium (chemical symbol He, atomic number 2) in 1868 through studies of the solar spectrum. Helium, from the Greek helios for the Sun, remains the only element so far discovered in space before being identified on Earth. Lockyer was also the founder of the prominent British scientific journal Nature in 1869.

While the center of astronomy was still in Western Europe, Europeans overseas were starting to leave their mark, above all in North America. The U.S. physicist Henry Rowland (1848-1901) did notable work in spectroscopy, and the American astronomer Vesto Slipher (1875-1969) was the first person to measure the enormous radial velocities of spiral nebulae. As the excellent reference book The Oxford Guide to the History of Physics and Astronomy states:

“In 1868, however, Huggins found what appeared to be a slight shift for a hydrogen line in the spectrum of the bright star Sirius, and by 1872 he had more conclusive evidence of the motion of Sirius and several other stars. Early in the twentieth century Vesto M. Slipher at the Lowell Observatory in Arizona measured Doppler shifts in spectra of faint spiral nebulae, whose receding motions revealed the expansion of the universe. Instrumental limitations prevented Huggins from extending his spectroscopic investigations to other galaxies. Astronomical entrepreneurship in America’s gilded age saw the construction of new and larger instruments and a shift of the center of astronomical spectroscopic research from England to the United States. Also, a scientific education became necessary for astronomers, as astrophysics predominated and the concerns of professional researchers and amateurs like Huggins diverged. George Ellery Hale, a leader in founding the Astrophysical Journal in 1895, the American Astronomical and Astrophysical Society in 1899, the Mount Wilson Observatory in 1904, and the International Astronomical Union in 1919, was a prototype of the high-pressure, heavy-hardware, big-spending, team-organized scientific entrepreneur.”

George Ellery Hale (1868-1938), a university-educated solar astronomer born in Chicago, represented the dawn of a new age, not only because he was American and the United States would emerge as a leading center of astronomical research (although scientifically and technologically speaking a direct extension of the European tradition), but at least as much because he personified the increasing professionalization of science and astronomy.

The telescope Galileo used in the early 1600s was a simple refractor. The sheer weight of the glass lens makes a refracting telescope larger than one meter in diameter impractical. The introduction of the reflecting mirror telescope by Newton in 1669 paved the way to virtually all modern telescopes. Hale built the largest telescope in the world four times: Once at Yerkes Observatory, then the 60-and 100-inch reflectors at Mt. Wilson and finally the 200-inch reflector at Mt. Palomar. As an undergraduate student at the Massachusetts Institute of Technology, Hale co-invented the spectroheliograph, an instrument to photograph outbursts of gas at the edge of the Sun, and discovered that sunspots were regions of relatively low temperatures and high magnetic fields. He hired Harlow Shapley and Edwin Hubble and encouraged research in astrophysics and galactic astronomy.

There is still room for non-professional astronomers; good amateurs can occasionally spot new comets before the professionals do. Yet it is a safe bet to say that never again will we have a situation like in the late eighteenth century when William Herschel, a musician by profession, was one of the leading astronomers of his age. From a world of a few enlightened and often wealthy gentlemen in the eighteenth century would emerge a world of trained scientists in the twentieth; the nineteenth century was a transitional period. As the example of William Huggins demonstrates, amateur astronomers were to enjoy a final golden age.

The English entrepreneur William Lassell (1799-1880) had made good money from brewing beer and used some of it to indulge his interest in astronomy, employing very good self-made instruments at his observatory near the city of Liverpool. Liverpool was the fastest-growing port in Europe, and the world’s first steam-hauled passenger railway ran from Liverpool to Manchester in 1830. The Industrial Revolution, where Britain played the leading role, was a golden age for the beer-brewing industry. The combination of beer and science is not unique; the seventeenth-century Polish astronomer Hevelius came from a brewing family, and the English scientific brewer James Joule seriously studied heat and the conservation of energy.

In 1846 William Lassell discovered Triton, the largest moon of Neptune, shortly after the planet had itself been mathematically predicted by the French mathematician Urbain Le Verrier and spotted by the German astronomer Johann Gottfried Galle. Lassel later discovered two moons around Uranus, Ariel and Umbriel; a satellite of Saturn, Hyperion, was spotted by him as well as the American father-and-son team William Bond (1789-1859) and George Bond (1825-1865). William Bond was a clockmaker in Boston who became a passionate amateur astronomer. In 1848, with his son George, he discovered Hyperion. They were among the first in the USA to use Daguerre’s photographic process for astrophotography.

The US astronomer Asaph Hall (1829-1907) discovered the two tiny moons of Mars, Deimos and Phobos, in 1877 and calculated their orbits. While only a few kilometers in diameter, the moons could be seen by viewers using smaller telescopes, which means their discovery owed as much to Hall’s observational skills as to his equipment. Asaph Hall was the son of a clockmaker and worked for a while with George Bond at the Harvard College Observatory.

In 2010, photos taken by the European Space Agency’s Mars Express spacecraft of Phobos, the larger of the two tiny, potato-shaped Martian moons, showed potential landing sites for Russia’s unmanned Phobos-Grunt mission, which is designed to bring samples of the Martian moon back to the Earth after 2012. The Russian Space Agency intends to include a Chinese Mars orbiter, Yinghuo-1, together with the mission. It will be China’s first interplanetary probe. China in 2003 became only the third nation to achieve human spaceflight, after the Soviet Union/Russia and the United States, and has plans for manned missions to the Moon.

The largely self-taught American astronomer Edward Barnard (1857-1923), originally a poverty-stricken photographer, made his own telescope and after some notable observations joined the initial staff of the Lick Observatory in 1887. He introduced wide-field photographic methods to study the structure of the Milky Way. The faint Barnard’s Star, which he discovered in 1916, had the largest proper motion of any known star. At a distance of about six light-years it is the closest neighboring star to the Sun next to the members of the Alpha Centauri system, around 4.4 light-years away. In 1892 he observed Amalthea, the first moon of Jupiter to be discovered since the four largest ones described by Galileo Galilei in 1610.

The Swiss natural philosopher Pierre Prévost (1751-1839), the son of a clergyman from Geneva, Switzerland, who served as a professor of physics at Berlin, showed in 1791 that all bodies radiate heat, regardless of their temperature.

 Early estimates of stellar surface temperatures gave results that were far too high. More accurate values were obtained by using the radiation laws of the Slovenian physicist Joseph Stefan from 1879 and the German physicist Wilhelm Wien from 1896. Stefan calculated the temperature of the Sun’s surface to 5400 °C, the most sensible value until then. The Stefan-Boltzmann Law, named after Stefan and his Austrian student Ludwig Boltzmann, suggests that the amount of radiation given off by a body is proportional to the fourth power of its temperature as measured in Kelvin units.

The surface temperature is not necessarily dependent upon the size of the star (the core temperature is a different matter). You can easily find red supergiants with many times the mass of the Sun, but with a surface temperature of less than 4000 K, compared to the Sun’s 5800 or so K. The surface temperature of a bright red star is approximately 3500 K, whereas blue stars can have ones of tens of thousands of degrees. Dark red stars have surface temperatures of about 2500 K. Blue stars are extremely hot and bright and live short lives by astronomical standards. The bright star Rigel in the constellation of Orion is a blue supergiant of an estimated 20 solar masses, shining with tens of thousands of times the Sun’s luminosity.

If you heat an iron rod with an intense flame it will first appear “red hot.” Heated a little more it will seem orange and feel hotter and then yellow after that. After more heating, the rod will appear white-hot and brighter still. If it doesn’t melt, further heating will make the rod appear blue and even brighter and progressively hotter. The same basic principle applies to stars as well. Stars, molten rock and iron bars are approximations of an important class of objects that physicists call blackbodies. An ideal blackbody absorbs all of the electromagnetic radiation that strikes it. Incoming radiation heats up the body, which then reemits the energy it has absorbed, but with different intensities at each wavelength than it received. This pattern of radiation emitted by blackbodies is independent of their chemical compositions.

“Ideal blackbodies have smooth blackbody curves, whereas objects that approximate blackbodies, such as the Sun, have more jagged curves whose variations from the ideal blackbody are caused by other physics. The total amount of radiation emitted by a blackbody at each wavelength depends only on the object’s temperature and how much surface area it has. The bigger it is, the brighter it is at all wavelengths. However, the relative amounts of different wavelengths (for example, the intensity of light at 750 nm compared to the intensity at 425 nm) depend on just the body’s temperature. So, by examining the relative intensities of an object’s blackbody curve, we are able to determine its temperature, regardless of how big or how far away it is. This is analogous to how a thermometer tells your temperature no matter how big you are.”