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History of Modern Astronomy

In old times the stars and astronomy were mainly considered for two reasons: Religious and philosophical contexts, and time determination.

Instruments and observing methods were restricted to positional measurements of celestial bodies, and this did not change through the middle ages. The view of the universe that days was the geocentric system established by Greek astronomer Ptolemy around 120 AD: A sphere with fixed stars on it rotates daily around the spherically shaped Earth, with Sun, Moon, and planets being guided around Earth by a complicated machinery of epicycles; many had even forgotten about the Earth's spherical shape.

The events that brought astronomy to the state of modern science were (a) the introduction of the heliocentric system, and (b) the invention of the telescope around 1600.

The Heliocentric System

While already considered by ancient Greek Aristarchus around 300 B.C., the heliocentric system was finally established in 1543 by Nicolaus Copernicus (1473-1543) from Poland when his book, De Revolutionibus ("On Revolutions") appeared. This model considered the Sun and no more the Earth to be the center of planetary motions, and the apparent annual motion of the Sun as an illusional effect caused by this motion, while the diurnal rotation of the stellar sky is explained by a rotation of the Earth around its axis. The observed apparent motion of the planets can be understood as their motion around the Sun, viewed from a moving Earth. However, as Copernicus kept the circular orbits, he also considered an epicycle system to describe planetary motion acurately.

After Copernicus, Danish astronomer Tycho Brahe (1546-1601) proposed a hybrid model of Moon and Sun orbiting the Earth and the other planets moving around the Sun, still needing epicycles for acurate description of their orbits. Strangely, he kept the idea that the sky and all planets encircle a static Earth daily, and got in conflict with Nikolaus Baer who thought Earth was rotating. Tycho also established the nature of comets as objects of translunar space and not atmospheric phenomena, as had been postulated by Aristotle, by measuring a lower limit of the distance of several times the Lunar distance for one comet, and observed a supernova in 1572, thus proving that the stellar skies are not so unchangable as people had believed previously.

German astronomer Johannes Kepler (1571-1630) used Brahe's Mars observations to establish that planets move on elliptical orbits around the Sun, and derived his three laws of planetary motion:

  1. The orbit of each planet is an ellipse with the Sun in one focus.
  2. The radius vector from Sun to planet sweeps equal areas at each time, meaning that the planet moves faster when closer to the Sun.
  3. The squares of the revolution periods are proportional to the cubes of the mean distances from the Sun for all planets.
The establishment of the Kepler laws of planetary motion was the last great achievement of the pre-telescopic era of astronomy, although Kepler himself had also developed a type of telescopes.

It was finally left to Galileo to give evidence for the heliocentric model with his telescopic discoveries of the moons of Jupiter and the phases of Venus. However, he got in serious trouble with the Roman Inquisition for his advocation of the Copernican system, and the Church authorities kept the old geocentric system of Ptolemy as their doctrine for a long time.

The first rigorous proof of the Earth's motion around the Sun came finally over a century later in 1729, when James Bradley discovered the aberration of light from the stars, a small apparent displacement caused by the combination of Earth's motion with the finite velocity of light (which had to be discovered previously, see below). The other predicted effect, stellar parallaxes, had to wait for their discovery until 1838, when Friedrich Wilhelm Bessel discovered the parallax of star 61 Cygni.

Early Telescopes and Telescopic Discoveries

Telescopes were invented around 1600, and about 1609 the first people began to use them for observing the sky. As already mentioned, Italian astronomer and physicist Galileo Galilei (1561-1642) was one of the first astronomers using a telescope. Having heard of of Hans Lippershey's invention, with which he had applied for patent 1608, Galileo constructed a scope consisted of a convex objective and a concave eyepiece lens (a scope now called of Galilean type). With this instrument, he discovered the four bright satellites of Jupiter, the phases of Venus, mountains on the Moon, spots on the Sun and what came finally out to be the rings of Saturn. Moreover, he found the Milky Way composed of faint stars, as he found the "nebula" Praesepe (now known as M44) to be a cluster of stars. Another astronomer who also began using a telescope for astronomical observations in 1609 was Thomas Harriott.

In the same year 1610, Nicholas-Claude Peiresc (1580-1637) discovered the Orion Nebula M42 around the star Theta Orionis. Simon Marius, who had independently discovered the four brigt Jovian moons about the same time as Galileo, and who gave them their names, found and described the Andromeda "Nebula" M31 in 1912 (this was actually an independent rediscovery as it had been longly found visually by Al Sufi in 964 AD).

As mentioned, Johannes Kepler had proposed another telescope type, consisted of two convex lenses, published in 1611; such an instrument was first constructed by Christopher Scheiner between 1613 and 1617. The Keplerian telescope became the dominant design of all major post-17th century refractors.

The first reflecting telescope was constructed by Isaac Newton in 1668, based partially on a design created in 1663 by James Gregory (1638-75); one reason was the intention to overcome chromatical aberration. In 1672, Jacques Cassegrain (1652-1712, also known as Guillaume or N. Cassegrain) proposed the telescope type named after him, but probably never constructed any; the first known Cassegrain telescope was built by James Short (1710-68). Other designs, or telescope types, were proposed about that time, such as a first idea of a schiefspiegler telescope of a Pater Zahn in 1685, but did not get any importance then.

In 1733, Chester Moore Hall invented the achromatic lens system by joining a crown glass lens and a flint glass lens, which allowed for minimizing chromatic aberration. John Dollond (1706-61) and others began to produce fine quality refractors with these achromatic objective lenses in 1757, while his eldest son, Peter Dollond (1730-1820), developed the achromatic triplet lens in 1765, placing convex lenses of crown glass on either side of a biconcave flint glass lens.

William Herschel (1738-1822) invented his kind of telescope, using one tilted mirror only, around 1780; he built a number of large telescopes after this principle, including an 48-inch constructed in 1789.

Celestial Mechanics

It was already evident from Kepler's laws of planetary motion that some force must be generated by the Sun and act on the planets. Isaac Newton (1643-1727) from England provided a theoretical framework to understand this force, in the form of a universally valid mechanical principle:
             F = G*m1*m2 / r^2
This is Newton's law of gravitation, which formed the foundation of treating not only the two-body problem of a planet moving around the Sun, but also of the many-body problem. Based on this theoretical grounds, a number of famous mathematicians developed celestial mechanics to a sophisticated science during the 18th and 19th century, among them Leonard Euler (1707-1783) from Switzerland, and Joseph Lagrange (1736-1813) and Pierre Simon Laplace (1749-1827) from France.

Studying the motion of Jupiter's moons, Ole (or Olaus) Roemer found in 1675 that they are observed in slightly deviating positions from what theory predicts, as the distance of Earth and Jupiter varies. He concluded that light was propagating with finite velocity.

The study of motion of solar system bodies was further stimulated by proving Edmond Halley's (1656-1742) prediction of the return of comet Halley in 1758 through Johann Georg Palitsch's (1723-1788) rediscovery, and other comet observations, the discovery of planet Uranus by William Herschel (1738-1822) in 1781 [a prediscovery observation of Uranus had been made by Flamsteed in 1690], and the discoveries of the first minor planets, the first being Ceres discovered in 1801 by Giuseppe Piazzi (1746-1826). Methods for determining orbits from few observations were developed by Carl Friedrich Gauss (1777-1855) and Wilhelm Olbers (1758-1840).

The ultimate fame of celestial mechanics was achieved by the discovery of planet Neptune in 1846 by Johann Gottfried Galle (1812-1910) and Heinrich d'Arrest () after mathematical predictions by Urbain Leverrier (1811-1877) in France and John Couch Adams (1819-1892) in England; Neptune had been seen but not recognized in prediscovery observations by Galileo in 1612, and by Challis () in 1845 when checking Adams' predictions.

The Discovery of the Stellar Universe

While already ancient Greek Hipparchus had found the precession of the skies around 130 BC (which we now know to be a result of perturbations of Earth's motion by Sun and Moon), ancients had assumed that all stars are fixed at a unique distance on a sphere (therefore the term "fixed stars"). The heliocentric system brought up doubts to this belief, and Newton's mechanics finally led to the breakdown of the belief in a "celestial sphere" with fixed stars on it. Instead, it was now assumed that stars are distributed throughout space in a vast range of distances (first documented in Thomas Digge's diagram of the Copernican System of 1576).

Evidence to support this view came from the discovery of "new" and variable stars, and of proper motions of stars by Edmond Halley in 1718.

The search for stellar parallaxes (and thus distance determinations) was longly unsuccessful, because the parallaxes are so small (and the distances so large). While lookin for this, James Bradley (1693-1762) discovered the aberration of light in 1725-26 (published 1729) from observations of the star Gamma Draconis (Eltanin), and in 1847 the Earth's nutation, a small deviation of Earth's axis caused by the Moon with a period of 18.6 years. Bradley correctly gave an upper limit of 1 arc second for the stellar parallax and thus a lower limit of 1 parsec (3.26 light years) for the distance of this star. Also, the great observer William Herschel was unsuccessful in this thread for all his life, and it remained to Friedrich Wilhelm Bessel to finally find the parallax of 0.3 arc seconds and thus the distance of 11.1 light years for 61 Cygni in 1838 (the nearest star, Alpha Centauri, is at 4.3 light years); Bessel had selected this star for its large proper motion of 5.21 arc seconds per year (still the fifth-largest known). Almost simultaneously, Wilhelm Struve in Pulkova found the parallax of 0.12 arc seconds for Alpha Lyrae (Vega, at 27 light years) and Thomas Henderson at the Cape Observatory that of Alpha Centauri (0.745 arc seconds).

Special types of stars had been detected: Binary and multiple stars as well as variables. Giovanni Batista Riccioli of Bologna discovered the nature of Mizar (Zeta Ursae Majoris) as a double star in 1650. In 1656, Christian Huygens found that the star Theta in the Orion, in the Orion Nebula M42, was actually a group of stars; he discovered three, the fourth Trapezium star was found in 1673 by Abbe Jean Picard (according to de Mairan), and independently by Huygens in 1684. Robert Hooke discovered Gamma Arietis in 1664 or 1665. Next, in the southern hemisphere Alpha Crucis (1685 by Father Fontenay at the Cape of Good Hope) and Alpha Centauri (1689 by Father Richaud from Pondicherry, India) were identified as double. In 1718, Gamma Virginis was found, and in 1719, James Bradley found the companion at Castor (Alpha Geminorum). A first catalog of 80 entries was compiled by Christian Mayer in 1779 and published in 1781 in Bode's "Jahrbuch für 1784", compiled with an 8-foot mural quadrant at power 60 to 80. Real systematic research was started in 1779 by William Herschel, who listed already 269 double stars in his early 1782 catalog, and about 700 in his 1785 catalog; he extended this number in later publications.

While suddenly occurring "new stars" (novae and supernovae) had been occasionally recorded through the centuries from various cultures, it was only Tychos supernova of 1572 and Kepler's from 1604, as well as the nova-like outburst of P Cygni in 1600, discovered by W.J. Blaeu, that became generally known for western astronomers. Variable stars of other type were discovered, namely Mira (Omicron Ceti) in 1596 by David Fabricius (1564-1617) and Algol (Beta Persei) around 1669 by Geminiano Montanari (1632-87), though ancient naming suggests that already the ancients had noted and were alarmed by Algols variability as it was called Ras Al Ghul or "Demon's Head" by the Arabs and Rosh ha Satan or "Satan's Head" by the Hebrews. Another nova occurred in Vulpecula in 1670, and Edmond Halley discovered the variability of peculiar Eta Carinae in 1677, Gottfried Kirch that of Chi Cygni in 1687, and J.-D. Maraldi that of R Hydrae in 1704, making up a total of 9 variable stars known in 1781 (in addition, John Flamsteed has perhaps seen, but not noticed, the supernova that created Cassiopeia A in 1667).

Roth lists the number of known variables as follows: 12 by 1786, 18 by 1844, 175 by 1890, 393 by 1896, 4,000 by 1912, 22,650 by 1970, and 28,450 by 1983.

Besides stars, star clusters and "nebulae" (all appearing as nebulous patches in the small telescopes of 17th and early 18th century observers) can be found in the sky; these are nowadays summarized under the term Deep Sky Objects. As described in more detail in the history of the discovery of the Deepsky objects, some few of these objects had been known since ancient times, but most of them have been discovered only with the aid of telescopes. Notable firsts:

Galileo discovers that the Milky Way is made of stars and the nature of Praesepe (M44) as (open) star cluster
Peiresc discovers the first (bright diffuse) gaseous nebula, the Orion Nebula M42
Abraham Ihle discovers the first globular cluster, M22 in Sagittarius
John Bevis discovers the first supernova remnant, the Crab Nebula (M1)
Le Gentil discovers M32, the first telescopic galaxy
La Caille discovers M83, the first galaxy beyond the Local Group, and the first extragalactic deepsky object, the Tarantula Nebula (NGC 2070) in the Large Magellanic Cloud
Messier discovers the first planetary nebula, the Dumbbell Nebula M27
Messier discovers M54 which is now known to be the first discovered extragalactic globular cluster
Darquier discovers the Ring Nebula M57 and first compares a planetary nebula with planets
Messier and Méchain discover the Virgo Cluster of Galaxies (then assumed to be a cluster of nebulae)
The understanding of the nature of these objects, however, had to wait for new observational methods and better understanding of physics.

William Herschel was the first who tried to make a physical model of the stellar universe on observational foundations, and therefore invented the method of stellar statistics to derive a first model of the Milky Way as an island universe (or galaxy). Previously, Johann Lambert (1728-77), Thomas Wright (1711-86), and Immanuel Kant (1724-1804) had hypothesized, on religious and philosophical grounds, that the Milky Way might be a thin flat system of stars, presumably a disk, and of some "nebulae" being other systems of the same kind (however, all their objects are really part of our Galaxy, mostly globular clusters). Herschel also determined the motion of the solar system with respect to the neighboring stars with remarkable good acuracy, and supposed that other "milky ways" should be in the universe, among them the nearest, the "Andromeda Nebula" M31. However, he significantly underestimated both the size of our Galaxy and the distance to M31, which he assumed to be at 2,000 times the distance of Sirius, and most of his other "milky way" candidates were nebulae within our Galaxy.

Catalogs of celestial objects

Important early star catalogs:
Johann Bayer (1572-1625), Uranometria; note this appeared still before the introduction of the telescope in astronomy in 1609! In this work, Bayer introduced the naming of stars for their constellation and by Greek letters in the approximate order of (apparent) brightness, e.g. Alpha Centauri.
Johann Hevelius (1611-1687), Sternverzeichnis
Edmond Halley (1656-1742) compiled the first southern star catalog
John Flamsteed (1646-1719), Historia Coelestis Britannica, edited by Edmond Halley. In this catalog, the labelling of stars by the so-called Flamsteed numbers (e.g., 21 Tauri) was introduced.
John Flamsteed, Stellarum Inerrantium Catalogus Britannicus. Flamsteed's own corrected and official version, without the numbers.
James Bradley (1693-1762), Star Catalog.
For historic catalogs of deepsky objects, refer to the history of the discovery of the Deepsky objects. Notably, the first more comprehensive and reliable compilation was Charles Messier's (1730-1817) Catalogue of Nebulae and Star Clusters from 1781 containing 103 entries. William Herschel cataloged another 2500 objects, and his early 1782 catalog contained already 269 double stars, while John Herschel's General Catalog (GC) of nonstellar objects of 1864 contained over 5,000 objects, while J.L.E. Dreyer's New General Catalogue (NGC), together with its two supplements (Index Catalog I and II, IC) summarize over 13,000 deep sky objects.

In the 18th century, better instruments allowed the compilatin of more acurate and larger catalogs. A milestone was the Bonner Durchmusterung, created 1852-59 under Friedrich Wilhelm Argelander (1799-1875), which contains positions and magnitudes for 320,000 stars. This catalog was extended southward by the Cordoba Durchmusterung, compiled 1885-1892. Other important catalogs compiled visually include the Harvard Revised Photometry and the Potsdammer Durchmusterung, both published 1907.

The pioneering work of photographic photometry was Karl Schwarzschild's (1873-1916) Göttinger Aktinometrie, compiled 1904-1908.

When spectroscopy came up, a first classification of 316 stars was published by the Italian Father Angelo Secchi (1818-1878) in 1867. A more comprehensive compilation of spectral classification was the Henry Draper Catalogue published 1918-24 at Harvard Observatory and containing data of 225,300 stars.


In the second half of the 19th century, the following developments lead to significant changes in astronomy, and especially the upcoming of astrophysics (then often called "New Astronomy"):
Stellar Photometry
Until early 19th century, stellar magnitudes were estimated approximately and inacurately by visual observers, to at most 0.2 magnitudes acuracy. In 1861, Karl Friedrich Zöllner (1834-82) in Berlin introduced the visual photometer. Further improvement was achieved by introducing photoelectric cells for photometry, and by measuring photographic plates.
Already in 1666, Isaac Newton had shown that sunlight can be decomposed into a spectrum when passing a prism. In 1802, British chemist and physicist William Hyde Wollaston (1766-1828) found dark lines in the Solar spectrum.

In 1818, Joseph Fraunhofer (1787-1826) was the first to take a good spectrum of the Sun and discovered 576 dark lines in it; he labelled the more prominent lines with letters A to K. He later discovered that the light from Moon and planets show the same spectral features as the solar spectrum, that the spectra of star differ from this spectrum, and developed the diffraction grating, one of his had 3,625 lines per centimeter.

In 1832, David Brewster showed that cold gasses produce dark absorption lines in continuous spectra. In 1847, John W. Draper found that hot solids emit light in continuous spectra while hot gasses produce line spectra. In 1859, Gustav Robert Kirchhoff (1824-87) and Robert Bunsen (1811-99) discovered that each chemical element (and compound) shows a characteristic spectrum of lines, which are at the same wavelengths in emission and absorption spectra. Thus, the chemical composition of a light source (including celestial bodies) can be determined from spectral analysis; Kirchoff published a study of the chemical constitution of the Sun im 1859.

Anders Jonas Angstrom (1818-74) published his map of the Solar spectrum with identifiication of lines corresponding chemical elements in 1863.

In 1864, British amateur William Huggins (1824-1910) published his investigations of spectra of stars and nebulae (thereby finding the gaseous nature of diffuse and planetary nebulae). The same year, Giovanni Batista Donati showed that comet spectra contain emission lines. The first spectrogram (photo of a spectrum) of a star, Vega (Alpha Lyrae), was obtained in 1872 by American amateur Henry Draper (1837-82).

Christian Doppler (1803-53) had discovered that moving bodies show shifted spectral lines, so that radial velocities can be determined spectroscopically with high acuracy. William Huggins stated in 1868 that because of this effect, spectral lines of moving celestial objects should appear shifted. The first measurements of this effect were obtained in 1888 by Hermann Carl Vogel (1841-1907).

Of the early spectral classifications schemes, that of Edward Charles Pickering (1846-1919) and Annie Cannon (1863-1941), used in their Henry Draper Catalogue, was finally adopted by the IAU.

Astronomical Photography
The first photo of the Moon was obtained in 1841 by J.W. Draper on Daguerre plates. The first solar eclipse photos were obtained on July 18, 1851. W.D. Bond obtained the first photos of stars in 1857. The major breakthrough was finally the invention of dry photographic plates by R.L. Maddox in 1871 which made durable photos possible.

The power of photography for every branch of astronomy was quickly demonstrated; early pioneering work was done by Isaac Roberts, Edward Emerson Barnard (1857-1923) and Max Wolf (1863-1932) especially for the Milky Way, star clusters, and nebulae.

Better large telescopes
During the 18th and early 19th century, small refractors and larger metal mirror reflectors (up to Lord Rosse's 72-inch Leviathan of 1845) were the telescopes available for observers.

Telescope optics was notably improved by Fraunhofer when he developed the achromatic objective in 1824, which led to the construction of larger refractors up to the Yerkes 102 cm.

The reflector techniques was significantly improved by the invention of glass mirrors by Steinheil in 1857 who built a 10 cm reflector, succeeded by Faucault's 33-cm and Lassell's 60-cm glass mirrors. Almost all big telescopes of the 20th century are reflectors with glass mirrors. The first telescope exceeding Lord Rosse's Leviathan of 1845 in aperture was the 100-inch Mount Wilson telescope constructed 1917, followed by the Palomar 200-inch in 1948, and the limitedly successful 6.1-meter Selenchukskaya telescope in 1976.

Milky Way, Nebulae, and Stellar Systems

After Galileo's discovery that Milky Way and some nebulous patches were composed of stars, it was generally assumed that all "nebulae" were actually distant star clusters and should be resolved in sufficiently powerful telescopes; a notable and exotic exception was Edmond Halley's view of "nebulae" as lucid "holes in space". William Herschel first found this view confirmed by his observations, as with his large telescopes, he could resolve also dense globular clusters, but later realized that some "nebulae", notably the Orion Nebula M42 and planetary nebulae, were probably made up of a "shining fluid of a nature totally unknown to us". William's son John Herschel, on the other hand, seriously doubted the existence of nebulae which were not made up of stars in the late 1840s.

In 1845, William Parsons, third Earl of Rosse (1800-67) discovered the spiral pattern of M51, and later of M99 and 13 other "nebulae" which were since known as "spiral nebulae".

The essential event marking the discovery of gaseous nebula came when William Huggins observed their spectra in 1864 and found them to be emission line spectra. Now there was a simple and unique criterion distinguishing them from star clusters, which like the stars composing them, show a continuous spectrum (with overlaid absorption and sometimes emission lines). Spiral "nebulae", however, show continuous spectra like stars.

It was known since Herschel that the Milky Way forms a system of stars one of which is our Sun. Since Kant and Herschel, it was speculated that there might be other similar stellar systems; some believed Rosse's spiral nebulae could be candidates. By 1900, Easton proposede a model of the Milky Way as a spiral nebula. Another fraction of astronomers, including astrophotographer Isaac Roberts who interpreted his photo of the Andromeda "nebula" M31, thought these nebulae were solar systems in formation (with the companions M32 and NGC 205 [M110] supposed as forming Jovian planets).

Stellar statistical methods, invented by Herschel and improved by H. von Seliger and J. Kapteyn, indicated that the Solar System was, presumably by chance, situated close to the center of the Milky Way Galaxy. In 1904, interstellar reddening and absorption were found; nevertheless, it was longly believed to be a minor effect only.

In 1912, Vesto M. Slipher of Lowell Observatory discovered the nature of the nebulae in the Pleiades star cluster M45 as reflection nebulae. In 1914, he found that the spiral and some elliptical "nebulae" are moving at very high radial velocities so that their membership in the Milky Way got questionable, and in 1915 he determined the rotational velocity of the edge-on "nebula" M104 to be about 300 km/s. The view that spiral "nebulae" might be galaxies like our Milky Way was stressed by Heber D. Curtis of Lick Observatory, also on the basis of nova observations and as absorption could explain why spirals "avoid" to be seen near the galactic plane, but opposed in particular by Adriaan van Maanen (1884-1946) who erroneously believed to have found internal proper motions in spirals which would have indicated observable rotation.

In 1912, Henrietta Leavitt found the period-luminosity relation of Cepheid variables in the Magellanic clouds. Using this relation, Harlow Shapley, in 1918, determined distances in the Milky Way, and in particular of globular clusters, which he found centered aroung a location in Sagittarius: He concluded that the center of the Galaxy should be located there, with the solar system lying in an outer region of Milky Way; however, as he significantly underestimated the influence of interstellar absorption, he overestimated the size of the Milky Way by a factor of about 3.

In 1924, Edwin Hubble resolved the outer part of the Andromeda "Nebula" M31 into stars and found novae and Cepheid variables, thus establishing its nature as an external star system or galaxy.

In 1926, Bertil Lindblad and Jan Oort developed the theory of kinematics and dynamics of the Milky Way Galaxy.

In 1929, Hubble derived his distance - redshift relation for galaxies, indicating the expansion of the universe.

In 1930, Robert Julius Trumpler (1886-1956) of Lick observatory found from investigations of open clusters that the interstellar absorption had been signficantly underestimated, and the Milky Way Galaxy was correspondingly smaller. In 1937, interstellar molecules (CO_2) were found as absorption lines.

In 1943, Carl Seyfert discovered that certain galaxies (now called Seyfert Galaxies) have "active" nuclei with peculiar nonthermal spectra. In 1944, Walter Baade discovered that the stellar population in different regions of galaxies varies and there are two different stellar populations: Young Population I in spiral arms and irregular galaxies, and old Population II stars in elliptical (and lenticular) galaxies, globular clusters, and the bulges and nuclei of spiral galaxies.

In 1951, the 21-cm radio radiation of neutral hydrogen was discovered. Observations of the Milky Way in this wavelength provided first direct evidence of the spiral structure of our Galaxy.

In 1952, Baade found that Cepheids of two classes exist: Type I Cepheids ("classical" Delta Cephei stars) which are members of population I and Type II Cepheids (W Virginis stars) which are 4 to 5 times fainter. This discovery implied that the intergalactic distance scale had to be revised, moving the galaxies to more than double distance away, and thus removing discrepancies of Milky Way size compared to external galaxies. Since, the distance scale had been subject to minor modifications on various occasions, last due to revision of the Cepheid distances found by the astrometric satellite Hipparcos in early 1997.

In 1963, the first quasar was discovered by Maarten Schmidt.

Stellar Evolution

Early models for stellar structure and evolution, though interesting in some cases, could not explain, e.g., why the Sun could emit an almost constant rate of energy for billions of years (although the Helmholtz-Kelvin model of a contracting star which radiates gravitational energy is interesting in the context of stellar formation and protostars). However, the true nature of stellar structure and evolution could be revealed only after the physical foundations had been established in the early 20th century.
Karl Schwarzschild modeled the solar atmosphere from theory of thermodynamical equilibrium; R. Emden published his theory of gaseous spheres; 1906-12 temperature-luminosity relation of stars discovered (Hertzsprung-Russell diagram)
Enjar Hertzsprung discovered the mass-luminosity relation
Theory of stellar atmospheres (C.H. Payne), stellar structure (A.S. Eddington) and convective transport (A. Unsöld) developed
Hypothesis of neutron stars proposed by W. Baade and F. Zwicky
Nuclear reactions proposed as stellar energy sources by H. Bethe and C.F. v. Weitzsäcker
Carbon formation reaction ("Triple-Alpha process") investigated by E.J. Öpik and E.E. Salpeter
1955 and later
Computer modelling of stellar structure and evolution, introduced by M. Schwarzschild and F. Hoyle.
An introduction to the current state of knowledge of stellar evolution is available.

Observations in the Invisible Light and Space Astronomy

As early as 1800, William Herschel discovered the infrared radiation beyond the red end of the visible spectrum with a thermometer. One year later in 1801, J.W. Ritter discovered UV radiation by demonstrating that they produce chemical effects. However, observation of this radiation was restricted to the Sun until the 20th century.

In 1931, K.G. Jansky discovered radio radiation from the Milky Way. In 1939, G. Reber found this radiation concentrated within the galactic plane and toward the galactic center. In 1942, J.S. Hey and J. Southward found the first extragalactic radio radiation.

Individual radio sources were identified in the early 1950s, and the first radio galaxies in 1954.

With upcoming space missions, astronomy became possible in those parts of the electromagnetic spectrum for which Earth's atmosphere is not transparent. In 1960, cosmic X-rays (fronm the Solar corona) were observed for the first time by an aerobee rocket. In 1965, the first cosmic X-rays were discovered (E.T. Byram, H. Friedman, T.A. Chubb); U.S. satellite Uhuru discovered 160 X-ray sources in 1970.

In 1963, radio astronomers discovered the first quasar (M. Schmidt), and in 1967, the first pulsar (J. Bell and A. Hewish).

Since, astronomical satellites have become a powerful tool to investigate astronomical objects in every spectral range; for more detail, look at the list of orbiting astronomical observatories (astronomy satellites).

Space Exploration

This sections is still preliminary, and a more detailed account of space exploration will be given elsewhere.
Sputnik 1 (USSR) first artificial satellite
Venera 1 (USSR) first mission to another planet (Venus)
Apollo 11 (USA) first men land on the Moon
Viking 1 and 2 (USA) first successful unmanned landing on Mars
Voyager 1 and 2 (USA) fly by Jupiter
Voyager 1 and 2 fly by Saturn
Voyager 2 flyby of Uranus
Voyager 2 flyby of Neptune
Hubble Space Telescope (USA/ESA) launched



Hartmut Frommert [contact]