The American astronomer Gerry Neugebauer (born 1932), son of the great Austrian historian of science Otto Neugebauer, did valuable pioneering work in infrared astronomy. He spent his entire career at the California Institute of Technology. Together with the US experimental physicist Robert B. Leighton (1919-1997), also at Caltech, he completed the first infrared survey of the sky. Leighton is also known for discovering five-minute oscillations in local surface velocities of the Sun, which opened up research into solar seismology. The American physicist Frank James Low (1933-2009) became a leader in the emerging field of infrared astronomy after inventing the gallium-doped germanium bolometer in 1961, which allowed the extension of observations to longer wavelengths than previously possible. He and his colleagues showed that Jupiter and Saturn emit more energy than they receive from the Sun.

Jupiter’s diameter is 142,984 kilometers, more than 11 times that of the Earth. It would take over one thousand Earths to fill up its volume. Jupiter alone contains almost two and a half times as much mass as the rest of the planets in our Solar System combined, but it would still have needed 75-80 times more mass to become a small star. The lowest mass that an object can have and still be hot enough to sustain the fusion of normal hydrogen into helium in its core is about 8% of the Sun’s mass. Jupiter, however, contains merely 0.001 solar masses.

Bodies with 13-75 times Jupiter’s mass fuse deuterium, a rare isotope of hydrogen, into helium, and those with between 60 and 75 times Jupiter’s mass also fuse lithium-7 nuclei (three protons and four neutrons) into helium. Yet this will only occur briefly in astronomical terms due to the limited supply of these materials. Such objects are called brown dwarfs or “failed stars” and are intermediate between planets and stars. Brown dwarfs are not literally brown. They were first hypothesized in 1963 by astronomer Shiv Kumar. The American astronomer Jill Tarter (born 1944) proposed the name in 1975. She later became the director of the Center for SETI Research, which looks for evidence of intelligent life beyond the Earth.

The astronomer Frank Drake, born 1930 in Chicago in the United States, in 1961 devised the Drake Equation, an attempt to calculate the potential number of extraterrestrial civilizations in our galaxy. He has participated in an on-going search for signals of intelligent origin. While this line of work was initially associated with searching for radio waves from other civilizations, more recently those engaged in these matters have started looking for other types of signals, above all optical SETI. Very brief, but powerful pulses of laser light from other planetary systems can potentially carry immense amounts of concentrated information across vast distances of many light-years. Obviously, if extraterrestrial civilizations do exist, it is quite conceivable that they may be scientifically more sophisticated than we are today and may possess some forms of communication technology that are totally unknown to humans.

The search for intelligent extraterrestrial life is not uncontroversial, especially when it comes to so-called “Active SETI” signals, where we beam signals into space in addition to passively recording signals we receive. The famous English astrophysicist Stephen Hawking believes that intelligent aliens are likely to exist, but fears that a visit by them to present-day humanity might have unfortunate consequences for us. “We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet,” he argues. It is possible to imagine an alien civilization of nomads, looking to conquer and colonize whatever planets they can reach, instead of peaceful interstellar “philosopher kings.” Others find it implausible that aliens would travel across vast astronomical distances merely to colonize us.

The possibility of life beyond the Earth has been discussed for centuries. The English bishop and naturalist John Wilkins (1614-1672), who proposed a decimal system of weights and measures that foreshadowed the metric system, in The Discovery of a World in the Moone (1638) suggested that the Moon is a habitable world. He was not the first person to entertain such views, which had been suggested by some ancient Greek authors. No lesser man than Johannes Kepler had written a story The Dream (Somnium) where a human observer is transported to the Moon. Wilkins worked in the turbulent age of Oliver Cromwell and the English Civil War and was associated with men who went on to found the Royal Society.

The author Bernard de Fontenelle (1657-1757), born in Rouen, Normandy, in northern France, in 1686 published Conversations on the Plurality of Worlds (Entretiens sur la pluralité des mondes), which supported the heliocentric model of Copernicus and spoke of the possibility of life on other planets. The colorful German (Hanoverian) storyteller Baron Münchhausen (1720-1797), who had fought against the Turks for the Russian army, in his incredible and unlikely tales allegedly claimed to have personally visited the Moon. In From the Earth to the Moon (1865) by the great French science fiction author Jules Verne, three men travel to the Moon in a projectile launched from a giant cannon. William Henry Pickering (1858-1938) from the USA, brother of Edward Pickering and otherwise a fine astronomer, in the 1920s believed he could observe swarms of insects on the Moon’s surface.

In the 1870s the Italian scholar Giovanni Schiaparelli had observed geological features on Mars which he called canali, “channels.” This was mistranslated into English as artificial “canals,” which fueled speculations about the possibility of intelligent life on that planet.

The English author H. G. Wells in 1898 published the influential science fiction novel The War of the Worlds, where the Earth is invaded by technologically superior Martians who eventually succumb not to our guns, but to our bacteria and microscopic germs, which we had evolved immunity against but they had not. In 1938, when commercial radio was in its first generation, a drama adoption of Wells’ novel caused panic in the USA, as thousands of radio listeners believed that it depicted a real, ongoing invasion. The man behind the broadcast, the American director Orson Welles (1915-1985), also wrote, directed, produced and acted in Citizen Kane from 1941, hailed as one of the best films from Hollywood’s Golden Age.

One of the earliest science fiction films, inspired by the writings of Jules Verne and H. G. Wells, was the black and white silent movie A Trip to the Moon from 1902 by the French filmmaker Georges Méliès (1861-1938). The Austrian-born motion-picture director Fritz Lang (1890-1976) created the costly silent film Metropolis in Germany in 1927. In the commercially successful Hollywood production E.T. the Extra-Terrestrial from 1982, directed by the influential American Jewish film director and producer Steven Spielberg (born 1946), a boy befriends a stranded, but friendly extraterrestrial and helps him to return home.

The American planetary scientist and science writer Carl Sagan (1934-1996), born in New York City, won enormous popularity as well as some criticism as a popularizer of astronomy and was a contributor to NASA’s Mariner, Viking, Voyager and Galileo expeditions to the planets. “He helped solve the mysteries of the high temperatures of Venus (answer: massive greenhouse effect), the seasonal changes on Mars (answer: windblown dust), and the reddish haze of Titan (answer: complex organic molecules).” The Ukrainian astrophysicist Iosif Shklovsky in the Soviet Union was one of the first major scientists to propose serious examination of the possibility of extraterrestrial life. His book Intelligent Life in the Universe was translated and expanded by Carl Sagan, whose father was a Russian Jewish immigrant.

From infrared radiation was discovered in 1800 and ultraviolet radiation the year after, European scientists mapped the electromagnetic spectrum – radio waves, microwaves, terahertz radiation, infrared radiation, visible light, UV, X-rays and gamma rays – until the French physicist Paul Villard had found gamma radiation in the year 1900. Astronomers began observing in all wavelengths in the twentieth century, primarily in the second half of it.

Typically, only 2% of the light striking film triggers a chemical reaction in the photosensitive material. Photographic film and plates have been replaced in favor of the highly efficient electronic light detectors called charge-coupled devices (CCDs). CCDs respond to 70% or more of the light falling on them, and their resolution is currently better than that of film. A CCD is divided into an array of small, light-sensitive squares called picture elements or pixels. A megapixel is one million pixels. This is the same basic technology that is used in digital cameras for the mass consumer market. The Canadian physicist Willard Boyle (born 1924) and the physicist George E. Smith (born 1930) from the USA invented the first charge-coupled device in 1969. They received the 2009 Nobel Prize in Physics in recognition of the tremendous importance their invention has had in the sciences, from astronomy to medicine.

Like radio and gamma ray astronomy, X-ray astronomy took off after WW2. Herbert Friedman (1916-2000), who spent most of his career at the US Naval Research Laboratory, using rocket-borne instruments found that the Sun weakly emits X-rays. The Nobel Prize-winning astrophysicist Riccardo Giacconi was born in Genoa in northern Italy in 1931 and earned his Ph.D. in cosmic ray physics at the University of Milan before moving to the USA. He there became one of the major pioneers in the discovery of cosmic X-ray sources, among them a number of suspected black holes. Giacconi and his American colleagues built instruments for X-ray observations that were launched into space. The first widely accepted black holes, such as the object called Cygnus X-1, were detected in the 1960s and 70s.

A black hole is an object with such a concentrated mass that no nearby object can escape its gravitational pull since the escape velocity, the speed required for matter to escape from its gravitational field, exceeds that of light. In the seventeenth century it had been established by Ole Rømer that light has a great, but finite speed, and Isaac Newton had introduced the concept of universal gravity. The idea that an object could have such a great mass that even light could not escape its gravitational pull was proposed independently in the late 1700s by the English natural philosopher John Michell (1724-1793) and the French mathematical astronomer Pierre-Simon Laplace, yet their ideas had little impact on later developments.

John Michell had studied at Cambridge University in England, where he taught Hebrew, Greek, mathematics and geology. He devised the famous experiment, successfully undertaken by Henry Cavendish in 1797-98 after his death, which measured the mass of the Earth. In addition to this, Michell is considered one of the founders of seismology. His interest for this subject was triggered by the powerful earthquake that destroyed the city of Lisbon, Portugal, in 1755. He showed that the focus of that earthquake was underneath the Atlantic Ocean.

Modern theories of black holes emerged after Einstein’s general theory of relativity from late 1915. The astrophysicist Karl Schwarzschild (1873-1916) was born in Frankfurt am Main in Germany to a Jewish business family and studied at the Universities of Strasbourg and Munich. From 1901 until 1909 he was professor at Göttingen and director of the Observatory there, and in 1909 he became director of the Astrophysical Observatory in Potsdam. On the outbreak of World War 1 in August 1914 he volunteered for German military service in Belgium and then Russia, where he contracted an illness that caused his death at the age of 42.

While on the Russian front, he completed the first two exact solutions of the Einstein field equations, which had been presented in November 1915. For a nonrotating black hole, the “Schwarzschild radius” (Rg) of an object of mass M is given by a formula where G is the universal gravitational constant and c is the speed of light: Rg = 2GM/c2. The Schwarzschild radius defines the spherical outer boundaries of a black hole, its event horizon. Ironically, Schwarzschild himself apparently did not believe in the physical reality of such objects.

According to scholar Ted Bunn, “Almost immediately after Einstein developed general relativity, Karl Schwarzschild discovered a mathematical solution to the equations of the theory that described such an object. It was only much later, with the work of such people as Oppenheimer, Volkoff, and Snyder in the 1930s, that people thought seriously about the possibility that such objects might actually exist in the Universe. (Yes, this is the same Oppenheimer who ran the Manhattan Project.) These researchers showed that when a sufficiently massive star runs out of fuel, it is unable to support itself against its own gravitational pull, and it should collapse into a black hole. In general relativity, gravity is a manifestation of the curvature of spacetime. Massive objects distort space and time, so that the usual rules of geometry don’t apply anymore. Near a black hole, this distortion of space is extremely severe and causes black holes to have some very strange properties.”

If the mass creating a black hole was not rotating then the black hole does not rotate, either. Nonrotating ones are called Schwarzschild black holes. When the matter that creates a black hole possesses angular momentum, that matter collapses to a ring-shaped singularity located inside the black hole between its center and the event horizon. Such rotating black holes are called Kerr black holes, after the mathematician Roy Kerr (born 1934) from New Zealand who calculated their structure in 1963. A black hole is empty except for the singularity.

The American physicist John Archibald Wheeler (1911-2008) is credited with having popularized the terms black hole and wormhole, a tunnel between two black holes which could hypothetically provide a shortcut between their end points. While a popular concept in science fiction literature, it has so far not been proven that wormholes actually exist. Wheeler received many prestigious awards, among them the Wolf Prize in Physics. He was also an influential teacher of many other fine American physicists, among them Richard Feynman (1918-1988) as well as Charles W. Misner (born 1932) and Kip Thorne (born 1940), who are both among the world’s leading experts on the astrophysical implications of general relativity.

The physicist Yakov B. Zel’dovich (1914-1987), born in Minsk into a Jewish family, played a major role in the development of thermonuclear weapons in the Soviet Union and was a pioneer in attempts to relate particle physics to cosmology. Together with Rashid Sunyaev (born 1943) he proposed the Sunyaev-Zel’dovich effect, an important method for determining absolute distances in space. Sunyaev has developed a model of disk accretion onto black holes and of X-radiation from matter spiralling into such a hole. Working in Moscow, Sunyaev led the team which built the X-ray observatory attached to the pioneering Soviet (and later Russian) MIR space station, which was constructed during the late 1980s and early 1990s.

The English theoretical physicist Stephen Hawking was born in Oxford in 1942 and studied at University College, Oxford. Hawking then went on to Cambridge University to do research in cosmology. In the 1970s he predicted that black holes, contrary to previous assumptions, can emit radiation and thus mass. This has become known as Hawking or Bekenstein-Hawking radiation after Jacob Bekenstein (born 1947), an Israeli physicist at the Hebrew University of Jerusalem. His work combined relativity, thermodynamics and quantum mechanics. Unlike Einstein’s theories, which have been experimentally confirmed numerous times, it is possible that Hawking’s ideas about black hole evaporation may never be directly observed by us.

Stephen Hawking, in collaboration with the English mathematical physicist Roger Penrose (born 1931), developed a new mathematical technique for analyzing the relation of points in spacetime. Hawking became a scientific celebrity, and his serious physical handicap contributed to the general public’s fascination with his person. His popular science book A Brief History of Time from 1988 has sold millions of copies in dozens of languages. In the twenty-first century, following the development of sophisticated electronic computers, complex computer simulations can be used to study the conditions in and around black holes. They are now believed to be dynamic, evolving, energy-storing and energy-releasing objects.

The Dutch-born American astronomer Maarten Schmidt (born 1929) earned his bachelor’s degree at the University of Groningen in the Netherlands and his Ph.D. under the great astronomer Jan Hendrik Oort at the University of Leiden in 1956. Schmidt was one of many prominent Dutch-born astronomers of the twentieth century, a respectable number for such a small nation. Some of them, like Oort, Willem de Sitter, Jacobus Kapteyn and Hendrik C. van de Hulst, remained in the Netherlands, whereas others like Sidney van den Bergh and Gerard Kuiper moved to North America. Sidney van den Bergh (born 1929) has worked on everything from star clusters to cosmology, but the research for which he is best known is in the classification of galaxies, the study of supernovae and the extragalactic distance scale.

Maarten Schmidt joined the California Institute of Technology. In 1963 he studied the spectrum of an object known as 3C 273 and found that it had a very high redshift, indicating that it was extremely far away from us. He investigated the distribution of such quasars (quasi-stellar objects) and discovered that they were more abundant when the universe was younger. The American radio astronomer Jesse Greenstein (1909-2002) collaborated with him in this work. Other quasars were soon found, but it was difficult to explain how they could generate enough energy to shine so brightly at distances of billions of light-years. Quasars were among the most distant, and by extension oldest, objects ever observed in the universe.

The English astrophysicist Donald Lynden-Bell (born 1935) was educated at the University of Cambridge in Britain where he eventually became professor of astrophysics. He made significant contributions to the theories of star motions, spiral structure in galaxies, chemical evolution of galaxies and the distributions and motions of galaxies and quasars. In 1969 he proposed that black holes are at the centers of many galaxies and provide the energy sources for quasars, powered by the collapse of great amounts of material into massive black holes.

A black hole can attract gas from its neighbors, which then swirls into it in the form of an extremely hot accretion disk. Matter that spirals into it emits copious quantities of X-rays and gamma-rays, which can be detected by us although the black hole itself cannot be directly observed since light cannot escape it. There are several classes of black holes; some are just a few solar masses, formed from the collapse of a large star. There may be an intermediate class of such bodies, too, but most if not all galaxies, including our own Milky Way, are believed to harbor supermassive black holes of millions or even billions of solar masses in their centers. These objects are believed to constitute a key force in shaping the lifecycle of galaxies.

Most astronomers today believe that quasars are created by supermassive black holes that are growing, perhaps forming when two large galaxies collide. Previously, black holes were generally seen as the endpoints of evolution, but a detailed survey has found that giant black holes were already common 13 billion years ago. The universe’s first, probably extremely massive stars collapsed after a few million years. In a remarkably short period of time and in a process that is not yet fully understood, smaller black holes apparently merged into supermassive centerpieces of star-breeding galaxies and evolved into galactic sculptors.

General relativity allows for the existence of gravitational waves, small distortions of spacetime geometry which propagate through space. However, just like black holes this was initially believed by most scientists to describe purely mathematical constructs, not actually existing physical phenomena. Significant gravitational waves are thought to be generated through the collision and merger of dense objects such as stellar black holes or neutron stars.

In 1974 the American astrophysicists Russell Hulse (born 1950) and Joseph Taylor (born 1941) discovered a pair of pulsars (neutron stars) in close orbit around each other. They shared the 1993 Nobel Prize in Physics for their studies of this pair, whose behavior deviates from that predicted by Newton’s theory of gravity. Einstein’s general theory of relativity predicts that they should lose energy by emitting gravitational waves in roughly the same manner as a system of moving electrical charges emits electromagnetic waves. These two extremely dense bodies are rotating faster and faster about each other in an increasingly tight orbit. The change is tiny, but noticeable, and is in agreement with what it should be according to the general theory of relativity. This is seen as an indirect proof of the existence of gravitational waves. We have to wait until later in the twenty-first century for a direct demonstration of their existence, or to revise our theories if it turns out that they do not exist.

The Polish astronomer Bohdan Paczynski (1940-2007) was born in Vilnius, Lithuania, educated at Warsaw University in Poland and in 1982 moved to Princeton University in the USA. He was a leading expert on the lives of stars. Because gravity bends light rays, or rather, appears to do so because it bends the fabric of space itself, an astronomical object passing in front of another can under certain conditions focus its light in a manner akin to a telescope lens. Paczynski showed that this effect could be applied to survey the stars in our galaxy. This is called gravitational microlensing. The possibility of gravitational lensing had been predicted by Einstein, but Paczynski worked out its technical underpinnings. He also championed the idea that gamma ray bursts originate billions of light-years away. Gravitational lensing has emerged as a highly useful tool in astronomical research.

Gamma ray bursts are short-lived but extremely powerful bursts which can briefly shine hundreds of times brighter than a regular supernova. They were discovered in the late 1960s, at the height of the Cold War, by military satellites designed to detect gamma radiation pulses from nuclear weapons tests. In 2008, NASA’s Swift satellite detected such an explosion 7.5 billion light-years away that was so powerful that its afterglow was briefly visible to the naked eye, the most distant object ever seen by human eyes without optical aid until then.

Gamma ray bursts are the most powerful explosions known in the universe. The light from the most distant such event yet recorded reached our world from more than 13 billion light-years away in 2009. That explosion, which lasted just a little more than a second, released roughly 100 times more energy than our Sun will release during its entire lifetime. Most likely, it originated from a dying star with far greater mass than the Sun in the very young universe.