The fifteenth century in Europe was remarkable in that two great intellectual movements began to sweep across the continent almost
simultaneously: the Renaissance and free scientific inquiry—particularly astronomy.
One of the founding fathers of modern astronomy, Nicolaus Copernicus, made waves for his rejection of the Earth as the literal center of the universe. Based on an elegant theoretical model of the geometry of the solar system and the motions of the planets, he eventually came up with the idea that the Sun was at the center of the solar system, not the Earth. Copernicus first presented his heliocentric theory in a preliminary thesis that was never published. He later expounded upon this thesis in a work called De revolutionibus
orbium coelestium (On the Revolutions of the Celestial Spheres), which wasn’t published until 1543, the year of his death.
After its appearance, De revolutionibus became the astronomy textbook of choice, quickly displacing Ptolemy's Almagest. The most remarkable thing about this tale was that the acceptance of the motion of the earth, as Copernicus wove it into his heliocentric theory over a period of some 30 years, overthrew a 1600- year-old tract that had enthralled the greatest minds for centuries.
Tycho Brahe, the last and greatest of all naked-eye observers, was a Danish noble. He studied at the University of Copenhagen. He is known for his very accurate astronomical observations. Tycho had studied the stars so assiduously that he knew by heart the positions of all the visible stars in the sky. His uncle sent him to Germany to complete his studies, and his fame grew across Europe as a young, energetic astronomer. It did not take much to convince Tycho about the need for precise observations because he had pursued the practice of very accurate observing over long periods of time.
Tycho had gained great fame at an early age, becoming so famous by the age of 26 years for his brilliant and accurate observations that the King of Denmark, Frederick II, invited hint to come back to Denmark to become the court astrologer. Because astrology in those days was in vogue among the nobility and Tycho himself was passionately devoted to it, he accepted the king's offer. Frederick was so pleased with Tycho that he offered him the island of Hven as the site for an observatory. Tycho accepted this offer and initiated the construction of the first and one of the most famous observatories in the history of astronomy—Uraniborg, or "City in the Sky".
When Christian IV, the new Danish king, deprived Tycho of the observatory and the benefits, Tycho accepted the invitation of the Holy Roman Emperor Rudolph II to come to Bohemia as the royal astronomer. There he was given an estate and castle at Benatky, near Prague. With a stipend of 3000 ducats, Tycho built another, much less pretentious observatory, and laid the basis for his collaboration with Kepler, which began in 1600, a year before Tycho died.
Tycho contributed two important discoveries to the understanding of two astronomical phenomena that were observed at that time. Tycho, on a walk near his home, suddenly saw a very bright star, near the constellation of Cassiopeia. As Tycho followed it, night after night, it began to fade. We know now that Tycho had observed a supernova, which is still called the 1572 Tycho nova. Tycho discussed the great comet of 1577 and conclusively proved that comets are not atmospheric phenomena but are instead produced in regions beyond the moon.
Outstanding among all the Copemicans was the Italian philosopher and Dominican monk Giordano Bruno, who proposed the truly revolutionary idea that every star in the sky is a sun and that the universe extends infinitely in all directions so that no center of the universe exists. Bruno paid with his life for these ideas, being burned at the stake by the Inquisition as a heretic.
A Copernican, Kepler was was particularly driven to find relations among what, to others, appeared to be disparate, unrelated concepts. He did not believe in coincidences in nature. Throughout his life Kepler sought the law or laws that bind the solar system together and determine the geometry of the orbits and the dynamics of planets. He was Tycho Brahe’s disciple. His greatest achievement is discovering a set of laws of planetary motion, describing the motions of the planets around the sun. That the planets move in elliptical orbits is one of the most remarkable discoveries in the history of science.
Astronomers after Copernicus continued his work, but none did so more faithfully than Galileo Galilei. An Italian astronomer, mathematician, physicist, and all-around scientist, Galileo was one of the first scientists to provide a mathematical description for the laws of nature. He also contributed significantly to society’s understanding of the physics of motion. Galileo designed enhanced telescopes (think of ’em as the Hubble Space Telescope of the Renaissance period) that were the most powerful of the day. That allowed him to perform the most-detailed astronomical observations of the time, including a study of Jupiter’s moons.
Galileo’s main concern, after he built his telescope, was to inform the public of his great astronomical discoveries, and he began by inviting the Venetian Senate to look through his "spyglass" from the tower of St. Marco. By that time he had built a 9-power telescope and its effect on the senators was spectacular. The Senate immediately doubled his salary and made his professorship at Padua permanent. His next step was to publish the first of his books, The Starry Messenger, which revolutionized the study of astronomy. He dedicated his book to Duke Cosimo II De Medici, his sponsor, whom Galileo tutored during childhood.
A student at Padua, Baldassarre Capra, published in Latin, Galileo's description (written in the Tuscan dialect) of the compass and claimed it as his own. Galileo was so enraged at this attempted theft that he wrote A Defense Against the Calumnies and Impostures of Baldessarre Capra. Galileo brought a legal action against Capra, which he won, and so Capra's book was banned. Galileo's writings brought him fame throughout Europe and he was sought out by students everywhere.
Galileo was somewhat secretive about his important discoveries such as the law of falling bodies, the concept of inertia, and the laws of projectiles. He communicated these ideas, as well as his cosmological concepts, only to those persons like Kepler, who, he knew would accept them or, at the very least, consider them with an open mind. The two corresponded regularly. Galileo first learned of Kepler through a common friend, Paulus Amberger, who delivered Kepler's book, Cosmic Mystery, to Galileo. Kepler, in a letter to his teacher, Maestlin, states that he had sent a copy of his book "to a mathematician Galileus Galileus, as he signs himself."
Galileo was under attack by the Aristotelians and clerics who saw his doctrines as a direct threat to their power and positions. They therefore turned to the Bible to find arguments against his support of the Copernican cosmology. Galileo was very skillful in demolishing the arguments of his opponents, but he could not defend himself from arrest and trial by the Roman Inquisition. This was preceded by the publication of Galileo's last great work, The Dialogue on the Two Chief Systems of the World. This was the first book of popular science ever written. Only with Galileo's death in 1643 did the struggle between science and religion end.
Various monarchs, in the tradition of King Frederick II of Denmark who funded Brahe's observatory Urania, and Emperor Rudolf II of
Bohemia who funded Brahe's observatory at Benathky, began to encourage and fund astronomical research and the construction of
observatories and telescopes in their countries. Most notable among these was Louis XIV (the "Sun King") of France, who established
the French Academy of Sciences. The French Academy of Sciences attracted scientists from all over the world, among them astronomers such as Johannes Hevelius of Danzig, Giovanni Cassini of Bologna, Olaus Roemer of Denmark, and Christian Huygens of Holland.
Isaac Newton was the dominant figure of 17th and early 18th century physical astronomy England under Newton, became the center
of research not only in experimental science (physics) but also, owing to Newton's great mathematical powers, in theoretical physics.
This primacy, of course, influenced the development of astronomy enormously. Here, however, we consider primarily his contributions to astronomy. His contributions to the optics of the telescope led to great improvements in telescope construction and in the quality
of the images produced by telescopes. He also formulated 3 fundamental laws of motion.
To evaluate the full impact of Newton's work on astronomy is difficult, if not impossible, but we are not off the mark if we say that
Newton ushered in the rise of modern astronomy. His discoveries in mathematics, physics, and optics were incredible stimuli to the
rapid development of theoretical astronomy, observational astronomy, and to the construction of observatories, either as adjuncts to
universities or as independent national institutions.
Edmund Halley improved on John Flamsteed's observations of stellar positions, becoming the first astronomer to demonstrate that "fixed stars" are not fixed but move with respect to each other and with respect to the solar system. Before becoming the Astronomer Royal, he went to the Island of St. Helena to catalog stars visible only in the southern latitudes. In his ocean journeys he discovered that his magnetic compass did not always point due north and from that observation deduced that the north magnetic pole does not coincide with the geographic North Pole. Halley also studied comets.
Halley was a devoted disciple of Newton and advanced his ideas and theories whenever and wherever he could. Concerned that the Newtonian truths might be lost, he persuaded Newton to write his famous Principia which Hailey published at his own expense.
James Bradley, who took over management of the Greenwich Observatory front Halley, was also a follower of Newton. His great contribution to observational astronomy was his insistence on the need for accuracy in such observations. He pointed out that such stellar observations are affected by certain properties of the Earth.
Not all of Newton's contemporaries worshipped him as devotedly as Halley and Bradley. Robert Hooke was one of the those who even went so far as to claim (unjustly) prior discovery of some of Newton's discoveries. He even claimed that he had prior knowledge of the law of gravity, but he could point to nothing that he had written or said that can be taken as a formulation of the law. Although a brilliant researcher Hooke's work did not advance astronomy to any great extent, even though he was a fervent supporter of it.
Christian Huygens proposed the wave theory of light, which marked him as one of the outstanding physicists of all time. Not only was Huygens an excellent mathematician and physicist, but he was also a very good astronomer. He developed a new method of grinding and polishing lenses which produced more accurate and better lenses.
Turning away from Newton's immediate contemporaries we come to Thomas Wright, who may be called the father of galactic astronomy. As a sailor he had ample time to study the heavens, particularly the Milky Way or galaxy. Many observers before Wright had discovered that as one turned toward the Milky Way the number of stars concentrated in a given area of the sky increased.
But Wright was the first to suggest that the stars in the Milky Way form a single system shaped like a lens, thick at the center and thin at the edges.
Leonhard Euler was probably the most productive mathematician of the 18th century. In his work on the lunar theory Euler laid the mathematical foundation for almost all future work that dealt with orbital theory. In particular, he initiated the theory of "successive approximation" and the theory of "perturbations," which encompass more than astronomical problems. Among his astronomical treatises we note his work on celestial mechanics, which stemmed from his analysis of the motions of the planets and comets. He discovered the “Euler equation”, an extension of Newton’s work.
Joseph Louis Lagrange is most famous for his "Mecanique Analytique" in which he extended Euler's dynamical equations in a form which is still used extensively in physics and astronomy. For the first time the concept of energy was introduced (specifically in the study of the motions of the planets).
Alexis Claude Clairaut, having published his first mathematical paper, The Differential Geometry of Space Curves, at the age of 18, he was elected to the French Academy by the time he entered the university. Interested as he was in space geometry, he turned his attention to the shape of the Earth, which he analyzed in his book The Theory of the Shape of the Earth. Clairaut proved that, owing to its rotation, the Earth is not a sphere but an ellipsoid, flattened at its poles. Clairaut achieved another success when he confirmed, by detailed calculations, Halley's prediction that Jupiter's gravitational pull on Halley's comet delays its periodic return.
Clairaut's scientific enterprises are associated with the activities of another famous mathematician, Pierre de Maupertuis. Though Maupertuis did not contribute much to astronomy, he discovered a remarkable principle which made him famous. This principle, known as the "principle of least action," states that a certain entity called "action" associated with the motion of a particle or a group of particles must be a minimum along the actual path of the particle. According to this point of view, a planet moves along an ellipse because its action along the ellipse is smaller than along any other path.
The attention of the French circle of astronomers and mathematicians now shifted to Pierre-Simon Laplace, whom we may consider as the father of cosmogony. He wrote a history of astronomy, Exposition du Système du Monde. First attracted to the dynamics of the solar system, he progressed from an analysis of the perturbations of the planets, proving that the solar system is highly stable, to his magnum opus, the origin of the solar system, based on his "nebular hypothesis" of the origin of stars. Laplace's boldness knew no bounds for he promulgated what we must accept as the first statement of a unified theory of science.
To the late 18th and early 19th century observational astronomers, making meaningful observations beyond the solar system meant constructing large telescopes and large observatories to house them. In this resurgence of observational astronomy, the lead, under the direction of the British astronomer, Sir William Herschel, was taken by England. He constructed the largest telescope up to that date. Charismatic and friendly, he was became famous for the discovery of the planet Uranus. Outside our solar system Herschel studied binary stars and star clusters.
Herschel began to study distant galaxies and, quite correctly, proposed that they are all "island universes" like the Milky Way. This was the beginning of rational cosmology, the attempt by astronomers to understand and describe the structure and manifestations of the entire universe in terms of Newton's laws. Observing the galaxies distributed throughout the space that was observable to him at the time, he reasoned correctly that they could not be fixed or suspended in space. The mutual force of gravity would cause them to move together, ultimately collapsing into a single sphere of matter. This would be the end of the universe.
Caroline Herschel became William's observing assistant, spending many hours peering through the best and largest of his telescopes. Devoting herself to searching for comets, at her brother's suggestion, she became the first female astronomer. In all, she discovered eight comets, adding to the fame of the Herschel family.
William Herschel's influence on astronomy particularly on stimulating observational astronomy, was immeasurable, for he attracted
the wealthy amateurs. The most famous such "wealthy amateur" was William Parsons, the third earl of Rosse in Ireland, who,
in 1845, built the largest telescope of that time for his own use. Rosse is most famous for having detected and sketched the detailed
spiral structure of what he called the "Whirlpool Nebula" (now known as the M51 galaxy).
While ever larger telescopes were being constructed by wealthy amateurs, a new technology—astronomical photography burst upon the world in 1870 and was to have as great an effect on 19th century astronomy as Galileo's telescope had on 17th century astronomy. The advent of photography freed observational astronomy from its dependence on the uncertainty of the observer's naked eye.
The Austrian physicist Christian Doppler discovered that the color (wavelength) of the light from the source changed slightly if the source was moving toward him or receding from him as compared to the wavelength if the source was not moving. He explained this effect—known as the Doppler effect— by reasoning that if a source of light and observer are approaching each other, the waves of light are crowded together so that the observer finds the wavelengths shortened and the light is therefore bluer. If, on the other hand, the observer and source are receding from each other, the waves are stretched out (wavelengths are longer) and the light is redder.
Photography very quickly permitted astronomers to measure stellar brightnesses and, from these measurements, to determine
stellar luminosities very accurately. Photographic astronomy, for the first time, permitted astronomers to introduce a precise magnitude scale of brightness. This was done by the British astronomer Norman Pogson.
Astronomers became interested in Mars owing to the announcement by the Italian astronomer Giovanni Schiaparelli that he had observed very long straight furrows or ravines which he called "canali" on the Martian surface. This word was unfortunately translated as "canals"— artifacts of intelligent beings. Since Schiaparelli was a highly respected astronomer his statement about "canali" on Mars was misinterpreted and blown up by the media out of all proportion. His "canali" became true canals (ie., artificial) in newspapers all over the world. This produced a Martian craze with observatories devoting themselves almost entirely to the study of Mars.
No one today doubts that physics and astronomy are interrelated and contribute to each other in very significant ways. But only in
the last half of the nineteenth century was the indispensability of physics to astronomy clearly recognized.
The final years of the 19th century saw the coming of age of astronomy as a scientific discipline in its own right and not merely as a branch of physics. This recognition by the science community and the academic community has been marked by the introduction of independent astronomy departments and astronomy faculties throughout the world.
Gustav Kirchhoff's law of radiation, states that the rate at which a body radiates energy at a given temperature and frequency (color) equals the rate at which it absorbs radiation at that frequency multiplied by the rate at which a perfect blackbody at the same temperature absorbs such radiation.
As the 19th century drew to a close, physicists faced the 20h century with a sense of great confidence, for it appeared to them that all that was left for physicists and astronomers to do was a kind of mopping up operation. With Newtonian dynamics, including Newtonian gravity, and the Maxwellian electromagnetic theory of light, everything in nature that one could think of seemed to be covered. And yet at this very time new experimental discoveries and two new theories were about to burst upon the science community.
Albert Einstein presented the theory of relativity in two steps: the special theory of relativity first appeared in one of Einstein's three famous papers that were published that year in Annalen der physik. The great importance of these three papers may be gauged by their evaluation, many years later, by the Nobel laureate Max Born, who stated that each of these papers opened up a "whole new branch of physics." The second step in the presentation of the theory of relativity appeared as the general theory of relativity.
The two great nonclassical physical theories, the quantum theory and the theory of relativity, that led to the nuclear model of the atom, also led to astrophysics, and to a rational cosmology. Astronomy began to change from a discipline concerned with the study of the positions, the motions, and the clustering of stars to the study of the structure of stars, to "astrophysics," which is the application of the laws of physics to the analysis of stellar structure. The aim here was, and still is, to lay down the blueprint for the theoretical structure of a model of a star.
Astronomers quickly discovered differences among stars in the same spectral class. These differences were first discovered by the Danish astronomer E. Hertzsprung. He discovered that the cooler yellow and red stars can themselves be divided into distinct groups: those that are intrinsically faint (low luminosity) and those that are intrinsically bright (large luminosity or small absolute magnitudes). Hertzsprung called the low luminosity stars "dwarfs" and large luminosity stars "giants." Thus the sun and Capella belong to the same spectral class (they are both G-type stars) but Capella, called a giant, is about 100 times as luminous as the sun.
Sir Arthur Eddington published a series of basic papers which initiated modern astrophysics. Eddington may properly be called the father of astrophysics. His most important contribution was his demonstration that in most stars (such as the sun) the heat is transported by radiation and that convection may play a role close to the center of the star and in its outer envelope.
While Eddington was laying down the foundations of modern astrophysics, his British contemporary, Sir James Jeans, like Eddington
an excellent theoretical physicist as well as an astronomer, was extending the domain in which physics and astrophysics overlap so that one could justifiably speak of "astrophysics" as a field of study in its own right with physics as important to it as astronomy. Jeans was best known as a physicist for his theoretical contribution to our understanding of the laws of radiation.
The lack of knowledge about nuclear energy, which Eddington had suggested as a source of the sun's power, held up the development
of astrophysics for some 20 years. The door to solving the stellar energy problem was opened by the discovery of the neutron by the British physicist James Chadwick. Up to that time the structure of the atomic nucleus was a deep mystery.
The existence of the neutron leads us to the existence of another particle, called the neutrino, which plays an important role in physics and astronomy—particularly in the generation of nuclear energy. The neutrino was introduced in 1930 by the theoretical physicist Wolfgang Pauli to account for problems associated with the decay of certain radioactive nuclei that decay by emitting electrons, which are called beta rays.
The H-R Diagram is a is a graphical tool that astronomers use to classify stars according to their luminosity, spectral type, color, temperature and evolutionary stage. That the various branches in the H-R diagram of the distribution of stars point to stellar evolution was first proposed by Henry Norris Russell.
Because we cannot watch individual stars evolve owing to their long lifespans, we must resort to observations of groups of stars which were born together but which evolved at different rates owing to their different masses so that each such group consists of stars in different stages of evolution. Where are such groups to be found? They are scattered throughout our galaxy and are called "local" or "galactic" clusters of stars, which we know must have been born together from the same primordial galactic cloud of gas and dust because they are moving together in the same direction in the galaxy.
The astrophysics of the evolution of the supergiant stars explains the presence, in galaxies, of stars that are really pulsating. Such stars, now called "cepheid variables," were named after the typical star in this category, alpha-cephei, first properly labeled as
a regular variable in 1784 by J. Goodricke. In 1912 the Harvard astronomer Henrietta Leavitt, studying the cepheid variables in the Magellanic clouds, discovered the famous period-luminosity law of cepheids which states that the luminosity of a cepheid variable is proportional to its pulsation period; the more slowly it pulsates, the greater its luminosity. This is physically reasonable.
The discovery of the cepheid period luminosity law opened up a new and very exciting branch of astronomy: the probing of the depths of space—particularly the distances of the most distant galaxies. All one has to do now to find the distance of a galaxy is to pick out a cepheid variable in the galaxy and measure its pulsation period, which can be done very accurately. From that measurement the luminosity of the cepheid can be found from the period-luminosity law. From this luminosity and the cepheid's observed brightness, the distance of this cepheid and, hence, of its galaxy can be found. Our own galaxy was beginning to be studied in depth with new technologies.
The study of the distant galaxies was dominated by Edwin P. Hubble who gave up a boxing and a law career to study astronomy. He joined the staff of the Mount Wilson Observatory and, a few years later, became its research director. Hubble's studies convinced him, and in time, all astronomers that the extragalactic galaxies are, indeed, aggregates of stars like our own. During his career Hubble collaborated with Milton Humason.
Because waves tend to spread out, physicists such as Werner Heisenberg discarded the classical idea that electrons can be described as being in a particular position in space. Instead, this concept of localization of action of a particle must be replaced by what has since been called Heisenberg's "uncertainty" or "indeterminacy" principle: if one tries to measure the precise position of a particle one loses all knowledge of its motion and vice versa.
This was also the period during which new windows into space were opened, stimulated by the discoveries of new kinds of "rays" coming from all parts of space. In particular, "cosmic rays," first correctly described by the Austrian physicist Victor Hess as emanating from
outer space and not from the earth, were among the most intriguing of these new kinds of rays. After a lengthy debate about the
nature of these particles everyone agreed that they are very energetic charged particles, primarily protons (intermixed with heavy nuclei) and not electromagnetic rays (e.g., X rays and gamma rays).
Astronomers studying galaxies was the recession of the distant galaxies, which led to the much larger and much more important cosmological study of the beginning and end of the universe. This great discovery began with the cosmic "shaking" discovered by Vesto M. Slipher that the spectral lines of the light from the stars in the galaxy M31 reveal an enormous Doppler shift toward the blue end of the spectrum. Over the next two decades Slipher studied the lines in the spectra of some 40 nebulae, finding that most of the spectral lines are shifted toward the red, meaning that these nebulae are receding from the sun.
The study of galactic clusters has led to a deeper insight into the importance of the "missing mass" (the dark matter) in the universe. That a great deal more matter is present in each cluster of galaxies is clear if one determines the mass: luminosity ratio in each cluster. Each cluster has much more mass than that amount deduced from the cluster's luminosity.
Jan Oort had become interested in constructing a model of the galaxy. He had analyzed star counts to show that the galaxy may be
pictured as consisting of layers of stars lying in planes parallel to the plane of the Milky Way. These planes thin out with increasing distance from the plane of the Milky Way, This led him to the study of the motions of the stars in the galactic solar neighborhood. He concluded that the galaxy is rotating. Oort first theorised the existence of a icy shell of objects that exist in the outermost reaches of the solar system.
But with the development of electromagnetic technology, other windows into space were discovered and used extensively. Radio astronomy was the most important of these new technologies because the optical part of the electromagnetic spectrum (to which the human eye responds) is only a small portion (one octave—from red to violet) of what we might use for peering into space. This goal was realized with the introduction of radio telescopes, which can pick up a much wider range of electromagnetic wavelengths.
Radio astronomy received a great boost after World War II owing to the rapid development of radar, which is essentially a radio
technology in which short wave radio signals are directed to and then reflected from the surfaces of nearby celestial objects such as the moon, the planets, comets, and meteors. One of the most remarkable astronomical discoveries made with radar waves reflected from the planets was that the surface of Venus is very hot.
The discovery of the hydrogen 21-centimeter (wavelength) radio line was another remarkable and extremely important event owing to its great usefulness in plotting the hydrogen atoms in our galaxy and in other galaxies. The existence of such a spectral line was first predicted by the Dutch astronomer H.C. van de Hulst during World War 2. This opens up the possibility of studying the galactic core of the Milky Way.
After world war 2 technological advancements allowed humankind to send artificial satellites, and evan humans into space. This, of course benefited astronomy. Space probes were also used to explore the cosmos. Today, the Hubble telescope offers invaluable information about the universe. Among the more important cosmological probes was the Cosmic Background Explorer (COBE) to determine the temperature of the universe, that is, the absolute (kelvin) temperature of intergalactic space.
The story of the birth of the universe and its evolution from chaos to order, told here, spans some 14 billion years. The big bang theory is the prevailing cosmological model for the formation of the universe. At its simplest, it talks about the universe as we know it starting with a small singularity, then inflating over the next 13.8 billion years to the cosmos that we know today.
Another cosmology that was vigorously supported initially by a number of leading cosmologists but is no longer as popular as it was initially, was introduced in the 1970s by the MIT physicist Alan Guth, who was concerned with certain esoteric features of the observable universe that seemed to contradict the big bang cosmology, Guth called his cosmology the "inflationary" universe cosmology.
Guth argued that the empty universe expanded initially by many orders of magnitude (a hundred thousand trillion trillion times) thus "supercooling" itself and then releasing vast amounts of energy which became the big bang.
The American Vela satellites were orbited to detect gamma ray flashes as signatures of nuclear bomb explosions because gamma rays are emitted by nuclei. These satellites did, indeed, record such flashes but it soon became clear that these gamma ray bursts were not produced by exploding bombs but were natural phenomena. This was announced in the prestigious Astrophysical Journal, These gamma ray bursts have been studied extensively since then and they have been found to occur throughout the universe.
Another mystery was presented to cosmologists by the discovery of certain radio sources which appeared like ordinary faint stars on photographic plates. But from the careful observations of a group of California astronomers it was discovered that these objects are at vast distances (billions of light years away) and, therefore, cannot be stars. These objects, now called "quasars," are small, compact extremely luminous objects, emitting as much energy per second as ten trillion suns or a few hundred galaxies.