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What was Johannes Kepler's profession? When and how did it begin?

What was Johannes Kepler known for?

Who did Johannes Kepler influence?

Where was Johannes Kepler born?

How and where did Johannes Kepler die?

Johannes Kepler, (born December 27, 1571, Weil der Stadt, Württemberg [Germany]-died November 15, 1630, Regensburg), German astronomer who discovered three major laws of planetary motion, conventionally designated as follows: (1) the planets move in elliptical orbits with the Sun at one focus; (2) the time necessary to traverse any arc of a planetary orbit is proportional to the area of the sector between the central body and that arc (the "area law"); and (3) there is an exact relationship between the squares of the planets' periodic times and the cubes of their mean distances from the Sun (the "harmonic law"). Kepler himself did not call these discoveries "laws," as would become customary after Isaac Newton derived them from a new and quite different set of general physical principles. He regarded them as celestial harmonies that reflected God's design for the universe. Kepler's discoveries turned Nicolaus Copernicus's Sun-centred system into a dynamic universe, with the Sun actively pushing the planets around in noncircular orbits. And it was Kepler's notion of a physical astronomy that fixed a new problematic for other important 17th-century world-system builders, the most famous of whom was Newton.

Among Kepler's many other achievements, he provided a new and correct account of how vision occurs; he developed a novel explanation for the behaviour of light in the newly invented telescope; he discovered several new, semiregular polyhedrons; and he offered a new theoretical foundation for astrology while at the same time restricting the domain in which its predictions could be considered reliable. A list of his discoveries, however, fails to convey the fact that they constituted for Kepler part of a common edifice of knowledge. The matrix of theological, astrological, and physical ideas from which Kepler's scientific achievements emerged is unusual and fascinating in its own right. Yet, because of the highly original nature of Kepler's discoveries, it requires an act of intellectual empathy for moderns to understand how such lasting results could have evolved from such an apparently unlikely complex of ideas. Although Kepler's scientific work was centred first and foremost on astronomy, that subject as then understood-the study of the motions of the heavenly bodies-was classified as part of a wider subject of investigation called "the science of the stars." The science of the stars was regarded as a mixed science consisting of a mathematical and a physical component and bearing a kinship to other like disciplines, such as music (the study of ratios of tones) and optics (the study of light). It also was subdivided into theoretical and practical categories. Besides the theory of heavenly motions, one had the practical construction of planetary tables and instruments; similarly, the theoretical principles of astrology had a corresponding practical part that dealt with the making of annual astrological forecasts about individuals, cities, the human body, and the weather. Within this framework, Kepler made astronomy an integral part of natural philosophy, but he did so in an unprecedented way-in the process, making unique contributions to astronomy as well as to all its auxiliary disciplines.

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Johannes Kepler | Biography, Discoveries, & Facts

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Johannes Kepler, German astronomer who discovered three major laws of planetary motion. His discoveries turned Nicolaus Copernicus's Sun-centered system into a dynamic universe, with the Sun actively pushing the planets around in noncircular orbits. Learn more about Kepler's life and discoveries in this article.

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Galileo Galilei

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Galileo (born February 15, 1564, Pisa [Italy]-died January 8, 1642, Arcetri, near Florence) Italian natural philosopher, astronomer, and mathematician who made fundamental contributions to the sciences of motion, astronomy, and strength of materials and to the development of the scientific method. His formulation of (circular) inertia, the law of falling bodies, and parabolic trajectories marked the beginning of a fundamental change in the study of motion. His insistence that the book of nature was written in the language of mathematics changed natural philosophy from a verbal, qualitative account to a mathematical one in which experimentation became a recognized method for discovering the facts of nature. Finally, his discoveries with the telescope revolutionized astronomy and paved the way for the acceptance of the Copernican heliocentric system, but his advocacy of that system eventually resulted in an Inquisition process against him.

Early life and career

Galileo was born in Pisa, Tuscany, on February 15, 1564, the oldest son of Vincenzo Galilei, a musician who made important contributions to the theory and practice of music and who may have performed some experiments with Galileo in 1588-89 on the relationship between pitch and the tension of strings. The family moved to Florence in the early 1570s, where the Galilei family had lived for generations. In his middle teens Galileo attended the monastery school at Vallombrosa, near Florence, and then in 1581 matriculated at the University of Pisa, where he was to study medicine. However, he became enamoured with mathematics and decided to make the mathematical subjects and philosophy his profession, against the protests of his father. Galileo then began to prepare himself to teach Aristotelian philosophy and mathematics, and several of his lectures have survived. In 1585 Galileo left the university without having obtained a degree, and for several years he gave private lessons in the mathematical subjects in Florence and Siena. During this period he designed a new form of hydrostatic balance for weighing small quantities and wrote a short treatise, La bilancetta ("The Little Balance"), that circulated in manuscript form. He also began his studies on motion, which he pursued steadily for the next two decades.

In 1588 Galileo applied for the chair of mathematics at the University of Bologna but was unsuccessful. His reputation was, however, increasing, and later that year he was asked to deliver two lectures to the Florentine Academy, a prestigious literary group, on the arrangement of the world in Dante's Inferno. He also found some ingenious theorems on centres of gravity (again, circulated in manuscript) that brought him recognition among mathematicians and the patronage of Guidobaldo del Monte (1545-1607), a nobleman and author of several important works on mechanics. As a result, he obtained the chair of mathematics at the University of Pisa in 1589. There, according to his first biographer, Vincenzo Viviani (1622-1703), Galileo demonstrated, by dropping bodies of different weights from the top of the famous Leaning Tower, that the speed of fall of a heavy object is not proportional to its weight, as Aristotle had claimed. The manuscript tract De motu ( On Motion), finished during this period, shows that Galileo was abandoning Aristotelian notions about motion and was instead taking an Archimedean approach to the problem. But his attacks on Aristotle made him unpopular with his colleagues, and in 1592 his contract was not renewed. His patrons, however, secured him the chair of mathematics at the University of Padua, where he taught from 1592 until 1610.

Although Galileo's salary was considerably higher there, his responsibilities as the head of the family (his father had died in 1591) meant that he was chronically pressed for money. His university salary could not cover all his expenses, and he therefore took in well-to-do boarding students whom he tutored privately in such subjects as fortification. He also sold a proportional compass, or sector, of his own devising, made by an artisan whom he employed in his house. Perhaps because of these financial problems, he did not marry, but he did have an arrangement with a Venetian woman, Marina Gamba, who bore him two daughters and a son. In the midst of his busy life he continued his research on motion, and by 1609 he had determined that the distance fallen by a body is proportional to the square of the elapsed time ( the law of falling bodies) and that the trajectory of a projectile is a parabola, both conclusions that contradicted Aristotelian physics.

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Galileo | Biography, Discoveries, Inventions, & Facts

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Galileo, the brilliant Italian polymath, revolutionized our understanding of the universe through his groundbreaking discoveries in astronomy and contributions to scientific methodology.

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Aristotle's Early Life

Aristotle was born in 384 B.C. in Stagira in northern Greece. Both of his parents were members of traditional medical families, and his father, Nicomachus, served as court physician to King Amyntus III of Macedonia. His parents died while he was young, and he was likely raised at his family's home in Stagira. At age 17 he was sent to Athens to enroll in Plato's Academy. He spent 20 years as a student and teacher at the school, emerging with both a great respect and a good deal of criticism for his teacher's theories. Plato's own later writings, in which he softened some earlier positions, likely bear the mark of repeated discussions with his most gifted student.

Did you know? Aristotle's surviving works were likely meant as lecture notes rather than literature, and his now-lost writings were apparently of much better quality. The Roman philosopher Cicero said that "If Plato's prose was silver, Aristotle's was a flowing river of gold."

When Plato died in 347, control of the Academy passed to his nephew Speusippus. Aristotle left Athens soon after, though it is not clear whether frustrations at the Academy or political difficulties due to his family's Macedonian connections hastened his exit. He spent five years on the coast of Asia Minor as a guest of former students at Assos and Lesbos. It was here that he undertook his pioneering research into marine biology and married his wife Pythias, with whom he had his only daughter, also named Pythias.

In 342 Aristotle was summoned to Macedonia by King Philip II to tutor his son, the future Alexander the Great -a meeting of great historical figures that, in the words of one modern commentator, "made remarkably little impact on either of them."

Aristotle and the Lyceum

Aristotle returned to Athens in 335 B.C. As an alien, he couldn't own property, so he rented space in the Lyceum, a former wrestling school outside the city. Like Plato's Academy, the Lyceum attracted students from throughout the Greek world and developed a curriculum centered on its founder's teachings. In accordance with Aristotle's principle of surveying the writings of others as part of the philosophical process, the Lyceum assembled a collection of manuscripts that comprised one of the world's first great libraries.

Aristotle's Works

It was at the Lyceum that Aristotle probably composed most of his approximately 200 works, of which only 31 survive. In style, his known works are dense and almost jumbled, suggesting that they were lecture notes for internal use at his school. The surviving works of Aristotle are grouped into four categories.

The "Organon" is a set of writings that provide a logical toolkit for use in any philosophical or scientific investigation. Next come Aristotle's theoretical works, most famously his treatises on animals ("Parts of Animals," "Movement of Animals," etc.), cosmology, the "Physics" (a basic inquiry about the nature of matter and change) and the "Metaphysics" (a quasi-theological investigation of existence itself).

Third are Aristotle's so-called practical works, notably the "Nicomachean Ethics" and "Politics," both deep investigations into the nature of human flourishing on the individual, familial and societal levels. Finally, his "Rhetoric" and "Poetics" examine the finished products of human productivity, including what makes for a convincing argument and how a well-wrought tragedy can instill cathartic fear and pity.

The Organon

"The Organon" (Latin for "instrument") is a series of Aristotle's works on logic (what he himself would call analytics) put together around 40 B.C. by Andronicus of Rhodes and his followers. The set of six books includes "Categories," "On Interpretation," "Prior Analytics," "Posterior Analytics," "Topics," and "On Sophistical Refutations." The Organon contains Aristotle's worth on syllogisms (from the Greek syllogismos, or "conclusions"), a form of reasoning in which a conclusion is drawn from two assumed premises. For example, all men are mortal, all Greeks are men, therefore all Greeks are mortal.

Metaphysics

Aristotle's "Metaphysics," written quite literally after his "Physics," studies the nature of existence. He called metaphysics the "first philosophy," or "wisdom." His primary area of focus was "being qua being," which examined what can be said about being based on what it is, not because of any particular qualities it may have. In "Metaphysics," Aristotle also muses on causation, form, matter and even a logic-based argument for the existence of God.

Rhetoric

To Aristotle, rhetoric is "the faculty of observing in any given case the available means of persuasion." He identified three main methods of rhetoric: ethos (ethics), pathos (emotional) and logos (logic). He also broke rhetoric into types of speeches: epideictic (ceremonial), forensic (judicial) and deliberative (where the audience is required to reach a verdict). His groundbreaking work in this field earned him the nickname "the father of rhetoric."

Poetics

Aristotle's "Poetics" was composed around 330 B.C. and is the earliest extant work of dramatic theory. It is often interpreted as a rebuttal to his teacher Plato's argument that poetry is morally suspect and should therefore be expunged from a perfect society. Aristotle takes a different approach, analyzing the purpose of poetry. He argues that creative endeavors like poetry and theater provides catharsis, or the beneficial purging of emotions through art.

Aristotle's Death and Legacy

After the death of Alexander the Great in 323 B.C., anti-Macedonian sentiment again forced Aristotle to flee Athens. He died a little north of the city in 322, of a digestive complaint. He asked to be buried next to his wife, who had died some years before. In his last years he had a relationship with his slave Herpyllis, who bore him Nicomachus, the son for whom his great ethical treatise is named.

Aristotle's favored students took over the Lyceum, but within a few decades the school's influence had faded in comparison to the rival Academy. For several generations Aristotle's works were all but forgotten. The historian Strabo says they were stored for centuries in a moldy cellar in Asia Minor before their rediscovery in the first century B.C., though it is unlikely that these were the only copies.

In 30 B.C. Andronicus of Rhodes grouped and edited Aristotle's remaining works in what became the basis for all later editions. After the fall of Rome, Aristotle was still read in Byzantium and became well-known in the Islamic world, where thinkers like Avicenna (970-1037), Averroes (1126-1204) and the Jewish scholar Maimonodes (1134-1204) revitalized Aritotle's logical and scientific precepts.

Aristotle in the Middle Ages and Beyond

In the 13th century, Aristotle was reintroduced to the West through the work of Albertus Magnus and especially Thomas Aquinas, whose brilliant synthesis of Aristotelian and Christian thought provided a bedrock for late medieval Catholic philosophy, theology and science.

Aristotle's universal influence waned somewhat during the Renaissance and Reformation, as religious and scientific reformers questioned the way the Catholic Church had subsumed his precepts. Scientists like Galileo and Copernicus disproved his geocentric model of the solar system, while anatomists such as William Harvey dismantled many of his biological theories. However, even today, Aristotle's work remains a significant starting point for any argument in the fields of logic, aesthetics, political theory and ethics.

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Aristotle (384-322 B.C.) was a Greek philosopher who made significant and lasting contributions to nearly every aspect of human knowledge, from logic to biology to ethics and aesthetics.

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Astronomer Nicolaus Copernicus was instrumental in establishing the concept of a heliocentric solar system, in which the sun, rather than the earth, is the center of the solar system.

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Milky Way Galaxy, large spiral system consisting of several hundred billion stars, one of which is the Sun. It takes its name from the Milky Way, the irregular luminous band of stars and gas clouds that stretches across the sky as seen from Earth. Although Earth lies well within the Milky Way Galaxy (sometimes simply called the Galaxy), astronomers do not have as complete an understanding of its nature as they do of some external star systems. A thick layer of interstellar dust obscures much of the Galaxy from scrutiny by optical telescopes, and astronomers can determine its large-scale structure only with the aid of radio and infrared telescopes, which can detect the forms of radiation that penetrate the obscuring matter.

This article discusses the structure, properties, and component parts of the Milky Way Galaxy. For a full-length discussion of the cosmic universe of which the Galaxy is only a small part, see cosmology. For the star system within the Galaxy that is the home of Earth, see solar system.

Milky Way Galaxy iStockphoto/Thinkstock

Major components of the Galaxy

Although most stars in the Galaxy exist either as single stars like the Sun or as double stars, there are many conspicuous groups and clusters of stars that contain tens to thousands of members. These objects can be subdivided into three types: globular clusters, open clusters, and stellar associations. They differ primarily in age and in the number of member stars.

The largest and most massive star clusters are the globular clusters, so called because of their roughly spherical appearance. The Galaxy contains more than 150 globular clusters (the exact number is uncertain because of obscuration by dust in the Milky Way band, which probably prevents some globular clusters from being seen). They are arranged in a nearly spherical halo around the Milky Way, with relatively few toward the galactic plane but a heavy concentration toward the centre. The radial distribution, when plotted as a function of distance from the galactic centre, fits a mathematical expression of a form identical to the one describing the star distribution in elliptical galaxies.

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Globular clusters are extremely luminous objects. Their mean luminosity is the equivalent of approximately 25,000 Suns. The most luminous are 50 times brighter. The masses of globular clusters, measured by determining the dispersion in the velocities of individual stars, range from a few thousand to more than 1,000,000 solar masses. The clusters are very large, with diameters measuring from 10 to as much as 300 light-years. Most globular clusters are highly concentrated at their centres, having stellar distributions that resemble isothermal gas spheres with a cutoff that corresponds to the tidal effects of the Galaxy. A precise model of star distribution within a cluster can be derived from stellar dynamics, which takes into account the kinds of orbits that stars have in the cluster, encounters between these member stars, and the effects of exterior influences. The American astronomer Ivan R. King, for instance, derived dynamical models that fit observed stellar distributions very closely. He finds that a cluster's structure can be described in terms of two numbers: (1) the core radius, which measures the degree of concentration at the centre, and (2) the tidal radius, which measures the cutoff of star densities at the edge of the cluster.

A key distinguishing feature of globular clusters in the Galaxy is their uniformly old age. Determined by comparing the stellar population of globular clusters with stellar evolutionary models, the ages of all those so far measured range from 11 billion to 13 billion years. They are the oldest objects in the Galaxy and so must have been among the first formed. That this was the case is also indicated by the fact that the globular clusters tend to have much smaller amounts of heavy elements than do the stars in the plane of the Galaxy, e.g., the Sun. Composed of stars belonging to the extreme Population II ( see below Stars and stellar populations), as well as the high-latitude halo stars, these nearly spherical assemblages apparently formed before the material of the Galaxy flattened into the present thin disk. As their component stars evolved, they gave up some of their gas to interstellar space. This gas was enriched in the heavy elements (i.e., elements heavier than helium) produced in stars during the later stages of their evolution, so that the interstellar gas in the Galaxy is continually being changed. Hydrogen and helium have always been the major constituents, but heavy elements have gradually grown in importance. The present interstellar gas contains elements heavier than helium at a level of about 2 percent by mass, while the globular clusters contain as little as 0.02 percent of the same elements.

Clusters smaller and less massive than the globular clusters are found in the plane of the Galaxy intermixed with the majority of the system's stars, including the Sun. These objects are the open clusters, so called because they generally have a more open, loose appearance than typical globular clusters.

Open clusters are distributed in the Galaxy very similarly to young stars. They are highly concentrated along the plane of the Galaxy and slowly decrease in number outward from its centre. The large-scale distribution of these clusters cannot be learned directly because their existence in the Milky Way plane means that dust obscures those that are more than a few thousand light-years from the Sun. By analogy with open clusters in external galaxies similar to the Galaxy, it is surmised that they follow the general distribution of integrated light in the Galaxy, except that there are probably fewer of them in the central areas. There is some evidence that the younger open clusters are more densely concentrated in the Galaxy's spiral arms, at least in the neighbourhood of the Sun where these arms can be discerned.

The brightest open clusters are considerably fainter than the brightest globular clusters. The peak absolute luminosity appears to be about 50,000 times the luminosity of the Sun, but the largest percentage of known open clusters has a brightness equivalent to 500 solar luminosities. Masses can be determined from the dispersion in the measured velocities of individual stellar members of clusters. Most open clusters have small masses on the order of 50 solar masses. Their total populations of stars are small, ranging from tens to a few thousand.

Open clusters have diameters of only 2 or 3 to about 20 light-years, with the majority being less than 5 light-years across. In structure they look very different from globular clusters, though they can be understood in terms of similar dynamical models. The most important structural difference is their small total mass and relative looseness, which result from their comparatively large core radii. These two features have disastrous consequences as far as their ultimate fate is concerned, because open clusters are not sufficiently gravitationally bound to be able to withstand the disruptive tidal effects in the Galaxy ( see star cluster: Open clusters). Judging from the sample of open clusters within 3,000 light-years of the Sun, only half of them can withstand such tidal forces for more than 200 million years, and a mere 2 percent have life expectancies as high as 1 billion years.

Measured ages of open clusters agree with the conclusions that have been reached about their life expectancies. They tend to be young objects; only a few are known to exceed 1 billion years in age. Most are younger than 200 million years, and some are 1 or 2 million years old. Ages of open clusters are determined by comparing their stellar membership with theoretical models of stellar evolution. Because all the stars in a cluster have very nearly the same age and chemical composition, the differences between the member stars are entirely the result of their different masses. As time progresses after the formation of a cluster, the massive stars, which evolve the fastest, gradually disappear from the cluster, becoming white dwarf stars or other underluminous stellar remnants. Theoretical models of clusters show how this effect changes the stellar content with time, and direct comparisons with real clusters give reliable ages for them. To make this comparison, astronomers use a diagram (the colour-magnitude diagram) that plots the temperatures of the stars against their luminosities. Colour-magnitude diagrams have been obtained for more than 1,000 open clusters, and ages are thus known for this large sample.

Because open clusters are mostly young objects, they have chemical compositions that correspond to the enriched environment from which they formed. Most of them are like the Sun in their abundance of the heavy elements, and some are even richer. For instance, the Hyades, which compose one of the nearest clusters, have almost twice the abundance of heavy elements as the Sun. It became possible in the 1990s to discover very young open clusters that previously had been entirely hidden in deep, dusty regions. Using infrared array detectors, astronomers found that many molecular clouds contained very young groups of stars that had just formed and, in some cases, were still forming.

Even younger than open clusters, stellar associations are very loose groupings of young stars that share a common place and time of origin but that are not generally tied closely enough together gravitationally to form a stable cluster. Stellar associations are limited strictly to the plane of the Galaxy and appear only in regions of the system where star formation is occurring, notably in the spiral arms. They are very luminous objects. The brightest are even brighter than the brightest globular clusters, but this is not because they contain more stars; instead it is the result of the fact that their constituent stars are very much brighter than the stars constituting globular clusters. The most luminous stars in stellar associations are very young stars of spectral types O and B. They have absolute luminosities as bright as any star in the Galaxy-on the order of one million times the luminosity of the Sun. Such stars have very short lifetimes, only lasting a few million years. With luminous stars of this type there need not be very many to make up a highly luminous and conspicuous grouping. The total masses of stellar associations amount to only a few hundred solar masses, with the population of stars being in the hundreds or, in a few cases, thousands.

The sizes of stellar associations are large; the average diameter of those in the Galaxy is about 250 light-years. They are so large and loosely structured that their self-gravitation is insufficient to hold them together, and in a matter of a few million years the members disperse into surrounding space, becoming separate and unconnected stars in the galactic field.

These objects are organizations of stars that share common measurable motions. Sometimes these do not form a noticeable cluster. This definition allows the term to be applied to a range of objects from the nearest gravitationally bound clusters to groups of widely spread stars with no apparent gravitational identity, which are discovered only by searching the catalogs for stars of common motion. Among the best known of the moving groups is the Hyades in the constellation Taurus. Also known as the Taurus moving cluster or the Taurus stream, this system comprises the relatively dense Hyades cluster along with a few very distant members. It contains a total of about 350 stars, including several white dwarfs. Its centre lies about 150 light-years away. Other notable moving stellar groups include the Ursa Major, Scorpius-Centaurus, and Pleiades groups. Besides these remote organizations, investigators have observed what appear to be groups of high-velocity stars near the Sun. One of these, called the Groombridge 1830 group, consists of a number of subdwarfs and the star RR Lyrae, after which the RR Lyrae variables were named.

Recent advances in the study of moving groups have had an impact on the investigation of the kinematic history of stars and on the absolute calibration of the distance scale of the Galaxy. Moving groups have proved particularly useful with respect to the latter because their commonality of motion enables astronomers to determine accurately (for the nearer examples) the distance of each individual member. Together with nearby parallax stars, moving-group parallaxes provide the basis for the galactic distance scale. Astronomers have found the Hyades moving cluster well suited for their purpose: it is close enough to permit the reliable application of the method, and it has enough members for deducing an accurate age.

One of the basic problems of using moving groups for distance determination is the selection of members. In the case of the Hyades, this has been done very carefully but not without considerable dispute. The members of a moving group (and its actual existence) are established by the degree to which their motions define a common convergent point in the sky. One technique is to determine the coordinates of the poles of the great circles defined by the proper motions and positions of individual stars. The positions of the poles will define a great circle, and one of its poles will be the convergent point for the moving group. Membership of stars can be established by criteria applied to the distances of proper-motion poles of individual stars from the mean great circle. The reliability of the existence of the group itself can be measured by the dispersion of the great circle points about their mean.

As radial velocities will not have been used for the preliminary selection of members, they can be subsequently examined to eliminate further nonmembers. The final list of members should contain only a very few nonmembers-either those that appear to agree with the group motion because of observational errors or those that happen to share the group's motion at the present time but are not related to the group historically.

The distances of individual stars in a moving group may be determined if their radial velocities and proper motions are known ( see below Stellar motions) and if the exact position of the radiant is determined. If the angular distance of a star from the radiant is λ and if the velocity of the cluster as a whole with respect to the Sun is V, then the radial velocity of the star, V r, is V r= T = V sin λ = 4.74 μ/ p = 4.74 μ cot λ/ V r. p where V cos λ. The transverse (or tangential) velocity, T, is given by p is the star's parallax in arc seconds. Thus, the parallax of a star is given by

The key to achieving reliable distances by this method is to locate the convergent point of the group as accurately as possible. The various techniques used (e.g., Charlier's method) are capable of high accuracy, provided that the measurements themselves are free of systematic errors. For the Taurus moving group, for example, it has been estimated that the accuracy for the best-observed stars is on the order of 3 percent in the parallax, discounting any errors due to systematic problems in the proper motions. Accuracies of this order were not possible by other means until the space-based telescope Hipparcos was able to measure highly precise stellar parallaxes for thousands of individual stars.

A conspicuous component of the Galaxy is the collection of large, bright, diffuse gaseous objects generally called nebulae. The brightest of these cloudlike objects are the emission nebulae, large complexes of interstellar gas and stars in which the gas exists in an ionized and excited state (with the electrons of the atoms excited to a higher than normal energy level). This condition is produced by the strong ultraviolet light emitted from the very luminous, hot stars embedded in the gas. Because emission nebulae consist almost entirely of ionized hydrogen, they are usually referred to as H II regions.

H II regions are found in the plane of the Galaxy intermixed with young stars, stellar associations, and the youngest of the open clusters. They are areas where very massive stars have recently formed, and many contain the uncondensed gas, dust, and molecular complexes commonly associated with ongoing star formation. The H II regions are concentrated in the spiral arms of the Galaxy, though some exist between the arms. Many of them are found at intermediate distances from the centre of the Milky Way Galaxy, with the largest number occurring at a distance of 10,000 light-years. This latter fact can be ascertained even though the H II regions cannot be seen clearly beyond a few thousand light-years from the Sun. They emit radio radiation of a characteristic type, with a thermal spectrum that indicates that their temperatures are about 10,000 kelvins. This thermal radio radiation enables astronomers to map the distribution of H II regions in distant parts of the Galaxy.

The largest and brightest H II regions in the Galaxy rival the brightest star clusters in total luminosity. Even though most of the visible radiation is concentrated in a few discrete emission lines, the total apparent brightness of the brightest is the equivalent of tens of thousands of solar luminosities. These H II regions are also remarkable in size, having diameters of about 1,000 light-years. More typically, common H II regions such as the Orion Nebula are about 50 light-years across. They contain gas that has a total mass ranging from one or two solar masses up to several thousand. H II regions consist primarily of hydrogen, but they also contain measurable amounts of other gases. Helium is second in abundance, and large amounts of carbon, nitrogen, and oxygen occur as well. Preliminary evidence indicates that the ratio of the abundance of the heavier elements among the detected gases to hydrogen decreases outward from the centre of the Galaxy, a tendency that has been observed in other spiral galaxies.

The gaseous clouds known as planetary nebulae are only superficially similar to other types of nebulae. So called because the smaller varieties almost resemble planetary disks when viewed through a telescope, planetary nebulae represent a stage at the end of the stellar life cycle rather than one at the beginning. The distribution of such nebulae in the Galaxy is different from that of H II regions. Planetary nebulae belong to an intermediate population and are found throughout the disk and the inner halo. There are more than 1,000 known planetary nebulae in the Galaxy, but more might be overlooked because of obscuration in the Milky Way region.

Another type of nebulous object found in the Galaxy is the remnant of the gas blown out from an exploding star that forms a supernova. Occasionally these objects look something like planetary nebulae, as in the case of the Crab Nebula, but they differ from the latter in three ways: (1) the total mass of their gas (they involve a larger mass, essentially all the mass of the exploding star), (2) their kinematics (they are expanding with higher velocities), and (3) their lifetimes (they last for a shorter time as visible nebulae). The best-known supernova remnants are those resulting from three historically observed supernovae: that of 1054, which made the Crab Nebula its remnant; that of 1572, called Tycho's Nova; and that of 1604, called Kepler's Nova. These objects and the many others like them in the Galaxy are detected at radio wavelengths. They release radio energy in a nearly flat spectrum because of the emission of radiation by charged particles moving spirally at nearly the speed of light in a magnetic field enmeshed in the gaseous remnant. Radiation generated in this way is called synchrotron radiation and is associated with various types of violent cosmic phenomena besides supernova remnants, as, for example, radio galaxies.

Dust clouds

The dust clouds of the Galaxy are narrowly limited to the plane of the Milky Way, though very low-density dust can be detected even near the galactic poles. Dust clouds beyond 2,000 to 3,000 light-years from the Sun cannot be detected optically, because intervening clouds of dust and the general dust layer obscure more distant views. Based on the distribution of dust clouds in other galaxies, it can be concluded that they are often most conspicuous within the spiral arms, especially along the inner edge of well-defined ones. The best-observed dust clouds near the Sun have masses of several hundred solar masses and sizes ranging from a maximum of about 200 light-years to a fraction of a light-year. The smallest tend to be the densest, possibly partly because of evolution: as a dust complex contracts, it also becomes denser and more opaque. The very smallest dust clouds are the so-called Bok globules, named after the Dutch American astronomer Bart J. Bok; these objects are about one light-year across and have masses of 1-20 solar masses.

More complete information on the dust in the Galaxy comes from infrared observations. While optical instruments can detect the dust when it obscures more distant objects or when it is illuminated by very nearby stars, infrared telescopes are able to register the long-wavelength radiation that the cool dust clouds themselves emit. A complete survey of the sky at infrared wavelengths made during the early 1980s by an unmanned orbiting observatory, the Infrared Astronomical Satellite (IRAS), revealed a large number of dense dust clouds in the Milky Way. Twenty years later the Spitzer Space Telescope, with greater sensitivity, greater wavelength coverage, and better resolution, mapped many dust complexes in the Milky Way. In some it was possible to view massive star clusters still in the process of formation.

Thick clouds of dust in the Milky Way can be studied by still another means. Many such objects contain detectable amounts of molecules that emit radio radiation at wavelengths that allow them to be identified and analyzed. More than 50 different molecules, including carbon monoxide and formaldehyde, and radicals have been detected in dust clouds.

The stars in the Galaxy, especially along the Milky Way, reveal the presence of a general, all-pervasive interstellar medium by the way in which they gradually fade with distance. This occurs primarily because of interstellar dust, which obscures and reddens starlight. On the average, stars near the Sun are dimmed by a factor of two for every 3,000 light-years. Thus, a star that is 6,000 light-years away in the plane of the Galaxy will appear four times fainter than it would otherwise were it not for the interstellar dust.

Another way in which the effects of interstellar dust become apparent is through the polarization of background starlight. Dust is aligned in space to some extent, and this results in selective absorption such that there is a preferred plane of vibration for the light waves. The electric vectors tend to lie preferentially along the galactic plane, though there are areas where the distribution is more complicated. It is likely that the polarization arises because the dust grains are partially aligned by the galactic magnetic field. If the dust grains are paramagnetic so that they act somewhat like a magnet, then the general magnetic field, though very weak, can in time line up the grains with their short axes in the direction of the field. As a consequence, the directions of polarization for stars in different parts of the sky make it possible to plot the direction of the magnetic field in the Milky Way.

The dust is accompanied by gas, which is thinly dispersed among the stars, filling the space between them. This interstellar gas consists mostly of hydrogen in its neutral form. Radio telescopes can detect neutral hydrogen because it emits radiation at a wavelength of 21 cm. Such radio wavelength is long enough to penetrate interstellar dust and so can be detected from all parts of the Galaxy. Most of what astronomers have learned about the large-scale structure and motions of the Galaxy has been derived from the radio waves of interstellar neutral hydrogen. The distance to the gas detected is not easily determined. Statistical arguments must be used in many cases, but the velocities of the gas, when compared with the velocities found for stars and those anticipated on the basis of the dynamics of the Galaxy, provide useful clues as to the location of the different sources of hydrogen radio emission. Near the Sun the average density of interstellar gas is 10 −21 gm/cm ³, which is the equivalent of about one hydrogen atom per cubic centimetre.

Even before they first detected the emission from neutral hydrogen in 1951, astronomers were aware of interstellar gas. Minor components of the gas, such as sodium and calcium, absorb light at specific wavelengths, and they thus cause the appearance of absorption lines in the spectra of the stars that lie beyond the gas. Since the lines originating from stars are usually different, it is possible to distinguish the lines of the interstellar gas and to measure both the density and velocity of the gas. Frequently it is even possible to observe the effects of several concentrations of interstellar gas between Earth and the background stars and thereby determine the kinematics of the gas in different parts of the Galaxy.

Companion galaxies

The Magellanic Clouds were recognized early in the 20th century as companion objects to the Galaxy. When American astronomer Edwin Hubble established the extragalactic nature of what we now call galaxies, it became plain that the Clouds had to be separate systems, both of the irregular class and more than 100,000 light-years distant. (The current best values for their distances are 163,000 and 202,000 light-years for the Large and Small Clouds, respectively.) Additional close companions have been found, all of them small and inconspicuous objects of the dwarf elliptical class. The nearest of these is the Sagittarius dwarf, a galaxy that is falling into the Milky Way Galaxy, having been captured tidally by the Galaxy's much stronger gravity. The core of this galaxy is about 90,000 light-years distant. Other close companions are the well-studied Carina, Draco, Fornax, Leo I, Leo II, Sextans, Sculptor, and Ursa Minor galaxies, as well as several very faint, less well-known objects. Distances for them range from approximately 200,000 to 800,000 light-years. The grouping of these galaxies around the Milky Way Galaxy is mimicked in the case of the Andromeda Galaxy, which is also accompanied by several dwarf companions.

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Milky Way Galaxy | Size, Definition, & Facts

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Milky Way Galaxy (sometimes simply called the Galaxy), large spiral system of about several hundred billion stars, one of which is the Sun. It takes its name from the Milky Way, the irregular luminous band of stars and gas clouds that stretches across the sky as seen from Earth.

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Are all galaxies the same shape?

What are the types of galaxies?

What is an example of a Sa galaxy?

galaxy, any of the systems of stars and interstellar matter that make up the universe. Many such assemblages are so enormous that they contain hundreds of billions of stars.

Nature has provided an immensely varied array of galaxies, ranging from faint, diffuse dwarf objects to brilliant spiral-shaped giants. Virtually all galaxies appear to have been formed soon after the universe began, and they pervade space, even into the depths of the farthest reaches penetrated by powerful modern telescopes. Galaxies usually exist in clusters, some of which in turn are grouped into larger clusters that measure hundreds of millions of light-years across. (A light-year is the distance traversed by light in one year, traveling at a velocity of 300,000 km per second [km/sec], or 650,000,000 miles per hour.) These so-called superclusters are separated by nearly empty voids, and this causes the gross structure of the universe to look somewhat like a network of sheets and chains of galaxies.

Galaxies differ from one another in shape, with variations resulting from the way in which the systems were formed and subsequently evolved. Galaxies are extremely varied not only in structure but also in the amount of activity observed. Some are the sites of vigorous star formation, with its attendant glowing gas and clouds of dust and molecular complexes. Others, by contrast, are quiescent, having long ago ceased to form new stars. Perhaps the most conspicuous activity in galaxies occurs in their nuclei, where evidence suggests that in many cases supermassive objects-probably black holes-lurk. These central black holes apparently formed several billion years ago; they are now observed forming in galaxies at large distances (and, therefore, because of the time it takes light to travel to Earth, at times in the far distant past) as brilliant objects called quasars.

The existence of galaxies was not recognized until the early 20th century. Since then, however, galaxies have become one of the focal points of astronomical investigation. The notable developments and achievements in the study of galaxies are surveyed here. Included in the discussion are the external galaxies (i.e., those lying outside the Milky Way Galaxy, the local galaxy to which the Sun and Earth belong), their distribution in clusters and superclusters, and the evolution of galaxies and quasars. For further details on the Milky Way Galaxy, see Milky Way Galaxy. For specifics about the components of galaxies, see star and nebula.

Historical survey of the study of galaxies

Early observations and conceptions

The dispute over the nature of what were once termed spiral nebulae stands as one of the most significant in the development of astronomy. On this dispute hinged the question of the magnitude of the universe: were we confined to a single, limited stellar system that lay embedded alone in empty space, or was our Milky Way Galaxy just one of millions of galaxies that pervaded space, stretching beyond the vast distances probed by our most powerful telescopes? How this question arose, and how it was resolved, is an important element in the development of our prevailing view of the universe.

Up until 1925, spiral nebulae and their related forms had uncertain status. Some scientists, notably Heber D. Curtis of the United States and Knut Lundmark of Sweden, argued that they might be remote aggregates of stars similar in size to the Milky Way Galaxy. Centuries earlier the German philosopher Immanuel Kant, among others, had suggested much the same idea, but that was long before the tools were available to actually measure distances and thus prove it. During the early 1920s astronomers were divided. Although some deduced that spiral nebulae were actually extragalactic star systems, there was evidence that convinced many that such nebulae were local clouds of material, possibly new solar systems in the process of forming.

It is now known that the nearest external galaxies are the Magellanic Clouds, two patchy irregular objects visible in the skies of the Southern Hemisphere. For years, most experts who regarded the Magellanic Clouds as portions of the Milky Way Galaxy system separated from the main stream could not study them because of their position. (Both Magellanic Clouds are too far south to be seen from most northern latitudes.) Moreover, the irregular shapes of the objects and their numerous hot blue stars, star clusters, and gas clouds did indeed make them resemble the southern Milky Way Galaxy.

The American astronomer Harlow Shapley, noted for his far-reaching work on the size and structure of the Milky Way Galaxy, was one of the first to appreciate the importance of the Magellanic Clouds in terms of the nature of spiral nebulae. To gauge the distance of the Clouds, he made use of the period-luminosity (P-L) relation discovered by Henrietta Leavitt of the Harvard College Observatory. In 1912 Leavitt had found that there was a close correlation between the periods of pulsation (variations in light) and the luminosities (intrinsic, or absolute, brightnesses) of a class of stars called Cepheid variables in the Small Magellanic Cloud. Leavitt's discovery, however, was of little practical value until Shapley worked out a calibration of the absolute brightnesses of pulsating stars closely analogous to the Cepheids, the so-called RR Lyrae variables. With this quantified form of the P-L relation, he was able to calculate the distances to the Magellanic Clouds, determining that they were about 75,000 light-years from Earth. The significance of the Clouds, however, continued to elude scientists of the time. For them, these objects still seemed to be anomalous, irregular patches of the Milky Way Galaxy, farther away than initially thought but not sufficient to settle the question of the nature of the universe.

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Galaxy | Definition, Formation, Types, Properties, & Facts

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Galaxy, any of the systems of stars and interstellar matter that make up the universe. Many such assemblages are so enormous that they contain hundreds of billions of stars. Virtually all galaxies appear to have been formed soon after the universe began, and they pervade all space that is viewable by modern telescopes.

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How is astronomy different from cosmology?

astronomy, science that encompasses the study of all extraterrestrial objects and phenomena. Until the invention of the telescope and the discovery of the laws of motion and gravity in the 17th century, astronomy was primarily concerned with noting and predicting the positions of the Sun, Moon, and planets, originally for calendrical and astrological purposes and later for navigational uses and scientific interest. The catalog of objects now studied is much broader and includes, in order of increasing distance, the solar system, the stars that make up the Milky Way Galaxy, and other, more distant galaxies. With the advent of scientific space probes, Earth also has come to be studied as one of the planets, though its more-detailed investigation remains the domain of the Earth sciences.

The scope of astronomy

Since the late 19th century, astronomy has expanded to include astrophysics, the application of physical and chemical knowledge to an understanding of the nature of celestial objects and the physical processes that control their formation, evolution, and emission of radiation. In addition, the gases and dust particles around and between the stars have become the subjects of much research. Study of the nuclear reactions that provide the energy radiated by stars has shown how the diversity of atoms found in nature can be derived from a universe that, following the first few minutes of its existence, consisted only of hydrogen, helium, and a trace of lithium. Concerned with phenomena on the largest scale is cosmology, the study of the evolution of the universe. Astrophysics has transformed cosmology from a purely speculative activity to a modern science capable of predictions that can be tested.

Its great advances notwithstanding, astronomy is still subject to a major constraint: it is inherently an observational rather than an experimental science. Almost all measurements must be performed at great distances from the objects of interest, with no control over such quantities as their temperature, pressure, or chemical composition. There are a few exceptions to this limitation-namely, meteorites (most of which are from the asteroid belt, though some are from the Moon or Mars), rock and soil samples brought back from the Moon, samples of comet and asteroid dust returned by robotic spacecraft, and interplanetary dust particles collected in or above the stratosphere. These can be examined with laboratory techniques to provide information that cannot be obtained in any other way. In the future, space missions may return surface materials from Mars, or other objects, but much of astronomy appears otherwise confined to Earth-based observations augmented by observations from orbiting satellites and long-range space probes and supplemented by theory.

Determining astronomical distances

A central undertaking in astronomy is the determination of distances. Without a knowledge of astronomical distances, the size of an observed object in space would remain nothing more than an angular diameter and the brightness of a star could not be converted into its true radiated power, or luminosity. Astronomical distance measurement began with a knowledge of Earth's diameter, which provided a base for triangulation. Within the inner solar system, some distances can now be better determined through the timing of radar reflections or, in the case of the Moon, through laser ranging. For the outer planets, triangulation is still used. Beyond the solar system, distances to the closest stars are determined through triangulation, in which the diameter of Earth's orbit serves as the baseline and shifts in stellar parallax are the measured quantities. Stellar distances are commonly expressed by astronomers in parsecs (pc), kiloparsecs, or megaparsecs. (1 pc = 3.086 × 10 ¹⁸ cm, or about 3.26 light-years [1.92 × 10 ¹³ miles].) Distances can be measured out to around a kiloparsec by trigonometric parallax ( see star: Determining stellar distances). The accuracy of measurements made from Earth's surface is limited by atmospheric effects, but measurements made from the Hipparcos satellite in the 1990s extended the scale to stars as far as 650 parsecs, with an accuracy of about a thousandth of an arc second. The Gaia satellite is expected to measure stars as far away as 10 kiloparsecs to an accuracy of 20 percent. Less-direct measurements must be used for more-distant stars and for galaxies.

Two general methods for determining galactic distances are described here. In the first, a clearly identifiable type of star is used as a reference standard because its luminosity has been well determined. This requires observation of such stars that are close enough to Earth that their distances and luminosities have been reliably measured. Such a star is termed a " standard candle." Examples are Cepheid variables, whose brightness varies periodically in well-documented ways, and certain types of supernova explosions that have enormous brilliance and can thus be seen out to very great distances. Once the luminosities of such nearer standard candles have been calibrated, the distance to a farther standard candle can be calculated from its calibrated luminosity and its actual measured intensity. (The measured intensity [ I] is related to the luminosity [ L] and distance [ d] by the formula I = L/4π d ².) A standard candle can be identified by means of its spectrum or the pattern of regular variations in brightness. (Corrections may have to be made for the absorption of starlight by interstellar gas and dust over great distances.) This method forms the basis of measurements of distances to the closest galaxies.

The second method for galactic distance measurements makes use of the observation that the distances to galaxies generally correlate with the speeds with which those galaxies are receding from Earth (as determined from the Doppler shift in the wavelengths of their emitted light). This correlation is expressed in the Hubble law: velocity = H × distance, in which H denotes Hubble's constant, which must be determined from observations of the rate at which the galaxies are receding. There is widespread agreement that H lies between 67 and 73 kilometres per second per megaparsec (km/sec/Mpc). H has been used to determine distances to remote galaxies in which standard candles have not been found. (For additional discussion of the recession of galaxies, the Hubble law, and galactic distance determination, see physical science: Astronomy.)

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Astronomy | Definition, History, Discoveries, & Facts

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Astronomy, science that encompasses the study of all extraterrestrial objects and phenomena. Since the late 19th century, astronomy has expanded to include astrophysics, the application of physical and chemical knowledge to an understanding of the nature of celestial objects.

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Geocentric vs Heliocentric Model of the Universe

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The most accepted theory about our universe is the 'Big Bang Theory'.
The theory state that the universe began as just a single point, then expanded and stretched to grow as large as it is right now, and, maybe, it could still be stretching.

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The Big Bang theory is an effort to explain what happened at the very beginning of our universe. Discoveries in astronomy and physics have shown beyond a reasonable doubt that our universe did in fact have a beginning. Prior to that moment there was nothing; during and after that moment there was something: our universe. The big bang theory is an effort to explain what happened during and after that moment.

According to the standard theory, our universe sprang into existence as "singularity" around 13.7 billion years ago. What is a "singularity" and where does it come from? Well, to be honest, we don't know for sure. Singularities are zones which defy our current understanding of physics. They are thought to exist at the core of "black holes." Black holes are areas of intense gravitational pressure. The pressure is thought to be so intense that finite matter is actually squished into infinite density (a mathematical concept which truly boggles the mind). These zones of infinite density are called "singularities." Our universe is thought to have begun as an infinitesimally small, infinitely hot, infinitely dense, something - a singularity. Where did it come from? We don't know. Why did it appear? We don't know.

After its initial appearance, it apparently inflated (the "Big Bang"), expanded and cooled, going from very, very small and very, very hot, to the size and temperature of our current universe. It continues to expand and cool to this day and we are inside of it: incredible creatures living on a unique planet, circling a beautiful star clustered together with several hundred billion other stars in a galaxy soaring through the cosmos, all of which is inside of an expanding universe that began as an infinitesimal singularity which appeared out of nowhere for reasons unknown. This is the Big Bang theory.

Big Bang Theory - Common Misconceptions
There are many misconceptions surrounding the Big Bang theory. For example, we tend to imagine a giant explosion. Experts however say that there was no explosion; there was (and continues to be) an expansion. Rather than imagining a balloon popping and releasing its contents, imagine a balloon expanding: an infinitesimally small balloon expanding to the size of our current universe.

Another misconception is that we tend to image the singularity as a little fireball appearing somewhere in space. According to the many experts however, space didn't exist prior to the Big Bang. Back in the late '60s and early '70s, when men first walked upon the moon, "three British astrophysicists, Steven Hawking, George Ellis, and Roger Penrose turned their attention to the Theory of Relativity and its implications regarding our notions of time. In 1968 and 1970, they published papers in which they extended Einstein's Theory of General Relativity to include measurements of time and space. 1, 2 According to their calculations, time and space had a finite beginning that corresponded to the origin of matter and energy." ³ The singularity didn't appear in space; rather, space began inside of the singularity. Prior to the singularity, nothing existed, not space, time, matter, or energy - nothing. So where and in what did the singularity appear if not in space? We don't know. We don't know where it came from, why it's here, or even where it is. All we really know is that we are inside of it and at one time it didn't exist and neither did we.

Big Bang Theory - Evidence for the Theory
What are the major evidences which support the Big Bang theory?

First of all, we are reasonably certain that the universe had a beginning.

Second, galaxies appear to be moving away from us at speeds proportional to their distance. This is called "Hubble's Law," named after Edwin Hubble (1889-1953) who discovered this phenomenon in 1929. This observation supports the expansion of the universe and suggests that the universe was once compacted.

Third, if the universe was initially very, very hot as the Big Bang suggests, we should be able to find some remnant of this heat. In 1965, Radioastronomers Arno Penzias and Robert Wilson discovered a 2.725 degree Kelvin (-454.765 degree Fahrenheit, -270.425 degree Celsius) Cosmic Microwave Background radiation (CMB) which pervades the observable universe. This is thought to be the remnant which scientists were looking for. Penzias and Wilson shared in the 1978 Nobel Prize for Physics for their discovery.

Finally, the abundance of the "light elements" Hydrogen and Helium found in the observable universe are thought to support the Big Bang model of origins.

Big Bang Theory - The Only Plausible Theory?
Is the standard Big Bang theory the only model consistent with these evidences? No, it's just the most popular one. Internationally renown Astrophysicist George F. R. Ellis explains: "People need to be aware that there is a range of models that could explain the observations....For instance, I can construct you a spherically symmetrical universe with Earth at its center, and you cannot disprove it based on observations....You can only exclude it on philosophical grounds. In my view there is absolutely nothing wrong in that. What I want to bring into the open is the fact that we are using philosophical criteria in choosing our models. A lot of cosmology tries to hide that." ⁴

In 2003, Physicist Robert Gentry proposed an attractive alternative to the standard theory, an alternative which also accounts for the evidences listed above. ⁵ Dr. Gentry claims that the standard Big Bang model is founded upon a faulty paradigm (the Friedmann-lemaitre expanding-spacetime paradigm) which he claims is inconsistent with the empirical data. He chooses instead to base his model on Einstein's static-spacetime paradigm which he claims is the "genuine cosmic Rosetta." Gentry has published several papers outlining what he considers to be serious flaws in the standard Big Bang model. ⁶ Other high-profile dissenters include Nobel laureate Dr. Hannes Alfvйn, Professor Geoffrey Burbidge, Dr. Halton Arp, and the renowned British astronomer Sir Fred Hoyle, who is accredited with first coining the term "the Big Bang" during a BBC radio broadcast in 1950.

Big Bang Theory - What About God?
Any discussion of the Big Bang theory would be incomplete without asking the question, what about God? This is because cosmogony (the study of the origin of the universe) is an area where science and theology meet. Creation was a supernatural event. That is, it took place outside of the natural realm. This fact begs the question: is there anything else which exists outside of the natural realm? Specifically, is there a master Architect out there? We know that this universe had a beginning. Was God the "First Cause"? We won't attempt to answer that question in this short article. We just ask the question:

Does God Exist?

1. Steven W. Hawking, George F.R. Ellis, "The Cosmic Black-Body Radiation and the Existence of Singularities in our Universe," Astrophysical Journal, 152, (1968) pp. 25-36.

2. Steven W. Hawking, Roger Penrose, "The Singularities of Gravitational Collapse and Cosmology," Proceedings of the Royal Society of London, series A, 314 (1970) pp. 529-548.

3. Mark Eastman, Chuck Missler, The Creator: Beyond Time and Space, (1996) p. 11.

4. W. Wayt Gibbs, "Profile: George F. R. Ellis," Scientific American, October 1995, Vol. 273, No.4, p. 55.

5. See https://www.halos.com/reports/ext-2003-022.pdf

6. See https://www.halos.com/reports/arxiv-1998-rosetta.pdf and https://www.halos.com/reports/ext-2003-021.pdf; see also https://www.halos.com/reports/arxiv-1998-redshift.pdf and https://www.halos.com/reports/arxiv-1998-affirmed.pdf

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The Big Bang theory is an effort to explain what happened at the very beginning of our universe. Discoveries in astronomy and physics have shown beyond a reasonable doubt that our universe did in fact have a beginning. Prior to that moment there was nothing; during and after that moment there was something: our universe.

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The New Astronomy: Crash Course History of Science #13

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The Big Bang & Formation of the Solar System

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Essential Question: How have theories of the formation and structure of the universe changed?

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Essential Question: How have theories of the formation and structure of the universe changed?

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Theories of the formation and structure of the universe have changed, because of people like Galileo and Copernicus who have used their findings and sharing it with others. This is still happening to this day, where asstronomers are finding new information and sharing with the world. Theories can change, because of new evindence or old evidence has been proved wrong or incorrect.

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People have been interested in our galaxy/universe forever. For example astronomer Aristotle was a Greek Philosopher/astronomer back in 384 BC - 322 BC, he  was the founder of the Lyceum. Another example is Ptolemy, he was an astronomer, astrologer and much more. He wrote several scientific treatises.

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Astronomy and the understanding of our solar system has come a long way, from where it started.

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Johannes Kepler created the Keplerian Telescope and is  best known for his laws of planetary motion. His books provided information for the foundations for Newton's theory of universal gravitation.

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Nicolaus Copernicus made the model for the Heliocentric model(A model of the solar system in which the sun is in the center) of our solar system.

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(Galileo proved his theory when he observed four moons around Jupiter.)

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For an example the Geocentric Model (A model of the solar system in which the earth is in the center) was accepted and was thought to be true. This was until Copernicus  proved this wrong.

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Theories have changed in many ways. The view and understanding of our universe have a lot to do with this.