1.
University of Strasbourg
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The University of Strasbourg in Strasbourg, Alsace, France, is the second largest university in France, with about 46,000 students and over 4,000 researchers. On 1 January 2009, the fusion of three universities reconstituted a united University of Strasbourg. With as many as 18 Nobel laureates, the university is now ranked among the best in the League of European Research Universities, the university emerged from a Lutheran humanist German Gymnasium, founded in 1538 by Johannes Sturm in the Free Imperial City of Strassburg. It was transformed to a university in 1621 and elevated to the ranks of a university in 1631. Among its earliest university students was Johann Scheffler who studied medicine and later converted to Catholicism and became the mystic and poet Angelus Silesius. The Lutheran German university still persisted even after the annexation of the City by King Louis XIV in 1681, during the German Empire the university was greatly expanded and numerous new buildings were erected because the university was intended to be a showcase of German against French culture in Alsace. In 1918, Alsace-Lorraine was returned to France, so a reverse exodus of Germanophone teachers took place, during the Second World War, when France was occupied, personnel and equipment of the University of Strasbourg were transferred to Clermont-Ferrand. In its place, the short-lived German Reichsuniversität Straßburg was created, the university campus covers a vast part near the center of the city, located between the Cité Administrative, Esplanade and Gallia bus-tram stations. Modern architectural buildings include, Escarpe, the Doctoral College of Strasbourg, Atrium, Pangloss, the student residence building for the Doctoral College of Strasbourg was designed by London-based Nicholas Hare Architects in 2007. The structures are depicted on the inner wall of the Esplanade university restaurant, accompanied by the names of their architects. The administrative organisms, attached to the university, are located in the Agora building
2.
Strasbourg
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Strasbourg is the capital and largest city of the Grand Est region of France and is the official seat of the European Parliament. Located close to the border with Germany in the region of Alsace. In 2014, the city proper had 276,170 inhabitants, Strasbourgs metropolitan area had a population of 773,347 in 2013, making it the ninth largest metro area in France and home to 13% of the Grand Est regions inhabitants. The transnational Eurodistrict Strasbourg-Ortenau had a population of 915,000 inhabitants in 2014, Strasbourg is the seat of several European institutions, such as the Council of Europe and the Eurocorps, as well as the European Parliament and the European Ombudsman of the European Union. The city is also the seat of the Central Commission for Navigation on the Rhine, Strasbourgs historic city centre, the Grande Île, was classified a World Heritage site by UNESCO in 1988, the first time such an honour was placed on an entire city centre. The largest Islamic place of worship in France, the Strasbourg Grand Mosque, was inaugurated by French Interior Minister Manuel Valls on 27 September 2012. Economically, Strasbourg is an important centre of manufacturing and engineering, as well as a hub of road, rail, the port of Strasbourg is the second largest on the Rhine after Duisburg, Germany. Before the 5th century, the city was known as Argantorati, a Celtic Gaulish name Latinized first as Argentorate, after the 5h century, the city became known by a completely different name Gallicized as Strasbourg. That name is of Germanic origin and means Town of roads, Strasbourg is situated on the eastern border of France with Germany. This border is formed by the River Rhine, which forms the eastern border of the modern city. The historic core of Strasbourg however lies on the Grande Île in the River Ill, which flows parallel to, and roughly 4 kilometres from. The natural courses of the two eventually join some distance downstream of Strasbourg, although several artificial waterways now connect them within the city. This section of the Rhine valley is an axis of north-south travel, with river traffic on the Rhine itself. The city is some 400 kilometres east of Paris, in spite of its position far inland, Strasbourgs climate is classified as Oceanic, with warm, relatively sunny summers and cold, overcast winters. Precipitation is elevated from mid-spring to the end of summer, but remains largely constant throughout the year, on average, snow falls 30 days per year. The highest temperature recorded was 38.5 °C in August 2003. The lowest temperature recorded was −23.4 °C in December 1938. Nonetheless, the disappearance of heavy industry on both banks of the Rhine, as well as effective measures of traffic regulation in and around the city have reduced air pollution
3.
France
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France, officially the French Republic, is a country with territory in western Europe and several overseas regions and territories. The European, or metropolitan, area of France extends from the Mediterranean Sea to the English Channel and the North Sea, Overseas France include French Guiana on the South American continent and several island territories in the Atlantic, Pacific and Indian oceans. France spans 643,801 square kilometres and had a population of almost 67 million people as of January 2017. It is a unitary republic with the capital in Paris. Other major urban centres include Marseille, Lyon, Lille, Nice, Toulouse, during the Iron Age, what is now metropolitan France was inhabited by the Gauls, a Celtic people. The area was annexed in 51 BC by Rome, which held Gaul until 486, France emerged as a major European power in the Late Middle Ages, with its victory in the Hundred Years War strengthening state-building and political centralisation. During the Renaissance, French culture flourished and a colonial empire was established. The 16th century was dominated by civil wars between Catholics and Protestants. France became Europes dominant cultural, political, and military power under Louis XIV, in the 19th century Napoleon took power and established the First French Empire, whose subsequent Napoleonic Wars shaped the course of continental Europe. Following the collapse of the Empire, France endured a succession of governments culminating with the establishment of the French Third Republic in 1870. Following liberation in 1944, a Fourth Republic was established and later dissolved in the course of the Algerian War, the Fifth Republic, led by Charles de Gaulle, was formed in 1958 and remains to this day. Algeria and nearly all the colonies became independent in the 1960s with minimal controversy and typically retained close economic. France has long been a centre of art, science. It hosts Europes fourth-largest number of cultural UNESCO World Heritage Sites and receives around 83 million foreign tourists annually, France is a developed country with the worlds sixth-largest economy by nominal GDP and ninth-largest by purchasing power parity. In terms of household wealth, it ranks fourth in the world. France performs well in international rankings of education, health care, life expectancy, France remains a great power in the world, being one of the five permanent members of the United Nations Security Council with the power to veto and an official nuclear-weapon state. It is a member state of the European Union and the Eurozone. It is also a member of the Group of 7, North Atlantic Treaty Organization, Organisation for Economic Co-operation and Development, the World Trade Organization, originally applied to the whole Frankish Empire, the name France comes from the Latin Francia, or country of the Franks
4.
Observatory
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An observatory is a location used for observing terrestrial or celestial events. Astronomy, climatology/meteorology, geology, oceanography and volcanology are examples of disciplines for which observatories have been constructed, historically, observatories were as simple as containing an astronomical sextant or Stonehenge. Astronomical observatories are mainly divided into four categories, space based, airborne, ground based, ground-based observatories, located on the surface of Earth, are used to make observations in the radio and visible light portions of the electromagnetic spectrum. Most optical telescopes are housed within a dome or similar structure, Telescope domes have a slit or other opening in the roof that can be opened during observing, and closed when the telescope is not in use. In most cases, the upper portion of the telescope dome can be rotated to allow the instrument to observe different sections of the night sky. Radio telescopes usually do not have domes, for optical telescopes, most ground-based observatories are located far from major centers of population, to avoid the effects of light pollution. The ideal locations for modern observatories are sites that have dark skies, a percentage of clear nights per year, dry air. At high elevations, the Earths atmosphere is thinner, thereby minimizing the effects of atmospheric turbulence, sites that meet the above criteria for modern observatories include the southwestern United States, Hawaii, Canary Islands, the Andes, and high mountains in Mexico such as Sierra Negra. Major optical observatories include Mauna Kea Observatory and Kitt Peak National Observatory in the USA, Roque de los Muchachos Observatory in Spain, and Paranal Observatory in Chile. Specific research study performed in 2009 shows that the best possible location for ground-based observatory on Earth is Ridge A — a place in the part of Eastern Antarctica. This location provides the least atmospheric disturbances and best visibility, beginning in 1930s, radio telescopes have been built for use in the field of radio astronomy to observe the Universe in the radio portion of the electromagnetic spectrum. Since the mid-20th century, a number of observatories have been constructed at very high altitudes, above 4. The largest and most notable of these is the Mauna Kea Observatory, the Chacaltaya Astrophysical Observatory in Bolivia, at 5,230 m, was the worlds highest permanent astronomical observatory from the time of its construction during the 1940s until 2009. It has now surpassed by the new University of Tokyo Atacama Observatory. As a result, the resolution of space telescopes such as the Hubble Space Telescope is often much smaller than a ground-based telescope with a similar aperture. However, all these advantages do come with a price, Space telescopes are much more expensive to build than ground-based telescopes. Due to their location, space telescopes are also difficult to maintain. The Hubble Space Telescope was serviced by the Space Shuttle while many other space telescopes cannot be serviced at all, Airborne observatories have the advantage of height over ground installations, putting them above most of the Earths atmosphere
5.
Franco-Prussian War
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The conflict was caused by Prussian ambitions to extend German unification and French fears of the shift in the European balance of power that would result if the Prussians succeeded. On 16 July 1870, the French parliament voted to declare war on the German Kingdom of Prussia, the German coalition mobilised its troops much more quickly than the French and rapidly invaded northeastern France. The German forces were superior in numbers, had training and leadership and made more effective use of modern technology, particularly railroads. The German states proclaimed their union as the German Empire under the Prussian king Wilhelm I, the Treaty of Frankfurt of 10 May 1871 gave Germany most of Alsace and some parts of Lorraine, which became the Imperial territory of Alsace-Lorraine. French determination to regain Alsace-Lorraine and fear of another Franco-German war, along with British apprehension about the balance of power, the causes of the Franco-Prussian War are deeply rooted in the events surrounding the unification of Germany. In the aftermath of the Austro-Prussian War of 1866, Prussia had annexed numerous territories and this new power destabilized the European balance of power established by the Congress of Vienna in 1815 after the Napoleonic Wars. France was strongly opposed to any further alliance of German states, in Prussia, some officials considered a war against France both inevitable and necessary to arouse German nationalism in those states that would allow the unification of a great German empire. Bismarck also knew that France should be the aggressor in the conflict to bring the southern German states to side with Prussia, many Germans also viewed the French as the traditional destabilizer of Europe, and sought to weaken France to prevent further breaches of the peace. The immediate cause of the war resided in the candidacy of Leopold of Hohenzollern-Sigmaringen, France feared encirclement by an alliance between Prussia and Spain. The Hohenzollern princes candidacy was withdrawn under French diplomatic pressure, releasing the Ems Dispatch to the public, Bismarck made it sound as if the king had treated the French envoy in a demeaning fashion, which inflamed public opinion in France. They also argue that he wanted a war to resolve growing domestic political problems, other historians, notably French historian Pierre Milza, dispute this. According to Milza, the Emperor had no need for a war to increase his popularity, the Ems telegram had exactly the effect on French public opinion that Bismarck had intended. This text produced the effect of a red flag on the Gallic bull, gramont, the French foreign minister, declared that he felt he had just received a slap. Napoleons new prime minister, Emile Ollivier, declared that France had done all that it could humanly and honorably do to prevent the war, a crowd of 15–20,000 people, carrying flags and patriotic banners, marched through the streets of Paris, demanding war. On 19 July 1870 a declaration of war was sent to the Prussian government, the southern German states immediately sided with Prussia. The French Army consisted in peacetime of approximately 400,000 soldiers, some of them were veterans of previous French campaigns in the Crimean War, Algeria, the Franco-Austrian War in Italy, and in the Mexican campaign. Under Marshal Adolphe Niel, urgent reforms were made, universal conscription and a shorter period of service gave increased numbers of reservists, who would swell the army to a planned strength of 800,000 on mobilisation. Those who for any reason were not conscripted were to be enrolled in the Garde Mobile, however, the Franco-Prussian War broke out before these reforms could be completely implemented
6.
German Empire
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The German Empire was the historical German nation state that existed from the unification of Germany in 1871 to the abdication of Kaiser Wilhelm II in 1918, when Germany became a federal republic. The German Empire consisted of 26 constituent territories, with most being ruled by royal families and this included four kingdoms, six grand duchies, five duchies, seven principalities, three free Hanseatic cities, and one imperial territory. Although Prussia became one of kingdoms in the new realm, it contained most of its population and territory. Its influence also helped define modern German culture, after 1850, the states of Germany had rapidly become industrialized, with particular strengths in coal, iron, chemicals, and railways. In 1871, it had a population of 41 million people, and by 1913, a heavily rural collection of states in 1815, now united Germany became predominantly urban. During its 47 years of existence, the German Empire operated as an industrial, technological, Germany became a great power, boasting a rapidly growing rail network, the worlds strongest army, and a fast-growing industrial base. In less than a decade, its navy became second only to Britains Royal Navy, after the removal of Chancellor Otto von Bismarck by Wilhelm II, the Empire embarked on a bellicose new course that ultimately led to World War I. When the great crisis of 1914 arrived, the German Empire had two allies, Italy and the Austro-Hungarian Empire, Italy, however, left the once the First World War started in August 1914. In the First World War, German plans to capture Paris quickly in autumn 1914 failed, the Allied naval blockade caused severe shortages of food. Germany was repeatedly forced to send troops to bolster Austria and Turkey on other fronts, however, Germany had great success on the Eastern Front, it occupied large Eastern territories following the Treaty of Brest-Litovsk. German declaration of unrestricted submarine warfare in early 1917 was designed to strangle the British, it failed, but the declaration—along with the Zimmermann Telegram—did bring the United States into the war. Meanwhile, German civilians and soldiers had become war-weary and radicalised by the Russian Revolution and this failed, and by October the armies were in retreat, Austria-Hungary and the Ottoman Empire had collapsed, Bulgaria had surrendered and the German people had lost faith in their political system. The Empire collapsed in the November 1918 Revolution as the Emperor and all the ruling monarchs abdicated, and a republic took over. The German Confederation had been created by an act of the Congress of Vienna on 8 June 1815 as a result of the Napoleonic Wars, German nationalism rapidly shifted from its liberal and democratic character in 1848, called Pan-Germanism, to Prussian prime minister Otto von Bismarcks pragmatic Realpolitik. He envisioned a conservative, Prussian-dominated Germany, the war resulted in the Confederation being partially replaced by a North German Confederation in 1867, comprising the 22 states north of the Main. The new constitution and the title Emperor came into effect on 1 January 1871, during the Siege of Paris on 18 January 1871, William accepted to be proclaimed Emperor in the Hall of Mirrors at the Palace of Versailles. The second German Constitution was adopted by the Reichstag on 14 April 1871 and proclaimed by the Emperor on 16 April, the political system remained the same. The empire had a parliament called the Reichstag, which was elected by universal male suffrage, however, the original constituencies drawn in 1871 were never redrawn to reflect the growth of urban areas
7.
Great refractor
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Great refractor refers to a large telescope with a lens, usually the largest refractor at an observatory with an equatorial mount. The preeminence and success of this style in observational astronomy was an era in telescope use in the 19th, Great refractors were large refracting telescopes using achromatic lenses. They were often the largest in the world, or largest in a region, despite typical designs having smaller apertures than reflectors, Great refractors offered a number of advantages and were favored for astronomy. It was not until the 20th century that they were superseded by large reflecting telescopes for professional astronomy. Great refractors were often used for observing stars and equipped with a Filar micrometer. Numerous discoveries of minor planets, satellites, the planet Neptune, and double stars were made, the choice of building large refractors over reflectors was a technological one. The difficulties of fabricating two disks of glass required to make a large achromatic lens were formidable. But reflecting telescopes had larger problems, the material their primary mirror was made of was speculum metal, a substance that only reflected up to 66 percent of the light that hit it and tarnished in months. They had to be removed, polished, and re-figured to the correct shape and this sometimes proved difficult, with the telescope mirrors sometimes having to be abandoned. Because of this, large refractors seemed to be the better choice, the Swiss Pierre Louis Guinand discovered and developed a way to make much larger crown and flint glass blanks. He worked with instrument maker Joseph von Fraunhofer to use technology for instruments in the early 19th century. The era of great refractors started with the first modern, achromatic, the first of these was the Dorpat Great Refractor, also known as the Fraunhofer 9-inch, at what was then Dorpat Observatory in the Governorate of Estonia. This telescope made by Fraunhofer had a 9 Paris inch aperture achromatic lens and it was also equipped with the first modern equatorial mount type called a German equatorial mount developed by Fraunhofer, a mount that became standard for most large refractors from then on. A Fraunhofer 9-inch at Berlin Observatory was used by Johann Gottfried Galle in the discovery of Neptune, refracting telescopes would quadruple in size by the end of the century, culminating with the largest practical refractor ever built, the Yerkes Observatory 40-inch aperture of 1895. To achieve its optical aperture it was slightly bigger physically. In addition to the lens, the rest of the telescope needed to be a practical and high precision instrument, for example, the Yerkes tube alone weighed 75 tons, and had to track stars just as accurately as a smaller instrument. The era slowly came to end as large reflecting telescopes superseded the great refractors, in 1856-57, Carl August von Steinheil and Léon Foucault introduced a process of depositing a layer of silver on glass telescope mirrors. Silvered glass mirrors were a vast improvement over speculum metal and made reflectors a practical instrument, Great refractors were admired for their quality, durability, and usefulness which correlated to features such as lens quality, mount quality, aperture, and also length
8.
World War I
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World War I, also known as the First World War, the Great War, or the War to End All Wars, was a global war originating in Europe that lasted from 28 July 1914 to 11 November 1918. More than 70 million military personnel, including 60 million Europeans, were mobilised in one of the largest wars in history and it was one of the deadliest conflicts in history, and paved the way for major political changes, including revolutions in many of the nations involved. The war drew in all the worlds great powers, assembled in two opposing alliances, the Allies versus the Central Powers of Germany and Austria-Hungary. These alliances were reorganised and expanded as more nations entered the war, Italy, Japan, the trigger for the war was the assassination of Archduke Franz Ferdinand of Austria, heir to the throne of Austria-Hungary, by Yugoslav nationalist Gavrilo Princip in Sarajevo on 28 June 1914. This set off a crisis when Austria-Hungary delivered an ultimatum to the Kingdom of Serbia. Within weeks, the powers were at war and the conflict soon spread around the world. On 25 July Russia began mobilisation and on 28 July, the Austro-Hungarians declared war on Serbia, Germany presented an ultimatum to Russia to demobilise, and when this was refused, declared war on Russia on 1 August. Germany then invaded neutral Belgium and Luxembourg before moving towards France, after the German march on Paris was halted, what became known as the Western Front settled into a battle of attrition, with a trench line that changed little until 1917. On the Eastern Front, the Russian army was successful against the Austro-Hungarians, in November 1914, the Ottoman Empire joined the Central Powers, opening fronts in the Caucasus, Mesopotamia and the Sinai. In 1915, Italy joined the Allies and Bulgaria joined the Central Powers, Romania joined the Allies in 1916, after a stunning German offensive along the Western Front in the spring of 1918, the Allies rallied and drove back the Germans in a series of successful offensives. By the end of the war or soon after, the German Empire, Russian Empire, Austro-Hungarian Empire, national borders were redrawn, with several independent nations restored or created, and Germanys colonies were parceled out among the victors. During the Paris Peace Conference of 1919, the Big Four imposed their terms in a series of treaties, the League of Nations was formed with the aim of preventing any repetition of such a conflict. This effort failed, and economic depression, renewed nationalism, weakened successor states, and feelings of humiliation eventually contributed to World War II. From the time of its start until the approach of World War II, at the time, it was also sometimes called the war to end war or the war to end all wars due to its then-unparalleled scale and devastation. In Canada, Macleans magazine in October 1914 wrote, Some wars name themselves, during the interwar period, the war was most often called the World War and the Great War in English-speaking countries. Will become the first world war in the sense of the word. These began in 1815, with the Holy Alliance between Prussia, Russia, and Austria, when Germany was united in 1871, Prussia became part of the new German nation. Soon after, in October 1873, German Chancellor Otto von Bismarck negotiated the League of the Three Emperors between the monarchs of Austria-Hungary, Russia and Germany
9.
Comet
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A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to evolve gasses, a process called outgassing. This produces an atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of collections of ice, dust. The coma may be up to 15 times the Earths diameter, if sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° across the sky. Comets have been observed and recorded since ancient times by many cultures, Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars, hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition, Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma and the tail, however, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System, the discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. As of November 2014 there are 5,253 known comets, however, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System is estimated to be one trillion. Roughly one comet per year is visible to the eye, though many of those are faint. Particularly bright examples are called Great Comets, the word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης, Κομήτης was derived from κομᾶν, which was itself derived from κόμη and was used to mean the tail of a comet. The astronomical symbol for comets is ☄, consisting of a disc with three hairlike extensions. The solid, core structure of a comet is known as the nucleus, cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are described as dirty snowballs after Fred Whipples model
10.
Variable star
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A variable star is a star whose brightness as seen from Earth fluctuates. Many, possibly most, stars have at least some variation in luminosity, an ancient Egyptian calendar of lucky and unlucky days composed some 3,200 years ago may be the oldest preserved historical document of the discovery of a variable star, the eclipsing binary Algol. This discovery, combined with supernovae observed in 1572 and 1604, proved that the sky was not eternally invariable as Aristotle. In this way, the discovery of variable stars contributed to the revolution of the sixteenth. The second variable star to be described was the eclipsing variable Algol, by Geminiano Montanari in 1669, chi Cygni was identified in 1686 by G. Kirch, then R Hydrae in 1704 by G. D. Maraldi. By 1786 ten variable stars were known, John Goodricke himself discovered Delta Cephei and Beta Lyrae. Since 1850 the number of variable stars has increased rapidly, especially after 1890 when it became possible to identify variable stars by means of photography. The latest edition of the General Catalogue of Variable Stars lists more than 46,000 variable stars in the Milky Way, as well as 10,000 in other galaxies, and over 10,000 suspected variables. The most common kinds of variability involve changes in brightness, but other types of variability also occur, by combining light curve data with observed spectral changes, astronomers are often able to explain why a particular star is variable. Variable stars are generally analysed using photometry, spectrophotometry and spectroscopy, measurements of their changes in brightness can be plotted to produce light curves. Peak brightnesses in the curve are known as maxima, while troughs are known as minima. By estimating the magnitude and noting the time of observation a visual lightcurve can be constructed. The American Association of Variable Star Observers collects such observations from participants around the world, from the light curve the following data are derived, are the brightness variations periodical, semiperiodical, irregular, or unique. What is the period of the brightness fluctuations, what is the shape of the light curve. From the spectrum the following data are derived, what kind of star is it, what is its temperature, is it a single star, or a binary. does the spectrum change with time. In very few cases it is possible to make pictures of a stellar disk and these may show darker spots on its surface. Combining light curves with spectral data often gives a clue as to the changes occur in a variable star. For example, evidence for a star is found in its shifting spectrum because its surface periodically moves toward and away from us
11.
Binary star
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A binary star is a star system consisting of two stars orbiting around their common barycenter. Systems of two or more stars are called multiple star systems and these systems, especially when more distant, often appear to the unaided eye as a single point of light, and are then revealed as multiple by other means. Research over the last two centuries suggests that half or more of visible stars are part of star systems. The term double star is used synonymously with binary star, however. Optical doubles are so called because the two stars close together in the sky as seen from the Earth, they are almost on the same line of sight. Nevertheless, their doubleness depends only on this effect, the stars themselves are distant from one another. A double star can be revealed as optical by means of differences in their measurements, proper motions. Most known double stars have not been studied closely to determine whether they are optical doubles or they are doubles physically bound through gravitation into a multiple star system. This also determines an empirical mass-luminosity relationship from which the masses of stars can be estimated. Binary stars are often detected optically, in case they are called visual binaries. Many visual binaries have long periods of several centuries or millennia. They may also be detected by indirect techniques, such as spectroscopy or astrometry, if components in binary star systems are close enough they can gravitationally distort their mutual outer stellar atmospheres. In some cases, these close binary systems can exchange mass, examples of binaries are Sirius, and Cygnus X-1. Binary stars are common as the nuclei of many planetary nebulae. This should be called a double star, and any two stars that are thus mutually connected, form the binary sidereal system which we are now to consider. By the modern definition, the binary star is generally restricted to pairs of stars which revolve around a common center of mass. Binary stars which can be resolved with a telescope or interferometric methods are known as visual binaries, for most of the known visual binary stars one whole revolution has not been observed yet, they are observed to have travelled along a curved path or a partial arc. The more general term double star is used for pairs of stars which are seen to be together in the sky
12.
Nebula
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A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, nebula was a name for any diffuse astronomical object, the Andromeda Galaxy, for instance, was once referred to as the Andromeda Nebula before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others. Most nebulae are of vast size, even millions of years in diameter. The Orion Nebula, the brightest nebula in the sky that occupies a region twice the diameter of the full Moon, can be viewed with the naked eye but was missed by early astronomers. Many nebulae are visible due to their fluorescence caused by the hot stars, while others are so diffuse they can only be detected with long exposures. Some nebulae are variably illuminated by T Tauri variable stars, Nebulae are often star-forming regions, such as in the Pillars of Creation in the Eagle Nebula. In these regions the formations of gas, dust, and other materials together to form denser regions, which attract further matter. The remaining material is believed to form planets and other planetary system objects. Around 150 AD, Claudius Ptolemaeus recorded, in books VII–VIII of his Almagest and he also noted a region of nebulosity between the constellations Ursa Major and Leo that was not associated with any star. The first true nebula, as distinct from a cluster, was mentioned by the Persian astronomer Abd al-Rahman al-Sufi. He noted a little cloud where the Andromeda Galaxy is located and he also cataloged the Omicron Velorum star cluster as a nebulous star and other nebulous objects, such as Brocchis Cluster. The supernova that created the Crab Nebula, the SN1054, was observed by Arabic, in 1610, Nicolas-Claude Fabri de Peiresc discovered the Orion Nebula using a telescope. This nebula was observed by Johann Baptist Cysat in 1618. However, the first detailed study of the Orion Nebula was not performed until 1659 by Christiaan Huygens, in 1715, Edmund Halley published a list of six nebulae. This number steadily increased during the century, with Jean-Philippe de Cheseaux compiling a list of 20 in 1746, from 1751 to 1753, Nicolas Louis de Lacaille cataloged 42 nebulae from the Cape of Good Hope, most of which were previously unknown. Charles Messier then compiled a catalog of 103 nebulae by 1781, his interest was detecting comets, the number of nebulae was then greatly expanded by the efforts of William Herschel and his sister Caroline Herschel. Their Catalogue of One Thousand New Nebulae and Clusters of Stars was published in 1786, a second catalog of a thousand was published in 1789 and the third and final catalog of 510 appeared in 1802. During much of their work, William Herschel believed that these nebulae were merely unresolved clusters of stars, in 1790, however, he discovered a star surrounded by nebulosity and concluded that this was a true nebulosity, rather than a more distant cluster
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