Misplaced Pages

#915084

83-598: The Drengin Empire has conquered most of the galaxy , driving the Terrans back to Earth , where humans have created an impenetrable defence around the homeworld. The Drengin have focused their attention on the other conquered races. Krindar I'Agohl, leader of the Korath clan, a specialist military faction feared in combat even against the Terrans, urges the annihilation of all non-Drengin species. Formerly few could survive against

166-407: A star system tend to be small fractions of a light-year, and are usually expressed in astronomical units . However, smaller units of length can similarly be formed usefully by multiplying units of time by the speed of light. For example, the light-second , useful in astronomy, telecommunications and relativistic physics, is exactly 299 792 458 metres or ⁠ 1 / 31 557 600 ⁠ of

249-452: A chain reaction of star-building that spreads throughout the gaseous region. Only when the available gas is nearly consumed or dispersed does the activity end. Starbursts are often associated with merging or interacting galaxies. The prototype example of such a starburst-forming interaction is M82 , which experienced a close encounter with the larger M81 . Irregular galaxies often exhibit spaced knots of starburst activity. A radio galaxy

332-456: A credit card or a service such as PayPal . A retail version of Dark Avatar is available for purchase at retail locations worldwide as Galactic Civilizations II: Gold Edition , which also includes Dread Lords and "Galactic Civilizations II: Dark Avatar". Initial reviews of Dark Avatar were highly favourable. The game received an aggregated score of 91% from Metacritic . Both GameSpy and IGN gave it Editor's Choice awards, while UGO rated

415-684: A debate took place between Harlow Shapley and Heber Curtis , the Great Debate , concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift. In 1922, the Estonian astronomer Ernst Öpik gave

498-536: A distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object. Using the new 100-inch Mt. Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables , thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of

581-418: A function of their radius (or distance from their cores) falls off more slowly than their smaller counterparts. The formation of these cD galaxies remains an active area of research, but the leading model is that they are the result of the mergers of smaller galaxies in the environments of dense clusters, or even those outside of clusters with random overdensities. These processes are the mechanisms that drive

664-462: A huge number of stars held together by gravitational forces, akin to the Solar System but on a much larger scale, and that the resulting disk of stars could be seen as a band on the sky from a perspective inside it. In his 1755 treatise, Immanuel Kant elaborated on Wright's idea about the Milky Way's structure. The first project to describe the shape of the Milky Way and the position of

747-412: A light-year. Units such as the light-minute, light-hour and light-day are sometimes used in popular science publications. The light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures. The Hayden Planetarium specifies the light month more precisely as 30 days of light travel time. Light travels approximately one foot in a nanosecond ; the term "light-foot"

830-429: A low portion of open clusters and a reduced rate of new star formation. Instead, they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst. In this sense they have some similarity to the much smaller globular clusters . The largest galaxies are

913-403: A mortal woman, the infant Heracles , on Hera 's breast while she is asleep so the baby will drink her divine milk and thus become immortal. Hera wakes up while breastfeeding and then realises she is nursing an unknown baby: she pushes the baby away, some of her milk spills, and it produces the band of light known as the Milky Way. In the astronomical literature, the capitalised word "Galaxy"

SECTION 10

#1732797934916

996-560: A part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy , began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way. For this reason they were popularly called island universes , but this term quickly fell into disuse, as

1079-417: A pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constant angular velocity . The spiral arms are thought to be areas of high-density matter, or " density waves ". As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of

1162-400: A plane, the majority of mass in spiral galaxies exists in a roughly spherical halo of dark matter which extends beyond the visible component, as demonstrated by the universal rotation curve concept. Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. In

1245-438: A reserve of cold gas that forms giant molecular clouds . Some galaxies have been observed to form stars at an exceptional rate, which is known as a starburst . If they continue to do so, they would consume their reserve of gas in a time span less than the galaxy's lifespan. Hence starburst activity usually lasts only about ten million years, a relatively brief period in a galaxy's history. Starburst galaxies were more common during

1328-435: A rotating bar structure in the center of this galaxy. With improved radio telescopes , hydrogen gas could also be traced in other galaxies. In the 1970s, Vera Rubin uncovered a discrepancy between observed galactic rotation speed and that predicted by the visible mass of stars and gas. Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter . Beginning in

1411-488: A single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered. Most of the information we have about dwarf galaxies come from observations of the local group , containing two spiral galaxies, the Milky Way and Andromeda, and many dwarf galaxies. These dwarf galaxies are classified as either irregular or dwarf elliptical / dwarf spheroidal galaxies . A study of 27 Milky Way neighbors found that in all dwarf galaxies,

1494-419: A smaller companion galaxy—that as the two galaxy centers approach, they start to oscillate around a center point, and the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water. For example, galaxy NGC 3923 has over 20 shells. Spiral galaxies resemble spiraling pinwheels . Though the stars and other visible material contained in such a galaxy lie mostly on

1577-528: Is 3C 236 , with lobes 15 million light-years across. It should however be noted that radio emissions are not always considered part of the main galaxy itself. A giant radio galaxy is a special class of objects characterized by the presence of radio lobes generated by relativistic jets powered by the central galaxy's supermassive black hole . Giant radio galaxies are different from ordinary radio galaxies in that they can extend to much larger scales, reaching upwards to several megaparsecs across, far larger than

1660-516: Is a galaxy with giant regions of radio emission extending well beyond its visible structure. These energetic radio lobes are powered by jets from its active galactic nucleus . Radio galaxies are classified according to their Fanaroff–Riley classification . The FR I class have lower radio luminosity and exhibit structures which are more elongated; the FR II class are higher radio luminosity. The correlation of radio luminosity and structure suggests that

1743-495: Is a spiral galaxy having the number 109 in the catalogue of Messier. It also has the designations NGC 3992, UGC 6937, CGCG 269–023, MCG +09-20-044, and PGC 37617 (or LEDA 37617), among others. Millions of fainter galaxies are known by their identifiers in sky surveys such as the Sloan Digital Sky Survey . Greek philosopher Democritus (450–370 BCE) proposed that the bright band on the night sky known as

SECTION 20

#1732797934916

1826-637: Is a system of stars , stellar remnants , interstellar gas , dust , and dark matter bound together by gravity . The word is derived from the Greek galaxias ( γαλαξίας ), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System . Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a thousand stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass . Most of

1909-562: Is also used to observe distant, red-shifted galaxies that were formed much earlier. Water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for infrared astronomy . The first non-visual study of galaxies, particularly active galaxies, was made using radio frequencies . The Earth's atmosphere is nearly transparent to radio between 5  MHz and 30 GHz. The ionosphere blocks signals below this range. Large radio interferometers have been used to map

1992-533: Is an odd name. In 1868 an English journal labelled the light-year as a unit used by the Germans. Eddington called the light-year an inconvenient and irrelevant unit, which had sometimes crept from popular use into technical investigations. Although modern astronomers often prefer to use the parsec , light-years are also popularly used to gauge the expanses of interstellar and intergalactic space. Distances expressed in light-years include those between stars in

2075-531: Is called the Galactic Center . The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter . Consequently, these galaxies also have

2158-618: Is filled with a tenuous gas (the intergalactic medium ) with an average density of less than one atom per cubic metre. Most galaxies are gravitationally organised into groups , clusters and superclusters . The Milky Way is part of the Local Group , which it dominates along with the Andromeda Galaxy . The group is part of the Virgo Supercluster . At the largest scale , these associations are generally arranged into sheets and filaments surrounded by immense voids . Both

2241-461: Is given by the Hubble sequence . Since the Hubble sequence is entirely based upon visual morphological type (shape), it may miss certain important characteristics of galaxies such as star formation rate in starburst galaxies and activity in the cores of active galaxies . Many galaxies are thought to contain a supermassive black hole at their center. This includes the Milky Way, whose core region

2324-668: Is often used to refer to the Milky Way galaxy, to distinguish it from the other galaxies in the observable universe . The English term Milky Way can be traced back to a story by Geoffrey Chaucer c.  1380 : See yonder, lo, the Galaxyë  Which men clepeth the Milky Wey ,  For hit is whyt. Galaxies were initially discovered telescopically and were known as spiral nebulae . Most 18th- to 19th-century astronomers considered them as either unresolved star clusters or anagalactic nebulae , and were just thought of as

2407-417: Is sometimes referred to as a flocculent spiral galaxy ; in contrast to the grand design spiral galaxy that has prominent and well-defined spiral arms. The speed in which a galaxy rotates is thought to correlate with the flatness of the disc as some spiral galaxies have thick bulges, while others are thin and dense. In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals ,

2490-598: Is the product of the J1900.0 mean tropical year and the defined speed of light. Abbreviations used for light-years and multiples of light-years are: The light-year unit appeared a few years after the first successful measurement of the distance to a star other than the Sun, by Friedrich Bessel in 1838. The star was 61 Cygni , and he used a 160-millimetre (6.2 in) heliometre designed by Joseph von Fraunhofer . The largest unit for expressing distances across space at that time

2573-583: The University of Nottingham , used 20 years of Hubble images to estimate that the observable universe contained at least two trillion ( 2 × 10 ) galaxies. However, later observations with the New Horizons space probe from outside the zodiacal light reduced this to roughly 200 billion ( 2 × 10 ). Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance

Galactic Civilizations II: Dark Avatar - Misplaced Pages Continue

2656-453: The observable universe . Most galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years ) and are separated by distances in the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 26,800 parsecs (87,400 ly) and is separated from the Andromeda Galaxy , its nearest large neighbour, by just over 750,000 parsecs (2.5 million ly). The space between galaxies

2739-603: The spectra invisible to humans (radio telescopes, infrared cameras, and x-ray telescopes ) allows detection of other galaxies that are not detected by Hubble. Particularly, surveys in the Zone of Avoidance (the region of sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies. A 2016 study published in The Astrophysical Journal , led by Christopher Conselice of

2822-519: The type-cD galaxies . First described in 1964 by a paper by Thomas A. Matthews and others, they are a subtype of the more general class of D galaxies, which are giant elliptical galaxies, except that they are much larger. They are popularly known as the supergiant elliptical galaxies and constitute the largest and most luminous galaxies known. These galaxies feature a central elliptical nucleus with an extensive, faint halo of stars extending to megaparsec scales. The profile of their surface brightnesses as

2905-579: The "Great Andromeda Nebula", as the Andromeda Galaxy, Messier object M31 , was then known. Searching the photographic record, he found 11 more novae . Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within this galaxy. As a result, he was able to come up with a distance estimate of 150,000  parsecs . He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies. In 1920

2988-525: The 1990s, the Hubble Space Telescope yielded improved observations. Among other things, its data helped establish that the missing dark matter in this galaxy could not consist solely of inherently faint and small stars. The Hubble Deep Field , an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion ( 1.25 × 10 ) galaxies in the observable universe. Improved technology in detecting

3071-432: The Hubble classification scheme, spiral galaxies are listed as type S , followed by a letter ( a , b , or c ) which indicates the degree of tightness of the spiral arms and the size of the central bulge. An Sa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, an Sc galaxy has open, well-defined arms and a small core region. A galaxy with poorly defined arms

3154-545: The IAU (1964) System of Astronomical Constants, used from 1968 to 1983. The product of Simon Newcomb 's J1900.0 mean tropical year of 31 556 925 .9747 ephemeris seconds and a speed of light of 299 792 .5 km/s produced a light-year of 9.460 530 × 10  m (rounded to the seven significant digits in the speed of light) found in several modern sources was probably derived from an old source such as C. W. Allen 's 1973 Astrophysical Quantities reference work, which

3237-583: The IAU for light-year is "ly", International standards like ISO 80000:2006 (now superseded) have used "l.y." and localized abbreviations are frequent, such as "al" in French, Spanish, and Italian (from année-lumière , año luz and anno luce , respectively), "Lj" in German (from Lichtjahr ), etc. Before 1984, the tropical year (not the Julian year) and a measured (not defined) speed of light were included in

3320-655: The IC ( Index Catalogue ), the CGCG ( Catalogue of Galaxies and of Clusters of Galaxies ), the MCG ( Morphological Catalogue of Galaxies ), the UGC ( Uppsala General Catalogue of Galaxies), and the PGC ( Catalogue of Principal Galaxies , also known as LEDA). All the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 (or "M109")

3403-649: The Korath in battle. Now none survive at all. However, there is something mysterious about the Korath. They do not resemble normal Drengin species, and without slavelings the Drengin Empire is weaker, for genocide is not their way. You, as the Drengin Dark Avatar, are sent to enslave the universe, find out the truth about the Korath, and stop the extermination of it. However, as the Drengin warlords begin to turn on each other, another faction called

Galactic Civilizations II: Dark Avatar - Misplaced Pages Continue

3486-515: The Krynn Consulate tries to convert the other races in a crusade to spread its religion. In the expansion, two races will die, two more will rise. On October 9, 2006, Stardock owner Brad Wardell announced that the release of Dark Avatar was pushed back to February 2007. As you guys gave us suggestions and new ideas we realized that we didn't want to have to wait until some future sequel to add in those features. So we're going to push

3569-608: The Large Magellanic Cloud in his Book of Fixed Stars , referring to "Al Bakr of the southern Arabs", since at a declination of about 70° south it was not visible where he lived. It was not well known to Europeans until Magellan 's voyage in the 16th century. The Andromeda Galaxy was later independently noted by Simon Marius in 1612. In 1734, philosopher Emanuel Swedenborg in his Principia speculated that there might be other galaxies outside that were formed into galactic clusters that were minuscule parts of

3652-536: The Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea . The word galaxy was borrowed via French and Medieval Latin from the Greek term for the Milky Way, galaxías (kúklos) γαλαξίας ( κύκλος ) 'milky (circle)', named after its appearance as a milky band of light in the sky. In Greek mythology , Zeus places his son, born by

3735-497: The Milky Way galaxy emerged. A few galaxies outside the Milky Way are visible on a dark night to the unaided eye , including the Andromeda Galaxy , Large Magellanic Cloud , Small Magellanic Cloud , and the Triangulum Galaxy . In the 10th century, Persian astronomer Abd al-Rahman al-Sufi made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud". In 964, he probably mentioned

3818-461: The Milky Way might consist of distant stars. Aristotle (384–322 BCE), however, believed the Milky Way was caused by "the ignition of the fiery exhalation of some stars that were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere , in the region of the World that is continuous with the heavenly motions ." Neoplatonist philosopher Olympiodorus

3901-431: The Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways. Toward the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae. In 1845, Lord Rosse examined the nebulae catalogued by Herschel and observed

3984-481: The Milky Way. Despite the prominence of large elliptical and spiral galaxies, most galaxies are dwarf galaxies. They are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, with only a few billion stars. Blue compact dwarf galaxies contains large clusters of young, hot, massive stars . Ultra-compact dwarf galaxies have been discovered that are only 100 parsecs across. Many dwarf galaxies may orbit

4067-402: The Milky Way. In 1926 Hubble produced a classification of galactic morphology that is used to this day. Advances in astronomy have always been driven by technology. After centuries of success in optical astronomy , infrared astronomy was attempted in the 1830s, but only blossomed in the early 1900s. Radio astronomy was born in the 1930s, and matured by the 1950s and 1960s. The problem

4150-488: The Sun was undertaken by William Herschel in 1785 by counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the Solar System close to the center . Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapley based on

4233-506: The Younger ( c.  495 –570 CE) was critical of this view, arguing that if the Milky Way was sublunary (situated between Earth and the Moon) it should appear different at different times and places on Earth, and that it should have parallax , which it did not. In his view, the Milky Way was celestial. According to Mohani Mohamed, Arabian astronomer Ibn al-Haytham (965–1037) made

SECTION 50

#1732797934916

4316-407: The active jets emitted from active nuclei. Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomena. Ultraviolet flares are sometimes observed when a star in a distant galaxy is torn apart from the tidal forces of a nearby black hole. The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of supermassive black holes at the cores of galaxies

4399-412: The cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the centre. Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane ; but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters , the present picture of

4482-749: The central mass is approximately 10 million solar masses , regardless of whether it has thousands or millions of stars. This suggests that galaxies are largely formed by dark matter , and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale. Interactions between galaxies are relatively frequent, and they can play an important role in galactic evolution . Near misses between galaxies result in warping distortions due to tidal interactions , and may cause some exchange of gas and dust. Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars of interacting galaxies usually do not collide, but

4565-442: The core, or else due to a tidal interaction with another galaxy. Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms. Our own galaxy, the Milky Way , is a large disk-shaped barred-spiral galaxy about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×10 ) stars and has a total mass of about six hundred billion (6×10 ) times

4648-399: The core, then merges into the spiral arm structure. In the Hubble classification scheme, these are designated by an SB , followed by a lower-case letter ( a , b or c ) which indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from

4731-564: The diameters of their host galaxies. Light year A light-year , alternatively spelled light year ( ly or lyr ), is a unit of length used to express astronomical distances and is equal to exactly 9 460 730 472 580 .8 km , which is approximately 5.88 trillion mi. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in vacuum in one Julian year (365.25 days). Despite its inclusion of

4814-476: The first attempt at observing and measuring the Milky Way's parallax, and he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere." Persian astronomer al-Biruni (973–1048) proposed the Milky Way galaxy was "a collection of countless fragments of the nature of nebulous stars." Andalusian astronomer Avempace ( d. 1138) proposed that it

4897-400: The formation of fossil groups or fossil clusters, where a large, relatively isolated, supergiant elliptical resides in the middle of the cluster and are surrounded by an extensive cloud of X-rays as the residue of these galactic collisions. Another older model posits the phenomenon of cooling flow , where the heated gases in clusters collapses towards their centers as they cool, forming stars in

4980-590: The galaxies' original morphology. If one of the galaxies is much more massive than the other, the result is known as cannibalism , where the more massive larger galaxy remains relatively undisturbed, and the smaller one is torn apart. The Milky Way galaxy is currently in the process of cannibalizing the Sagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy . Stars are created within galaxies from

5063-457: The game an A. Reviewers cited the significant increase in customization options, a better artificial intelligence , and an improvement in graphics quality as decisive factors. GameSpy later named it their Game of the Month for February 2007. Criticism revolved around "uninspired" ground combat, a generic technology tree , and the continued lack of multiplayer support. Galaxy A galaxy

SECTION 60

#1732797934916

5146-448: The gas and dust within the two forms interacts, sometimes triggering star formation. A collision can severely distort the galaxies' shapes, forming bars, rings or tail-like structures. At the extreme of interactions are galactic mergers, where the galaxies' relative momentums are insufficient to allow them to pass through each other. Instead, they gradually merge to form a single, larger galaxy. Mergers can result in significant changes to

5229-470: The higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars. A majority of spiral galaxies, including the Milky Way galaxy, have a linear, bar-shaped band of stars that extends outward to either side of

5312-513: The light-year is the product of the Julian year (365.25 days, as opposed to the 365.2425-day Gregorian year or the 365.24219-day Tropical year that both approximate) and the speed of light ( 299 792 458  m/s ). Both of these values are included in the IAU (1976) System of Astronomical Constants , used since 1984. From this, the following conversions can be derived: The abbreviation used by

5395-466: The mass in a typical galaxy is in the form of dark matter , with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies. Galaxies are categorised according to their visual morphology as elliptical , spiral , or irregular . The Milky Way is an example of a spiral galaxy. It is estimated that there are between 200 billion ( 2 × 10 ) to 2 trillion galaxies in

5478-421: The mass of the Sun. Recently, researchers described galaxies called super-luminous spirals. They are very large with an upward diameter of 437,000 light-years (compared to the Milky Way's 87,400 light-year diameter). With a mass of 340 billion solar masses, they generate a significant amount of ultraviolet and mid-infrared light. They are thought to have an increased star formation rate around 30 times faster than

5561-438: The mental picture of the approximate transit time for light, but he refrained from using the light-year as a unit. He may have resisted expressing distances in light-years because it would reduce the accuracy of his parallax data due to multiplying with the uncertain parameter of the speed of light. The speed of light was not yet precisely known in 1838; the estimate of its value changed in 1849 ( Fizeau ) and 1862 ( Foucault ). It

5644-474: The process, a phenomenon observed in clusters such as Perseus , and more recently in the Phoenix Cluster . A shell galaxy is a type of elliptical galaxy where the stars in its halo are arranged in concentric shells. About one-tenth of elliptical galaxies have a shell-like structure, which has never been observed in spiral galaxies. These structures are thought to develop when a larger galaxy absorbs

5727-506: The release date of the expansion pack to around the 1-year anniversary of the release of Galactic Civilizations II: Dread Lords New features include: Dark Avatar is being distributed by electronic download, however one has the option of ordering a disk as well to negate the need to re-download should it need to be uninstalled for whatever reason. Currently all purchase options are priced in US dollars, although international customers can pay using

5810-493: The same general area, such as those belonging to the same spiral arm or globular cluster . Galaxies themselves span from a few thousand to a few hundred thousand light-years in diameter, and are separated from neighbouring galaxies and galaxy clusters by millions of light-years. Distances to objects such as quasars and the Sloan Great Wall run up into the billions of light-years. Distances between objects within

5893-439: The sources in these two types of galaxies may differ. Radio galaxies can also be classified as giant radio galaxies (GRGs), whose radio emissions can extend to scales of megaparsecs (3.26 million light-years). Alcyoneus is an FR II class low-excitation radio galaxy which has the largest observed radio emission, with lobed structures spanning 5 megaparsecs (16×10 ly ). For comparison, another similarly sized giant radio galaxy

5976-522: The sphere of the fixed stars." Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study it and discovered it was composed of a huge number of faint stars. In 1750, English astronomer Thomas Wright , in his An Original Theory or New Hypothesis of the Universe , correctly speculated that it might be a rotating body of

6059-576: The spiral structure of Messier object M51 , now known as the Whirlpool Galaxy. In 1912, Vesto M. Slipher made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher discovered that the spiral nebulae have high Doppler shifts , indicating that they are moving at a rate exceeding the velocity of the stars he had measured. He found that the majority of these nebulae are moving away from us. In 1917, Heber Doust Curtis observed nova S Andromedae within

6142-498: The universe that extended far beyond what could be seen. These views "are remarkably close to the present-day views of the cosmos." In 1745, Pierre Louis Maupertuis conjectured that some nebula -like objects were collections of stars with unique properties, including a glow exceeding the light its stars produced on their own, and repeated Johannes Hevelius 's view that the bright spots were massive and flattened due to their rotation. In 1750, Thomas Wright correctly speculated that

6225-442: The universe's early history, but still contribute an estimated 15% to total star production. Starburst galaxies are characterized by dusty concentrations of gas and the appearance of newly formed stars, including massive stars that ionize the surrounding clouds to create H II regions . These stars produce supernova explosions, creating expanding remnants that interact powerfully with the surrounding gas. These outbursts trigger

6308-724: The word universe implied the entirety of existence. Instead, they became known simply as galaxies. Millions of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy , the Magellanic Clouds , the Whirlpool Galaxy , and the Sombrero Galaxy . Astronomers work with numbers from certain catalogues, such as the Messier catalogue , the NGC ( New General Catalogue ),

6391-442: The word "year", the term should not be misinterpreted as a unit of time . The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist contexts and popular science publications. The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years). As defined by the International Astronomical Union (IAU),

6474-416: Was composed of many stars that almost touched one another, and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects were near. In the 14th century, Syrian-born Ibn Qayyim al-Jawziyya proposed the Milky Way galaxy was "a myriad of tiny stars packed together in

6557-405: Was confirmed through X-ray astronomy. In 1944, Hendrik van de Hulst predicted that microwave radiation with wavelength of 21 cm would be detectable from interstellar atomic hydrogen gas; and in 1951 it was observed. This radiation is not affected by dust absorption, and so its Doppler shift can be used to map the motion of the gas in this galaxy. These observations led to the hypothesis of

6640-432: Was not yet considered to be a fundamental constant of nature, and the propagation of light through the aether or space was still enigmatic. The light-year unit appeared in 1851 in a German popular astronomical article by Otto Ule . Ule explained the oddity of a distance unit name ending in "year" by comparing it to a walking hour ( Wegstunde ). A contemporary German popular astronomical book also noticed that light-year

6723-493: Was that air only allows visible light and radio waves to pass, with a little bit of near infrared. The first ultraviolet telescope was launched in 1968, and since then there's been major progress in all regions of the electromagnetic spectrum . The dust present in the interstellar medium is opaque to visual light. It is more transparent to far-infrared , which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail. Infrared

6806-440: Was the astronomical unit , equal to the radius of the Earth's orbit at 150 million kilometres (93 million miles). In those terms, trigonometric calculations based on 61 Cygni's parallax of 0.314 arcseconds, showed the distance to the star to be 660 000 astronomical units (9.9 × 10  km; 6.1 × 10  mi). Bessel added that light takes 10.3 years to traverse this distance. He recognized that his readers would enjoy

6889-438: Was updated in 2000, including the IAU (1976) value cited above (truncated to 10 significant digits). Other high-precision values are not derived from a coherent IAU system. A value of 9.460 536 207 × 10  m found in some modern sources is the product of a mean Gregorian year (365.2425 days or 31 556 952  s ) and the defined speed of light ( 299 792 458  m/s ). Another value, 9.460 528 405 × 10  m ,

#915084