The Universe is all of space and time and their contents, including planets, stars and all other forms of matter and energy. While the spatial size of the entire Universe is unknown, it is possible to measure the size of the observable universe, estimated to be 93 billion light years in diameter. In various multiverse hypotheses, a universe is one of many causally disconnected constituent parts of a larger multiverse, which itself comprises all of space and time and its contents; the earliest scientific models of the Universe were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth at the center of the Universe. Over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the center of the Solar System. In developing the law of universal gravitation, Isaac Newton built upon Copernicus' work as well as observations by Tycho Brahe and Johannes Kepler's laws of planetary motion. Further observational improvements led to the realization that the Sun is one of hundreds of billions of stars in the Milky Way, one of at least hundreds of billions of galaxies in the Universe.
Many of the stars in our galaxy have planets. At the largest scale galaxies are distributed uniformly and the same in all directions, meaning that the Universe has neither an edge nor a center. At smaller scales, galaxies are distributed in clusters and superclusters which form immense filaments and voids in space, creating a vast foam-like structure. Discoveries in the early 20th century have suggested that the Universe had a beginning and that space has been expanding since and is still expanding at an increasing rate; the Big Bang theory is the prevailing cosmological description of the development of the Universe. Under this theory and time emerged together 13.799±0.021 billion years ago and the energy and matter present have become less dense as the Universe expanded. After an initial accelerated expansion called the inflationary epoch at around 10−32 seconds, the separation of the four known fundamental forces, the Universe cooled and continued to expand, allowing the first subatomic particles and simple atoms to form.
Dark matter gathered forming a foam-like structure of filaments and voids under the influence of gravity. Giant clouds of hydrogen and helium were drawn to the places where dark matter was most dense, forming the first galaxies and everything else seen today, it is possible to see objects that are now further away than 13.799 billion light-years because space itself has expanded, it is still expanding today. This means that objects which are now up to 46.5 billion light-years away can still be seen in their distant past, because in the past when their light was emitted, they were much closer to the Earth. From studying the movement of galaxies, it has been discovered that the universe contains much more matter than is accounted for by visible objects; this unseen matter is known as dark matter. The ΛCDM model is the most accepted model of our universe, it suggests that about 69.2%±1.2% of the mass and energy in the universe is a cosmological constant, responsible for the current expansion of space, about 25.8%±1.1% is dark matter.
Ordinary matter is therefore only 4.9% of the physical universe. Stars and visible gas clouds only form about 6% of ordinary matter, or about 0.3% of the entire universe. There are many competing hypotheses about the ultimate fate of the universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will be accessible; some physicists have suggested various multiverse hypotheses, in which our universe might be one among many universes that exist. The physical Universe is defined as all of their contents; such contents comprise all of energy in its various forms, including electromagnetic radiation and matter, therefore planets, stars and the contents of intergalactic space. The Universe includes the physical laws that influence energy and matter, such as conservation laws, classical mechanics, relativity; the Universe is defined as "the totality of existence", or everything that exists, everything that has existed, everything that will exist.
In fact, some philosophers and scientists support the inclusion of ideas and abstract concepts – such as mathematics and logic – in the definition of the Universe. The word universe may refer to concepts such as the cosmos, the world, nature; the word universe derives from the Old French word univers, which in turn derives from the Latin word universum. The Latin word was used by Cicero and Latin authors in many of the same senses as the modern English word is used. A term for "universe" among the ancient Greek philosophers from Pythagoras onwards was τὸ πᾶν, tò pân, defined as all matter and all space, τὸ ὅλον, tò hólon, which did not include the void. Another synonym was ho kósmos. Synonyms are found in Latin authors and survive in modern languages, e.g. the German words Das All and Natur for Universe. The same synonyms are found in English, such as everything, the cosmos, the world (as in the many-worlds interpr
A dwarf galaxy is a small galaxy composed of about 100 million up to several billion stars, a small number compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy. Dwarf galaxies' formation and activity are thought to be influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their composition. Current theory states that most galaxies, including dwarf galaxies, form in association with dark matter, or from gas that contains metals. However, NASA's Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases with low metallicity; these galaxies were located in the Leo Ring, a cloud of hydrogen and helium around two massive galaxies in the constellation Leo. Because of their small size, dwarf galaxies have been observed being pulled toward and ripped by neighbouring spiral galaxies, resulting in galaxy merger.
There are many dwarf galaxies in the Local Group. A 2007 paper has suggested that many dwarf galaxies were created by galactic tides during the early evolutions of the Milky Way and Andromeda. Tidal dwarf galaxies are produced when their gravitational masses interact. Streams of galactic material are pulled away from the parent galaxies and the halos of dark matter that surround them. A 2018 study suggests that some local dwarf galaxies formed early, during the Dark Ages within the first billion years after the big bang. More than 20 known dwarf galaxies orbit the Milky Way, recent observations have led astronomers to believe the largest globular cluster in the Milky Way, Omega Centauri, is in fact the core of a dwarf galaxy with a black hole at its centre, at some time absorbed by the Milky Way. Elliptical galaxy: dwarf elliptical galaxy Dwarf spheroidal galaxy: Once a subtype of dwarf ellipticals, now regarded as a distinct type Irregular galaxy: dwarf irregular galaxy Spiral galaxy: dwarf spiral galaxy Magellanic type dwarfs Blue compact dwarf galaxies Ultra-compact dwarf galaxies In astronomy, a blue compact dwarf galaxy is a small galaxy which contains large clusters of young, massive stars.
These stars, the brightest of which are blue, cause the galaxy. Most BCD galaxies are classified as dwarf irregular galaxies or as dwarf lenticular galaxies; because they are composed of star clusters, BCD galaxies lack a uniform shape. They consume gas intensely, which causes their stars to become violent when forming. BCD galaxies cool in the process of forming new stars; the galaxies' stars are all formed at different time periods, so the galaxies have time to cool and to build up matter to form new stars. As time passes, this star formation changes the shape of the galaxies. Nearby examples include NGC 1705, NGC 2915, NGC 3353 and UGCA 281. Ultra-compact dwarf galaxies are a class of compact galaxies with high stellar densities, discovered in the 2000s, they are thought to be on the order of 200 light years across. It is theorised that these are the cores of nucleated dwarf elliptical galaxies that have been stripped of gas and outlying stars by tidal interactions, travelling through the hearts of rich clusters.
UCDs have been found in the Virgo Cluster, Fornax Cluster, Abell 1689, the Coma Cluster, amongst others. In particular, an unprecedentedly large sample of ~ 100 UCDs has been found in the core region of the Virgo cluster by the Next Generation Virgo Cluster Survey team; the first relatively robust studies of the global properties of Virgo UCDs suggest that UCDs have distinct dynamical and structural properties from normal globular clusters. An extreme example of UCD is M60-UCD1, about 54 million light years away, which contains 200 million solar masses within a 160 light year radius. M59-UCD3 is the same size as M60-UCD1 with a half-light radius, rh, of 20 parsecs but is 40% more luminous with an apparent relative magnitude of −14.6. This makes M59-UCD3 the densest known galaxy. Based on stellar orbital velocities, two UCD in the Virgo Cluster are claimed to have supermassive black holes weighing 13% and 18% of the galaxies' masses. Galaxy morphological classification – System for categorizing galaxies based on appearance List of nearest galaxies Pea galaxy – Possibly a type of Luminous Blue Compact Galaxy, undergoing high rates of star formation Milky Way Satellite Galaxies SPACE.com article on "hobbit galaxies" Science article on "hobbit galaxies"
The Magellanic Clouds are two irregular dwarf galaxies visible in the Southern Celestial Hemisphere. Because both show signs of a bar structure, they are reclassified as Magellanic spiral galaxies; the two galaxies are: Large Magellanic Cloud 160,000 light-years away Small Magellanic Cloud 200,000 light years away The Magellanic Clouds have been known since the first millennium in Western Asia. The first preserved mention of the Large Magellanic Cloud is by the polymath Ibn Qutaybah, in his book on Al-Anwaa: "وأسفل من سهيل قدما سهيل. وفى مجرى قدمى سهيل، من خلفهما كواكب زهر كبار، لا ترى بالعراق، يسميها أهل تهامة الأعبار And below Canopus, there are the feet of Canopus, on their extension, behind them bright big stars, not seen in Iraq, the people of Tihama call them al-a`baar." Al Sufi, a professional astronomer, in 964 CE, in his Book of Fixed Stars, mentioned the same quote, but with a different spelling. Under Argo Navis, he quoted that "unnamed others have claimed that beneath Canopus there are two stars known as the'feet of Canopus', beneath those there are bright white stars that are unseen in Iraq nor Najd, that the inhabitants of Tihama call them al-Baqar, Ptolemy did not mention any of this so we do not know if this is true or false.".
Both Ibn Qutaybah and Al-Sufi were quoting from the former's contemporary and famed scientist Abu Hanifa Dinawari's lost work on Anwaa. Abu Hanifa was quoting earlier sources, which may be just travelers stories, hence Al-Sufi's comments about their veracity. In Sri Lanka, from ancient times, these clouds have been referred to as the Maha Mera Paruwathaya meaning "the great mountain", as they look like the peaks of a distant mountain range. In Europe, the Clouds were first reported by 16th century Italian authors Peter Martyr d'Anghiera and Andrea Corsali, both based on Portuguese voyages. Subsequently, they were reported by Antonio Pigafetta, who accompanied the expedition of Ferdinand Magellan on its circumnavigation of the world in 1519–1522. However, naming the clouds after Magellan did not become widespread until much later. In Bayer's Uranometria they are designated as nubecula major and nubecula minor. In the 1756 star map of the French astronomer Lacaille, they are designated as le Grand Nuage and le Petit Nuage.
Herschel in 1847 from Cape Observatory South Africa spent 4 years writing a 400-page report detailing over a thousand of the many stars and clusters which constitute the cloud which appeared to be a separate more distant group to the usual stars in the Milky Way, an early indication of separate galaxy. The Large Magellanic Cloud and its neighbour and relative, the Small Magellanic Cloud, are conspicuous objects in the southern hemisphere, looking like separated pieces of the Milky Way to the naked eye. 21° apart in the night sky, the true distance between them is 75,000 light-years. Until the discovery of the Sagittarius Dwarf Elliptical Galaxy in 1994, they were the closest known galaxies to our own; the LMC lies about 160,000 light years away, while the SMC is around 200,000. The LMC is about twice the diameter of the SMC. For comparison, the Milky Way is about 100,000 ly across; the total mass of these two galaxies is uncertain. Only a fraction of their gas seems to have coalesced into stars and they both have large dark matter halos.
One recent estimate of the total mass of the LMC is about 1/10 that of the Milky Way. That would make the LMC rather a large galaxy in the current observable universe. Since the size of nearby galaxies are skewed, the average mass can be a misleading statistic. In terms of rank, the LMC appears to be the fourth most massive member of over 50 galaxies in the local group. Suggesting that the Magellanic cloud system is not a part of the Milky Way is evidence that the SMC has been in orbit about the LMC for a long time; the Magellanic system seems most similar to the distinct NGC 3109 system, on the edge of the Local Group. Astronomers have long assumed that the Magellanic Clouds have orbited the Milky Way at their current distances, but evidence suggests that it is rare for them to come as close to the Milky Way as they are now. Observation and theoretical evidence suggest that the Magellanic Clouds have both been distorted by tidal interaction with the Milky Way as they travel close to it; the LMC maintains a clear spiral structure in radio-telescope images of neutral hydrogen.
Streams of neutral hydrogen connect them to the Milky Way and to each other, both resemble disrupted barred spiral galaxies. Their gravity has affected the Milky Way as well. Aside from their different structure and lower mass, they differ from our galaxy in two major ways. First, they are gas-rich, they are more metal-poor than the Milky Way. Both are noted for their nebulae and young stellar populations, but as in our own galaxy their stars range from the young to the old, indicating a long stellar formation history; the Large Magellanic Cloud was host galaxy to a supernova, the brightest observed in over four centuries. Measurements with the Hubble Space Telescope
The parsec is a unit of length used to measure large distances to astronomical objects outside the Solar System. A parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond, which corresponds to 648000/π astronomical units. One parsec is equal to 31 trillion kilometres or 19 trillion miles; the nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun. Most of the stars visible to the unaided eye in the night sky are within 500 parsecs of the Sun; the parsec unit was first suggested in 1913 by the British astronomer Herbert Hall Turner. Named as a portmanteau of the parallax of one arcsecond, it was defined to make calculations of astronomical distances from only their raw observational data quick and easy for astronomers. For this reason, it is the unit preferred in astronomy and astrophysics, though the light-year remains prominent in popular science texts and common usage. Although parsecs are used for the shorter distances within the Milky Way, multiples of parsecs are required for the larger scales in the universe, including kiloparsecs for the more distant objects within and around the Milky Way, megaparsecs for mid-distance galaxies, gigaparsecs for many quasars and the most distant galaxies.
In August 2015, the IAU passed Resolution B2, which, as part of the definition of a standardized absolute and apparent bolometric magnitude scale, mentioned an existing explicit definition of the parsec as 648000/π astronomical units, or 3.08567758149137×1016 metres. This corresponds to the small-angle definition of the parsec found in many contemporary astronomical references; the parsec is defined as being equal to the length of the longer leg of an elongated imaginary right triangle in space. The two dimensions on which this triangle is based are its shorter leg, of length one astronomical unit, the subtended angle of the vertex opposite that leg, measuring one arc second. Applying the rules of trigonometry to these two values, the unit length of the other leg of the triangle can be derived. One of the oldest methods used by astronomers to calculate the distance to a star is to record the difference in angle between two measurements of the position of the star in the sky; the first measurement is taken from the Earth on one side of the Sun, the second is taken half a year when the Earth is on the opposite side of the Sun.
The distance between the two positions of the Earth when the two measurements were taken is twice the distance between the Earth and the Sun. The difference in angle between the two measurements is twice the parallax angle, formed by lines from the Sun and Earth to the star at the distant vertex; the distance to the star could be calculated using trigonometry. The first successful published direct measurements of an object at interstellar distances were undertaken by German astronomer Friedrich Wilhelm Bessel in 1838, who used this approach to calculate the 3.5-parsec distance of 61 Cygni. The parallax of a star is defined as half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun. Equivalently, it is the subtended angle, from that star's perspective, of the semimajor axis of the Earth's orbit; the star, the Sun and the Earth form the corners of an imaginary right triangle in space: the right angle is the corner at the Sun, the corner at the star is the parallax angle.
The length of the opposite side to the parallax angle is the distance from the Earth to the Sun (defined as one astronomical unit, the length of the adjacent side gives the distance from the sun to the star. Therefore, given a measurement of the parallax angle, along with the rules of trigonometry, the distance from the Sun to the star can be found. A parsec is defined as the length of the side adjacent to the vertex occupied by a star whose parallax angle is one arcsecond; the use of the parsec as a unit of distance follows from Bessel's method, because the distance in parsecs can be computed as the reciprocal of the parallax angle in arcseconds. No trigonometric functions are required in this relationship because the small angles involved mean that the approximate solution of the skinny triangle can be applied. Though it may have been used before, the term parsec was first mentioned in an astronomical publication in 1913. Astronomer Royal Frank Watson Dyson expressed his concern for the need of a name for that unit of distance.
He proposed the name astron, but mentioned that Carl Charlier had suggested siriometer and Herbert Hall Turner had proposed parsec. It was Turner's proposal. In the diagram above, S represents the Sun, E the Earth at one point in its orbit, thus the distance ES is one astronomical unit. The angle SDE is one arcsecond so by definition D is a point in space at a distance of one parsec from the Sun. Through trigonometry, the distance SD is calculated as follows: S D = E S tan 1 ″ S D ≈ E S 1 ″ = 1 au 1 60 × 60 × π
An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, they are chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. Collectively they are thought to make up about a quarter of all galaxies; some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force. Irregular galaxies may contain abundant amounts of dust; this is not true for dwarf irregulars. Irregular galaxies are small, about one tenth the mass of the Milky Way galaxy. Due to their small sizes, they are prone to environmental effects like crashing with large galaxies and intergalactic clouds. There are three major types of irregular galaxies: An Irr-I galaxy is an irregular galaxy that features some structure but not enough to place it cleanly into the Hubble sequence. Subtypes with some spiral structure are called Sm galaxies Subtypes without spiral structure are called Im galaxies.
An Irr-II galaxy is an irregular galaxy that does not appear to feature any structure that can place it into the Hubble sequence. A dI-galaxy is a dwarf irregular galaxy; this type of galaxy is now thought to be important to understand the overall evolution of galaxies, as they tend to have a low level of metallicity and high levels of gas, are thought to be similar to the earliest galaxies that populated the Universe. They may represent a local version of the faint blue galaxies known to exist in deep field galaxy surveys; some of the irregular galaxies of the Magellanic type, are small spiral galaxies that are being distorted by the gravity of a larger neighbor. The Magellanic Cloud galaxies were once classified as irregular galaxies; the Large Magellanic Cloud has since been re-classified as type SBm a type of barred spiral galaxy, the barred Magellanic spiral type. The Small Magellanic Cloud remains classified as an irregular galaxy of type Im under current Galaxy morphological classification, although it does contain a bar structure.
Dwarf galaxy Dwarf elliptical galaxy Peculiar galaxy Galaxy morphological classification Irregular galaxy at Encyclopædia Britannica
The Orion Arm is a minor spiral arm of the Milky Way some 3,500 light-years across and 10,000 light-years in length, containing the Solar System, including the Earth. It is referred to by its full name, the Orion–Cygnus Arm, as well as Local Arm, Orion Bridge, the Local Spur and Orion Spur; the arm is named for the Orion constellation, one of the most prominent constellations of Northern Hemisphere winter. Some of the brightest stars and most famous celestial objects of the constellation are within it as shown on the interactive map below; the arm is between the Carina -- the Perseus Arm. Long thought to be a minor structure, namely a "spur" between the two arms mentioned, evidence was presented in mid 2013 that the Orion Arm might be a branch of the Perseus Arm, or an independent arm segment. Within the arm, the Solar System is close to its inner rim, in a relative cavity in the arm's Interstellar Medium known as the Local Bubble, about halfway along the arm's length 8,000 parsecs from the Galactic Center.
The Orion Arm contains a number of Messier objects: The Butterfly Cluster The Ptolemy Cluster Open Cluster M23 Open Cluster M25 The Dumbbell Nebula Open Cluster M29 Open Cluster M34 Open Cluster M35 Open Cluster M39 Winnecke 4 Open Cluster M41 The Orion Nebula The De Mairan's Nebula The Beehive Cluster The Pleiades Open Cluster M46 Open Cluster M47 Open Cluster M48 Open Cluster M50 The Ring Nebula Open Cluster M67 M73 The Little Dumbbell Nebula Diffuse Nebula M78 Open Cluster M93 The Owl Nebula Messier Objects in the Milky Way A 3D map of the Milky Way Galaxy
The Carina–Sagittarius Arm is thought to be a minor spiral arm of the Milky Way galaxy. Each spiral arm is a diffuse curving streamer of stars that radiates from the galactic center; these gigantic structures are composed of billions of stars and thousands of gas clouds. The Carina–Sagittarius Arm is one of the most pronounced arms in our galaxy as a large number of HII regions, young stars and giant molecular clouds are concentrated in it; the Milky Way is a barred spiral galaxy, consisting of a central crossbar and bulge from which two major and several minor spiral arms radiate outwards. The Carina–Sagittarius Arm lies between two major spiral arms—the Scutum–Centaurus Arm the near part of, visible looking inward i.e. toward the galactic centre with the rest beyond the galactic centre and the Perseus Arm, similar in size and shape but locally positioned outward. It is named for its proximity to the Sagittarius and Carina constellations as seen in the night sky from Earth, in the direction of the galactic center.
The arm dissipates near its middle, shortly after reaching its maximal angle, viewed from our solar system, from the galactic centre of about 80°. Extending from the galaxy's central bar is the Sagittarius Arm. Beyond the dissipated zone it is the Carina Arm. In 2008, infrared observations with the Spitzer Space Telescope showed that the Carina–Sagittarius Arm has a relative paucity of young stars, in contrast with the Scutum-Centaurus Arm and Perseus Arm; this suggests. These two appear to be concentrations of gas, sparsely sprinkled with pockets of newly formed stars. A number of Messier objects and other objects visible through an amateur's telescope or binoculars are found in the Sagittarius Arm: M11, the Wild Duck Cluster in Scutum Open Cluster M26 in Scutum M16, the Eagle Nebula in Serpens M17, the Omega Nebula in Sagittarius Open Cluster M18 in Sagittarius Globular Cluster M55 in Sagittarius M24, the Sagittarius Star Cloud Open Cluster M21 in Sagittarius M8, the Lagoon Nebula in Sagittarius NGC 3372, the Carina Nebula in Carina http://members.fcac.org/~sol/chview/chv5.htm Messier Objects in the Milky Way