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58-657: The Omega Nebula is an H II region in the constellation Sagittarius . It was discovered by Philippe Loys de Chéseaux in 1745. Charles Messier catalogued it in 1764. It is by some of the richest starfields of the Milky Way , figuring in the northern two-thirds of Sagittarius. This feature is also known as the Swan Nebula , Checkmark Nebula , Lobster Nebula , and the Horseshoe Nebula , and catalogued as Messier 17 or M17 or NGC 6618 . The Omega Nebula

116-465: A cold molecular gas, which originated from the same parent GMC. Magnetic fields are produced by these weak moving electric charges in the ionised gas, suggesting that H II regions might contain electric fields . A number of H II regions also show signs of being permeated by a plasma with temperatures exceeding 10,000,000 K, sufficiently hot to emit X-rays. X-ray observatories such as Einstein and Chandra have noted diffuse X-ray emissions in

174-591: A large telescope, and nebulae that could be resolved into stars, now known to be galaxies external to our own. Confirmation of Herschel's hypothesis of star formation had to wait another hundred years, when William Huggins together with his wife Mary Huggins turned his spectroscope on various nebulae. Some, such as the Andromeda Nebula , had spectra quite similar to those of stars , but turned out to be galaxies consisting of hundreds of millions of individual stars. Others looked very different. Rather than

232-659: A moon of Saturn . In 1851 he discovered Ariel and Umbriel , two moons of Uranus . In 1855, he built a 48-inch (1,200 mm) telescope, which he installed in Malta because of the observing conditions that were better than in often-overcast England. While in Malta his astronomical observing assistant was Albert Marth . On his return to the UK after several years in Malta, he moved to Maidenhead and operated his 24-inch (610 mm) telescope in an observatory there. The 48-inch telescope

290-537: A number of star-forming regions, notably the Orion Nebula, Messier 17, and the Carina Nebula. The hot gas is likely supplied by the strong stellar winds from O-type stars, which may be heated by supersonic shock waves in the winds, through collisions between winds from different stars, or through colliding winds channeled by magnetic fields. This plasma will rapidly expand to fill available cavities in

348-538: A period of several million years, a cluster of stars will form in an H II region, before radiation pressure from the hot young stars causes the nebula to disperse. The precursor to an H II region is a giant molecular cloud (GMC). A GMC is a cold (10–20  K ) and dense cloud consisting mostly of molecular hydrogen . GMCs can exist in a stable state for long periods of time, but shock waves due to supernovae , collisions between clouds, and magnetic interactions can trigger its collapse. When this happens, via

406-526: A planetary system like the Solar System . H II regions vary greatly in their physical properties. They range in size from so-called ultra-compact (UCHII) regions perhaps only a light-year or less across, to giant H II regions several hundred light-years across. Their size is also known as the Stromgren radius and essentially depends on the intensity of the source of ionising photons and

464-452: A process of collapse and fragmentation of the cloud, stars are born (see stellar evolution for a lengthier description). As stars are born within a GMC, the most massive will reach temperatures hot enough to ionise the surrounding gas. Soon after the formation of an ionising radiation field, energetic photons create an ionisation front, which sweeps through the surrounding gas at supersonic speeds. At greater and greater distances from

522-413: A strong continuum with absorption lines superimposed, the Orion Nebula and other similar objects showed only a small number of emission lines . In planetary nebulae , the brightest of these spectral lines was at a wavelength of 500.7  nanometres , which did not correspond with a line of any known chemical element . At first it was hypothesized that the line might be due to an unknown element, which

580-462: A suspicion that some real change may have taken place in the relative brightness of this portion compared with the rest of the nebula; seeing that a figure of it made on June 25, 1837, expresses no such diffusion, but represents the arc as breaking off before it even attains fully to the group of small stars at the [western] angle of the Omega. … Under these circumstances the arguments for a real change in

638-528: A thousand stars in formation on its outer regions. It is also one of the youngest clusters known, with an age of just 1 million years. The luminous blue variable HD 168607 , in the south-east part of the nebula, is generally assumed to be associated with it; its close neighbor, the blue hypergiant HD 168625 , may be too. The Swan portion of M17, the Omega Nebula in the Sagittarius nebulosity

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696-612: Is actually a thin layer of ionised gas on the outer border of the OMC-1 cloud. The stars in the Trapezium cluster , and especially θ Orionis , are responsible for this ionisation. The Large Magellanic Cloud , a satellite galaxy of the Milky Way at about 50 kpc ( 160 thousand light years ), contains a giant H II region called the Tarantula Nebula . Measuring at about 200 pc ( 650 light years ) across, this nebula

754-464: Is also emitted, but at approximately 1/3 of the intensity of H-alpha. Most of the rest of an H II region consists of helium , with trace amounts of heavier elements. Across the galaxy, it is found that the amount of heavy elements in H ;II regions decreases with increasing distance from the galactic centre. This is because over the lifetime of the galaxy, star formation rates have been greater in

812-610: Is at high vacuum by laboratory standards. Physicists showed in the 1920s that in gas at extremely low density , electrons can populate excited metastable energy levels in atoms and ions , which at higher densities are rapidly de-excited by collisions. Electron transitions from these levels in doubly ionized oxygen give rise to the 500.7 nm line. These spectral lines , which can only be seen in very low density gases, are called forbidden lines . Spectroscopic observations thus showed that planetary nebulae consisted largely of extremely rarefied ionised oxygen gas (OIII). During

870-429: Is between 5,000 and 6,000 light-years from Earth and it spans some 15 light-years in diameter. The cloud of interstellar matter of which this nebula is a part is roughly 40 light-years in diameter and has a mass of 30,000 solar masses. The total mass of the Omega Nebula is an estimated 800 solar masses . It is considered one of the brightest and most massive star-forming regions of our galaxy. Its local geometry

928-557: Is credited with the discovery of the Orion Nebula in 1610. Since that early observation large numbers of H II regions have been discovered in the Milky Way and other galaxies. William Herschel observed the Orion Nebula in 1774, and described it later as "an unformed fiery mist, the chaotic material of future suns". In early days astronomers distinguished between "diffuse nebulae " (now known to be H II regions), which retained their fuzzy appearance under magnification through

986-407: Is said to resemble a barber's pole . The first attempt to accurately draw the nebula (as part of a series of sketches of nebulae) was made by John Herschel in 1833, and published in 1836. He described the nebula as such: The figure of this nebula is nearly that of a Greek capital omega , Ω, somewhat distorted , and very unequally bright. ... Messier perceived only the bright eastern branch of

1044-460: Is similar to the Orion Nebula except that it is viewed edge-on rather than face-on. The open cluster NGC 6618 lies embedded in the nebulosity and causes the gases of the nebula to shine due to radiation from these hot, young stars ; however, the actual number of stars in the nebula is much higher – up to 800, 100 of spectral type earlier than B9, and 9 of spectral type O , plus over

1102-490: Is sometimes confusion between the identical spoken forms of "H II" and "H 2 ". A few of the brightest H II regions are visible to the naked eye . However, none seem to have been noticed before the advent of the telescope in the early 17th century. Even Galileo did not notice the Orion Nebula when he first observed the star cluster within it (previously cataloged as a single star, θ Orionis, by Johann Bayer ). The French observer Nicolas-Claude Fabri de Peiresc

1160-481: Is the case for NGC 604 , a giant H II region in the Triangulum Galaxy . For a H II region which cannot be resolved , some information on the spatial structure (the electron density as a function of the distance from the center, and an estimate of the clumpiness) can be inferred by performing an inverse Laplace transform on the frequency spectrum. Notable Galactic H II regions include

1218-537: Is the most massive and the second-largest H II region in the Local Group . It is much bigger than the Orion Nebula, and is forming thousands of stars, some with masses of over 100 times that of the sun— OB and Wolf-Rayet stars . If the Tarantula Nebula were as close to Earth as the Orion Nebula, it would shine about as brightly as the full moon in the night sky. The supernova SN 1987A occurred in

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1276-420: Is there represented as too much elongated in a vertical direction and as bearing altogether too large a proportion to [the eastern] streak and to the total magnitude of the object. The nebulous diffusion, too, at the [western] end of that arc, forming the [western] angle and base-line of the capital Greek omega (Ω), to which the general figure of the nebula has been likened, is now so little conspicuous as to induce

1334-474: The spiral arms , while in irregular galaxies they are distributed chaotically. Some galaxies contain huge H II regions, which may contain tens of thousands of stars. Examples include the 30 Doradus region in the Large Magellanic Cloud and NGC 604 in the Triangulum Galaxy . The term H II is pronounced "H two" by astronomers. "H" is the chemical symbol for hydrogen, and "II" is

1392-461: The 20th century, observations showed that H II regions often contained hot, bright stars . These stars are many times more massive than the Sun, and are the shortest-lived stars, with total lifetimes of only a few million years (compared to stars like the Sun, which live for several billion years). Therefore, it was surmised that H II regions must be regions in which new stars were forming. Over

1450-647: The December 1946 Harvard Observatory Centennial Symposia that these globules were likely sites of star formation. It was confirmed in 1990 that they were indeed stellar birthplaces. The hot young stars dissipate these globules, as the radiation from the stars powering the H II region drives the material away. In this sense, the stars which generate H II regions act to destroy stellar nurseries. In doing so, however, one last burst of star formation may be triggered, as radiation pressure and mechanical pressure from supernova may act to squeeze globules, thereby enhancing

1508-485: The Omega Nebula. SOFIA's composite image revealed that blue areas (20 microns) near the center indicate gas heated by massive stars, while green areas (37 microns) trace dust warmed by massive stars and newborn stars. Nine previously unseen protostars were discovered primarily in the southern regions. Red areas near the edges represent cold dust detected by the Herschel Space Telescope (70 microns), and

1566-526: The Orion Nebula, the Eta Carinae Nebula , and the Berkeley 59 / Cepheus OB4 Complex . The Orion Nebula, about 500  pc (1,500 light-years) from Earth, is part of OMC-1 , a giant molecular cloud that, if visible, would be seen to fill most of the constellation of Orion . The Horsehead Nebula and Barnard's Loop are two other illuminated parts of this cloud of gas. The Orion Nebula

1624-473: The Roman numeral for 2. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionised—H II is H in other sciences—III for doubly-ionised, e.g. O III is O , etc. H II, or H , consists of free protons . An H I region consists of neutral atomic hydrogen, and a molecular cloud of molecular hydrogen, H 2 . In spoken discussion with non-astronomers there

1682-562: The death of his father, William Lassell was apprenticed to a merchant in Liverpool from 1814 to 1821. He later made his fortune as a beer brewer , which afforded him the means to pursue his passion for astronomy . He built an observatory at his house " Starfield " in West Derby , a suburb of Liverpool . There he had a 24-inch (610 mm) aperture metal mirror reflector telescope (aka the "two-foot" telescope), for which he pioneered

1740-403: The denser central regions, resulting in greater enrichment of those regions of the interstellar medium with the products of nucleosynthesis . H II regions are found only in spiral galaxies like the Milky Way and irregular galaxies . They are not seen in elliptical galaxies . In irregular galaxies, they may be dispersed throughout the galaxy, but in spirals they are most abundant within

1798-769: The density of the region. Their densities range from over a million particles per cm in the ultra-compact H II regions to only a few particles per cm in the largest and most extended regions. This implies total masses between perhaps 100 and 10 solar masses . There are also "ultra-dense H II" regions (UDHII). Depending on the size of an H II region there may be several thousand stars within it. This makes H II regions more complicated than planetary nebulae, which have only one central ionising source. Typically H II regions reach temperatures of 10,000 K. They are mostly ionised gases with weak magnetic fields with strengths of several nanoteslas . Nevertheless, H II regions are almost always associated with

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1856-399: The density within them. The young stars in H II regions show evidence for containing planetary systems. The Hubble Space Telescope has revealed hundreds of protoplanetary disks ( proplyds ) in the Orion Nebula. At least half the young stars in the Orion Nebula appear to be surrounded by disks of gas and dust, thought to contain many times as much matter as would be needed to create

1914-401: The distance from Earth to large H II regions is considerable, with the nearest H II ( California Nebula ) region at 300 pc (1,000 light-years); other H II regions are several times that distance from Earth. Secondly, the formation of these stars is deeply obscured by dust, and visible light observations are impossible. Radio and infrared light can penetrate the dust, but

1972-585: The gas inside it to millions of degrees, producing bright X-ray emissions. The total mass of the hot gas in NGC ;604 is about 6,000 Solar masses. As with planetary nebulae, estimates of the abundance of elements in H II regions are subject to some uncertainty. There are two different ways of determining the abundance of metals (metals in this case are elements other than hydrogen and helium) in nebulae, which rely on different types of spectral lines, and large discrepancies are sometimes seen between

2030-437: The gas is converted into stars rather than the normal rate of 10% or less. Galaxies undergoing such rapid star formation are known as starburst galaxies . The post-merger elliptical galaxy has a very low gas content, and so H II regions can no longer form. Twenty-first century observations have shown that a very small number of H II regions exist outside galaxies altogether. These intergalactic H II regions may be

2088-421: The hot young stars will eventually drive most of the gas away. In fact, the whole process tends to be very inefficient, with less than 10 percent of the gas in the H II region forming into stars before the rest is blown off. Contributing to the loss of gas are the supernova explosions of the most massive stars, which will occur after only 1–2 million years. Stars form in clumps of cool molecular gas that hide

2146-429: The ionising star, the ionisation front slows, while the pressure of the newly ionised gas causes the ionised volume to expand. Eventually, the ionisation front slows to subsonic speeds, and is overtaken by the shock front caused by the expansion of the material ejected from the nebula. The H II region has been born. The lifetime of an H II region is of the order of a few million years. Radiation pressure from

2204-407: The least suspicion of the existence of the fainter horseshoe arc attached to the [eastern] extremity of Messier's streak. Dr. Lamont has given a figure of this nebula, accompanied by a description. In this figure [our Fig. 4], the nebulous diffusion at the [western] angle and along the [western] base-line of the Omega is represented as very conspicuous; indeed, much more so than I can persuade myself it

2262-494: The material from which they are forming are often seen in silhouette against the rest of the ionised nebula. Bart Bok and E. F. Reilly searched astronomical photographs in the 1940s for "relatively small dark nebulae", following suggestions that stars might be formed from condensations in the interstellar medium; they found several such "approximately circular or oval dark objects of small size", which they referred to as "globules", since referred to as Bok globules . Bok proposed at

2320-590: The molecular clouds due to the high speed of sound in the gas at this temperature. It will also leak out through holes in the periphery of the H II region, which appears to be happening in Messier ;17. Chemically, H II regions consist of about 90% hydrogen. The strongest hydrogen emission line, the H-alpha line at 656.3 nm, gives H II regions their characteristic red colour. (This emission line comes from excited un-ionized hydrogen.) H-beta

2378-560: The most massive stars in the resulting star cluster disperse the gases of the H II region, leaving a cluster of stars which have formed. H II regions can be observed at considerable distances in the universe, and the study of extragalactic H II regions is important in determining the distances and chemical composition of galaxies . Spiral and irregular galaxies contain many H II regions, while elliptical galaxies are almost devoid of them. In spiral galaxies, including our Milky Way , H II regions are concentrated in

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2436-416: The nascent stars. It is only when the radiation pressure from a star drives away its 'cocoon' that it becomes visible. The hot, blue stars that are powerful enough to ionize significant amounts of hydrogen and form H II regions will do this quickly, and light up the region in which they just formed. The dense regions which contain younger or less massive still-forming stars and which have not yet blown away

2494-410: The nebula might seem to have considerable weight. Nevertheless, they are weakened or destroyed by a contrary testimony entitled to much reliance. Mr. Mason ... expressly states that both the nebulous knots were well seen by himself and his coadjutor Mr. Smith on August 1, 1839, i.e., two years subsequent to the date of my last drawing. Neither Mr. Mason, however, nor any other observer, appears to have had

2552-423: The nebula now in question, without any of the attached convolutions which were first noticed by my father. The chief peculiarities which I have observed in it are – 1. The resolvable knot in the eastern portion of the bright branch, which is, in a considerable degree, insulated from the surrounding nebula; strongly suggesting the idea of an absorption of the nebulous matter; and, 2. The much feebler and smaller knot at

2610-431: The northwestern end of the same branch, where the nebula makes a sudden bend at an acute angle . A second, more detailed sketch was made during his visit to South Africa in 1837. The nebula was also studied by Johann von Lamont and separately by an undergraduate at Yale College , Mr Mason, starting from around 1836. When Herschel published his 1837 sketch in 1847, he wrote: In particular the large horseshoe-shaped arc …

2668-577: The outskirts of the Tarantula Nebula. Another giant H II region— NGC 604 is located in M33 spiral galaxy, which is at 817 kpc (2.66 million light years). Measuring at approximately 240 × 250 pc ( 800 × 830 light years ) across, NGC 604 is the second-most-massive H II region in the Local Group after the Tarantula Nebula, although it is slightly larger in size than the latter. It contains around 200 hot OB and Wolf-Rayet stars, which heat

2726-403: The remnants of tidal disruptions of small galaxies, and in some cases may represent a new generation of stars in a galaxy's most recently accreted gas. H II regions come in an enormous variety of sizes. They are usually clumpy and inhomogeneous on all scales from the smallest to largest. Each star within an H II region ionises a roughly spherical region—known as a Strömgren sphere —of

2784-495: The results derived from the two methods. Some astronomers put this down to the presence of small temperature fluctuations within H II regions; others claim that the discrepancies are too large to be explained by temperature effects, and hypothesise the existence of cold knots containing very little hydrogen to explain the observations. The full details of massive star formation within H II regions are not yet well known. Two major problems hamper research in this area. First,

2842-559: The spiral arms. A large spiral galaxy may contain thousands of H II regions. The reason H II regions rarely appear in elliptical galaxies is that ellipticals are believed to form through galaxy mergers. In galaxy clusters , such mergers are frequent. When galaxies collide, individual stars almost never collide, but the GMCs and H II regions in the colliding galaxies are severely agitated. Under these conditions, enormous bursts of star formation are triggered, so rapid that most of

2900-429: The surrounding gas, but the combination of ionisation spheres of multiple stars within a H II region and the expansion of the heated nebula into surrounding gases creates sharp density gradients that result in complex shapes. Supernova explosions may also sculpt H II regions. In some cases, the formation of a large star cluster within an H II region results in the region being hollowed out from within. This

2958-474: The surrounding gas. H II regions—sometimes several hundred light-years across—are often associated with giant molecular clouds . They often appear clumpy and filamentary, sometimes showing intricate shapes such as the Horsehead Nebula . H II regions may give birth to thousands of stars over a period of several million years. In the end, supernova explosions and strong stellar winds from

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3016-551: The use of an equatorial mount for easy tracking of objects as the Earth rotates. He ground and polished the mirror himself, using equipment he constructed. The observatory was later (1854) moved further out of Liverpool, to Bradstones . In 1846, Lassell discovered Triton , the largest moon of Neptune , just 17 days after the discovery of Neptune itself by German astronomer Johann Gottfried Galle , using his self-built instrument. In 1848, he independently co-discovered Hyperion ,

3074-519: The white star field was observed by the Spitzer Space Telescope (3.6 microns). These observations suggest that parts of the nebula formed separately, contributing to its distinctive swan-like shape. H II region The regions may be of any shape because the distribution of the stars and gas inside them is irregular. The short-lived blue stars created in these regions emit copious amounts of ultraviolet light that ionize

3132-588: The youngest stars may not emit much light at these wavelengths . William Lassell William Lassell (18 June 1799 – 5 October 1880) was an English merchant and astronomer . He is remembered for his improvements to the reflecting telescope and his ensuing discoveries of four planetary satellites. William Lassell was born in Bolton , Lancashire, on 18 June 1799. He received his early education in Bolton and later attended Rochdale Academy.. After

3190-700: Was also a Fellow of the Royal Society of Literature (FRSL). He was furthermore elected an honorary Fellow of the Royal Society of Edinburgh (HonFRSE) and of the Society of Sciences of Upsala, and received an honorary LL.D. degree from the University of Cambridge in 1874. Lassell died in Maidenhead in 1880 and is buried at St. Luke's Church. Upon his death, he left a fortune of £80,000 (roughly equivalent to £10,100,000 in 2023). His telescope

3248-605: Was dismantled and was eventually scrapped. The 24-inch telescope was later moved to Royal Observatory, Greenwich in the 1880s, but eventually dismantled. Lassell was a Fellow of the Royal Astronomical Society (FRAS) from 1839, won the Gold Medal of the Royal Astronomical Society in 1849, and served as its president for two years starting in 1870. He was elected a Fellow of the Royal Society (FRS) in 1849 and won their Royal Medal in 1858. Lassell

3306-582: Was his intention it should appear. Sketches were also made by William Lassell in 1862 using his four-foot telescope at Malta , and by M. Trouvelot from Cambridge, Massachusetts , and Edward Singleton Holden in 1875 using the twenty-six inch Clark refractor at the United States Naval Observatory . In January 2020, the Stratospheric Observatory for Infrared Astronomy ( SOFIA ) provided new insights into

3364-534: Was named nebulium —a similar idea had led to the discovery of helium through analysis of the Sun 's spectrum in 1868. However, while helium was isolated on earth soon after its discovery in the spectrum of the sun, nebulium was not. In the early 20th century, Henry Norris Russell proposed that rather than being a new element, the line at 500.7 nm was due to a familiar element in unfamiliar conditions. Interstellar matter, considered dense in an astronomical context,

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