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This is a composite image of the central region of our Milky Way galaxy. Credit: NASA/JPL-Caltech/ESA/CXC/STScI.

Any considerable and connected part of a space or surface studied in astronomy is an entity in regional astronomy.


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Def. any "considerable and connected part of a space or surface; specifically, a tract of land or sea of considerable but indefinite extent; a country; a district; in a broad sense, a place without special reference to location or extent but viewed as an entity for geographical, social or cultural reasons"[1] is called a region.

Theoretical regional astronomy

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Galactic-Center region

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At the top of this article is a composite image. "In celebration of the International Year of Astronomy 2009, NASA's Great Observatories -- the Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray Observatory -- have produced a matched trio of images of the central region of our Milky Way galaxy. Each image shows the telescope's different wavelength view of the galactic center region, illustrating the unique science each observatory conducts."[2]

"In this spectacular image, observations using infrared light and X-ray light see through the obscuring dust and reveal the intense activity near the galactic core. Note that the center of the galaxy is located within the bright white region to the right of and just below the middle of the image. The entire image width covers about one-half a degree, about the same angular width as the full moon."[2]

X-ray astronomy

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This is a Chandra X-ray Observatory image of the Galactic Central region. Credit: NASA/CXC.
This is a 400 by 900 light-year mosaic of several Chandra X-ray Observatory images of the Galactic center region. Credit: NASA/UMass/D. Wang et al.

“From August 23 through September 24, 2003, the INTEGRAL Observatory conducted a deep survey of the Galactic-Center region with a record-breaking sensitivity at energies above 20 keV.”[3]

At right is an image of the Galactic central region using the Chandra X-ray Observatory.

"X-rays detected by Chandra expose a wealth of exotic objects and high-energy features [Figure 3 (bottom frame of poster)]. In this image, pink represents lower energy X-rays and blue indicates higher energy. Hundreds of small dots show emission from material around black holes and other dense stellar objects. A supermassive black hole -- some four million times more massive than the Sun -- resides within the bright region in the lower right. The diffuse X-ray light comes from gas heated to millions of degrees by outflows from the supermassive black hole, winds from giant stars, and stellar explosions. This central region is the most energetic place in our galaxy."[2]

The second image at right is a "400 by 900 light-year mosaic of several Chandra images of the central region of our Milky Way galaxy ... [It] reveals hundreds of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog of multimillion-degree gas. The supermassive black hole at the center of the galaxy is located inside the bright white patch in the center of the image. The colors indicate X-ray energy bands - red (low), green (medium), and blue (high)."[4]

Visual astronomy

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This is a visible image of the Galactic Central region from the Hubble Space Telescope. Credit: NASA/ESA/STScI.

"Although best known for its visible-light images, Hubble also observes over a limited range of infrared light [Figure 2 (middle frame of poster)]. The galactic center is marked by the bright patch in the lower right. Along the left side are large arcs of warm gas that have been heated by clusters of bright massive stars. In addition, Hubble uncovered many more massive stars across the region. Winds and radiation from these stars create the complex structures seen in the gas throughout the image.This sweeping panorama is one of the sharpest infrared pictures ever made of the galactic center region."[2]

Infrared astronomy

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This is an infrared image of the Galactic Central region using the Spitzer Space Telescope. Credit: NASA/JPL-Caltech.

"Spitzer's infrared-light observations provide a detailed and spectacular view of the galactic center region [Figure 1 (top frame of poster)]. The swirling core of our galaxy harbors hundreds of thousands of stars that cannot be seen in visible light. These stars heat the nearby gas and dust. These dusty clouds glow in infrared light and reveal their often dramatic shapes. Some of these clouds harbor stellar nurseries that are forming new generations of stars. Like the downtown of a large city, the center of our galaxy is a crowded, active, and vibrant place."[2]

Radio astronomy

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This is a radio image of the central region of the Milky Way galaxy. Credit: NRL/SBC Galactic Center Radio Group.

At right is a radio image of the central region of the Milky Way galaxy. The arrow indicates a supernova remnant which is the location of a newly-discovered transient, the bursting low-frequency radio source GCRT J1745-3009.

Active galactic nucleus

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This image of Messier 100 is from the NASA/ESA Hubble Space Telescope. Credit: ESA/Hubble & NASA.
This is a colour-composite image of the central 5,500 light-years wide region of the spiral galaxy NGC 1097, obtained with the NACO adaptive optics on the VLT. Credit: European Southern Observatory.

"This [visual] image [at right] from the NASA/ESA Hubble Space Telescope, the most detailed made to date, shows the bright core of the galaxy and the innermost parts of its spiral arms. Messier 100 has an active galactic nucleus — a bright region at the galaxy’s core caused by a supermassive black hole that is actively swallowing material, which radiates brightly as it falls inwards."[5] Bold added.

"Messier 100 is a perfect example of a grand design spiral galaxy, a type of galaxy with prominent and very well-defined spiral arms. These dusty structures swirl around the galaxy’s nucleus, and are marked by a flurry of star formation activity that dots Messier 100 with bright blue, high-mass stars."[5]

"The galaxy’s spiral arms also host smaller black holes, including the youngest ever observed in our cosmic neighbourhood, the result of a supernova observed in 1979."[5]

"Messier 100 is located in the direction of the constellation of Coma Berenices, about 50 million light-years distant."[5]

"This image, taken with the high resolution channel of Hubble’s Advanced Camera for Surveys demonstrates the continued evolution of Hubble’s capabilities over two decades in orbit. This image, like all high resolution channel images, has a relatively small field of view: only around 25 by 25 arcseconds."[5]

The visual data is centered at 555 nm (blue), the visual + infrared is in green, and additional infrared centered at 814 nm is red.[5]

At lower right is a "[c]olour-composite image of the central 5,500 light-years wide region of the spiral galaxy NGC 1097 [45 million light years away], obtained with the NACO adaptive optics on the VLT. More than 300 star forming regions - white spots in the image - are distributed along a ring of dust and gas in the image. At the centre of the ring there is a bright central source where the active galactic nucleus and its super-massive black hole are located. The image was constructed by stacking J- (blue), H- (green), and Ks-band (red) [infrared] images. North is up and East is to the left. The field of view is 24 x 29 arcsec2, i.e. less than 0.03% the size of the full moon!"[6]

Galactic halo

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The NASA/ESA Hubble Space Telescope has produced this beautiful image of the globular cluster Messier 56 (also known as M 56 or NGC 6779). Credit: NASA & ESA. Acknowledgement: Gilles Chapdelaine.
This NASA/ESA Hubble Space Telescope image shows a compact and distant globular star cluster that lies in one of the smallest constellations in the night sky, Delphinus (The Dolphin). Credit: ESA/Hubble & NASA.

"The NASA/ESA Hubble Space Telescope has produced this beautiful image [at right] of the globular cluster Messier 56 (also known as M 56 or NGC 6779), which is located about 33 000 light years away from the Earth in the constellation of Lyra (The Lyre). The cluster is composed of a large number of stars, tightly bound to each other by gravity."[7]

"Astronomers typically infer important properties of globular clusters by looking at the light of their constituent stars. But they have to be very careful when they observe objects like Messier 56, which is located close to the Galactic plane. This region is crowded by “field-stars”, in other words, stars in the Milky Way that happen to lie in the same direction but do not belong to the cluster. These objects can contaminate the light, and hence undermine the conclusions reached by astronomers."[7] Bold added.

"A tool often used by scientists for studying stellar clusters is the colour-magnitude (or Hertzsprung-Russell) diagram. This chart compares the brightness and colour of stars – which in turn, tells scientists what the surface temperature of a star is."[7]

"By comparing high quality observations taken with the Hubble Space Telescope with results from the standard theory of stellar evolution, astronomers can characterise the properties of a cluster. In the case of Messier 56, this includes its age, which at 13 billion years is approximately three times the age of the Sun. Furthermore, they have also been able to study the chemical composition of Messier 56. The cluster has relatively few elements heavier than hydrogen and helium, typically a sign of stars that were born early in the Universe’s history, before many of the elements in existence today were formed in significant quantities."[7]

"Astronomers have found that the majority of clusters with this type of chemical makeup lie along a plane in the Milky Way’s halo. This suggests that such clusters were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system as had been previously thought."[7]

"This image consists of visible [blue, centered at 606 nm] and near-infrared [red, centered at 814 nm] exposures from Hubble’s Advanced Camera for Surveys. The field of view is approximately 3.3 by 3.3 arcminutes."[7]

At lower right the "NASA/ESA Hubble Space Telescope image shows a compact and distant globular star cluster that lies in one of the smallest constellations in the night sky, Delphinus (The Dolphin). Due to its modest size, great distance and relatively low brightness, NGC 7006 is often ignored by amateur astronomers. But even remote globular clusters such as this one appear bright and clear when imaged by Hubble’s Advanced Camera for Surveys."[8] The visual portion is centered at 606 nm (blue), a visual + infrared is green, and the infrared is centered at 814 nm (red).[8]

"NGC 7006 resides in the outskirts of the Milky Way. It is about 135 000 light-years away, five times the distance between the Sun and the centre of the galaxy, and it is part of the galactic halo. This roughly spherical region of the Milky Way is made up of dark matter, gas and sparsely distributed stellar clusters."[8] Bold added.

"Like other remote globular clusters, NGC 7006 provides important clues that help astronomers to understand how stars formed and assembled in the halo. The cluster now pictured by Hubble has a very eccentric orbit indicating that it may have formed independently, in a small galaxy outside our own that was then captured by the Milky Way."[8]

"Although NGC 7006 is very distant for a Milky Way globular cluster, it is much closer than the many faint galaxies that can be seen in the background of this image. Each of these faint smudges is probably accompanied by many globular clusters similar to NGC 7006 that are too faint to be seen even by Hubble."[8]

"This image was taken using the Wide Field Channel of the Advanced Camera for Surveys, in a combination of visible and near-infrared light. The field of view is a little over 3 by 3 arcminutes."[8]

Local Void

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This is a Hubble Space Telescope image of NGC 6503, which sits at the edge of a giant, hollowed-out region of space called the Local Void. Credit: ESA/Hubble and NASA.
Map shows voids and superclusters within 500 million light years from Milky Way, where the Local Void is in the yellow circle. Credit: Richard Powell.{{free media}}
A rendition of the area surrounding the Local Void, with our galaxy at the center of the three arrows. Credit: R. Brent Tully and Richard Fisher.{{fairuse}}

"NGC 6503 sits at the edge of a giant, hollowed-out region of space called the Local Void. The Hercules and Coma galaxy clusters, as well as our own Local Group of galaxies, circumscribe this vast, sparsely populated region. Estimates for the void’s diameter vary from 30 million to more than 150 million light-years — so NGC 6503 does not have a lot of galactic company in its immediate vicinity."[9]

The Local Void is a vast, empty region of space, lying adjacent to our own Local Group.[10][11] The Local Void is now known to be composed of three separate sectors, separated by bridges of "wispy filaments".[11] The precise extent of the void is unknown, but it is at least 150 million light years across[12] and may have a long dimension of up to 70 Mpc (230 million light years).[11] The Local Void also appears to have significantly fewer galaxies than expected from standard cosmology.[13]

The Milky Way sits in a large, flat array of galaxies called the Local Sheet, which bounds the Local Void.[10] The Local Void extends approximately 60 megaparsecs, beginning at the edge of the Local Group.[14] It is believed that the distance from Earth to the centre of the Local Void must be at least 23 megaparsecs (75 Mly).[11]

The size of the Void was calculated due to an isolated dwarf galaxy located inside it.

The Void may be growing and the Local Sheet, which makes up one wall of the void, is rushing away from the void's centre at 260 kilometres per second.[15]

"Fresh starbirth infuses the galaxy NGC 6503 [at right] with a vital pink glow in this image from the NASA/ESA Hubble Space Telescope. This galaxy, a smaller version of the Milky Way, is perched near a great void in space where few other galaxies reside."[9]

The Local Void is surrounded uniformly by matter in all directions, except for one sector in which there is nothing.

The Milky Way's velocity away from the Local Void is 270 kilometres per second (600,000 mph).[10][12]

"This new image [at right] from Hubble’s Advanced Camera for Surveys displays, with particular clarity, the pink-coloured puffs marking where stars have recently formed in NGC 6503's swirling spiral arms. Although structurally similar to the Milky Way, the disc of NGC 6503 spans just 30 000 light-years, or just about a third of the size of the Milky Way, leading astronomers to classify NGC 6503 as a dwarf spiral galaxy."[9]

"NGC 6503 lies approximately 17 million light-years away in the constellation of Draco (the Dragon)."[9]

"This Hubble image was created from exposures taken with the Wide Field Channel of the Advanced Camera for Surveys. The filters were unusual, which explains the peculiar colour balance of this picture. The red colouration derives from a 28-minute exposure through a filter that just allows the emission from hydrogen gas ([H-alpha,] F658N [, 658 nm]) to pass and which reveals the glowing clouds of gas associated with star-forming regions. This was combined with a 12-minute exposure through a near-infrared filter (F814W) [814 nm], which was coloured blue for contrast. The field of view is 3.3 by 1.8 arcminutes."[9] A combination of H-alpha and infrared is also used and is green in color.[9]

While "the Milky Way is surrounded by other galaxies and cosmic structures, our galaxy also sits at the edge of a large, empty region."[16]

The "motion of 18,000 galaxies [was measured] to develop a map that shows the boundaries between where matter is present and where it is absent in order to outline the edge of the Local Void."[17]

The "Milky Way, our largest neighboring galaxy Andromeda and smaller surrounding galaxies deviate from the speed of expansion by 1.3 million mph (600 kilometers per second)."[17]

"Galaxies tend to move towards denser areas in the universe, pulled by the gravity of surrounding bodies in space, while moving away from the less populated regions. Therefore, [...] at least half of this deviation is a combination of the gravitational tug by the Virgo Cluster, a nearby cluster of galaxies, and the expansion of the Local Void as it grows emptier while the universe continues to expand."[18]

H I region

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This is a sky plot of all astronomical objects within the ±2° error circle of Centaurus XR-4. Credit: Aladin at SIMBAD.

An H I region is an interstellar cloud composed of neutral atomic hydrogen (H I), in addition to the local abundance of helium and other elements.

The degree of ionization in an H I region is very small at around 10−4 (i.e. one particle in 10,000). The temperature of an H I region is about 100 K,[19] and it is usually considered as isothermal, except near an expanding H II region.[20]

SIMBAD contains some 8,542 entries of the astronomical object type 'HI' (H I region, otype='HI').

These regions are non-luminous, save for emission of the 21-cm (1,420 MHz) region spectral line. Mapping H I emissions with a radio telescope is a technique used for determining the structure of spiral galaxies.

At right is a sky plot of all astronomical objects in the SIMBAD database within the ±2° error circle of the X-ray source Centaurus XR-4. The HI regions are symbolized by blue triangles.

H II regions

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An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place.

Visual astronomy

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This unique image shows AB7, one of the highest excitation nebulae in the Magellanic Clouds (MCs), two satellite galaxies of our own Milky Way. Credit: European Southern Observatory.

"This unique image shows AB7, one of the highest excitation nebulae in the Magellanic Clouds (MCs), two satellite galaxies of our own Milky Way. AB7 is a binary star, consisting of one WR-star — highly evolved massive star - and a mid-age massive companion of spectral type O. These exceptional stars have very strong stellar winds: they continuously eject energetic particles — like the "solar wind" from the Sun — but some 10 to 1,000 million times more intensely than our star! These powerful winds exert an enormous pressure on the surrounding interstellar material and forcefully shape those clouds into "bubbles", well visible in the photos by their blue colour. AB7 is particularly remarkable: the associated huge nebula and HeII region indicate that this star is one of the, if not the, hottest WR-star known so far, with a surface temperature in excess of 120,000 degrees ! Just outside this nebula, a small network of green filaments is visible — they are the remains of another supernova explosion."[21]

The blue features are from He III, the green from O III, and the red are from H I.[21]

Infrared astronomy

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This is an image of NGC 3603, the only massive, galactic "HII-region". Credit: ESO.

At right is an image from the European Southern Observatory.

"These images of the NGC 3603 region were obtained in three near-IR filter bands (Js [blue], H [green] and Ks [red]) with the ISAAC instrument at the ANTU telescope at the VLT at Paranal. NGC 3603 is located in the Carina spiral arm of the Milky Way galaxy at a distance of about 20,000 light-years (6-7 kpc). It is the only massive, galactic "HII-region" (so denoted by astronomers because part of its hydrogen is ionized) in which a central cluster of strongly UV-radiating stars of types O and B that ionize the nebula can be studied at visual and near-infrared wavelengths. This is because the line-of-sight is reasonably free of dust in this direction; the dimming in near-infrared radiation due to intervening matter between the nebula and us is only about a factor of 2 (compared to 80 in visible light). The total mass of the hot O and B stars in NGC 3603 is over 2,000 solar masses. Together, the more than fifty heavy and bright O stars in NGC 3603 have about 100 times the ionizing power of the well-known Trapezium cluster in the Orion Nebula. In fact, the star cluster in NGC 3603 is in many respects very similar to the core of the large, ionizing cluster in the approximately eight times more distant Tarantula Nebula in the Large Magellanic Cloud. An important conclusion of studying this region is that there are lots of sub-solar mass stars in NGC 3603. Contrary to several theoretical predictions, these low-mass stars do form in violent starbursts. The overall age of stars in the contraction phase that are located in the innermost region of NGC 3603 was found to be 300 000 to 1 000 000 years. The counts clearly show that this cluster is well populated in sub-solar mass stars."[22]

Molecule forming regions

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This is a colour-composite annotated image of part of the Galactic Plane seen by the ATLASGAL survey, divided into sections. Credit: ESO/APEX & MSX/IPAC/NASA.

The “[w]arm molecular envelope of M giants and Miras [is] a new molecule forming region”[23]

At right is a "[c]olour-composite annotated image of part of the Galactic Plane seen by the ATLASGAL survey, divided into sections."[24]

"The total size of the image is approximately 42 degrees by 1.75 degrees."[24]

"Some of the most prominent features visible in the image are (from left to right, top to bottom):"[24]

  • "Messier 20 (the Trifid Nebula): A nebula containing an open cluster of stars as well as a stellar nursery. The name “Trifid” refers to the way that dense dust appears to divide it into three lobes at visible wavelengths."[24]
  • "Sagittarius B2 (Sgr B2): One of the largest clouds of molecular gas in the Milky Way, this dense region lies close to the Galactic Centre and is rich in many different interstellar molecules."[24]
  • "Galactic Centre: The centre of the Milky Way, home to a supermassive black hole more than four million times the mass of our Sun. It is about 25 000 light-years from Earth."[24]
  • "NGC 6357: A diffuse nebula containing the open cluster Pismis 24, home to several very massive stars."[24]
  • "NGC 6334: An emission nebula also known as the “Cat’s Paw Nebula”."[24]
  • "RCW 120: A region where an expanding bubble of ionised gas about ten light-years across is causing the surrounding material to collapse into dense clumps that are the birthplaces of new stars."[24]
  • "The Norma Arm: The region of somewhat brighter emission extending over about 10 degrees on the right-hand side of the image corresponds to the position of the Norma Arm, one of the spiral arms of the Milky Way."[24]

Infrared astronomy

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In the image at the top of the "Molecular forming regions", the submillimetre-wavelength data is "overlaid on a view of the region in infrared light, from the Midcourse Space Experiment (MSX) in green and blue."[24]

Submillimeter astronomy

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In the image at the top of the "Molecular forming regions", "the ATLASGAL submillimetre-wavelength data are shown in red"[24]. The wavelength used is 870 µm.[24]

Nuclear transition regions

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Def. “the energy range from 10 keV to about 100 MeV, where nuclear-transition lines are expected”[25] is called a nuclear transition region.

Dusty regions

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This is a Hubble Space Telescope image of Arp 220. Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University).
RCW 120 is a region of hot gas and glowing dust. Credit: NASA/JPL-Caltech/GLIMPSE-MIPSGAL Teams.

"Arp 220 [at right] appears to be a single, odd-looking galaxy, but is in fact a nearby example of the aftermath of a collision between two spiral galaxies. It is the brightest of the three galactic mergers closest to Earth, about 250 million light-years away in the constellation of Serpens, the Serpent. The collision, which began about 700 million years ago, has sparked a cracking burst of star formation, resulting in about 200 huge star clusters in a packed, dusty region about 5,000 light-years across (about 5 percent of the Milky Way's diameter). The amount of gas in this tiny region equals the amount of gas in the entire Milky Way Galaxy. The star clusters are the bluish-white bright knots visible in the Hubble image. Arp 220 glows brightest in infrared light and is an ultra-luminous infrared galaxy. Previous Hubble observations, taken in the infrared at a wavelength that looks through the dust, have uncovered the cores of the parent galaxies 1,200 light-years apart. Observations with NASA s Chandra X-ray Observatory have also revealed X-rays coming from both cores, indicating the presence of two supermassive black holes. Arp 220 is the 220th galaxy in Arp's Atlas of Peculiar Galaxies."[26] Bold added.

At lower right is a "glowing emerald nebula seen by NASA's Spitzer Space Telescope ... Named RCW 120, this region of hot gas and glowing dust can be found in the murky clouds encircled by the tail of the constellation Scorpius. The ring of dust is actually glowing in infrared colors that our eyes cannot see, but show up brightly when viewed by Spitzer's infrared detectors. At the center of this ring are a couple of giant stars whose intense ultraviolet light has carved out the bubble, though they blend in with other stars when viewed in infrared."[27]

"The green ring is where dust is being hit by winds and intense light from the massive stars. The green color represents infrared light coming from tiny dust grains called polycyclic aromatic hydrocarbons. These small grains have been destroyed inside the bubble. The red color inside the ring shows slightly larger, hotter dust grains, heated by the massive stars."[27]

"This bubble is far from unique. ... Spitzer has found that such bubbles are common and can be found around O stars throughout our Milky Way galaxy. The small objects at the lower right area of the image may themselves be similar regions seen at much greater distances across the galaxy."[27]

"RCW 120 can be found slightly above the flat plane of our galaxy, located toward the bottom of the picture. The green haze seen here is the diffuse glow of dust from the galactic plane."[27]

"This is a three-color composite that shows infrared observations from two Spitzer instruments. Blue represents 3.6-micron light and green shows light of 8 microns, both captured by Spitzer's infrared array camera. Red is 24-micron light detected by Spitzer's multiband imaging photometer."[27]

Star-forming regions

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This image is a near-infrared, colour-coded composite image of a sky field in the south-western part of the galactic star-forming region Messier 17. Credit: European Southern Observatory.

A star-forming region is a region of the celestial sphere within which predominately very young stellar objects (YSOs) are located and their formation is likely occurring.

A discovery by the Herschel Space Observatory infrared telescope, in conjunction with other ground based telescopes, determined that black patches of space in certain areas encompassing a star formation are not dark nebulae but actually vast holes of empty space.

“The exact cause of this phenomenon is still being investigated, although it has been hypothesized that narrow jets of gas from some of the young stars in the region punctured the sheet of dust and gas, as well as, powerful radiation from a nearby mature star may have helped to create the hole.

This is a previously unknown and unexpected step in the star-forming process.[28]

This picture of the star formation region NGC 3582 was taken using the Wide Field Imager at ESO's La Silla Observatory in Chile. Credit: ESO, Digitized Sky Survey 2 and Joe DePasquale.

At right is an image of the star-forming region NGC 3582 taken primarily using red astronomy (red is colored red and H-alpha also colored red). Infrared is in blue and combined infrared + red is in green.[29]

"The image reveals giant loops of gas ejected by dying stars that bear a striking resemblance to solar prominences."[29]


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At top right "is a near-infrared, colour-coded composite image of a sky field in the south-western part of the galactic star-forming region Messier 17. In this image, young and heavily obscured stars are recognized by their red colour. Bluer objects are either foreground stars or well-developed massive stars whose intense light ionizes the hydrogen in this region. The diffuse light that is visible nearly everywhere in the photo is due to emission from hydrogen atoms that have (re-)combined from protons and electrons. The dark areas are due to obscuration of the light from background objects by large amounts of dust — this effect also causes many of those stars to appear quite red. A cluster of young stars in the upper-left part of the photo, so deeply embedded in the nebula that it is invisible in optical light, is well visible in this infrared image. Technical information : The exposures were made through three filtres, J (at wavelength 1.25 µm; exposure time 5 min; here rendered as blue), H (1.65 µm; 5 min; green) and Ks (2.2 µm; 5 min; red); an additional 15 min was spent on separate sky frames. The seeing was 0.5 - 0.6 arcsec. The objects in the uppermost left corner area appear somewhat elongated because of a colour-dependent aberration introduced at the edge by the large-field optics. The sky field shown measures approx. 5 x 5 arcmin 2 (corresponding to about 3% of the full moon). North is up and East is left."[30]

Stellar active regions

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This image from the TRACE satellite shows numerous flares from a stellar active region. Credit: NASA.

A stellar active region is "[a] localized, transient volume of [a stellar] atmosphere in which plages, [star]spots, faculae, flares, etc., may be observed. Active regions are the result of enhanced magnetic fields; they are bipolar and may be complex if the region contains two or more bipolar groups."[31]

A stellar active region on a star's surface can form a bright spot which intensifies and grows. An active region may have a coronal portion.

Most stellar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars are commonly called starspots and both light and dark spots have been measured.[32]

Active regions are highly dynamic sources of explosive events.

[Flickerings, brightenings, small explosions, bright points, flares and mass eruptions are observed very frequently, especially in active regions.

"Plages are formed in the inner parts of flux loops emerging from below. ... In the early stages of active region growth the appearance of the group is symmetric, while a few days later the f spot may disappear, leaving an extensive plage."[33]

"[M]ajor changes in active regions only take place in the following ways:

  1. [starspot] formation and break up;
  2. flux outflow from [starspots];
  3. new flux emergence; and
  4. magnetic reconnection."[33]

The interconnections of active regions are arcs connecting zones of opposite magnetic field, in different active regions. Significant variations of these structures are often seen after a flare. Some other features of this kind are helmet streamers—large cap-like coronal structures with long pointed peaks that usually overlie sunspots and active regions. Coronal streamers are considered as sources of the slow solar wind.[34]

Most coronal mass ejections originate from active regions on a star's surface. Near a stellar maxima a star such as the Sun produces about three CMEs every day, whereas near stellar minima there is about one CME every five days.[35]


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  1. Dominant groups occur more often in regional astronomy than other astronomies.

See also

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  1. wikt:User:Vildricianus:Vildricianus (25 March 2006). region. San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2012-09-10. 
  2. 2.0 2.1 2.2 2.3 2.4 Sue Lavoie; Karen Boggs (November 10, 2009). PIA12348: Great Observatories' Unique Views of the Milky Way. Pasadena, California USA: NASA, JPL. Retrieved 2013-03-14. 
  3. M. G. Revnivtsev; R. A. Sunyaev; D. A. Varshalovich; V. V. Zheleznyakov; A. M. Cherepashchuk; A. A. Lutovinov; E. M. Churazov; S. A. Grebenev et al. (June 2004). "A hard X-ray survey of the Galactic-Center region with the IBIS telescope of the INTEGRAL observatory: A catalog of sources". Astronomy Letters 30 (6): 382-9. doi:10.1134/1.1764884. Retrieved 2012-05-30. 
  4. Megan Watzke (January 9, 2002). Chandra takes in bright lights, big city of Milky Way. Huntsville, Alabama, USA: NASA Marshall Space Flight Center. Retrieved 2013-03-14. 
  5. 5.0 5.1 5.2 5.3 5.4 5.5 ESA/Hubble; NASA (January 16, 2012). Core of Messier 100 in super high res. ESA/Hubble & NASA. Retrieved 2013-03-14. 
  6. ESO05 (October 17, 2005). The Centre of the Active Galaxy NGC 1097. Paranal: European Southern Observatory. Retrieved 2013-03-15. 
  7. 7.0 7.1 7.2 7.3 7.4 7.5 Gilles Chapdelaine (August 20, 2012). A collection of ancient stars. NASA & ESA. Retrieved 2013-03-14. 
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Further reading

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{{Principles of radiation astronomy}}