# Stars/Star-forming regions

A star-forming region is an area in space within which very young stellar objects predominate and are likely being formed.

## Radiation

"RCW 79 is seen in the southern Milky Way, 17,200 light-years from Earth in the constellation Centaurus. The bubble is 70-light years in diameter, and probably took about one million years to form from the radiation and winds of hot young stars."
"The balloon of gas and dust is an example of stimulated star formation. Such stars are born when the hot bubble expands into the interstellar gas and dust around it. RCW 79 has spawned at least two groups of new stars along the edge of the large bubble. Some are visible inside the small bubble in the lower left corner. Another group of baby stars appears near the opening at the top."
"NASA's Spitzer Space Telescope easily detects infrared light from the dust particles in RCW 79. The young stars within RCW79 radiate ultraviolet light that excites molecules of dust within the bubble. This causes the dust grains to emit infrared light that is detected by Spitzer and seen here as the extended red features."

## Regions

Any considerable and connected part of a space or surface studied in astronomy is an entity in regional astronomy.
Def. "[a]ny 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" is called a region.

## Young stellar objects

"This image, the first to be released publicly from VISTA, the world’s largest survey telescope, shows the spectacular star-forming region known as the Flame Nebula, or NGC 2024, in the constellation of Orion (the Hunter) and its surroundings. In views of this evocative object in visible light the core of the nebula is completely hidden behind obscuring dust, but in this VISTA view, taken in infrared light, the cluster of very young stars at the object’s heart is revealed. The wide-field VISTA view also includes the glow of the reflection nebula NGC 2023, just below centre, and the ghostly outline of the Horsehead Nebula (Barnard 33) towards the lower right. The bright bluish star towards the right is one of the three bright stars forming the Belt of Orion. The image was created from VISTA images taken through J, H and Ks filters in the near-infrared part of the spectrum. The image shows about half the area of the full VISTA field and is about 40 x 50 arcminutes in extent. The total exposure time was 14 minutes."

## Minerals

"This image from NASA's Spitzer Space Telescope reveals the complex life cycle of young stars, from their dust-shrouded beginnings to their stellar debuts. The stellar nursery was spotted in a cosmic cloud sitting 21,000 light-years away in the Cepheus constellation."
"A star is born when a dense patch gas and dust collapses inside a cosmic cloud. In the first million years of a star's life, it is hidden from visible-light view by the cloud that created it. Eventually as the star matures, its strong winds and radiation blow away surrounding material and the star fully reveals itself to the universe."
"The first stages of stellar life are represented by the greenish yellow dot located in the center of the image (just to the right of the blue dot). Astronomers suspect that this source is less than a million years old because spectra of the region (right bottom graph) reveal a deep absorption feature due to silicate dust (crushed crystalline grains that are smaller than sand) indicating that the star is still deeply embedded inside the cosmic cloud that collapsed to form it. Wisps of green surrounding the star and its nearby environment illustrate the presence of hot hydrogen gas."
"Above and to the left of the central greenish yellow dot, a large, bright pinkish dot reveals a more mature star on the verge of emerging from its natal cocoon. Although this star is still shrouded by its birth material, astronomers use Spitzer, a temperature-sensitive infrared telescope, to see the surrounding gas and dust that is being heated up by the star."
"The region's oldest and fully exposed stars can be seen as bunches of blue specks located just left of the concave ridge. Energetic particles and ultraviolet photons from nearby star clusters etched this arc into the cloud by blowing away surrounding dust and gas."
"Spectral observations of the ridge (right top graph) and reddish-white dot, or "mature star" (right middle graph), indicate the presence of carbon rich molecules called polycyclic aromatic hydrocarbons (PAHs), which are found on barbecue grills and in automobile exhaust on Earth."
"The featured image is a four-channel composite, where blue indicates emission at 3.6 microns, green corresponds to 4.5 microns, and red to 5.8 and 8.0 microns. The image was taken by Spitzer's Infrared Array Camera (IRAC). Spectra of the region were obtained with the telescope's Infrared Spectrometer (IRS) instrument."

## Theoretical star-forming regions

Star formation is a general astrophysics hypothesis that stars as detected with radiation astronomy are formed by an isolated process rather than being a secondary condition within large coronal clouds. Star formation should also account for the formation of single, binary, and multiple star systems.

## Objects

"Resembling a nightmarish beast rearing its head from a crimson sea, this monstrous object is actually an innocuous pillar of gas and dust. Called the Cone Nebula (and cataloged NGC 2264) is so named because, in ground-based images, it has a conical shape. This giant pillar resides in a turbulent star-forming region. NASA's Hubble Space Telescope imaged the "Cone Nebula," a nebula close to home. It exhibits a craggy-looking mountaintop of cold gas and dust that is a cousin to Hubble's iconic "pillars of creation" in the Eagle Nebula, photographed in 1995." Red color is 658 nm for Hα, 814 nm I band infrared is green, and 435nm is blue.

## Emptiness

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. The star is V280 Orionis.

## Local Voids

"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."
The Local Void is a vast, empty region of space, lying adjacent to our own Local Group.
The Local Void is now known to be composed of three separate sectors, separated by bridges of "wispy filaments". The precise extent of the void is unknown, but it is at least 150 million light years across and may have a long dimension of up to 70 Mpc (230 million light years). The Local Void also appears to have significantly fewer galaxies than expected from standard cosmology.
The Milky Way sits in a large, flat array of galaxies called the Local Sheet, which bounds the Local Void. The Local Void extends approximately 60 megaparsecs, beginning at the edge of the Local Group. It is believed that the distance from Earth to the centre of the Local Void must be at least 23 megaparsecs (75 Mly).
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.
"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."
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).
"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."
"NGC 6503 lies approximately 17 million light-years away in the constellation of Draco (the Dragon)."
"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." A combination of H-alpha and infrared is also used and is green in color.

## Gamma rays

A gamma-ray burst (GRB) may have an afterglow at longer wavelengths. Specifically, GRB 000418 has a "very red afterglow [which is] further evidence for dust extinction ... [where] the GRB was associated with a dusty star-forming region."

## X-rays

"This painterly portrait of a star-forming cloud, called NGC 346, is a combination of multiwavelength light from NASA's Spitzer Space Telescope (infrared), the European Southern Observatory's New Technology Telescope (visible), and the European Space Agency's XMM-Newton space telescope (X-ray)."
"The infrared observations highlight cold dust in red, visible data show glowing gas in green, and X-rays show very warm gas in blue. Ordinary stars appear as blue spots with white centers, while young stars enshrouded in dust appear as red spots with white centers."
"The colorful picture demonstrates that stars in this region are being created by two different types of triggered star formation -- one involving wind, and the other, radiation. Triggered star formation occurs when massive stars spur new, smaller stars into existence. The first radiation-based mechanism is demonstrated near the center of the cloud. There, radiation from the massive stars is eating away at the surrounding dust cloud, creating shock waves that compress gas and dust into new stars. This compressed material appears as an arc-shaped orange-red filament, while the new stars within this filament are still blanketed with dust and cannot be seen."
"The second wind-based mechanism is at play higher up in the cloud. The isolated, pinkish blob of stars at the upper left was triggered by winds from a massive star located to the left of it. This massive star blew up in a supernova explosion 50,000 years ago, but before it died, its winds pushed gas and dust together into new stars. While this massive star cannot be seen in the image, a bubble created when it exploded can be seen near the large, white spot with a blue halo at the upper left (this white spot is actually a collection of three stars)."
"NGC 346 is the brightest star-forming region in the Small Magellanic Cloud, an irregular dwarf galaxy that orbits our Milky Way galaxy, 210,000 light-years away."

## Ultraviolets

"Just in time for the holidays: a Hubble Space Telescope picture postcard of hundreds of brilliant blue stars wreathed by warm, glowing clouds. The festive portrait is the most detailed view of the largest stellar nursery in our local galactic neighborhood."
"The massive, young stellar grouping, called R136, is only a few million years old and resides in the 30 Doradus Nebula, a turbulent star-birth region in the Large Magellanic Cloud (LMC), a satellite galaxy of our Milky Way. There is no known star-forming region in our galaxy as large or as prolific as 30 Doradus."
"Many of the diamond-like icy blue stars are among the most massive stars known. Several of them are over 100 times more massive than our Sun. These hefty stars are destined to pop off, like a string of firecrackers, as supernovas in a few million years."
"The image, taken in ultraviolet, visible, and red light by Hubble's Wide Field Camera 3, spans about 100 light-years. The nebula is close enough to Earth that Hubble can resolve individual stars, giving astronomers important information about the stars' birth and evolution."
"The brilliant stars are carving deep cavities in the surrounding material by unleashing a torrent of ultraviolet light, and hurricane-force stellar winds (streams of charged particles), which are etching away the enveloping hydrogen gas cloud in which the stars were born. The image reveals a fantasy landscape of pillars, ridges, and valleys, as well as a dark region in the center that roughly looks like the outline of a holiday tree. Besides sculpting the gaseous terrain, the brilliant stars can also help create a successive generation of offspring. When the winds hit dense walls of gas, they create shocks, which may be generating a new wave of star birth."
"The movement of the LMC around the Milky Way may have triggered the massive cluster's formation in several ways. The gravitational tug of the Milky Way and the companion Small Magellanic Cloud may have compressed gas in the LMC. Also, the pressure resulting from the LMC plowing through the Milky Way's halo may have compressed gas in the satellite. The cluster is a rare, nearby example of the many super star clusters that formed in the distant, early universe, when star birth and galaxy interactions were more frequent. Previous Hubble observations have shown astronomers that super star clusters in faraway galaxies are ubiquitous."
"The LMC is located 170,000 light-years away and is a member of the Local Group of Galaxies, which also includes the Milky Way."
"The Hubble observations were taken Oct. 20-27, 2009. The blue color is light from the hottest, most massive stars; the green from the glow of oxygen; and the red from fluorescing hydrogen."

## Opticals

"This wide-field view shows a rich region of dust clouds and star formation in the southern constellation of Vela. Close to the centre of the picture the jets of the Herbig-Haro object HH 46/47 can be seen emerging from a dark cloud in which infant stars are being born (to zoom in use the zoomable version of this image). This view was created from images forming part of the Digitized Sky Survey 2."

## Visuals

"The image composite compares an infrared image taken by NASA's Spitzer Space Telescope to a visible-light picture of the same region (inset). While the infrared view, dubbed "Mountains of Creation," reveals towering pillars of dust aglow with the light of embryonic stars (white/yellow), the visible-light view shows dark, barely-visible pillars. The added detail in the Spitzer image reveals a dynamic region in the process of evolving and creating new stellar life."
"Why do the pictures look so different? The answer has two parts. First, infrared light can travel through dust, while visible light is blocked by it. In this case, infrared light from the stars tucked inside the dusty pillars is escaping and being detected by Spitzer. Second, the dust making up the pillars has been warmed by stars and consequently glows in infrared light, where Spitzer can see it. This is a bit like seeing warm bodies at night with infrared goggles. In summary, Spitzer is both seeing, and seeing through, the dust."
"The Spitzer image was taken by the infrared array camera on Spitzer. It is a 4-color composite of infrared light, showing emissions from wavelengths of 3.6 microns (blue), 4.5 microns (green), 5.8 microns (orange), and 8.0 microns (red). The visible-light image is from California Institute of Technology's Digitized Sky Survey."

## Blues

"RCW 108 [in the image at the right] is a molecular cloud that is in the process of being destroyed by intense ultraviolet radiation from heavy and hot stars in the nearby stellar cluster NGC 6193, seen to the left in the photo. A series of images were obtained with the Wide Field Imager (WFI) of areas in the Milky Way band, including some in which interstellar nebulae of gas and dust are seen. Each frame records 8184 x 8196, or over 67 million, pixels in a sky field of 32 x 32 arcmin 2. The photo shows the RCW 108 complex of bright and dark nebulae in the southern association Ara OB1, a star-forming region in the constellation Ara (the Altar), deep in the southern sky. The resolution in this image has been degraded by reducing the number of pixels in one direction from about 8000 to 3000 in the "High-Resolution version", in order to make the image transportable over the web without incurring completely unacceptable transfer times. Still, it is very large, even in the highly compressed jpeg format, reflecting the great amount of details visible."
"This colour picture is a composite made from 12 separate images, obtained with the WFI on 27 March 1999. The blue component corresponds to the B filter, the green to the V filter, and the red to the H-alpha filter. The images in each filter are the composite of 4 individual frames obtained with the telescope pointing at slightly different positions on the sky, so that the parts of the sky falling in the gaps between the 8 individual 2k x 4k CCDs in any given frame are recorded on the others. The monochromatic images are then produced by superimposing the individual frames, correcting for the telescope offsets ; this ensures that the complete field is well covered. This procedure is not simple, as the observing conditions may change slightly from exposure to exposure, resulting in small differences. Finally, the combined images in each filter are aligned and colour-coded to produce the colour picture."
"For the processing of this large photo (8k x 8k; 256 Mbytes), a minimum of contrast correction was made and very faint lines may still be perceived in some places where the individual frames were joined. It may also be noted that there is a slight misalignment of the individual colours in stellar images at the extreme corners of the large field. This is due to the effect of differential atmospheric refraction, i.e. light rays of different colours are bent differently in air."
"The exposure time was 300 sec for each frame in H-alpha, and 60 sec in B and V. East is to the left and North to the top."

## Oranges

"Looking like an elegant abstract art piece painted by talented hands, this picture is actually a NASA/ESA Hubble Space Telescope image of a small section of the Carina Nebula. Part of this huge nebula was documented in the well-known Mystic Mountain picture (heic1007a) and this picture takes an even closer look at another piece of this bizarre astronomical landscape (heic0707a)."
"The Carina Nebula itself is a star-forming region about 7500 light-years from Earth in the southern constellation of Carina (The Keel: part of Jason’s ship the Argo). Infant stars blaze with a ferocity so severe that the radiation emitted carves away at the surrounding gas, sculpting it into strange structures. The dust clumps towards the upper right of the image, looking like ink dropped into milk, were formed in this way. It has been suggested that they are cocoons for newly forming stars."
"The Carina Nebula is mostly made from hydrogen, but there are other elements present, such as oxygen and sulphur. This provides evidence that the nebula is at least partly formed from the remnants of earlier generations of stars where most elements heavier than helium were synthesised."
"The brightest stars in the image are not actually part of the Carina Nebula. They are much closer to us, essentially being the foreground to the Carina Nebula’s background."
"This picture was created from images taken with Hubble’s Wide Field Planetary Camera 2. Images through a blue filter (F450W) were coloured blue and images through a yellow/orange filter (F606W) were coloured red. The field of view is 2.4 by 1.3 arcminutes."
"This false-color image [at the lower right] taken by NASA's Spitzer Space Telescope shows the "South Pillar" region of the star-forming region called the Carina Nebula. Like cracking open a watermelon and finding its seeds, the infrared telescope "busted open" this murky cloud to reveal star embryos (yellow or white) tucked inside finger-like pillars of thick dust (pink). Hot gases are green and foreground stars are blue. Not all of the newfound star embryos can be easily spotted."
"Though the nebula's most famous and massive star, Eta Carinae, is too bright to be observed by infrared telescopes, the downward-streaming rays hint at its presence above the picture frame. Ultraviolet radiation and stellar winds from Eta Carinae and its siblings have shredded the cloud to pieces, leaving a mess of tendrils and pillars. This shredding process triggered the birth of the new stars uncovered by Spitzer."
"The inset visible-light picture of the Carina Nebula shows quite a different view. Dust pillars are fewer and appear dark because the dust is soaking up visible light. Spitzer's infrared detectors cut through this dust, allowing it to see the heat from warm, embedded star embryos, as well as deeper, more buried pillars."
"Eta Carinae is a behemoth of a star, with more than 100 times the mass of our Sun. It is so massive that it can barely hold itself together. Over the years, it has brightened and faded as material has shot away from its surface. Some astronomers think Eta Carinae might die in a supernova blast within our lifetime."
"Eta Carinae's home, the Carina Nebula, is located in the southern portion of our Milky Way galaxy, 10,000 light-years from Earth. This colossal cloud of gas and dust stretches across 200 light-years of space. Though it is dominated by Eta Carinae, it also houses the star's slightly less massive siblings, in addition to the younger generations of stars."
"This image was taken by the infrared array camera on Spitzer. It is a three-color composite of invisible light, showing emissions from wavelengths of 3.6 microns (blue), 4.5 microns (green), 5.8 microns (orange), and 8.0 microns (red). The visible-light picture is from the National Optical Astronomy Observatory."

## Reds

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.
"The image reveals giant loops of gas ejected by dying stars that bear a striking resemblance to solar prominences."
"The gas in the clouds of NGC 6559 [at lower right], mainly hydrogen, is the raw material for star formation ... When a region inside this nebula gathers enough matter, it starts to collapse under its own gravity. The center of the cloud grows ever denser and hotter, until thermonuclear fusion begins and a star is born. The hydrogen atoms combine to form helium atoms, releasing energy that makes the star shine. ... In regions where it is very dense, the dust completely blocks the light behind it, as is the case for the dark isolated patches and sinuous lanes to the bottom left-hand side and right-hand side of the image".
"The Danish 1.54-metre telescope located at ESO’s La Silla Observatory in Chile has captured a striking image of NGC 6559, an object that showcases the anarchy that reigns when stars form inside an interstellar cloud. This region of sky includes glowing red clouds of mostly hydrogen gas, blue regions where starlight is being reflected from tiny particles of dust and also dark regions where the dust is thick and opaque."
"The two colors of the cloud represent a pair of nebulas. Once the young stars are born, they "energize" the hydrogen surrounding them, ESO officials said. The gas then creates the red wispy cloud — known to astronomers as an "emission nebula" — in the center of the image."
"These young stars are usually of spectral type O and B, with temperatures between 10 000 and 60 000 K, which radiate huge amounts of high energy ultraviolet light that ionises the hydrogen atoms."
"The blue section of the photo — representing a "reflection nebula" — shows light from the newly formed stars in the cosmic nursery being reflected in all directions by the particles of dust made of iron, carbon, silicon and other elements in the interstellar cloud."
NGC 6559 is planetary nebula located at a distance of about 5000 light-years from Earth, in the constellation of Sagittarius.
"NGC 6559 is a cloud of gas and dust located at a distance of about 5000 light-years from Earth, in the constellation of Sagittarius (The Archer). The glowing region is a relatively small object, just a few light-years across, in contrast to the one hundred light-years and more spanned by its famous neighbour, the Lagoon Nebula (Messier 8, eso0936). Although it is usually overlooked in favour of its distinguished companion, NGC 6559 has the leading role in this new picture."
"The Milky Way fills the background of the image with countless yellowish older stars. Some of them appear fainter and redder because of the dust in NGC 6559."
"This eye-catching image of star formation was captured by the Danish Faint Object Spectrograph and Camera (DFOSC)".

## Infrareds

At 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."
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!"

## Submillimeters

"A team of astronomers has used ALMA (the Atacama Large Millimeter/submillimeter Array) to pinpoint the locations of over 100 of the most fertile star-forming galaxies in the early Universe."
"The best map so far of these distant dusty galaxies was made using the Atacama Pathfinder Experiment (APEX), but the observations were not sharp enough to unambiguously identify these galaxies in images at other wavelengths. ALMA needed just two minutes per galaxy to pinpoint each one within a comparatively tiny region 200 times smaller than the broad APEX blobs, and with three times the sensitivity."
"This image shows a selection of the galaxies as seen in the sharp new observations by ALMA (in red). The ALMA observations, at submillimetre wavelengths, are overlaid on an infrared view of the region as seen by the IRAC camera on the Spitzer Space Telescope (coloured blue)."

## Gaseous objects

With respect to "gaseous objects with mass below the H-burning limit. ... the hydrogen-rich gaseous objects with mass below the minimum main sequence mass of ~ 0.08 Msun. ... the minimum mass of a gaseous fragment may be as low as 0.001 Msun."
"The first stage is the initial gravitational collapse phase during which the object moves more or less vertically downward in the H-R diagram. The color of the object is red (or very red) during this stage."
"During [the second evolutionary] stage the object, still luminous, starts sliding down on its cooling curve, and its color gets redder and redder as the surface temperature decreases with time."
"The saying "X" marks the spot holds true in this NASA Hubble Space Telescope (HST) image where Hubble-X marks the location of a dramatic burst of star formation, very much like the Orion Nebula in our Milky Way galaxy, but on a vastly greater scale."
"Hubble-X is a glowing gas cloud, one of the most active star-forming regions within galaxy NGC 6822. The name Hubble-X does not refer to the shape of the gas cloud, but rather is derived from a catalog of objects in this particular galaxy. The "X" is actually a Roman numeral designation. The galaxy lies in the constellation Sagittarius at a distance of only 1,630,000 light-years and is one of the Milky Way's closest neighbors. The intense star formation in Hubble-X occurred only about 4 million years ago, a small fraction of the approximate 10 billion year age of the universe."
"Giant gas clouds in NGC 6822 have held a special attraction for astronomers since their discovery by the visual observer E. E. Barnard in 1881. Edwin P. Hubble, after whom the HST is named, used the then-new 100-inch telescope at Mount Wilson Observatory in 1925 to make the first detailed photographic investigation of NGC 6822. The Hubble image reveals details too fine to be resolved from telescopes on the ground."
"Stars form in groups from enormous clouds of gas and dust called giant molecular clouds. Once star formation begins in a molecular cloud, its rate accelerates until the process is stopped when one or more very massive hot stars are formed. At that point the clouds change from near darkness into the brightly glowing objects such as seen in Hubble-X. It is the intense ultraviolet radiation from the massive stars that causes the residual gas to glow. Radiation and gas outflows, called stellar winds, then cause the gas to disperse, bringing further star formation to an abrupt end."
"The Hubble-X image was taken with Hubble's Wide Field Planetary Camera 2 (WFPC2) in September 1997, by astronomers C. Robert O'Dell of Vanderbilt University, Paul W. Hodge of the University of Washington, and R. C. Kennicutt, Jr. of Steward Observatory at the University of Arizona."
"The image shows a nearly circular bright cloud at the core of Hubble-X. The cloud's diameter is about 110 light-years, and contains many thousands of newly formed stars in a central cluster. The brightest of these young stars are easily visible in the Hubble image, where they appear as numerous bright white dots."
"Hubble-X is many times brighter and larger than the Orion Nebula, the brightest nearby star formation region in our own Milky Way galaxy. In fact, the tiny cloud just below Hubble-X, barely resolved even by HST, has about the same size and brightness as the Orion Nebula."

## Rocky objects

"This image of the star formation region NGC 6334 is one of the first scientific images from the ArTeMiS instrument on APEX. The picture shows the glow detected at a wavelength of 0.35 millimetres coming from dense clouds of interstellar dust grains. The new observations from ArTeMiS show up in orange and have been superimposed on a view of the same region taken in near-infrared light by ESO’s VISTA telescope at Paranal."

## Dusty regions

"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." 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."
"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."
"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."
"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."
"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."

## Hydrogens

"This image from the NASA/ESA Hubble Space Telescope shows Sh 2-106, or S106 for short. This is a compact star forming region in the constellation Cygnus (The Swan). A newly-formed star called S106 IR is shrouded in dust at the centre of the image, and is responsible for the surrounding gas cloud’s hourglass-like shape and the turbulence visible within. Light from glowing hydrogen [Hα] is coloured blue in this image." Radiation at 1.1 µm J band in the infrared is colored cyan, 1.6 µm H band in the infrared is colored red.

## H I regions

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, and it is usually considered as isothermal, except near an expanding H II region.
H I regions of the ISM contain the cold neutral medium (CNM). The CNM constitutes 1-5 % by volume of the ISM, ranges in size from 100-300 pc, has a temperature between 50 and 100 K, with an atom density of 20-50 atoms/cm$^3$. The CNM has hydrogen in the neutral atomic state and emits the 21 cm line.
The warm neutral medium (WNM) is 10-20 % of the ISM, ranges in size from 300-400 pc, temperature between 6000 and 10000 K, is composed of neutral atomic hydrogen, has a density of 0.2-0.5 atoms/cm$^3$, and emits the hydrogen 21 cm line.
Also, within the H I regions is the warm ionized medium (WIM), constituting 20-50 % by volume of the ISM, with a size around 1000 pc, a temperature of 8000 K, an atom density of 0.2-0.5 atoms/cm$^3$, of ionized hydrogen, emitting the hydrogen alpha line and exhibiting pulsar dispersion.
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

An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place.
At top right is an image in three-color infrared of an H II region excited by a cluster of young, hot stars. The region is in Messier 17 (M 17). A large silhouette disc occurs to the southwest of the cluster center. This image is obtained with the ISAAC near-infrared instrument at the 8.2-m VLT ANTU telescope at Paranal.

## Molecule forming regions

The “[w]arm molecular envelope of M giants and Miras [is] a new molecule forming region”
At right is a "[c]olour-composite annotated image of part of the Galactic Plane seen by the ATLASGAL survey, divided into sections."
"The total size of the image is approximately 42 degrees by 1.75 degrees."
"Some of the most prominent features visible in the image are (from left to right, top to bottom):"
* "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."
* "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."
* "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."
* "NGC 6357: A diffuse nebula containing the open cluster Pismis 24, home to several very massive stars."
* "NGC 6334: An emission nebula also known as the “Cat’s Paw Nebula”."
* "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."
* "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."

## Molecular clouds

A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H$_2$).
Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H$_2$ is CO (carbon monoxide). The ratio between CO luminosity and H$_2$ mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.
Such clouds make up < 1% of the ISM, have temperatures of 10-20 K and high densities of 10$^2$ - 10$^6$ atoms/cm$^3$. These clouds are astronomical radio and infrared sources with radio and infrared molecular emission and absorption lines.

## Giant molecular clouds

"A vast assemblage of molecular gas with a mass of approximately 10$^3$–10$^7$ times the mass of the Sun[See, e.g., Table 1 and the Appendix] is called a giant molecular cloud (GMC). GMCs are ≈15–600 light-years in diameter (5–200 parsecs). Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average density of a GMC is 10$^2$–10$^3$ particles per cubic centimetre. Although the Sun is much denser than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun. The substructure of a GMC is a complex pattern of filaments, sheets, bubbles, and irregular clumps.
The densest parts of the filaments and clumps are called "molecular cores", whilst the densest molecular cores are, unsurprisingly, called "dense molecular cores" and have densities in excess of 10$^4$–10$^6$ particles per cubic centimeter. Observationally molecular cores are traced with carbon monoxide and dense cores are traced with ammonia. The concentration of dust within molecular cores is normally sufficient to block light from background stars so that they appear in silhouette as dark nebulae.
GMCs are so large that "local" ones can cover a significant fraction of a constellation; thus they are often referred to by the name of that constellation, e.g. the Orion Molecular Cloud (OMC) or the Taurus Molecular Cloud (TMC). These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt. The most massive collection of molecular clouds in the galaxy forms an asymmetrical ring around the galactic center at a radius of 120 parsecs; the largest component of this ring is the Sagittarius B2 complex. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in interstellar space.

## Materials

"Located 1,000 light-years from Earth in the constellation Perseus, a reflection nebula called NGC 1333 epitomizes the beautiful chaos of a dense group of stars being born. Most of the visible light from the young stars in this region is obscured by the dense, dusty cloud in which they formed. With NASA's Spitzer Space Telescope, scientists can detect the infrared light from these objects. This allows a look through the dust to gain a more detailed understanding of how stars like our sun begin their lives."
"The young stars in NGC 1333 do not form a single cluster, but are split between two sub-groups. One group is to the north near the nebula shown as red in the image. The other group is south, where the features shown in yellow and green abound in the densest part of the natal gas cloud. With the sharp infrared eyes of Spitzer, scientists can detect and characterize the warm and dusty disks of material that surround forming stars. By looking for differences in the disk properties between the two subgroups, they hope to find hints of the star- and planet-formation history of this region."
"The knotty yellow-green features located in the lower portion of the image are glowing shock fronts where jets of material, spewed from extremely young embryonic stars, are plowing into the cold, dense gas nearby. The sheer number of separate jets that appear in this region is unprecedented. This leads scientists to believe that by stirring up the cold gas, the jets may contribute to the eventual dispersal of the gas cloud, preventing more stars from forming in NGC 1333."
"In contrast, the upper portion of the image is dominated by the infrared light from warm dust, shown as red."

## Meteorites

"The Raman spectra of some [interplanetary dust particle] IDPs also show red photoluminescence that is similar to the excess red emission seen in some astronomical objects and that has also been attributed to [polycyclic aromatic hydrocarbons] PAH s and hydrogenated amorphous carbon. Moreover, a part of the carbonaceous phase in IDPs and meteorites contains deuterium to hydrogen ratios that are greater than those for terrestrial samples."

## Interstellar medium

In astronomy, the interstellar medium (or ISM) is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space.
The ISM consists of about 0.1 to 1 particles per cm$^3$ and is typically composed of roughly 70% hydrogen by mass, with most of the remaining gas consisting of helium. This medium has been chemically enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their main sequence lifetime. Higher density regions of the interstellar medium form clouds, or diffuse nebulae, where star formation takes place.
At left is a visual negative of the LW Cassiopeia Nebula (ISM). Within the image are H II regions (red +). stars (red *), X-ray sources (X), infrared objects (red diamond), molecular clouds (MolCld), reflection nebulae (RfNeb), dark nebulae (DkNeb), and the interstellar medium (ISM).

## Rho Ophiuchi complex

The variety of colors seen in the image at right from NASA's Wide-field Infrared Survey Explorer (WISE) "represents different wavelengths of infrared light. The bright white nebula in the center of the image is glowing due to heating from nearby stars, resulting in what is called an emission nebula. The same is true for most of the multi-hued gas prevalent throughout the entire image, including the bluish, bow-shaped feature near the bottom right. The bright red area in the bottom right is light from the star in the center--Sigma Scorpii--that is reflected off of the dust surrounding it, creating what is called a reflection nebula. And the much darker areas scattered throughout the image are pockets of cool, dense gas that block out the background light, resulting in absorption (or 'dark') nebulae. ... The bright pink objects just left of center are young stellar objects--baby stars just beginning to form. Many of them are still enveloped in their own tiny compact nebulae. In visible light, these baby stars are completely hidden in the dark nebula that surrounds them. Also seen in this image are some of the oldest stars in our Milky Way galaxy. The first cluster, M80, is on the far right edge of the image towards the top. The second, NGC 6144, is found close to the bottom edge near the center. They both appear as small densely compacted groups of blue stars. Globular clusters such as these typically harbor some of the oldest stars known, some as old as 13 billion years, born soon after the universe formed."
"The colours used in this image represent specific wavelengths of infrared light. Blue and cyan (blue-green) represent light emitted at wavelengths of 3.4 and 4.6 microns, which is predominantly from stars. Green and red represent light from 12 and 22 microns, respectively, which is mostly emitted by dust. Distance: ~400 light years (Rho Ophiuchi). Size: ~14 light years across (Rho Ophiuchi in this image). Position of object (J2000): RA = 16h 25m 17s; Dec = -24° 14’ 49”. Constellation: Ophiucus and Scorpius. Field of View: 1.967x1.948 degrees. Orientation: North is 0.12966 degrees left of up. Colour Mapping: Blue=3.4 microns; Cyan=4.6 microns; Green=12 microns; Red=22 microns."
Over half of the mass of the complex is concentrated around the L1688 cloud, and this is the most active star-forming region. There are embedded infrared sources within the complex. A total of 425 infrared sources have been detected near the L1688 cloud. These are presumed to be young stellar objects, including 16 classified as protostars, 123 T Tauri stars with dense circumstellar disks, and 77 weaker T Tauri stars with thinner disks. The last two categories of stars have estimated ages ranging from 100,000 to a million years. The first brown dwarf to be identified in a star-forming region was Rho Oph J162349.8-242601, located in the Rho Ophiuchi cloud.

## The Ghost Head Nebula

The Ghost Head Nebula [NGC 2080] ... is a star-forming region and emission nebula ... in the southern constellation of Dorado. It belongs to the Large Magellanic Cloud, a satellite galaxy to the Milky Way, which is at a distance of 168,000 light years.
The Ghost Head Nebula has a diameter of 50 light-years and is named for the two distinct white patches it possesses, called the "eyes of the ghost". The western patch, called A1, has a bubble in the center which was created by the young, massive star it contains. The eastern patch, called A2, has several young stars in a newly formed cluster, but they are still obscured by their originating dust cloud. These stars together have begun to create a bubble in the nebula with their outpourings of material, called stellar wind.
The presence of stars also greatly influences the color of the nebula. The western portion of the nebula has a dominant oxygen emission line because of a powerful star on the nebula's outskirts; this colors it green. The rest of the nebula's outskirts have a red hue due to the ionization of hydrogen. Because both hydrogen and oxygen are ionized in the central region, which appears pale yellow; when hydrogen is energized enough to emit a second wavelength of light, it appears blue, as in the area surrounding A1 and A2.

## NGC 2363

"Clusters of stars and a fishhook-shaped cloud of luminescent gases glow brilliantly in NGC 2363, a giant star-forming region in the Magellanic galaxy NGC 2366."
"The brightest star visible on this image (at the tip of the fishhook) is a rare class called an erupting Luminous Blue Variable (LBV). This monstrous star (30 to 60 times as massive as the Sun) is in a very unstable, eruptive phase of its life."
NGC 2363 is 13 million light years distant in the constellation Camelopardalis. It is the large, blue-white nebula at the upper right in the second image of NGC 2366.

## Dragonfish nebula

The Dragonfish nebula is a "turbulent region, jam-packed with stars, is home to some of the most luminous massive stars in our Milky Way galaxy. It is located approximately 30,000 light-years away in the Crux constellation."
"The massive stars have blown a bubble in the gas and dust, carving out a shell of more than 100 light-years across (seen in lower, central part of image). This shell forms the "toothy mouth" of the Dragonfish, and the two bright spots make it up its beady eyes."
"The infrared light in this region is coming from the gas and dust that are being heated up by the unseen central cluster of massive stars. The bright spots along the shell, including the "eyes," are possible smaller regions of newly formed stars, triggered by the compression of the gas and dust by winds from the central, massive stars."
"Infrared light in this image was captured by the infrared array camera on Spitzer, at wavelengths of 3.6 microns (blue); 4.5 microns (green); and 8.0 microns (red). The data were captured before Spitzer ran out of its liquid coolant in 2009, and began its "warm" mission."

## Milky Way bar end

The image at right is an artist's conception of the Milky Way galaxy as seen from far Galactic North (in the constellation Coma Berenices. The image is annotated with arms (color-coded) as well as distances from the Solar System and galactic longitude with corresponding constellations.
The region where the Scutum–Centaurus Arm arm connects to the bar of the galaxy is rich in star-forming regions. In 2006 a large cluster of new stars containing 14 red supergiant stars was discovered there and named RSGC1. In 2007 a cluster of approximately 50,000 newly formed stars named RSGC2 was located only a few hundred light years from RSGC1; it contains 26 red supergiant stars, the largest grouping of such stars known. Other clusters in this region include RSGC3 and Alicante 8.

## Magellanic clouds

"This VISTA image shows the spectacular 30 Doradus star-forming region, also called the Tarantula Nebula. At its core is a large cluster of stars known as R 136, in which some of the most massive stars known are located. This infrared image, made with ESO’s VISTA survey telescope, is from the VISTA Magellanic Cloud Survey. The project will scan a vast area — 184 square degrees of the sky (corresponding to almost one thousand times the apparent area of the full Moon), including our nearby neighbouring galaxies, the Large and Small Magellanic Clouds. The end result will be a detailed study of the star formation history and three-dimensional geometry of the Magellanic system."
"This image was created from images taken through Y, J and Ks filters in the near-infrared part of the spectrum (coloured blue, green and red respectively). The exposure times were 40, 47 and 81 minutes per filter respectively. The image covers a region of sky about 52 by 70 arcminutes."

## Starburst galaxy

"The presence of ERE has been established spectroscopically in ... the starburst galaxy M82 (Perrin, Darbon, & Sivan 1995)."
"This mosaic image [at right] is the sharpest wide-angle view ever obtained of M82. The galaxy is remarkable for its bright blue disk, webs of shredded clouds, and fiery-looking plumes of glowing hydrogen blasting out of its central regions."
"Throughout the galaxy's center, young stars are being born 10 times faster than they are inside our entire Milky Way Galaxy. The resulting huge concentration of young stars carved into the gas and dust at the galaxy's center. The fierce galactic superwind generated from these stars compresses enough gas to make millions of more stars."
"In M82, young stars are crammed into tiny but massive star clusters. These, in turn, congregate by the dozens to make the bright patches, or "starburst clumps," in the central parts of M82. The clusters in the clumps can only be distinguished in the sharp Hubble images. Most of the pale, white objects sprinkled around the body of M82 that look like fuzzy stars are actually individual star clusters about 20 light-years across and contain up to a million stars."
"The rapid rate of star formation in this galaxy eventually will be self-limiting. When star formation becomes too vigorous, it will consume or destroy the material needed to make more stars. The starburst then will subside, probably in a few tens of millions of years."
"Located 12 million light-years away, M82 appears high in the northern spring sky in the direction of the constellation Ursa Major, the Great Bear. It is also called the "Cigar Galaxy" because of the elliptical shape produced by the oblique tilt of its starry disk relative to our line of sight."
"The observation was made in March 2006, with the Advanced Camera for Surveys' Wide Field Channel. Astronomers assembled this six-image composite mosaic by combining exposures taken with four colored filters that capture starlight from visible and infrared wavelengths as well as the light from the glowing hydrogen filaments."

## Star-forming galaxies

"[T]he rate of star birth peaked around 11 billion years ago, just 2.7 billion years after the Big Bang that created the universe. It has been dropping ever since, and the rate now stands at just ... one-thirtieth its historic high".
Blue star-forming galaxies are more like spiral galaxies. Spiral arms contain a great many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so remarkable.

## Technology

"This STS-80 onboard photograph [at the right] shows the Orbiting Retrievable Far and Extreme Ultraviolet Spectrometer-Shuttle Pallet Satellite II (ORFEUS-SPAS II), photographed during approach by the Space Shuttle Orbiter Columbia for retrieval. Built by the German Space Agency, DARA, the ORFEUS-SPAS II, a free-flying satellite, was dedicated to astronomical observations at very short wavelengths to: investigate the nature of hot stellar atmospheres, investigate the cooling mechanisms of white dwarf stars, determine the nature of accretion disks around collapsed stars, investigate supernova remnants, and investigate the interstellar medium and potential star-forming regions. Some 422 observations of almost 150 astronomical objects were completed, including the Moon, nearby stars, distant Milky Way stars, stars in other galaxies, active galaxies, and quasar 3C273. The STS-80 mission was launched November 19, 1996."

## Hypotheses

* Stars can form outside galaxies.