INFRARED - LONGER THAN RED

This set of 24 colorful slides features familiar astronomical objects as seen at infrared wavelengths. The set was compiled by Dr. Ian Gatley and Suzanne Jacoby of the National Optical Astronomy Observatory. Here we present thumbnails of the included images along with their associated captions.

Clicking on the images will open a larger image in a new window.

Hot Spots at the Kitt Peak 4-Meter Telescope

4.0 Meter telescope with IR image inset

The Nicholas U. Mayall 4-m telescope at Kitt Peak National Observatory is seen here in visual light, with a 10 micron infrared image of the telescope inserted on the left hand side of the picture. The small color infrared image was taken in 1992 with a thermal video camera, where differences in (computer generated) color represent differences in temperature. Oil lubricated bearings support the moving part of the telescope. The bearings appear in the infrared image as bright red, representing a 15 degree centigrade increase in temperature over those areas shown in blue.

In the infrared image, you can see the elevated walkway as a dark blue horizontal line just below the hand railing. The bright yellow and red areas below the walkway are a relatively warm area in the building where several oil pipes run. The grid at the base of the telescope is called the Cassegrain cage; it holds electronic observing equipment at the Cassegrain focus of the telescope. In the optical image, the Cassegrain cage is shown as the white screened area nestled in the blue horseshoe.

The moving part of the telescope, the blue horseshoe and tube supported by hydrostatic bearings, weighs 375 tons. To move this huge instrument so easily, enormous care was taken to minimize friction in the bearing system. This was done in part through the use of oil-lubricated hydrostatic bearings perpendicular to the polar axis of the telescope. These bearings are seen as light blue slanted supports on either side of the blue horseshoe. They are quite easy to pick out in the infrared image - they are seen as bright red, representing a 15-degree C temperature increase over areas shown as medium blue in the same image. The lubricating oil is heated by the pressure system required to deliver the oil to the bearings and the friction of the oil passing between the bearing surface and horseshoe. The hot oil in turn heats up the horseshoe and surrounding air, which causes temperature differences within the dome and adversely affects the quality of images seen with the telescope. Cooling the oil to ambient temperature started in 1995, a procedure which continues to remove the greatest single source of undesirable heat in the dome.



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True Color Image of M17, the Horseshoe Nebula

True Color Image of M17, the Horseshoe Nebula

This beautiful nebula is known by many names, including M17, NGC 6618, the Omega Nebula, the Swan Nebula, and the Horseshoe Nebula. It is also known to many backyard astronomers as a star formation region located in the constellation of Sagittarius. In this image taken in visual light, we see bright clouds and lanes of opaque dust and gas. Massive stars are forming in a cool cloud containing molecular hydrogen gas. Radiation from the newly formed, hot stars produces enough energy to ionize hydrogen. This glowing, ionized gas in turn releases energy which produces the beautiful emission nebula. The streaks around some of the brighter stars are an artifact of the electronic Charge Coupled Device (CCD) detector used in the observations.

This is a true color image, made by combining three CCD frames taken through different filters placed in front of the detector. These filters correspond to the primary colors blue, green, and red. By combining the resultant three frames, it is possible to recreate what we would expect to see if the object were bright enough for us to use ordinary color film.

M17 appears in several slides in this set and will be used as a case study to demonstrate analysis techniques used by infrared astronomers. Note the dark areas of this image, those areas where no light or information is present. The young stars that ionize the nebula are embedded in material trapped between two V-shaped slabs of gas and dust. These areas are largely opaque in visible light, but will look quite different when probed with the tools available in the infrared.


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M17 at 2.2 microns, old technology

M17 at 2.2 microns, old technology

The dramatic evolution in detector technology and the resulting improvements in observational capabilities are shown in this picture and the next slide. Both slides show M17, the well known emission nebula. Here is an image taken in the near infrared 2.2 micron K band and displayed in false color. This picture is the result of five nights of observation with a small 58x62 pixel Indium Antimonide (InSb) array on the KPNO 2.1-m telescope. Many individual images have been pieced together in a mosaic to form the single picture, and you can still see the individual squares. Once you get past the distraction of the individual squares, you will notice an abundance of information and individual stars showing in the areas that were completely opaque in the previous visual light image of M17.


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M17 at 2.2 microns, new technology

M17 at 2.2 microns, new technology

This image of M17 was taken in 1993 with a relatively large 256x256 SBRC Indium Antimonide (InSb) array on the KPNO 1.3-m telescope. Nine images have been seamlessly mosaiced together an a 3 by 3 grid. Since each sub-image is actually a pair of two-second exposures, this represents only 36 seconds of exposure time. This was taken in the single color K band and is displayed in black and white (in contrast with the artificial color of the previous M17 frame M17, also a K band image). Increased sensitivity of the newer detectors result in this image requiring 36 seconds of telescope time, where the older image required five entire nights of time on a larger telescope. In addition, you'll notice the absence of edges in the mosaiced frame - you can't tell where the individual images intersect, a result of both the short exposure time and the increased stability in detector performance.

In the picture you can see details in the clouds, there are threads and pillars of cooler material (seen in absorption) and dust throughout the nebulosity. Notice also the different number of background stars between the upper left and lower right corners of the picture, and the sharp boundaries at the edge of the dust cloud. The dust cloud at the lower right is so dense even 2.2 micron infrared radiation is blocked.



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M17 in JHK

M17 in JHK

This image of M17 was taken with a Platinum Silicide (PtSi) detector at the KPNO 1.3-m by Ian Gatley and Mike Merrill in 1990. It reveals many of the details that make this object popular both aesthetically and scientifically, a popularity reflected in its multiplicity of names (see an earlier caption). M17 is one of the most active sites of formation in our Galaxy, and its HII region is one of the strongest thermal radio sources in the Milky Way. The associated gas cloud, known as M17SW, is one of the densest and most massive molecular cloud cores known, with more than 100 young OB stars found in the area. This particular image is a color composite, with the infrared J, H and K-bands represented as blue, green, and red, respectively. The stellar sources which excite the powerful HII regions and heat the dust in the adjacent molecular cloud are heavily obscured and cannot be seen at optical wavelengths (cf the visual image in this collection). In fact, the differential extinction in M17 is very large, corresponding to almost 20 magnitudes of visual extinction.
There are some truly exquisite features in this picture, such as the prominent filament of dark material projecting down into the HII region from the northern bar. Notice the red edges of the nebulosity, where the longer wavelength K-band is most prominent. As you move along any given ray of radiation from a young star in the nebula, the most excited species will be encountered first, maybe ionized helium. The next thing you'll get is ionized hydrogen, then a boundary or ionization front, beyond which you'll find neutral hydrogen and then molecular hydrogen. The nebula can be unpeeled like an onion, with the excitation decreasing as you move out through the layers. An interesting technical article by Lada et al. can be found in the Astrophysical Journal for 1991, volume 374, page 553.



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M17 components: stars and not stars

M17 components: stars and not stars

It is especially interesting to disentangle the distribution of stars from the distribution of dust and gas in a star formation region like M17, and this is only possible with infrared astronomy. Here you can see processed images of M17 showing the distribution of stars on the left, and everything but stars, the distribution of gas and dust, on the right. Note the cluster of bright stars on the right hand side of the nebula. This picture was made by spatially filtering the previous JHK image of M17, that is, by separating out everything that had the same shape as a stellar image. As a result, you can see a large amount of patchy extinction from the dust envelope around M17, mainly around the edges of the nebula.



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Brackett series emission in M17

Brackett series emission in M17

This image gives another look at M17, this time using the Brackett series of emission lines from atomic hydrogen. The Brackett alpha line is at 4.05 microns and Brackett gamma is at 2.17 microns. What is shown here is the intensity ratio of these two lines, which is a measure of the extinction, or obscuration by dust. This false color picture is normalized by using white in the southwest at a position of intermediate extinction, and then using a set of colors from blue (low extinction) to red (high extinction) to cover the full range of possible obscuration. Thus, the blue patch in the upper center is a region of low extinction, which corresponds to where the nebula is brightest at optical wavelengths. There are extraordinary, heavily reddened clumps down the ionization front on the southwest of the HII region, where the molecular cloud is densest, and also, very conspicuously, along the northern bar. We would not normally expect to see such heavy reddening along an ionization front, but it has been known for some time that M17 contains dense molecular material seen in projection against the ionized gas. In other words, this picture tells us a great deal about the three-dimensional structure of the nebula.



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M17 at 3.3 microns

M17 at 3.3 microns

Another picture of M17 illustrates a different technique used to reveal aspects of the physical conditions in the nebula. As we have seen in earlier images, the nebula contains hot OB stars which give off high energy ultraviolet radiation. The small dust grains which are thus irradiated absorb part of the energy and fluoresce in the infrared at 3.3 microns. This is a longer, or redder, wavelength than was used in previous images. The 3.3 micron radiation has another distinction, in that it can be observed during the daytime.
Studying 3.3 micron radiation gives us an additional way to trace the abundant structure in the photo-dissociation regions on the ionization fronts of M17, in addition to studying molecular hydrogen emission. The vast majority of the diffuse emission here (and in the other pictures) is in fact from polycyclic aromatic hydrocarbons (PAHs), which are a very fine tracer of the neutral envelope around the HII region. We can see a faint halo of emission from the edges of the neutral area which protrudes into the HII region from the northern bar (seen in the earlier JHK image). Similar details are present throughout, and there is a close correlation between the dark, neutral globules seen in absorption at JHK and corresponding emission features seen in this PAH image. We can trace the photo-dissociation regions with arc second resolution.


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NGC 2024 in JHK

NGC 2024 in JHK

This picture shows three separate images of the star formation region NGC 2024, found in the Horsehead Nebula region of Orion. This is a galactic HII region, an area dominated by the presence of ionized hydrogen. The dark patches are dust lanes, in which an enormous amount of extinction is taking place, so that light emitted by sources embedded within the region is absorbed by the dust. The bright star near the center is named Grasdalen's star, after its discoverer. The bright extended area on the south side of the image, in which the star is embedded, is an ionization front, containing hydrogen which has been ionized by absorbing energy from the light sources. The shape of the front and the distance between the ionization boundary and the energy source gives information about the strength of the source and the properties of the intervening material.

These images were taken through different filters, revealing light emitted from the object at different wavelengths (different colors). The left-most image is the K band image, centered at 2.2 microns, the central image is in the H band, at 1.6 microns, and the far-right image shows the J band, at 1.2 microns. As we move from left to right, we see light of successively shorter wavelengths. The longest wavelength is attenuated least as it leaves the object, and therefore we see furthest into NGC 2024 in the left-most image. We put these three pieces together in the next slide to make a color picture, the method used for all JHK composite images used in this set.


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NGC 2024 IR Color Composite

NGC 2024 IR Color Composite

As with most objects, NGC 2024 looks quite different in optical and infrared wavelengths. This composite photograph shows an optical image of the nebula on the left, and an infrared image on the right. The two images are displayed at the same scale and orientation, although the infrared image shows a wider area of the nebula.

NGC 2024, also known as Orion B, is a concentration of star formation activity within the Orion Nebula in our Galaxy. The optical image shows a dark lane of dust running vertically through the frame, which obscures our view of whatever the area might contain. The infrared image reveals the young stars embedded in this region, because the infrared radiation passes through the intervening dust without interacting. The color infrared image was made by combining the three separate images shown in the previous picture, using the same relation we use for all JHK composites (blue is J, green is H, and red is K). Variations in extinction and emission within the region are seen as different colors, enabling us to analyse the structure within star formation regions.


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Orion - demonstration of large-format detector

Orion - demonstration of large-format detector

This is the Great Nebula in Orion, also known as the Orion A complex, seen in the near infrared with a resolution of 1.3 arc seconds per pixel (again, blue represents the 1.2 micron J band, green shows the 1.6 micron H band, and red is the 2.2 micron K band). The stacked left panels (each 1024 x 1024 pixels in size) contain M42 (also known as OMC 1), including the Trapezium cluster and the Becklin-Neugebauer source, in the lower panel, and in the upper panel, M43 and OMC 2. The views from three generations of near IR arrays are depicted across the top, dramatizing how increasing the breadth of field enhances both the quality and the quantity of the information in an image. The upper left 1024 x 1024 pixel panel represents the size of arrays currently under development. The 256 x 256 pixel outline within this panel (centered on OMC 2) and its enlargement to the right, represent the current widely deployed generation. Within the 256 x 256 enlargement, the 58 x 62 outline and its enlargement to the right represent the first generation of arrays to be widely available. The dramatic improvement in capability flows from right to left.


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Orion in Optical and Infrared

Orion in Optical and Infrared

The familiar Orion Nebula, M42, is shown here in optical (left) and infrared (right) light. The infrared image is a J=blue, H=green, K=red false color composite. The large area of emission in the lower portion of the nebula is properly called M42, while the smaller bright area above it is M43, and the red area shown in the upper portion of the infrared image is known as OMC2, the Orion Molecular Cloud 2. Orion is a well studied star forming area, due in part to its close proximity to Earth. What we see visually as M42 is actually a blister or bubble of ionized hydrogen on the near side of a giant molecular cloud. Within the cloud, seen only in the infrared image, are very young stars to the upper right of the Trapezium cluster. Some of these stars, such as the well known Becklin-Neugebauer object, are on the verge of nuclear fusion. The OMC2 region is also seen only in the infrared image. This region gives off bright emission lines of molecular hydrogen, resulting from collisional excitation of the gas by a hot stellar wind. Infrared observations reveal a wealth of features unseen in visible light, and more importantly show how the structure of the Orion Nebula is intimately tied to the cycle of star birth inside the gas cloud.


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Orion components: gas, dust, stars

Orion components: gas, dust, stars

This collection of near infrared views of the Great Nebula in Orion (the Orion A complex) shows the difference between the distributions of dust emission (left), ionized gas (middle), and starlight (right). The left panel depicts 3.3 micron radiation from small dust particles irradiated by ultraviolet light, using a purple false-color representation. The central panel depicts the 2.17 micron radiation from ionized hydrogen, using a green false-color representation. The right panel is the same JHK composite color image used in the previous slide. In all three panels, we can see the familiar fields of M42, M43, and OMC2. Notice the pockets around the young, hot stars in the central image. These are pockets of ionized gas created by ultraviolet radiation from the included stars stripping electrons from hydrogen in the cloud. In fact, all over this field, there are outflows from young stellar objects, holes where the HII regions are systematically destroying the molecular hydrogen through ultraviolet irradiation, and filaments of fluorescent gas throughout the region. All of this exquisite detail is hidden within the Great Nebula in Orion when seen only in visible light!


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Orion OMC2 JHK

Orion OMC2 JHK

This is an enlarged view of the region in Orion which includes the molecular cloud OMC2, or Orion Molecular Cloud 2, shown as a JHK color composite image. OMC2 is a small, dense cloud located within the much larger Orion star forming complex. OMC2 is extremely young, and is known to contain a compact cluster of infrared sources as shown here. The five discrete infrared sources seen as bright spots in the red nebulosity are named IRS1-5; IRS1 is on the lower right of the horizontal bar of nebulosity below center to the right of this frame. IRS1 exhibits colors and structure indicative of a circumstellar dust shell resulting from a bipolar outflow. Throughout this image, and especially with source IRS2 at the upper right of the embedded OMC2 sources, you can see cavities and flow patterns in the nebulosity, giving clues to the structure and dynamics of this active star forming region.


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W3 star formation region in JHK

W3 star formation region in JHK

This is the star formation region known as W3, revealed by the infrared color composite technique of mapping the J, H, and K bands at 1.2, 1.6, and 2.2 microns, into blue, green, and red respectively. This object was one of the first in which the importance of infrared observations of star forming regions was demonstrated. W3 is a region of radio continuum with a core of multiple compact and ultra-compact HII regions and infrared sources. These sources show characteristics suggesting W3 is not experiencing its first burst of star formation. It is possible that very compact sources in W3 are objects in which the protostellar core has become hot enought to cause hydrogen ionization. The source W3-IRS5, shown very near the center of the frame, is one of the highest mass protostars known; it is now known to be a double source, as this image confirms.


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NGC 7538 in JHK

NGC 7538 in JHK

NGC 7538 is another area of star formation within our Galaxy, and this is a color composite image, with J, H and K mapped to blue, green, and red respectively. This picture shows a group of infrared sources in a dense molecular cloud, with substantial mass motions in the ionized gas. The optically visible part of NGC 7538 is the bluer region to the northwest. The redder infrared sources are seen to the south and east. The very extended infrared reflection nebula around IRS9 was one of the first examples of infrared reflection nebulosity known; more recently, infrared reflection nebulosity is commonplace. The horizontal streaks are due to bright stars and detector imperfections.


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Sharpless 106 JHK

Sharpless 106 JHK

The Sharpless 106 complex is, at 600 parsecs or a little over 2000 light years, a relatively nearby region of star formation. Seen here using the usual J=blue, H=green, K=red false color representation with a resolution of 1.3 arc seconds per pixel, it appears that the central source within the bipolar nebula is embedded within a dense ring of obscuring gas and dust. A powerful stellar wind is blowing off the central B0 star, causing material to flow rapidly outward from the source in two opposing directions. The outflow in this object is extended along the axis from north-east (top left) to south-west (bottom right). Looking at the neutral envelope, we can see that the so-called `central source' is severely displaced from the centroid of this outer envelope. It is not clear why this is so: perhaps the central source is drifting with respect to where it was when the mass-loss began. Perhaps there is another source of outflow responsible for the larger cavity. Polarization data show that the central source, called IRS4, is indeed inside the cavity and illuminates its walls. The emission in the Brackett alpha and gamma lines shows the extinction and therefore reveals the orientation of the outflow, confirming that the northern lobe is receding. We can also see reddening all the way down the left hand side, consistent with the notion that the source is drifting into the wall of the cavity, where the molecular cloud is denser.


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Sharpless 106 molecular and ionized gas

Sharpless 106 molecular and ionized gas

This view of the Sharpless 106 complex compares the near infrared two-micron radiation from the molecular (left) and from the ionized (right) gas. The left panel is a composite color image of three lines of molecular hydrogen representing red, green, and blue. The right panel is a composite color image of emission from ionized hydrogen (red) and from iron (green and blue). These images are built up by stepping a spectrograph slit, which gives essentially a line scan through the object, in the north-south direction, then measuring the different line strengths at each location, and reorganizing those results into images.
Looking more closely at the ionized gas using the Brackett gamma line, we can see that the southern lobe is extended parallel to the walls of the outer envelope, while the northern lobe twists and goes up and to the right. Comparing the dust image with the ionized gas image reveals a distinctly three-dimensional appearance, in that the ionized gas towards the northwest crosses over the limb-brightened edge of the dust cavity.


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NGC 2023 at 2.12 microns

NGC 2023 at 2.12 microns

This picture shows NGC 2023 as seen in molecular hydrogen emission at 2.12 microns using the KPNO 1.3-m telescope. This is a reflection nebula, seen at optical wavelengths in reflected light from the central star. This central star is not as hot as the stars in M17, so that the ultraviolet (UV) radiation it gives off is at a slightly longer wavelength and is not energetic enough to ionize hydrogen throughout the nebula. However, the UV illumination is sufficient to induce molecules of the gas to fluoresce, which they do at infrared wavelengths. This image of fluorescent molecular hydrogen maps density variations in the material, and shows how the nebula is clearly not circularly symmetric but is built up in sheets. The higher the density of gas, the more quickly the incident UV radiation will be absorbed, and the brighter the region will appear. Less dense regions require a greater length to use up the same amount of radiation, and will therefore appear larger and be less bright (bigger and more diffuse). If we deconvolve the observed appearance by imagining rays emitted by the central star in all directions, we can build up an actual three-dimensional model of the nebula.

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NGC 2023 spectra

NGC 2023 spectra

These are two micron grism spectra of selected regions in the NGC 2023 reflection nebula. A larger view of this object was shown in the previous slide. To generate this image, a mask containing multiple slits at the positions indicated with yellow lines on the right panel was employed to produce simultaneously the spectra seen in the left panel. Each spectrum covers a range from 2.0 to 2.4 microns. These spectra are dominated by various emission lines of molecular hydrogen. The ratio of these lines reveals the physical mechanism at work in the nebula, which is fluorescent emission from molecular hydrogen irradiated by ultraviolet light from the hot central B star.


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Four Planetary Nebulae at 2.12 microns

Four Planetary Nebulae at 2.12 microns

These images of planetary nebulae were obtained with the NOAO Near-Infrared Cryogenic Optical Bench (COB). Each picture was taken through a filter centered at 2.12 microns and having a narrow band-pass, or width, of only 1% of the wavelength. This filter isolates the energy released from a particular transition of molecular hydrogen. Clockwise from lower right the nebulae shown are NGC 2346, NGC 6720, NGC 6772, and NGC 6781. The field sizes in arc seconds are 221, 257, 228, and 221, respectively.
Notice the delicate filamentary structure seen outside the regions of the nebulae which are optically brighter. It has been known for decades that halos of ionized gas exist around planetary nebula. These halos can extend half a parsec beyond the central star and are the result of gas being ionized by radiation from the central star. In general, molecular hydrogen halos outline the optical halos quite closely. In these evolved planetary nebulae, the molecular hydrogen halos are probably due to shock excitation. Despite appearances, planetary nebulae might be not spherical or circular, but actually bipolar or bow-tie-shaped and the various shapes we see are due to viewing at a range of angles. An interesting technical article by Kastner et al. can be found in the Astrophysical Journal for 1994, volume 421, page 600.


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Dumbbell Nebula at 2.12 microns

Dumbbell Nebula at 2.12 microns

The Dumbbell Nebula (M 27, or NGC 6853) as seen in molecular hydrogen emission at 2.12 microns. Compared to the familiar image of this object in visible light, e.g., the July 1993 cover of Sky & Telescope Magazine, M27 in the infrared seems stark and perhaps bare. Here you will see the radial filamentary structure, which appears well correlated with the emission from neutral gas seen at optical wavelengths. There is a bright ridge of h4 emission that runs through the central star diagonally across the frame and considerable structure along the individual filaments.


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Molecular Hydrogen Doppler Image of the Galactic Center

Molecular Hydrogen Doppler Image of the Galactic Center

Infrared observations reveal areas obscured by dust, such as our Galactic Center. However, infrared techniques are not limited to just taking pictures of obscured regions, and this image illustrates the use of a technique called Doppler imaging, which is capable of determining motions. The Doppler effect refers to the change in frequency of emissions coming from a moving object. In the current use, we observe the Doppler effect for light as it changes the frequency (and thus the wavelength) of the 2.12 micron emission line of molecular hydrogen. By measuring this change in wavelength at different positions in the image, we can build up a map of the gas velocity along our line of sight. This picture translates the measured speed into color, showing the gas moving towards us as blue, while the gas moving away from us is shown as red. Gas that is neither coming nor going appears as green (the white spot in the center was caused by a bright star known as IRS7 being too bright to measure). This image was taken at the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea in Hawaii, using an instrument called the CGS4 Spectrograph. The pattern of red and blue (that is, of the motions) is consistent with a model which places the molecular gas in a ring rotating around the Galactic Center.


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Galactic Center in JHK

Galactic Center in JHK

This is a picture of the center of our own Galaxy, the Milky Way. The center is located in the constellation of Sagittarius, and forms part of what is called the Sagittarius A complex. This is a composite image, with the standard infrared bands mapped to optical colors in the usual way (blue: 1.2 micron J band, green: 1.6 micron H band, red: 2.2 micron K band). Due to very heavy obscuration by dust, the Galactic Center is not visible at optical wavelengths, but the infrared radiation passes through easily, revealing a large population of stars packed very densely together. Dark patches and dark thread-like structures visible in the picture are regions where the dust is too dense even for the infrared. The actual center itself is located in the middle in the brightest part of the image. This picture covers a region some 36 arc minutes square: the molecular hydrogen doppler image in the previous slide covers a much smaller area at the center.


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Daniel Washburn, working with the NOAO Educational Outreach Office and supported by NASA through the Arizona Space Grant Consortium, contributed greatly to this World Wide Web presentation of Infrared, Longer than Red.

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