Another valuable Internet site that has links to nearly all of the spaceborne telescopes and other instruments used for astronomical observations over the entire spectrum is at a site maintained by Danish Astronomers.
Most "star gazers" feel more comfortable looking at the luminous
bodies of the Universe - stars and galaxies - as they appear in optical imagery. But, there is generally much more "illuminating" information about celestial bodies in images depicting energy distribution and intensity of radiation associated with other parts of the spectrum. The usefulness of examining bodies outside the Milky Way at different wavelengths was earlier demonstrated in the multispectral images of the Crab Nebula shown on page I-3 of the Introduction.
Astronomers at NASA's Goddard Space Flight Center have assembled images taken at various regions of the spectrum by instruments (ground and space based telescopes, etc.) looking at our galaxy the Milky Way, as depicted in this montage (this is not a view of the M.W. taken externally but one looking towards its center and beyond towards its far edge; thus that part of the M.W. lying behind us [further from the center] is not included. Below, each image is identified by its imaging wavelength or wavelength interval of the spectrum, together with a brief description of the principal information that is associated with data collected from that region. Starting from the top:
(1) Atomic Hydrogen (1420 Mhz): Picks out radiation from excited neutral hydrogen in interstellar gas and dust clouds. (2) Radio Continuum (480 Mhz): Signal produced by fast-moving electrons; good for spotting sites of now diminished supernovae. (3) Molecular Hydrogen (115 GHz): Shows distribution of molecular hydrogen associated with carbon monoxide in cold interstellar matter.
(4) Radio Continuum (2.4-2.7 GHz): Caused by high energy electrons and associated warm, ionized gases.
(5) Far-Infrared (12-100 µm): Radiation emanates from dust heated by stellar radiation; emphasizes active star-forming regions. (6)Mid-Infrared (6.8-10.8 µm): Due to excitation of complex molecules in interstellar clouds and in cooler reddish stars.
(7) Near-Infrared (1.25-3.5 µm): Reveals temperatures, mainly of Giant, relatively cool stars, and shows the galactic core; dust is "transparent" in this spectral region and does not obscure many luminous features. (8)Visible Light (0.4-0.5 µm): Displays primarily nearby stars and thin ionized gas; dark areas cold.
(9) X-Rays (0.25-1.5 kiloelectron-volts): Reveals gases heated by shock waves from supernovae.
(10) Gamma-Rays (300 megaelectron-volts): Pinpoints high energy sources coming from pulsars or phenomena stemming from cosmic-rays. More information about these images is presented at this Goddard Multiwavelength Astronomy site. This idea of imaging cosmological entities at different wavelengths can be further enforced by looking at the montage of five views of the star Centaurus A in the wavelength regions indicated on each panel. A point to be kept in mind in looking at images below, as well as on preceding and subsequent pages: Images acquired in any of the specific regions of the EM spectrum do not necessarily look the same - some may appear notably different than others because of the way in which the image is processed and displayed (for example, different filters may be used or the image values for intensity may be rendered in color-coded levels assigned different colors). We'll start our survey of cosmic images and data sets obtained at different wavelengths by examining the phenomena obtained using the highest EM energy sources (shortest wavelengths; highest frequencies) - gamma rays. Gamma radiation is observable over the entire sky. It may appear as a diffuse glow or as localized (point) sources. Several modes of generation of the gamma rays have been considered: black hole influence; neutron star attracting infalling material; supernovae; dark matter itself; collisions between antimatter and matter. One way in which this radiation is observed is in short-live gamma ray bursts. On April 5, 1991 NASA launched the Compton Gamma Ray Observatory (CGRO) as a complement to the HST that extends coverage into the short wavelength, high energy end of the EM spectrum. It carried four instruments that could measure radiation whose energies range from 30 MeV to 30 GeV. This huge (central part nearly the size of a school bus) sensor platform has been one of the most productive astronomical observatories orbited so far. It is shown in this artist's drawing:
The individual range of coverage by the CGRO sensors is shown in this plot: The acronyms stand for BATSE = Burst and Transient Source experiment; COMPTEL = Imaging Compton Telescope; EGRET = Energetic Gamma Ray Experiment Telescope; OSSE = Oriented Scintillation Spectral Telescope. (The CGRO was named to honor Dr. Arthur Holly Compton, an eminent physicist, Nobel Laureate, and Chancellor of Washington University when the writer [NMS] was a graduate student there). The CGRO was designed to measure radiation associated with stars and galaxies which result from high energy, usually nuclear processes. It looked particularly at supernovas, quasar and pulsar emissions, black hole accretions and other powerful stellar processes (next paragraph). CGRO discovered a new class of energetic objects, called blazars, that give off energy in the 30 MeV-30-GeV range, but actually produce detectable energy over the entire spectrum. Redshift studies (page 20-9) indicate most blazars are far from Earth and therefore quite old. While distant blazars look like bright single stars, they are actually associated with galaxies in an advanced stage of inflow of copious amounts of stars and gas/dust into supermassive black holes (with masses billions greater than the Sun), in so doing generating huge amounts of energy release. This means that the high luminosity correlated with high energy release persists over long-term telescope viewing. Electric and magnetic fields usually carry the luminescent materials as directional jets; thus for one to be seen from Earth our detectors must fall within the cone of a jet to been seen. That mechanism is depicted below, and beneath it is an image of the starlike blazar jet emanating from Markanian 421.
One prime astronomical target of the CGRO was to search for Gamma Ray Bursts (GRBs), which are huge releases of energy that are short-lived and variable, are widespread in the celestial sphere and occur mainly in galaxies. Here is a map of those bursts measured over time by CGRO; the local effects of the Milky Way bursts have been removed. These GRBs will be discussed in more detail at the bottom of page page 20-6. For now we will show one example of a GRB imaged by the telescope at the European Southern Observatory in South America. The BATSE instrument has produced the following map of GRBs across the sky: Another GRB was detected in the vicinity of the star Vela. Here is a plot of associated energy levels: A plot based on frequencies (which can be converted to energies) was obtained by the CGRO for the pulsar Geminga: A great deal of information about gamma rays comes from studies within the Milky Way. The CGRO has produced this image showing a generalized gamma ray energy distribution over the entire M.W. disk: A somewhat more refined version puts the highest levels of radiation in brown. More detail within this general halo is brought out by special processing, which indicates regions of strong gamma rays that may be pulsars or other concentrated but steady sources: The OSSE instrument on CGRO picked up an unusual distribution of gamma ray energy, shown in this figure as the red glow above the Milky Way plane. It has been interpreted as a region in which antimatter (electrons are positively charged [positrons] and protons have a negative charge.) has interacted with conventional matter, releasing a huge amount of energy. It may seem surprising that telescopes can pick up evidence of radioactivity associated with stellar or galactic material. One radioactive isotope, Al26, is fairly aboundant in galactic gas and dust. With a rather short half-life, when it decays it produces abundant gamma radiation. Here is a CGRO image made by the COMPTEL instrument which shows the distribution of this radiation associated with Al26 decay. A novel application of the CGRO EGRET was devised by R. Petry, who accumulated 7 years of data when the Observatory was programmed to look at the Earth itself. The four panels shown below are imagery taken at different gamma ray intensity levels, each assigned a color, with the fourth (lower right) panel being a composite. Much of the radiation is associated with cosmic rays but about 60% is of terrestrial origin. The Compton Gamma Ray
Observatory was a major achievement guided by astrophysicists and operated by NASA Goddard. You can learn more about its results, with many additional images, at the CGRO site. On June 4, 2000 the CGRO was deliberatly decelerated so as to enter the atmosphere over the Pacific, as its orbital decay (adjustment fuel exhausted) meant it might fall to Earth at any time soon, possibly threatening populated areas. An ESA satellite called Integral has made a variety of observations of the M.W.'s galactic center, where the gamma radiation is most intense. This image shows variations in intensity within the center. Another Integral image is designed to pick out individual major sources of gamma radiation in the inner Milky Way. Some of these might be gamma ray bursts but most last longer and may be pulsars. A more powerful, higher resolution gamma ray observor, GLAST (Gamma ray Large Area Space Telescope), is scheduled for launch in 2005. Several mechanisms account for this x-ray generation. Most prevalent
is excitation into ionized states of intragalactic gases between stars or gases between galaxies
that, in the tenuous void separating the stellar bodies, are traveling at such
high velocities that they represent temperatures in excess of 1,000,000 °K
capable of producing strong x-ray responses. The second Rosat image portrays x-ray variations spread over the entire Coma
supercluster, comprised of well over 1000 bright galaxies, located some 300,000,000
light years away. X-ray intensities vary from strong in reds to decreasingly
weaker in greens to blues and purples. The interstellar gases emitting this
radiation make up about 10% of the total mass of the supercluster, along with
2% more in the stars found in the individual galaxies as determined from
optical measurements; the remainder of the mass is presently unaccounted for
after inventories across the spectrum are related to their sources, so that
the bulk of the mass is presumed associated with dark matter (see page 20-9). Thus,
examining both galaxies and intergalactic regions using radiation at wavelengths
both shorter and longer than the visible helps to quantify the distribution
of the entire mass of the Universe. A star in the Vela galaxy is a typical example. As seen by Rosat, it looked like this: When variations in x-ray intensities are determined and displayed by color coding, this results: CSRO provided this image in which the energy levels are color-shaded: That energy spectrum can be quantified for this object, as shown in this plot: In September of 1999, NASA, guided by scientists from several nations, launched the Chandra X-ray (Telescope) (CXO).
Named after the late S. Chandrasekhar, a reknown astronomer from India, Chandra is managed by the Marshall Space Center.
Its length, when fully deployed, is 13.6 m (45 ft). It carries 4 sensors: a charge-coupled imaging spectrometer, a High Resolution camera, and High and Low Energy gratings.Its spatial resolution is 8 times greater than the best previous X-ray observatory and can pick out objects 20 times fainter as sources of x-radiation. Here is Chandra in space, as photographed from the Space Shuttle from which it was launched:
Its astronomical targets include quasars, supernova and other high energy-emitting objects. Here is an example of an image of a ring of x-radiation associated with the remnants of a supernova in the Constellation Tucane:
Chandra has made images of regions of more recent star formations (sometimes as bursts) in the Milky Way. This one is striking indeed. The Milky Way galaxy has a powerful x-ray source at its center probably associated with material infall into a Black Hole, as imaged thusly: Imaging in the x-ray region of the spectrum commonly picks up a completely different picture of the object being imaged in the visible or other regions (see page I-3). This is exemplified by this artist's reworking of the visible and x-ray renditions of Eta Carinae in the Milky Way galaxy. The Chandra view shows an outer ring of high energy particles not having any obvious counterpart in the HST image.
When the Carina nebula is imaged using narrow band filters, it looks like this. The blue image used to construct a color composite results from excitation of oxygen in the nebular gases; green represents hydrogen in this rendition; red is associated with sulphur ions. We emphasize these points with these three views: Visible (top); X-Ray (middle); Radio Wave (bottom) segments of the EM spectrum, for a exploded star in the nearby Large Magellanic Cloud (a nebular satellite galaxy) Because Chandra measures x-radiation from its targets over a range of wavelengths, individual elements which give off x-ray spectra at specific wavelengths can be detected and mapped. This has been done for the supernova Cassiopeia A. An HST optical image of this exploding star looks like this: Here is a four panel set of Chandra images of Cassiopeia A (see also page 20-6). The upper left is color density map of the broad band radiation from Cassiopeia A. The upper right focuses on Silicon emission lines; the lower left on Calcium; and the lower right on iron. Thus Chandra is an adept tool for determining the distribution in the expelled material of various elements that were produced by nuclear burning in the star.
Chandra has explored our Milky Way galaxy as well. This next image shows part of the central core region of the galaxy (about 400 light years wide) in which a number of very bright objects, seen in x-radiation, correspond to high energy emissions where interstellar gases are drawn into white dwarfs, neutron stars, and possible black holes. becoming continuously "ignited". A spectacular image of part of this central region was made by the Advanced CCD Spectrometer on Chandra: Chandra scored a first in late summer of 2003. As it monitored a galaxy in Perseus (as imaged on the left), it also detected a signal that is best interpreted as evidence of sound waves passing through the galaxy. Calculations show the sound to be in the "musical note" of B flat, but 57 octaves below the lowest octave on a standard piano. The sound waves were rendered into an image (on the right): In December of 1999, the European Space Agency launched an even more powerful x-ray telescope known as XMM-Newton (XMM stands for X-ray Multi-Mirror). Here are two colorized images, the first showing the variations in x-ray intensities in several of the Hickson group of stars and the second showing details of a supernova explosion in the nearby Large Magellanic Cloud: XMM-Newton has demonstrated that large x-ray energy bursts also associate with the starbursts that mark development of young stars. Here is an image of NGC253, some 8 million light years from Earth; the inset on the left is a closer look at its center. XMM-Newton has taken a series of images over a period of days that can be sequenced to give a movie-like effect of the expansion and dissipation of materials during a burst. We will cite the Internet connection on which this rendition was accessed, with the proviso that it may no longer be active. So, click on starburst to see if the "show goes on". Satellites began to examine the UV region of the sky with the OAO series (OAO-3 was named Copernicus) in the late 1960's. The follow-on International Ultraviolet Explorer (IUE) program began in 1978. Copernicus led to maps of bright UV stars such as this:
The ultraviolet (UV) region of the spectrum, from 70-2000 (0.007 - 0.2 µm) (Far) to 2000-4000 Angstroms (0.2 - 0.4 µm) (Near), has provided interesting images of stellar bodies, including the Sun. It also contains many diagnostic spectral lines helpful in determining elemental composition. This next image shows the Earth as imaged by EUVE (Extreme UltraViolet Explorer, launched in 1992 and operating until February, 2001; imaging from 70 to 760 Angstroms). It shows excited helium (yellow) and hydrogen (orange) in an auroral field extending well beyond the solid Earth. Looking outward into space, the EUVE provided this image of the Vela Supernova: One of the first UV telescopes is the IUE (International Ultraviolet Explorer) launched jointly by ESA and NASA in 1978; it operated into 1996. This is a UV image of the galactic source NGC1680: The Ultraviolet Imaging Telescope (UIT) was flown as part of Astro-1 and Astro-2 lab packages on Shuttle STS-35 and STS-67 in the mid-1990s. The telescope covers the UV range between 1200 and 3200 Angstroms. It is particularly adept at recognizing hot, young stars which give out strong UV radiation. The difference in appearance between visible and ultraviolet images is pronounced in this UIT view of the galaxy M94:
This next image shows three galaxies in UV (top) and Visible (bottom); note the structure of the spiral arms as brought out by molecular hydrogen excitation
In this UIT image, the globular cluster Omega Centauri in visible light appears to consist of mainly red to orange stars, typical of older stellar bodies. But, the UV on the right shows that there are also many younger, hotter stars. Launched on June 24, 1999, FUSE (Far Ultraviolet Spectroscopic Explorer) gathers spectra in the interval 910 - 1180 Angstroms. The program is run out of Johns Hopkins University, with NASA, French, and Canadian partners.
Excitation of molecular and elemental species in a star's atmosphere or a galaxy en masse in this interval provides valuable information about stellar processes. Here is a typical spectral plot obtained by FUSE from observing a galaxy. Observations through the FUSE telescope can be converted to images, such as this: FUSE's primary goal has been to trace the history of the early Universe by monitoring the distribution of Hydrogen (H), Deuterium (D), and Helium (He) in the intergalactic medium. Preliminary results indicate that helium, formed in the first minute of the Big Bang, and then dispersed during the expansion, will prove a sensitive indicator (it is also well monitored by FUSE) of the inhomogenieties in the expanding Universe following the initial explosion. This is a genralized diagram of the ratio of D to H since the Big Bang, as Deuterium is converted to He through H fusion. FUSE will try to obtain data that lead to a more specific plot. JPL has developed GALEX (Galaxy Evolution Explorer), which was launched on April 28, 2003. Designed to gather imagery in the far and near ultraviolet (FUV and NUV), it will concentrate on monitoring distant galaxies and stars (out to at least 10 billion l.y.) to determine the conditions under which they had formed in the early years of the Universe. This image of Galaxy M101 was made by combining the FUV (blue) and NUV (green) images with an image (oranage) obtained separately by the Digital Sky Survey telescope: Here are a trio of images of Galaxy M51, one in the UV (Galex), the middle from an optical telescope, and the one on the right in the Near-IR (2Mass project) A final look at a galaxy imaged by GALEX entirely in the UV: NGC 1232 is shown here as a partial false color composite made from two UV bands: Thus, the UV is proving to be an optimum segment of the EM spectrum to study conditions in the so-called empty space which actually contains hot interstellar gas. CHIPS (Cosmic Hot Interstellar Plasma Spectrometer) is an astronomy satellite to be launched in December 2002. It will measure the diffuse extreme ultraviolet glow that will better define the properties and physical processes associated with the interstellar medium. A review by Univ. of Calif-Berkeley scientists of the mission's purpose and some results is available at this
UC-Berkeley site. In 2004. NASA, along with several cooperating Universities and organizations, launched SWIFT, a telescope observatory satellite whose prime mission is to search for Gamma Ray Bursts and then examine their sites in UV and Visible light. SWIFT carries a gamma-ray detector, an X-ray detector, and a detector whose operational range includes parts of the UV and Visible. Below is the first image, of the Pinwheel Galaxy, made by this third instrument: The UV carries to the Visible spectral range. Just beyond the Visible is the Infrared, extending from about 1 to 1000 µm. Much of the interval coincides with the thermal IR which you studied in Section 9. Hot stars are strong emitters in the IR and can be studied both as images and from their spectra. Other astronomical features amenable to IR observations include properties of accretionary disks and interstellar clouds, the structure of the H II type stars (those in an early stage of development that contain significant ionized hydrogen in the inner part of the hydrogen gas cloud that is the source of their nuclear fuel), and the dynamics of the Milky Way. Viewing galaxies and regions of heavy dust densities in the Infrared has a distinct benefit compared with seeing the same features in the Visible. This image pair vets this statement (read its caption): Small dark interstellar dust that obscures stars in the Visible are called Bok Globules (discovered by a Dutch astronomer of that name. They represent nebular gas and dust nearing the protostar phase (see page 20-5); such molecular hydrogen clouds are very cold (-263°C) and generally because of their small size (about a parsec) produce only one to several stars. These globules (some of which can be nearly spherical) stand out best in images that extend into the Near IR, as exemplified by this photo taken through the Anglo-Australian Telescope (AAT):
These two photos (acquired by ESA's New Technology Telescope) show details of a Bok Globule in Barnard 68. The left image is made from three bands in the visible; the right image consists of bands at 1.25 µm = Blue; 1.65 µm = green; and 2.16 µm = red, which renders the cloud now partially transparent so that stars behind it become visible. A galaxy (NGC2024) that is still largely shrouded by dust looks much like a visible image in this version made by the NICMOS camera on HST. The color composite consists of Blue = J band (1.6 µm); Red = K band (2.2 µm); and Green = J and K combined. One of the first infrared-dedicated satellites was IRAS (Infrared Astronomical Satellite) launched in January of 1983. Its sensors were tuned to the 12, 25, 60, and 100 µm IR wavelengths. During its lifetime, IRAS discovered more than 350,000 previously undetected IR objects in the sky. This color composite of the interstellar "cirrus" clouds made up of gas and dust grains in the Milky Way that occupy a wide field centered on the North Celestial Pole is constructed from Blue = 12; Green = 60; Red = 100 µm. On a grander scale, look at this IRAS image of the now familiar neighbor, the Andromeda Galaxy, with color-codes indicating variations in thermal emission at 12 µm. IRAS obtained IR imagery of the Milky Way. In this next image, the central part of the galactic plane is displayed. Much of the yellows and oranges is thermal radiation from dust that obscures individual stars. Other IR observatories have since been placed in space. ISO, the Infrared Space Observatory, was operated by ESA from November '95 until May '98. The instruments include an IR camera, a spectrometer, and a polarimeter. The spectral range was 2.5 to 240 µm. This is a colorized image of the Whirlpool Galaxy. A short IR wavelength plot of radiation received from NGC6543 shows peaks correlated with argon, neon, hydrogen, and sulphur which occurs in the dust and gas nebula associated with this, the Antennae galaxy. The star GL2591 is surrounded by a dense cloud. Spectra in the Short Wave IR interval sampled by IS0's spectrometer disclose water ice, carbon dioxide ice and silicate particles in the dust grains within the enclosing material. On August 24, 2003 NASA launched a major new telescope - one of the Big Four of the Great Observatory series in its current astronomy programs - SIRTF (Space Infrared Telescope Facility). Following a contest to rename this powerful new sky-searcher, it is now called the Spitzer Space Telescope (SST), named after the pioneer astronomer Lyman Spitzer. SST is comparable in its capabilities to the HST and Chandra. The orbit is heliocentric and Earth-trailing. Its instruments operate in the infrared between 3 and 180 µm, which includes much of the thermal infrared spectral region. Its primary mission will be to peer through cosmic clouds and dust (usually, transparent in the infrared) to look back in time to see galaxies and stars in their earlier stages of development. A preview of the SST is given at a JPL lecture. Access this at von Karman lectures, entering Webcast into the Format box, and choosing the topic "The Space Infrared Telescope Facility", June 12, 2003, clicking on RealPlayer.
Here is a sketch of the SST in orbit:
First data were released on December 18 of 2003. This panel of four images is typical of first results: The upper left panel shows the spiral galaxy M81 in a false color thermal infrared composite. Upper right is a Haro-Herbig star seen as a thermal object (in visible light it is masked by clouds). The lower left panel is a view of Comet
Schwassmann-Wachman; the remaining panel shows the (ordinarily indistinct in visible light) Dark Globule IC1396. Lets take a closer look at three of these. This illustration enlarges the Dark Globule IC1396 and shows on the right this same image as sensed by two of the instruments on SST. As stated above, one powerful attribute of the SST is its ability to "see" through thick clouds of dust. A region within the Milky Way about 10000 l.y. away contains a great clot of dark dust in which almost no stars are visible. When viewed in the IR by SST stars and glowing gas were revealed; among these were some very large stars, thus demonstrating the stars up to 100 times more massive than the Sun are still forming in our galaxy and, by inference, probably throughout the Universe (thus big stars can be recent in time of formation, even though not long-lived). This ability to penetrate unresolved dust or close-spaced, very distant galaxies is making Spitzer a powerful tool for studying galaxies formed early in Universe history. The left image below shows what appears to be a uniform nebula imaged in the infrared from the ground by the United Kingdom's SCUBA instrument; nothing beyond is visible. When the SST looked at the same area, at different IR wavelengths, faint galaxies, very far away and hence seen now in their earlier stages, are now resolved (the bright orange feature is a nearby star in the line of sight. As has already been demonstrated elsewhere in this Section, combining stellar images made at different wavelengths by different instruments can be symbiotically informative. Here is a color composite of band images made by the SST (in the IR) and Chandra (X-ray region) of the expanding Tarantula Nebula: On Valentine's Day, February 14, 2004, the Internet contained one of the most beautiful images returned by SST. It has been named the "Rose Nebula", for reasons obvious in this image: This gas-dust cloud, NGC 7129, lies 3300 l.y. from Earth. The rose tint comes from heated dust, the green from carbon monoxide. The colors involved in the image are from these wave bands: 3.6 µm = blue; 4.5 µm = green; 5.8 µm = orange; 8.0 µm = red. The SST has a spectrometer that can recognize chemical components of a star and its surrounding cloud. For HH 46/47, shown again in the inset, the spectral curve contains strong absorption bands that indicate the presence of water ice, methyl alcohol, silicate particles, and carbon dioxide. Now, lets move farther out to longer wavelengths in the EM spectrum. Astronomical objects, in particular galaxies and supernovae, emit the gamut of radiation across the spectrum. Galaxies are usually strong emitters of microwave radiation, in particular in the radio region. Radio waves are generated by excitation of neutral hydrogen. A good general review of radio astronomy has been prepared by the Haystack group at MIT.
The specialized field of radio astronomy utilizes large "dish" antennas to capture the long wavelength radiation. One of the first radio wave monitors is the famed Arecibo site in Puerto Rico, in which the parabolic receiver is embedded in a limestone sink in the jungle. The dish, 305 meters (just over 1000 ft) wide, is fixed in orientation and must use the rotation of the Earth to examine parts of the astronomical heavens. The largest movable telescope in the world is the 100 meter radio antenna facility at Effelsberg in Germany. It can both rotate and swing up and down.
Resolution of celestial targets from which radio waves emanate can be improved by developing a synthesized aperture by means of electronically hooking together individual radio telescopes. A major facility in the National Radio Astronomy Observatory group is the Y shaped array of 27 radio telescopes, each 25 m (81 ft) in diameter, located in the flats 70 miles west of Socorro, New Mexico. This creates an effective resolution of 36 km (22 miles). This Very Large Array (VLA) mode uses principles of Interferometry to process the signals from each telescope as a unit. More background information on radio interferometry can be found at these Australian and Canadian Web sites. In essence, the same signals are received almost simultaneously at different receivers; when added together these may be out of phase and may cancel out or reenforce at specific wavelengths; computer processing allows a new interference signal to be produced. Radio telescopes separated by hundreds and even thousands of kilometers can be tied together by electronic wiring or radio signals to each other to produce an array called VLBI (Very Long Baseline Interferometry). The effect of integrating the telescoope signals is to increase the resolution significantly, so that smaller features in radio objects can be discriminated. One of the major tasks of radio astronomy was to survey the sky at 21 cm to pick up the distribution of neutral hydrogen in the Milky Way and the halo around our galaxy. Here is the result: More details about the central region of the Milky Way appear in this radio telescope image made at 90 cm. Still another set of information about the Milky Way is evident when viewed at 408 cm. Whole galaxies are imaged at the 21 cm H wavelength. Here is M81: In the early days of radio astronomy, many radio sources in deep space were discovered but when the same region was examined by optical telescopy often no obvious galaxy or other stellar body was found at first. Later observations at non-radio wavelengths have now detected the astronomical feature, usually a galaxy (many galaxies are very strong radio wave emitters). One of the best examples of powerful energy emitters in which visible images do not detect any obvious sources is Cygnus A, from a galactic center about 700 million light years away. Cygnus A is the strongest radio wave emitter in our part of the Universe. Consider these images:
In the above image, the upper left shows a visible light image (star groupings in bright blue) but with no obvious galactic shape; however as colorized in red are two distant lobes representing radio wave signals associated with Cygnus A. The lower left image is another radio wave rendition of signals received at 6 cm. The lower right, made by HST, reveals some strong radiation coming from the central region of Cygnus A. When that region is examined through a ground-based x-ray telescope, again it shows below as an expanded area of matter giving off high amounts of energy at short wavelengths.
The inset at the lower right is another radio wave image (note that there is a corresponding area for each lobe in the x-ray image.
Here is a galaxy seen in the Infrared, on which is superimposed the intensity contours associated with two radio sources in the limbs that once seemed isolated from this distinct galaxy. This next image shows an L-Band image of the Starburst Galaxy; this was made at the Jodrell Bank Radio Telescope Observatory near Manchester, England, one of the premier facilities in the field. The signals were obtained from the MERLIN (Multi-Element Radio Linked Interferometer Network). A longer wavelength radio image acquired by the MERLIN VLBI system shows the binary star pair SS433. Contour lines show the extent of radio wave activity outside the central region occupied by the star pair.
The Red Giant Star Betelguese (see next page) has been imaged within the microwave region (outside the main radio interval) at 7 mm. Under these conditions it was possible to measure a temperature profile (right) in the expanded gas envelope (photosphere) around the star. Supernovae (see page 20-6) are strong sources of radio waves. They expand so rapidly that time lapse images taken months apart can monitor their spread and the changes in shape of the radio wave field. Here is such a sequence for Supernova SN1993J in the galaxy M81. The images on the left were taken at 3.6 cm; those on the right at 6.0 cm. As more radio telescope images of galaxies have accumulated, a distinct pattern has been found with galaxies having a powerful central black hole (see page 20-6). One or more powerful jets (material being expelled at speeds approaching that of light) are the hallmark of this type. Below is a series of panels picturing different radio galaxies that show these lobes of ejected materials. The reader might have had a thought during this review of radio astronomy: Why not put a radio telescope in space? But, wouldn't the antenna be much larger than is commonly on satellites. The answer is "No" if the VLBI concept (above) is employed. The Japanese Space Program has developed and launched HALCA (Highly Advanced Laboratory for Communications and Astronomy) in February 1997 as the kingpin in their VSOP (VLBI Space Observatory Project) program. The radio satellite has a 25 m antenna and looks like this: HALCA's orbit is elliptical, with its perigee (closest approach) at 1000 km and apogee (farthest) at 20000 km. When coupled electronically with one or more radio telescopes on the ground, the effective diameter of the joint system is greater than that of the Earth itself (12755 km). This creates a very high resolution radio wave detector (in some applications, 1000x better than the HST) when used in the Interferometer mode. Although HALCA experienced some trouble in 1999, it did send back considerable data and proved the concept of using multiple integrated radio receivers to achieve exceptional resolution. Here are three images of quasars (see page 20-6) at considerable distances from Earth that illustrate one of the ways in which HALCA data can be displayed: Plans to put other radio telescopes in space are now active. Principles behind using multiple radio telescope satellites, in formation flying, to increase resolution are examined on JPL's Starlight program site. JPL has a brief synopsis of the forthcoming Space Interferometry Mission (SIM) Access it through the JPL Video Site, then the pathway Format-->Video -->Search to bring up the list that includes "The Keys to the Stellar Kingdom", January 29, 2003. To start it, once found, click on the blue RealVideo link. Since we have introduced the specialized technique of interferometry on this page, it is now appropriate to revert back to imaging in the visible spectrum to mention the CHARA (Center for High Resolution Astronomy; operated by Georgia State Univ. astronomers) project which is now commencing operation at the famed Mt. Wilson Observatory (in the mountains north of Los Angeles), shown here: The large central observatory dome houses the famed 100 inch Hooker telescope that Edwin Hubble used to track down galaxies outside the Milky Way and to measure redshifts, laying the foundation for the Big Bang model. In the above picture are several of the 6 auxiliary optical telescopes tied to the main telescopes. Working in pairs, and later in larger combinations, light from separate components of the array must be combined and synchronized to produce interferometric images in which the waves reenforce rather than cancel. This multiple system produces a baseline (at optimum, 1080 feet) that greatly increases the angular resolution of the central telescope, thus providing images that are expected to exceed the Hubble Space Telescope in sharpness. To get the signals from two or more telescopes into coincidence (the light arrives at any two pairs at slightly different times), one beam is sent through an optical pipe that contain movable mirrors mounted on rails (the "delay line"). The mirror(s) are moved until the extra distance traveled by light to the second telescope (relative to the first) is just compensated enough (equalized) to bring the two signals into phase. This delicate adjustment is made through a computer program that controls pathway adjustments. One of the most unusual observatory systems now operating productively is AMANDA-II (Antarctic Muon and Neutrino Detector Array), designed to not only detect neutrons but to locate them in the celestial sphere and possibly associate any concentrations of neutrons with discreet sources. The AMANDA is a series of more than 600 glass optical detectors buried in the solid ice 1.5 km below the surface. Here is the first preliminary map of results:
So far, some of these blue dot point sources have been matched with galaxies; others have yet to be correlated with known sources. The neutrinos seem to be generating in the interiors of large galaxies, particularly those with suspected supermassive Black Holes. However, the thick blue band near the equator relates to the Milky Way, which demonstrates other possible stellar sources. With this examination of space observatories that collect data over different parts of the spectrum, we now return to the exposition of aspects of Cosmology by looking at the origin and evolution of individual stars.(1)
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Chandra has imaged a stricking example of a bow shock wave developed in a cluster of stars in a small galaxy known as IE0657-6. Gases within the cluster are extremely hot, around 100 million degrees Centigrade. The bow shock (right side in the next image) was produced from a collision with a subcluster of stars whose gases were around 70 x 106 degrees C. In time much of the gases will be blown out from the present configuration.
