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. Likewise, we repeat the Net address for space observatories that was given on page 20-2. A list, many with Web links, of both ground- and space-based telescope observatories is accessed at this Wikipedia site.
The entire EM spectrum has been utilized to study galaxies, stars, and other astronomical phenomena. As a reminder, here is still another diagram that depicts the full spectrum and its subdivisions. On this page we will examine examples of astronomical images beginning at the shortest wavelengths (Gamma Ray region) and ending at the longest (radio waves):
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. In fact, because dust often obscures phenomena within a galaxy when viewed in the visible, the ability to penetrate that dust using other wavelengths reveals many aspects or characteristics of the composition and structure of galaxies that greatly expand our knowledge of the nature of both galaxies and intergalactic space. The usefulness of examining bodies outside the Milky Way at different wavelengths was earlier demonstrated in the multispectral images of the Crab Nebula shown near the bottom of page I-4 of the Introduction. Both ground and space observatories are operating, or will be activated later, in parts of the EM spectrum beyond the traditional visible light range.
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; also, that part of the M.W. lying in the halo behind us is not included. The layout in each image is reconstructed as continuous; since Earth is within the galaxy, both the parts towards its center and those beyond can be seen by looking outward from various points and times from the Earth. 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. First shown is an image of the M.W. that serves as a reference map on which useful star markers are plotted. Then, starting from the top, which is also the longest wavelengths, is a full microwave image of the M.W., with those beneath arranged by decreasing wavelengths:
(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. 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. Perhaps you noticed that one part of the EM Spectrum was omitted in the above M.W. sequence, namely, the Ultraviolet. As we shall see below, this segment has useful information.
A point to be kept in mind in looking at images below, as well as on preceding and subsequent pages: Images acquired by the same or different telescopes for 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). Thus, the same target in the sky, such as a specific star or galaxy, may show up with distinct differences when the image processing choice changes parameters. To illustrate, look at the different-appearing renditions that result when wavebands in discrete spectral regions, such as various parts of the infrared, are utilized, as shown by this M81 panel: 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. The Soviet space program launched the International Astrophysical Observatory "Granat" in late 1989. Its seven instruments monitored stars and galaxies in the Gamma Ray and X-ray regions of the spectrum. It operated until 1998. 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 in St. Louis. 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 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: 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 abundant 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. 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), was successfully launched on June 11, 2008. (It has since been renamed the Fermi Gamma Ray Telescope.) Here is a drawing of the spacecraft with its two principal instruments. You can check its status by going to the GLAST website. The objectives of GLAST are as follows: 1. To understand the mechanisms of particle acceleration in AGNs, pulsars, and SNRs. This understanding is a key to solving the mysteries of the formation of jets, the extraction of rotational energy from spinning neutron stars, and the dynamics of shocks in SNRs. 2. Resolve the gamma-ray sky: unidentified sources and diffuse emission.Interstellar emission from the Milky Way and a large number of unidentified sources are prominent features of the gamma-ray sky. 3. Determine the high-energy behavior of gamma-ray bursts and transients. Variability has long been a powerful method to decipher the workings of objects in the Universe on all scales. Variability is a central feature of the gamma-ray sky. 4. Probe dark matter and early Universe. Observations of gamma-ray AGN serve to probe supermassive black holes through jet formation and evolution studies, and provide constraints on the star-formation rate at early epochs through photon-photon absorption over extragalactic distances. There are also the possibilities of observing monoenergetic gamma-ray "lines" above 30 GeV from supersymmetric dark matter interaction; detecting decays of relics from the very early Universe, such as cosmic strings or evaporating primordial black holes; or even using gamma-ray bursts to detect quantum gravity effects. Scientific results from GLAST/Fermi have been notably slow in release, although data gathering began in August. Prior to launch, this simulated Gamma Ray sky, centered on the Crab Nebula, anticipates the eventual data displays. The points of bright light are various gamma-ray bursts, most being orders of magnitude more energetic than the views associated with stars seen in visible light. Now, let us look at high energy radiations in the X-ray region. The first focused telescopes that operated in this region were the three HEAO (High Energy Astronomical Observatory) satellites operated by Goddard Space Flight Center and flown in the 1970-1980s:
HEAO-1 found many X-ray sources within the Milky Way and also outside our galaxy (some sources have yet to be identified with specific visible sources: HEAO-2, renamed the Einstein X-ray Observatory, gathered data between October, 1978 and April, 1981. The spacecraft had these components:
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
Here are two parts of the sky imaged by the Einstein X-ray Observatory:
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 next 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.
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: 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 (see page 20-6 for more information about supernovae). An HST optical image of this exploding star looks like this: Here is a four panel set of Chandra images of Cassiopeia A. The upper left is color density map of the broad band radiation from this supernova. 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. A notable discovery made by XMM-Newton is a huge (109 solar masses) concentration of fast-moving (1000 km/sec) gas, shown in this image: This mass of gas has been likened to a "mega-comet" (but it is not a true comet in the Solar System sense) some 5 million light years in length. The gas appears red to yellow, representing the mapping of data into entropy values. The feature is within the galaxy cluster Abell 3266. Its discoverers believe it to be held together by Dark Matter. 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 dynamic rendition can be accessed - with the proviso that it may no longer be active. So, click on starburst to see if the "show goes on". Still on the drawing boards (not fully funded) is the Constellation-X Observatory that will be the most sophisticated X-ray satellite yet; launch will be in 2017 or later. Satellites began to examine the UV region of the sky with the OAO series (OAO-3 was named Copernicus) in the late 1960's. Copernicus produced 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. In the 1970s, the Soviet space program installed UV Observatories, named Orion 1 and Orion 2, on a Salyut and Soyuz spacecraft respectively.
This next image shows the Earth as imaged by EUVE (Extreme UltraViolet Explorer, a free flying observatory 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 the UIT image below, 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. Deuterium was detected in this FUSE UV image of star AE Aurigae, which in visible light is shrouded by dust:
Preliminary results from FUSE indicate that Helium, formed in the first minute of the Big Bang, and then dispersed during the expansion, will prove a sensitive indicator of the inhomogenieties in the expanding Universe following the initial explosion. This is a generalized diagram of the ratio of D to H since the Big Bang, as Deuterium is converted to He through H fusion (thus, decreasing the ratio). 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: 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 (called CHIPSat) launched on January 12, 2003. It conducts an all-sky spectroscopic survey of the diffuse background at wavelengths from 90 to 260 Å with a peak resolution of about 0.5 eV. CHIPS data is helping scientists determine the electron temperature, ionization conditions, and cooling mechanisms of the million-degree plasma believed to fill the local interstellar bubble. The majority of the luminosity from diffuse million-degree plasma is expected to emerge in the poorly-explored CHIPS band, making CHIPS data of relevance in a wide variety of Galactic and extragalactic astrophysical environments. Thus, it has measured 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. Using CHIPS data, in part, this map of the local bubble around the Sun extending to nearby stars has been published: 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: SWIFT made this time lapse set of images of a short-lived burst from a neutron star, SGB J1550-5418, in which X-rays have excited dust surrounding it, so that a dispersed cloud of material is evident as an expanding ring: This next pair of dual images shows one of the typical gamma-ray burst sequences sought by SWIFT. In the top image, the left view shows SN 2007uy (labelled) and a star above it that will become SN 2008D, seen on the left view of the lower image. This burst is evident in the visible image (right; lower) near the "top" of galaxy 2770.
Chandra has imaged a striking 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 are around 70 x 106° C. In time much of the gases will be blown out from the present configuration.
The UV is particularly suited to spotting active star formation in galaxies. The Pinwheel image has numerous super-bright regions of young stars.
The UV carries into 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 20-2); 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. 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.
An entire 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.
The Infrared region of the EM Spectrum has a treasuretrove of information. Hot stars are often shrouded in dust but some IR bands are "transparent" relative to that blocking material, allowing the stellar thermal source to shine through. 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. Here it is at the dirt-free facility at Goddard Space Flight Center - at the time just a hundred meters from my office:
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.
One of the finest IRAS images is this 25, 60, 100 µm color composite showing the nebular material around Lambda Ori:
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.
Here is a painting of the ISO:
These ISO images of the familiar Andromeda galaxy are typical of this observatory's products.
ISO can monitor the spectra of various celestial objects. This example shows the results for an interstellar cirrus-like nebula; CarbonII is a major constituent:
A 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 Dark Globule IC1396 (ordinarily indistinct in visible light). 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, demonstrating that 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 (arrows), 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. One class of galaxies has proved especially amenable to study using infrared wavelengths. This is the so-called ULIRGs, for Ultraluminous Infrared Galaxies. These galaxies are marked by high production rates of stars. The radiation from the stars heats up the dense clouds of dust and gas, causing them to glow brightly in the IR. Here are images of ULIRGs made by the SST: Multispectral images made by different space telescopes covering separated regions of the EM spectrum can lead to some striking images, as seen in this view of Galaxy M82. The Chandra image is rendered blue; the Hubble image of excited Hydrogen is shown in green; and the Spitzer ST image is assigned 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.
On February 22, 2006 NASDA, the Japonese Space Program, launched ASTRO-F, an Infrared Observatory. Nicknamed "AKARI" ("Night"), the spacecraft looked like this:
Its main sensor has 8 infrared channels. Each produces its characteristic expression of thermal energy distribution, as exemplified:
AKARI produces high resolution imagery. An idea of how much improved are the resulting images can be gained by comparing an AKARI image of galaxy IC4954 (left) with one of that galaxy made by IRAS in 1983. The striking difference between them confirms what has been promulgated throughout this Section: namely, improving spatial resolution is a strong argument for building and launching still more astronomical observatories.
Infrared imagery has proved especially sensitive to detecting Deep Field galaxies. These have been redshifted by the Universe's expansion so as to radiate in the infrared. This image shows some of the stars at great distances from Earth that glow brightly in the IR:
The Herschel Observatory, shown below, was launched on May 14, 2009. It operates mainly in the far infrared. Herschel has been placed at the Lagrangian point L2 (where the gravity of the Sun just balances that of Earth) about 150,000 km from Earth. Read about it on its ESA website.
Photodetector Array Camera and Spectrometer (PACS), a camera and a low- to medium-resolution spectrometer for wavelengths up to about 205 micrometres. It uses two bolometer detector arrays in the camera and two photo-conductor detector arrays in the spectrometer.
Spectral and Photometric Imaging Receiver (SPIRE), a camera and a low- to medium-resolution spectrometer for wavelengths longer than 200 micrometres. It uses five detector arrays: three to take images of infrared sources in three different infrared 'colours' and two to fully analyse the longer infrared light being released from the source.
Heterodyne Instrument for the Far Infrared (HIFI), a highly accurate spectrometer that can be used to obtain information about the chemical composition, kinematics, and physical environment of infrared sources.
The first image from Herschel released to the public is shown below. It is the Whirlpool galaxy, near the Milky Way. Contrary to the usual convention, red denotes relatively cooler temperatures while blue is warmer. An indication of the improvement in resolution afforded by Herschel is gained from this comparison of galaxy M51 as seen by Spitzer (left) and by Herschel's PACS (right): Similar improvements are evident when Herschel's SPIRE instrument is compared with its Spitzer equivalent, using galaxy M74 as the target.
Herschel is spending much of its observing time looking at the Milky Way as it appears in the Infrared. The gases between stars are generally quite cold but the sensitive PACS and SPIRE instruments can detect low temperature outputs, giving a better understanding of how the gas is distributed. Here is a small area within the M.W. as seen individually by these instruments, and then as an image made by combining them:
Herschel's HIFI is also working well, as evidenced by this data set that shows detection of carbon in a galactic medium:
Herschel in mid-summer of 2009 was still in the instrument readiness phase but routine data gathering is expected by November.
Now in the planning stage, another IR satellite will be WISE, the Widefield Infrared Survey Explorer. Launch is scheduled for the second half of 2009. Read about it at this University of California-Berkeley website.
Now, lets move still 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. 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. Within the Milky Way, regions of more intense radio wave sources can be identified and mapped. These regions have been called "radio blobs", referring to their somewhat diffuse nature. The blobs consist mostly of ionized Hydrogen gas (plasma). The green areas in nearby galaxy NGC3603 exemplify radio blob distribution. Most galaxies are so far away that their internal localized sources of radio waves cannot be resolved. 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. Superimposed, 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 image, made by HST, reveals some strong radiation coming from the central region of Cygnus A. 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 (M82); 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) array. 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.
Radio telescopy can identify various organic compounds in space. A spectacular result, using MERLIN, was found in observing a star in the Milky Way from which a gas plume 463 billion kilometers (288 billion miles) long emanates. A large part of that plume is composed of methanol (methyl alcohol; not "ethyl", the drinkable kind - Distillers of the World, Sorry!): The Red Giant Star Betelguese (see page 20-2) 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 have to 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. JPL has a brief synopsis of the forthcoming Space Interferometry Mission (SIM) on its SIM web site. 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. Astronomy and Cosmology are live and well! Space Observatories are being planned for the next decade or so. Here is a partial list - Click on each to go to its Web Home Page:
Herschel;
James Webb Space Telescope; Kepler; Laser Interferometer Space Antenna; Nuclear Spectroscopic Telescope Array; Planck Surveyor; Single Aperture Far Infrared Observatory; Space Interferometry Mission; Supernova Acceleration Probe;
Wide-Field Infrared Survey Explorer
Ground telescopes are being improved, as new ones are built. Here are several:Atacama Large Millimeter Array; Large Binocular Telescope Interferometer; Large Synoptic Survey Telescope; Pan-STARRS; Square Kilometer Array (radio telescope)
With this examination of observatories that collect data over different parts of the spectrum, we now return to the exposition of aspects of Cosmology by looking next at the some special topics relating to galaxies.
