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. The most frequently cited result is this image showing the Gamma Ray sky associated with the Milky Way:
Two other images are described in their captions:
Another achievement has been to gain a better insight into the nature and origin of cosmic rays. Fermi has determined that a principal component of this radiation is energetic, high speed protons that are accelerated by magnetic fields associated with supernova bursts.
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:
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):