Electromagnetic Radiation (a.k.a. Light):

Maxwell (1850's) showed that light is energy carried in the form of opposite but supporting electric and magnetic fields in the shape of waves, i.e. self-propagating electromagnetic waves.

The wavelength of the light determines its characteristics. For example, short wavelengths are high energy gamma-rays and x-rays, long wavelengths are radio waves. The whole range of wavelengths is called the electromagnetic spectrum.

Our eyes only see over the following range of wavelengths:

Gamma-Ray Space Missions:

The Vela-5A nuclear test detection satellite was part of a program run jointly by the Advanced Research Projects of the U.S. Department of Defense and the U.S. Atomic Energy Commission, managed by the U.S. Air Force, to verfy the Atmospheric Test Ban Treaty.

It and its twin, Vela-5B, were placed 180 degrees apart in nearly circular orbits at a geocentric distance of 118,000 km on 23 May 1969; the orbital period was 112 hours. The x-ray and gamma-ray detector was located 90 degrees from the spin axis, and so covered the celestial sphere twice per satellite orbit. Data were telemetered in 1-sec count accumulations.

In 1973, the Vela satellites discovered brief bursts of gamma-rays of cosmic origin coming from random directions on the sky. Networks of satellites carrying gamma-ray burst detectors were established in the inner solar system in the 1970's and 1980's (every space probe was mounted with a small gamma-ray detector) to produce source locations for these transients using a method analogous to triangulation of ships at sea. However, inspection of the relatively small locations produced no obvious counterparts to the gamma-ray bursts at any other wavelength.

The mystery of gamma-ray bursts has been prolonged by the impracticality of building and launching a gamma-ray telescope with focusing optics that could maneuver and point to the unknown direction of a burst while it is still in progress. This was partially resolved by the launch of the Compton Gamma-Ray Observatory (CGRO) in the late 1980's.

One of CGRO's experiments is EGRET which produced an all-sky map at gamma-ray energies above 100 MeV in galactic coordinates. The diffuse emission, which appears brightest along the galactic plane, is primarily due to cosmic ray interactions with the interstellar medium. The Vela, Geminga, and Crab pulsars are clearly visible as bright knots of emission in the galactic plane in the right portion of the image.

The burst-monitoring experiment on CGRO, BATSE (Burst And Transient Source Experiment), localizes bursts by comparing the burst's different intensities as measured by its eight detectors pointing to the eight octants of the sky.

As can be seen from the uniform distribution of burst positions in this recent figure, no preferred direction for the burst sources is apparent. If the burst sources were associated with our own Milky Way Galaxy, then a distribution concentrated toward the Galactic plane (represented by the central horizontal line in the figure -- the "galactic equator"), like that seen by CGRO's EGRET instrument. Such reasoning led some gamma-ray astrophysicists to believe that bursts are coming either from very local sources (just outside the Solar System) or the very distant reaches of the Universe. In the later case, these brief events would be, momentarily, much brighter than entire galaxies.

Energy Release by Human Experience
    1 ergs = energy to make a mosiquito jump
 10^3 ergs = ball drop
10^10 ergs = hit by truck
10^15 ergs = smart bomb
10^20 ergs = H bomb
10^26 ergs = killer asteroid
10^40 ergs = Death Star 

Energy Release by Astronomical Experience
10^33 ergs/s = Sun
10^39 ergs/s = nova
10^41 ergs/s = SN
10^45 ergs/s = galaxy

10^52 ergs/s = GRB

X-ray Space Missions:

The HEAO-2 spacecraft, also known as the Einstein Observatory, carried what was then the largest focusing x-ray telescope. HEAO-2's telescope was a 4-in-1 apparatus, four telescopes nested within each other. Each comprised a primary mirror and a secondary mirror to focus X-rays and magnify the image.

X-ray telescopes are the only way we can observe extremely hot matter with temperatures of millions of degrees Celsius. It takes gigantic explosions, or intense magnetic or gravitational fields to energize particles to these high temperatures. Where do such conditions exist? In an astonishing variety of places, ranging from the vast spaces between galaxies to the bizarre, collapsed worlds of neutron stars and black holes.

X-rays do not reflect off mirrors the same way that visible light does. Because of their high energy, x-ray photons penetrate into the mirror in much the same way that bullets slam into a wall. Likewise, just as bullets ricochet when they hit a wall at a grazing angle, so too will x-rays ricochet off mirrors (see diagram below). These properties mean that x-ray telescopes must be very different from optical telescopes. The mirrors have to be precisely shaped and aligned nearly parallel to incoming x-rays. Thus they look more like barrels than the familiar dish shape of optical telescopes.

The Chandra X-ray Observatory, launched by Space Shuttle Columbia on July 23, 1999, is NASA's newest Great Observatory. Chandra detects and images X-ray sources that are billions of light years away. The mirrors on Chandra are the largest, most precisely shaped and aligned, and smoothest mirrors ever constructed. If the surface of Earth was as smooth as the Chandra mirrors, the highest mountain would be less than six feet tall! The images Chandra makes are twenty-five times sharper than the best previous X-ray telescope. This focusing power is equivalent to the ability to read a newspaper at a distance of half a mile. Chandra's improved sensitivity is making possible more detailed studies of black holes, supernovae, and dark matter. Chandra will increase our understanding of the origin, evolution, and destiny of the universe.

The X-ray nebula (Left) shown in the Chandra image is about 40% as large as the optical nebula (Right). The X rays are more concentrated toward the center than the optical emission. Also, it seems that the x-ray image is NOT centered on the pulsar, the star remnant from the supernova. There are all sorts of interesting loops and knots in the x-ray image, some of which appear to correlate with optical features and some that do not.

UV Space Missions:

The Extreme Ultraviolet Explorer (EUVE) is a NASA-funded astronomy mission operating in the relatively unexplored extreme ultraviolet (70-760) band. The science payload, which has been designed and built at the Space Sciences Laboratory at the University of California, Berkeley, consists of three grazing incidence scanning telescopes and an extreme ultraviolet (EUV) spectrometer/deep survey instrument. The science payload is attached to a Multi-Mission Modular spacecraft.

The EUVE mission, which launched on June 7, 1992 on a Delta II rocket from Cape Canaveral, is the culmination of nearly thirty years of effort at the University of California at Berkeley to create the field of EUV Astronomy. EUVE opens up this last unexplored spectral window in astrophysics. The first six months of the mission were dedicated to mapping the EUV sky with the scanning telescopes.

This artist's conception of the EUVE satellite in orbit around the Earth, displays the spacecraft with its fully deployed rectangular solar panel arrays which give the EUVE spacecraft its source of power. The EUVE orbits the Earth in a near-circular orbit high above the Earth's surface at an approximate altitude of 530 km (330 miles).

The EUVE telescopes detected a total of 739 sources. Most of these sources (268 of them) are cool stars of spectral type F,G,K, and M, with active coronae that emit in the EUV. EUVE detected 104 white dwarf stars, 24 hot stars with early spectral types, 16 cataclysmic variables, 36 active nuclei of galaxies beyond our own Milky Way, and a few each of other astronomical objects that include planetary nebulae, neutron stars, novae, supernova remnants and solar system objects.

Infrared Space Missions:

The InfraRed Astronomical Satellite (IRAS), a NASA Explorer mission, conducted the first survey of the sky at thermal infrared wavelengths in 1983. A collaborative effort between the US, the Netherlands and the UK, IRAS opened a new chapter in astronomical exploration. Utilizing a 57-cm diameter telescope cryogenically cooled to a temperature of 4 K, IRAS circled the Earth in a 900-km polar orbit and operated for 10 months before its liquid helium was exhausted.

The entire sky, as seen in infrared wavelengths and projected at 0.5 degree resolution, as observed by IRAS. The bright horizontal band is the plane of the Milky Way, with the center of the Galaxy located at the center of the image. The colors represent infrared emission detected in three of the telescope's four wavelength band (blue is 12 microns, green is 60 microns, and red is 100 microns). Hotter material radiates at the shorter infrared wavelengths. The hazy, horizontal "S-shaped" feature that crosses the image is the faint heat emitted by dust in the plane of our Solar System. Among the discrete celestial objects seen in the photo are regions of star formation in the constellation Ophiuchus (above the Galactic Center) and Orion (the two brightest spots below the Galactic plane at the far right). The Large Magellanic Cloud, a companion galaxy to the Milky Way, is the relatively isolated spot located below the plane, right of center. Black stripes are regions of the sky not observed by IRAS.

The familiar winter constellation of Orion takes presents a spectacular contrast between the visible-light view (left) and the appearance as seen by IRAS (right). The IRAS false-colored mosaic covers a region 30x24 degrees in extent, and is a composite of 12-, 60-, and 100-micron data. New processing techniques have been used to enhance faint details and to remove instrumental artifacts. The warmest features -- the stars -- are brightest at 12 microns, and are coded as blue. The cooler interstellar dust is more luminous at 60- (green) and 100-microns (red).

This figure shows an artists impression by the Alcatel company of the Darwin spacecraft, operating some 1.5 million kilometres from the Earth, in the direction away from the Sun. Alcatel started in December 1997 a contract with ESA to perform a study of this concept. The infrared light from the target planetary system comes down from the top right of the picture into the six individual telescopes, each of which has a collecting mirror of 1.5 metres diameter. In this version, called the `free-flyer', the six individual telescopes are mounted are separate spacecraft. The central spacecraft holds the hub with the optics which combines the light from the individual telescopes. The individual spacecraft are moved around by very precise small rocket-type engines on each of them.

In interferometry, one gets some of the information one would get from a telescope as big as the separation between the individual telescopes. For Darwin, this would be between 50 and 500 metres. To get enough of this information to build up a good picture, one has to move the individual telescopes around to different relative positions and repeat the `exposures'. The optical distances between the telescopes and the central hub have to be controled to about a billionth of a meter. The actual distances between the spacecraft are measured with lasers to within a few millionths of a metre. There is a seventh, small, spacecraft, shown below the telescopes and the hub. This is used both in the distance measuring, and for the communications with Earth.

This is a simulation of how Darwin would detect Earth-like planets orbiting a nearby star. We have simulated looking at the inner part of our own Solar System, placed around a star 10 parsecs (33 light-years) away from the Sun. This is the picture that Darwin would make after a 10-hour `exposure'. You can see the three bright objects, which are Venus, Earth and Mars. As you can see they are clearly distinguishable from the background. In practice, one would take a series of such exposures spread over a few months and watch the planets orbit the star.


  • Why do we need space telescopes to observe certain regions of the spectrum? What are the advantages to those regions?
  • What were key missions in the gamma-ray and x-ray regions? What were their key discoveries?
  • What science can be done in the infrared?