Appendix A-4


Selected Results Summarized Historically

Cygnus X-1

1964 Strong source Cygnus X-1 discovered with an X-ray telescope carried on a sounding rocket.
1971 Uhuru satellite monitored time dependence of Cyg X-1 and found periodicity matching that of visible-light spectral changes in a faint blue star in the direction of the X-ray source. A radio source also found at this position. The spectral changes revealed that the blue star has a dark companion star so massive that, according to current theory, it cannot be a neutron star and presumably is a black hole.
1973 X-ray telescope on a sounding rocket found very rapid intensity changes in Cyg X-1 of type predicted to accompany accretion of matter onto a black hole
1975 Analysis of X-ray intensity changes of Cyg X-1 showed that flares of about 0.5 seconds duration occur randomly, a phenomenon not found in other X-ray sources.

pphoto of a large doughnut shaped cloud of orange gases in space
The Planetary Nebula in Aquarius, a huge circular cloud of dust amid the stars. (Copyright 1965 by California Institute of Technology and Carnegie Institution of Washington.)

Gamma Ray Bursts

1975 Physicists announced that celestial gamma ray bursts of unknown origin, arriving from random directions in space, were discovered with the Vela satellites, a set of Earth-orbiting spacecraft designed to monitor the Limited Test Ban Treaty. The bursts were first recorded in 1967, but were not immediately recognized as extraterrestial events.
1974 Study of a gamma ray burst found in data recorded in 1972 by an instrument on Apollo 16 showed that it had more than five prominent intensity peaks. Earlier known bursts had one or two peaks.
1978 Statistics of the occurrence of gamma ray bursts of weak in tensity, as measured with balloon-borne instrumentation, showed that the bursts must arise in sources located inside our Milky Way galaxy.
1979 Interplanetary network of gamma ray detectors on nine spacecraft observed the strong est gamma ray burst yet on March 5th, with a remarkably fast rise time of less than one thousandth of a second. The source was pinned down to the direction of a supernova remnant in the Large Magellanic Cloud, companion galaxy of the Milky Way. The implication, if this identification is correct, is that the burst came from a neutron star that remained from the supernova. An eight second recurrence of intensity peaks was observed, perhaps to be interpreted as the rotation period of the neutron star. Astronomers cautioned that the burst's properties were exceptional and that it might represent a different phenomenon than the other observed gamma ray bursts.

Satellite image of celestial object number SS 433
The mysterious object SS 433, glimpsed from Earth by the Einstein observatory satellite.

Cosmic Rays

1912 Cosmic rays discovered in a balloon experiment. However, it was thought that they were gamma rays rather than subatomic particles.
1929 Cosmic rays found to be electrically charged, thanks to measurements from the newly developed Geiger-Muller counter.
1936 Muon recognized as a cosmic ray that reaches the ground. The first discovery of an unstable subatomic particle, this finding launched the discipline of elementary particle physics and showed that the primary cosmic rays which don't reach the ground are something other than muons.
1939 Primary cosmic rays found to be positively charged, with the flux of particles from the west slightly greater than that from the east, as would be predicted for positively charged particles moving in the Earth's magnetic field.
1960 Balloon experiments at high altitude discovered that about one percent of the cosmic rays are relativistic electrons.
1964 The flux of cosmic rays studied over a complete solar cycle was found to vary so that the flux reaching the Earth is reduced by as much as 20 percent when the Sun is in its active phase.
1974 Balloon measurements determined the average amount of matter traversed by cosmic rays in space between their sources and the Earth (five grams per square centimeter) by measuring the ratio of the number of light nuclei in the cosmic rays (lithium, beryl lium, boron) to the number of heavy nuclei (carbon, oxygen). Light nuclei are probably produced when the heavier ones collide with interstellar matter, thus the larger the ratio, the more matter has been traversed by the heavy nuclei.
1977 The age of cosmic rays was found to be about 20 million years from two kinds of experiments. First, the IMP-7 and IMP-8 satellites measured the ratio of the beryllium-10 and beryllium-9 isotopes in cosmic rays; both are produced by the interaction of carbon and oxygen nuclei with the interstellar matter, but beryllium-10 is unstable, with a half-life of 2.5 million years. Second, balloon borne telescopes observed the energy spectrum of relativistic cosmic ray electrons and found it to bend at 20 GeV, a result predicted for a 20-million-year exposure to galactic magnetic fields and the micro wave background radiation.
1979 Launch of HEAO-3 satellite with two large cosmic ray experiments, one to study isotopes and one to observe very heavy nuclei. A new theory of cosmic ray variations due to phenomena of the solar wind and interplanetary magnetic fields was advanced on the basis of data from many inter planetary spacecraft.


1962 The position of radio source 3C273 was determined when it was occulted by the Moon, and a faint star-like object with a jet was photographed at the position. Its spectra showed unusual emission lines.
1963 The spectral lines were identified as familiar hydrogen lines but redshifted by about 16 percent of their wavelength, so that 3C273 must be extremely far away and hence thousands of times more luminous than a galaxy.
1970 Sounding rocket telescope discovered X-rays from 3C273.
1972 Variability of infrared radiation from 3C273 was reported; the time scale of variation is too short for the radiation to arise from hot dust grains, so the emission mechanism must be nonthermal.
1978 COS-13 satellite observations revealed a weak gamma ray source in the direction of 3C273. X-ray observations of the quasar OX 169 showed that its intensity changed by a factor of six in only six hours. If (as one theory maintains) the source of the X-rays is matter accreting onto a black hole, then this rapid change implies that the black hole has a mass at least a million times greater than that of the Sun.
1979 An X-ray quasar reported to have a redshift of 3.2, corresponding to a velocity of recession of 97 percent of the speed of light. (The most distant known quasar has a redshift of 3.5.)

Photo of the Andromedea Galaxy
The Great Galaxy in Andromeda, our nearest galactic neighbor in space. (The two smaller Magellanic Clouds are actually satellites of our own galaxy.) (Copyright 1959 by California Institute of Technology and Carnegie Institution of Washington.)


1967 Discovery of the first radio pulsar, CP 1919, with a pulse period of 1.337 seconds and a pulse width of 0.04 second.
1968 Publication of theory that pulsars are rotating, magnetized neutron stars. Observations showed that the pulsars are gradually slowing down.
1969 Visible light pulses were discovered from the pulsar in the Crab Nebula. Then, discovery of X-ray pulses from this object showed that the power radiated in the form of X-rays is more than 10,000 times the luminosity of the Sun at all wavelengths.
1972 Gamma ray pulses at high energies observed from the Crab Nebula pulsar. There are two gamma ray pulses per pulse period, mimicking the X-ray pulse behavior, and they are exactly in phase with the radio pulses. Radio studies of many pulsars were sensitive enough to study them pulse by pulse; these investigations discovered a rich variety of phenomena, including drifting subpulses, so-called "giant pulses" and "nulling", when an entire pulse fails to occur.
1975 SAS-2 satellite discovered gamma ray pulses from the pulsar in the Vela X supernova remnant. Radio observations of this object had shown one pulse per period, although there are two gamma ray pulses in the same interval, neither one coinciding with the phase of the radio pulse. Also, the first radio pulsar in a bi nary system was discovered; continued monitoring over several years suggested that the orbital period is slowly changing, perhaps as the system loses energy in the form of gravitational waves, as pre dicted from the General Theory of Relativity.
1977 Optical pulses discovered from the Vela pulsar. Again, two pulses seen per pulse period, but not coincident with either of the gamma ray pulses nor with the radio pulse.

Interstellar and Intergalactic Media

1951 Detection of 21-cm(8.3-inch) wavelength radio emission from interstellar atomic hydrogen.
1956 Theory that there exists a galactic corona was proposed.
1969 Model of cold clouds and warm intercloud medium in pressure balance was proposed. The average density of interstellar matter was found to be one atom per cubic centimeter and thought to be equally divided between thin, warm gas and cool, denser clouds.
1972 Another Orbiting Astronomical Observatory (OAO-3), also called Copernicus, was launched, carrying the first in strumentation for ultraviolet spectroscopy of stars from Earth orbit.
1973 Copernicus results showed that: (1) The ratio of deuterium to hydrogen in interstellar gas is very low, a result that is evidence for the "Open Universe" theory, according to which the expansion of the universe will never end; (2) many of the heavier elements are much less abundant in the interstellar gas than in the stars that supply mass to the gas, which indicates that atoms of these elements have condensed to the solid state and are present in interstellar dust grains.
1974 Copernicus data lead to the theory that low-density cavities in interstellar space are caused by supernova explosions and are filled with hotter gas than the surrounding regions. It was suggested that a series of supernovae may cause 1962 cavities that connect to form tunnels of hot gas, threading through the cold clouds and warm intercloud medium.
1975 White dwarf stars were detected with an extreme ultraviolet telescope on the Apollo-Soyuz mission. The results show that there is very little gas (less than I atom per 100 hundred cubic centimeters) in the vicinity of the Sun, out to at least 200 light years.
1976 According to Copernicus results, the bulk of the neutral interstellar gas is contained in small dense clouds rather than uniformly distributed.
1979 Discovery of an X-ray "super bubble" in the constellation Cygnus, made with an X-ray telescope on HEAO-1. Discovery by IUE of a corona of 100,000 C gas around the Milky Way galaxy, as theorized in 1956. The corona perhaps is fed with hot material by super bubbles expanding from the galactic plane. Discovery by IUE of coronae around the Magellanic Clouds; coronae must be very common around galaxies.

Cosmic Background Radiation

1946 Prediction made that a cosmic background radiation exists due to the origin of the universe in the Big Bang.
1962 X-ray background radiation discovered by an instrument on a sounding rocket.
1965 Cosmic microwave back ground discovered with a radio telescope.
1968 Measurements of the isotropy of the X-ray background showed that the radiation must come from very great distances, because the universe is homogeneous and isotropic only on very large scales.
1977 Anisotropy found in the microwave background. The measurements indicate that our galaxy is moving at about 300 to 400 kilometers per second (190 to 250 miles per second) in the direction of the constellation Leo.
1979 The diffuse X-ray background at energies around 1 keV was found to be dominated by emission from many very distant individual objects, per haps quasars and young galaxies.

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