X-Ray Astronomy
X-ray astronomy is a product of the space age, and is one of the fastest-growing fields of astronomy. It involves not only X-ray stars, but galaxies, pulsars, quasars, and black holes as well.
X-rays had been discovered quite by accident in 1895 by German physicist Wilhelm Röntgen. He noticed that barium crystals, which he had stored across the room, fluoresced whenever he turned on a cathode-ray discharge tube. He correctly surmised that a previously unknown form of radiation of very short wavelengths, which he called X-rays, caused the crystals to glow.
Stars radiate energy in many other wavelengths than visible light, which is only one small part of the electromagnetic spectrum. At the long-wavelength end are low-energy infrared and radio wavelength regimes; at shorter wavelengths than the visible part of the spectrum are high-energy ultraviolet radiation, X-rays, and gamma rays.
There are two broad areas of X-ray astronomy. The first involves study of soft X-rays located at the long-wavelength end of the X-ray spectrum, overlapping longer ultraviolet wavelengths. The other involves study of hard X-rays at the short-wavelength end of the electromagnetic spectrum, overlapping shorter gamma ray wavelengths.
The ozone layer of the earth's atmosphere shields the planet from high-energy radiation, preventing such radiation from reaching the surface. That is fortunate, since such rays can be fatal to living organisms. Hard X-rays can be detected by instruments aboard balloons at high altitudes, as Victor Franz Hess discovered in 1911, but the only way to detect soft X-rays is to get above the atmosphere. American physicist Herbert Friedman (1916-) accomplished this in the late 1940s. He used German V-2 rockets that had been captured during World War II. Friedman placed X-ray detectors on board and launched them above the atmosphere. He was rewarded by discovering that the sun emits X-rays.
Shortly after this discovery, a group of physicists from the Massachusetts Institute of Technology founded a company called American Science and Engineering, Inc. (ASEI). Its purpose was to study high-energy physics. When physicist Riccardo Giacconi (1931-) joined the company in 1959, he and his colleagues concentrated on extraterrestrial X-ray sources and worked with NASA to further Friedman's pioneering work. Giacconi and ASEI devised a very sensitive X-ray telescope that could detect weak X-rays from specific areas of the sky. NASA placed this X-ray detector aboard an Arobee rocket for a six minute flight on the night of June 18-19, 1962, that resulted in the first discovery of X-rays from interstellar space. (To be sure, there had been numerous equipment and rocket malfunctions before this success.) The detector had discovered emissions of X-rays coming from the constellation of Scorpius. Since this was the first source located in that constellation, it was named Scorpius X-1.
Additional X-ray sources were discovered in other parts of space on later flights. One was located at the Crab Nebula in Taurus, where a pulsar (neutron star) was later identified. Another was found in Cygnus, called Cygnus X-1, where the X-rays are thought to be produced by a black hole in a binary star system. The first X-ray galaxy, M87 (also known as Virgo A), was discovered in 1966. Astronomers believe most galaxies, including the Milky Way, emit weak X-rays. One galaxy, identified as NGC 5128 (also known as Centaurus A), is extremely strange, emitting ten times as much X-ray radiation as a normal galaxy. Other strong X-ray sources include the quasar 3C 273 and the Seyfert galaxy NGC 4151.
During the 1960s, Giacconi and Friedman actively urged NASA to build more complex X-ray satellites that could be placed in the Earth's orbit, and in December 1970, the X-ray satellite Uhuru was launched. It produced an extensive map of the X-ray sky. Later in the 1970s and early 19802, three High Energy Astronomical Observatories (HEAO's) were placed in orbit. X-ray observations are very important in sorting out a wide variety of problems with great cosmological implications. X-rays offer clues about energy production deep in stellar interiors, the outermost layers of the atmospheres of stars, the large-scale structure of the universe, and the strange properties of such compact exotic objects as pulsars and black holes. The European EXOSAT satellite increased the X-ray database during its mission in the mid-1980s.
New technology led to a number of X-ray missions during the 1990s. In 1990 the Broad-Band X-ray Telescope (BBXRT) went into space for nine days aboard the space shuttle Columbia, and despite some hardware difficulties performed a important observations of a variety of X-ray-emitting objects. The most ambitious X-ray observatory ever built, the Advanced X-ray Astronomy Facility (AXAF) is scheduled for launch in late 1998, and it is expected to advance our view of the X-ray sky in the same way that the Hubble Space Telescope provided unprecedented views of the heavens in visual and ultraviolet light.
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