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Universe Teeming with Black Holes
Date: Wed, 14 Mar 2001 18:34:21 UTC
Origin: HQ DEEPEST X-RAYS EVER REVEAL UNIVERSE TEEMING WITH BLACK HOLES
For the first time, astronomers believe they have proof black holes of all sizes once ruled the universe. NASA's Chandra X-ray Observatory provided the deepest X-ray images ever recorded, and those pictures deliver a novel look at the past 12 billion years of black holes. Two independent teams of astronomers today presented images that'contain the faintest X-ray sources ever detected, which include an abundance of active super-massive black holes. "The Chandra data show us that giant black holes were much more active in the past than at present," said Riccardo Giacconi, of Johns Hopkins University, Baltimore, MD, and Associated Universities, Inc., Washington, DC. The exposure is known as "Chandra Deep Field South" since it is located in the Southern Hemisphere constellation of Fomax. "In this million-second image, we also detect relatively faint X-ray emission from galaxies, groups, and clusters of galaxies." The images, known as the Chandra Deep Fields, were obtained during many long exposures over the course of more than a year. Data from the Chandra Deep Field South will be placed in a public archive for scientists beginning today. "For the first time, we are able to use X-rays to look back to a time when normal galaxies were several billion years younger," said Ann Hornchemeier, Pennsylvania State University, University Park. The group's 500,000-second exposure included the Hubble Deep Field North, allowing scientists the opportunity to combine the power of Chandra and the Hubble Space Telescope, two of NASA's Great Observatories. The Penn State team recently acquired an additional 500,000 seconds of data, creating another one-million-second Chandra Deep Field, located in the constellation of Ursa Major. The images are called Chandra Deep Fields because they are comparable to the famous Hubble Deep Field in being able to see further and fainter objects than any image of the universe taken at Х-ray wavelengths. Both Chandra Deep Fields are comparable in observation time to the Hubble Deep Fields, but cover a much larger area of the sky. "In essence, it is like seeing galaxies similar to our own Milky Way at much earlier times in their lives," Hornschemeier added. "These data will help scientists better understand star formation and how stellar-sized black holes evolve." Combining infrared and X-ray observations, the Penn State team also found veils of dust and gas are common around young black holes. Another discovery to emerge from the Chandra Deep Field South is the detection of an extremely distant X-ray quasar, shrouded in gas and dust. "The discovery of this object, some 12 billion light years away, is key to understanding how dense clouds of gas form galaxies, with massive black holes at their centers," said Colin Norman of Johns Hopkins University. The Chandra Deep Field South results were complemented by the extensive use of deep optical observations supplied by the European Southern Observatory in Garching, Germany. More information is available on the Internet at: "http://chandra.harvard.edu" and "http://chandra.nasa.gov".
More About Black Holes
When a star runs out of nuclear fuel, it will collapse. If the core, or central region of the star, has amass that is greater than three suns, no known nuclear forces can prevent the core from forming a black hole.
Anything that comes within a certain distance of the black hole, called the event horizon, cannot escape, not even light. The radius of the event horizon (proportional to the mass) is very small, only 30 kilometers for a non-spinning black hole with the mass often suns.
Since a black hole cannot be directly observed, astronomers must use circumstantial evidence to prove its existence. The bottom line is that the observations must. imply that a sufficiently large amount of matter is compressed into a sufficiently small region of space so that no other explanation is possible.
How can black holes be located? X-ray observations are extremely useful for finding black holes. The extreme gravity around black holes will produce X-rays when infalling gas is heated to millions of degrees. The best places to look for black holes are regions where large supplies of gas are available, such as double star systems, star forming regions, or the centers of galaxies.
Have different types of black holes been discovered? There is strong evidence for two types of black holes: stellar black holes with masses of a dozen or so suns, and supermassive black holes with masses of many millions of suns. Stellar black holes are formed as a natural consequence of the evolution of massive stars (see 1st paragraph). The ongin of supermassive black holes is a mystery. They are found only in the centers of galaxies. It is not known whether they formed in the initial collapse of the gas cloud that formed the galaxy, or from the gradual growth of a stellar mass black hole, or from the merger of a centrally located cluster of black holes, or by some other mechanism.
How do astronomers determine the mass of black holes? The mass of a stellar black hole can. be deduced by observing the orbital acceleration of a star as it .orbits its unseen companion. Likewise, the mass of a supermassive black hole can be determined by using the orbital acceleration of gas clouds swirling around the central black hole. When orbital acceleration cannot be used to establish the mass of a black hole, astronomers can place a lower limit on its mass by measuring the X-ray luminosity due to matter falling into a black hole. The pressure of the X-rays must be less than the pull of the black holers gravity. In the case of the black hole discovered in M82 this limits its mass to greater than 500 suns. The M82 black hole is much larger than known stellar black holes, and much smaller than supermassive black holes, thus it is called a "mid-mass" black hole.
What is the significance of a third type of black hole? Astrophysicists had come to believe that galactic centers were the "only piaces where conditions were right for the formation and growth of large or very large black holes. The discovery of a large, mid-mass black hole away from the galaxy's center shows that somehow-and it is not an easy task theoretically - black holes much more massive than ordinary stellar black holes can form in dense star clusters. Current possible explanations for the formation of mid-mass black holes includes such exotica as black hole mergers or the collapse of a hyperstar. An intriguing implication is that mid-mass black holes could prove to be a common feature in star forming regions of galaxies.
ELECTRONICS AND MICROELECTRONICS
I. The intensive effort of electronics to increase the reliability and performance of its products while reducing their size and cost has led to the results that hardly anyone would have dared to predict.
The evolution of electronic technology is sometimes called a revolution. What we have seen has been a steady quantitative evolution: smaller and smaller electronic components performing increasingly complex electronic functions at ever higher speeds. And yet there has been a true revolution: a quantitative change in technology has given rise to qualitative change in human capabilities.
It all began with the development of the transistor.
Prior to the invention of the transistor in 1947 its function in an electronic circuit could be performed only by a vacuum tube. Tubes came in so many shapes and sizes and performed so many functions that in 1947 it seemed audacious to think that the transistor would be able to compete except in limited applications.
The first transistors had no striking advantage in size over the smallest tubes and they were more costly. The one great advantage the transistor had over the best vacuum tubes was exceedingly low power consumption. Besides they promised greater reliability and longer life. However it took years to demonstrate other transistor advantages.
With the invention of the transistor all essential circuit functions could be carried out inside solid bodies. The goal of creating electronic circuits with entirely solid-state components had finally been realized.
Early transistors, which were often described as being a size of a pea, were actually enormous on the scale at which electronic events take place, and therefore they were very slow. They could respond at a rate of a few million times a second; this was fast enough to serve in radio and hearing-aid circuits but far below the speed needed for high-speed computers or for microwave communication systems.
It was, in fact, the effort to reduce the size of transistors so that they could operate at higher speed that gave rise to the whole technology of microelectronics.
A microelectronics technology has shrunk transistors and other circuit elements to dimensions almost invisible to unaided eye.
The point of this extraordinary miniaturization is not so much to make circuits small per se as to make circuits that are rugged, long-lasting, low in cost and capable of performing electronic functions at extremely high speeds. It is known that the speed of response depends primarily on the size of transistor: the smaller the transistor, the faster it is.
The second performance benefit resulting from microelectronics stems directly from the reduction of distances between circuit components. If a circuit is to operate a few billion times a second the conductors that tie the circuit together must be measured in fractions of an inch. The microelectronics technology makes close coupling attainable.
It may be helpful if we say a few words about four of the principal devices found in electronic circuits: resistors, capacitors, diodes and transistors. Each device has a particular role in controlling the flow the electrons so that the completed circuit performs some desired function.
During the past decade the performance of electronic systems increased manifold by the use of ever larger numbers of components and they continue to evolve. Modern scientific and business computers, for example, contain 109 elements; electronic switching systems contain more than a million components.
The tyrany of numbers - the problem of handling many discrete electronic devices - began, to concern the scientists as early as 1950. The overall reliability of the electronic system is universally related to the number of individual components. A more serious shortcoming was that it was once the universal practice to manufacture each of the components separately and then assemble the complete device by wiring the components together with metallic conductors. It was no good: the more components and interactions, the less reliable the system.
The development of rockets and space vehicles provided the final impetus to study the problem. However, many attempts were largely unsuccessful.
What ultimately provided the solution was the semiconductor integrated circuit, the concept of which had begun to take shape a few years after the invention of the transistor. Roughly between 1960 and 1963 a new circuit technology became a reality. It was microelectronics development that solved the problem.
The advent of microelectronic circuits has not, for the most part, changed the nature of the basic functional units: microelectronic devices are also made up of transistors, resistors, capacitors, and similar components. The major difference is that all these elements and their interconnections are now fabricated on single substrate in single series of operations.
II. Several key developments were required before the exciting potential of integrated circuits could be realized.
The development of microelectronics depended on the invention of techniques for making the various functional units on or in a crystal of semiconductor materials. In particular, a growing number of functions have been given over the circuit elements that perform best: transistors. Several kinds of microelectronic transistors have been developed, and for each of them families of associated circuit elements and circuit patterns have evolved.
It was the bipolar transistor that was invented in 1948 by John Bardeen, Walter H.Brattain and William Shockley of the Bell Telephone Laboratories. In bipolar transistors charge carries of both polarities are involved in their operation. They are also known as junction transistors. The npn and pnp transistors make up the class of devices called junction transistors.
A second kind of transistor was actually conceived almost 25 years before the bipolar devices, but its fabrication in quality did not become practical until the early 1960's. This is the field-effect transistor. The one that is common in microelectronics is the metal-oxide-semiconductor field-effect transistor. The term refers to the three materials employed in its constrution and is abbreviated MOSFET.
The two basic types of transistor, bipolar and MOSFET, divide microelectronic circuits into two large families. Today the greatest density of circuit elements per chip can be achieved with the newer MOSFET technology.
An individual integrated circuit (IC) on a chip now can embrace more electronic elements than most complex piece of electronic equipment that could be built in 1950.
In the first 15 years since the inception of integrated circuits, the number of transistors that could be placed on a single chip (with tolerable yield) has doubled every year. The 1980 state of art is about 70K density per chip. Nowadays we can put a million transistors on a single chip.
The first generation of commercially produced microelectronic devices are now referred to as small-scale integrated circuits (SSI). They included a few gates. The circuitry defining a logic array had to be provided by external conductors.
Devices with more than about 10 gates on a chip but fewer than about 200 are medium-scale integrated circuits (MSI). The upper boundary of medium-scale integrated circuits techology is marked by chips that contain a complete arithmetic and logic unit. This unit accepts as inputs two operands and can perform any one of a dozen or so operations on them. The operations include additions, subtraction, comparison, logical "and" and "or" and shifting one bit to the left or right.
A large-scale integrated circuit (LSI) contains tens of thousands of elements, yet each element is so small that the complete circuit is typically less than a quarter of an inch on a side.
Integrated circuits are evolving from large-scale to very-large-scale (VLSI) and water-scale integration (WSI).
The change in scale can be measured by counting the number of transistors that can be fitted onto a chip.
Continued evolution of the microcomputer will demand further increases in packing density.
There appeared a new mode of integrated circuits, microwave integrated circuits. In broadest sense, a microwave integrated circuit is any combination of circuit functions which are packed together without a user accessible interface.
The evolution of microwave integrated circuits must begin with the development of planar transmission lines.
As we moved into the 1970's, stripline and microstrip assemblies became commonplace and accepted as the everyday method of building microwave integrated circuits. New forms of transmission lines were on the horizon, however. In 1974 new integrated-circuit components in a transmission line called fineline appeared. Other more exotic techniques, such as dielectric waveguide integrated circuits emerge. Major efforts currently are directed at such areas as image guide, co-planar waveguide, fineline and dielectric waveguide, all with emphasis on techniques which can be applied to monolithic integrated circuits. These monolithic circuits encompass all of the traditional microwave functions of analog circuits as well as new digital applications.
Microelectronic technique will continue to displace other modes. As the limit of optical resolution is now being reached, new lighographic and fabrication techniques will be required. Circuit patterns will have to be formed with radiation having wavelength shorter than those of light, and fabrication techniques capable of greater definition will be needed.
Electronics has extended man's intellectual power. Microelectronics extends that power still further.
