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[FONT="]This spectrum can reveal much information regarding the source of the light. The color indicates the temperature of a star. By combining the type of star, identified by observing lots of other stars with similar characteristics, and our models of stellar processes with a measurement of the star's luminosity, it is possible to calculate the distance to the star. We can even determine the chemical composition of the star by observing any emission or absorption lines in the spectra. Furthermore, these lines are very distinctive, and if they appear in the correct relation to each other but have been Doppler-shifted towards the red or blue ends of the spectrum, a measurement of the star's speed relative to the earth can be obtained. The only distinguishing feature of a black hole is its gravity, however, and searching for a black hole with an optical telescope is next to impossible. A black hole does not give off any light. It's too small to observe by blocking out stars behind it. It could act as a gravitational lens, but to do so it would have to be directly in line with the Earth and some bright object, and even then there would be no way to distinguish between a black hole or a very dim star. Still, there was on promising method proposed by Russian astronomers Zel'dovich and Guseinov in 1964. If the black hole was in a binary system with another, normal star, the light curve of the system would give it away. Binary systems comprise about half of all known stars, so it is not unlikely that a black hole might be found next to a normal star. In a spectroscopic binary system, the stars rotate about their center of mass and the light will be Doppler shifted. The light curve of a star is a graph of the intensity or Doppler-shift of light from the star versus time. Here the light curve of the visible companion can yield much information. The period of rotation about the center of mass can be determined by inspection of the Doppler-shifted light curve itself, and the mass of the visible star is given by the type of star and how luminous it is. All that is then needed is a reasonable estimation of the inclination i of the system, and several important things can be calculated. The mass function f(M) = M2^3 sin i / (M1 +M2)^2 gives a relation between the masses of the two bodies, and the semi-major axis a1=AM2/(M1+M2)^2 sin i (where A is the separation of the centers of mass) gives the size of the orbit, which can also be related to the rotational velocities of the stars. A spectroscopic binary with no visible companion would be a candidate for a black hole, and if the dim star's mass is determined to be greater than that of the visible star, it would be a promising candidate. However, this method consists of many uncertainties. Although there were no hard cases for black holes any scientist¡¦s search, there arose another way a black hole might show itself. If the black hole were in a gaseous nebula, the gas would fall into the black hole. The inherent magnetic fields of the gas create turbulence, generating heat, which is in turn transformed into electromagnetic radiation. The luminosity of the gas could oscillate rapidly due to the turbulence, and such rapid oscillations would give the black hole away. Another Soviet scientist, Schwarzmann, developed the "Multichannel Analyzer of Nanosecond Pulses of Brightness Variation" in an effort to detect these oscillations, but that method also proved fruitless. X-ray novas are a special class of X-ray binaries where the system contains a late-type optical companion (a star near the end of its life) and a compact object, which can be either a neutron star or a black hole . Usually the spectrum of the companion in this type of system is very weak compared to that of the gas, but in X-ray novae the fraction of light from X-ray heating is negligible, and we have an excellent opportunity to study the system in detail. If the accretion disk is due to a black hole, then understanding the companion star in detail will also allow understanding of the processes of X-ray emission. Several X-ray satellites detected Muscae 1991 and calculations began to pinpoint an optical companion. To do this, the exact position of the X-ray source must be known. If there is a star in the visible range at that same position, it is most likely related to the X-ray star, and the light curve can then be studied in detail. In this case, a companion was found. The similarities of Muscae 1991 with one of the best black hole candidates, V616 Mon, make it seem realistic that it might be a black hole. The evolution of the light curves, the decay rate in magnitude of the novae, and variations in brightness on the order of a day are all similar in the two systems.
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[FONT="]The spectrum of the nova, its various emission lines and other spectroscopic details, also does not resemble a classical nova in the same stages, but instead resembles that of the black hole candidates Cen X-4 and V616 Mon. As it is not a classical nova, the distance to Muscae 1991 must be estimated from a known linear relation of the width of the NaD line to distance. This gives a result of ~1.4 kpc (kiloparsecs), which returns some typical values for low mass X-ray binaries and justifies confidence in its validity. Using this distance and the spectral features of the binary, the companion star seems to be a late main sequence star, which is in agreement with current theories of low-mass X-ray binaries. What this all boils down to is that the binary X-ray nova Muscae 1991 behaves very similarly to other black hole candidates in the galaxy, and gives a picture of the nova as a burst of gravitational potential energy released as matter from the disk accreted onto the compact object. The large amounts of energy released in the nova as X-rays indicates the companion is at least a neutron star and possibly a black hole, but no obvious conclusions can be made as to Muscae 1991's containing a black hole. Cygnus X-1 is accepted as a black hole by most astronomers, there is still nothing about it that demands unequivocally to be accepted as such. Cygnus X-1 is the best X-ray astronomy can give us. But X-rays and visible light are not the only ways of probing the sky. Radio astronomy was also discovered accidentally. In the 1930's, a technician trying to clear up intercontinental phone calls discovered radio waves coming from the Milky Way. Curiously enough, nobody really seemed to care very much; an amateur built the world's first radio telescope. A modest 9 meters in size, it had extremely poor resolution, and the larger dishes that were to slowly follow did not fare much better. As in X-ray astronomy, the astronomers couldn't do anything really useful with cosmic radio waves until they could identify an optical counterpart. Since radio waves are on the order of meters long, diffraction effects would require unreasonably large dishes to acquire any decent resolution. To counter this, astronomers came up with radio interferometry. At first the bodies that shone most brightly in the sky could not be associated with an optical counterpart. As radio telescopes improved, the error boxes for these sources shrank until, in 1953, a team at Cambridge had a sufficiently accurate estimate that other astronomers at the Palomar 5-meter optical telescope could identify the radio source Cyngus A with an optical source. This source turned out to be a galaxy, and once it's redshift, and hence distance, were measured, it was found that this galaxy's radio luminosity was millions of times brighter than that of an ordinary galaxy. The first radio galaxy had been found. Now that the technology was in place, more and more of these galaxies were discovered and they began to be studied in great detail. The results troubled astronomers; radio galaxies had two lobes of radio emissions with the dim optical galaxy in the center. These lobes stretched out millions of light-years, indicating a stable source of emission, and conservative estimates of the energy involved in their production was on the order of 10^61 ergs, as much energy as would be released in ten billion supernovas. Radio galaxies were among the first in what are today classified as AGN - active galactic nuclei. Other types of AGN include Seyfert galaxies, N galaxies, BL Lacertae objects, and quasars. They all demonstrate violent behavior that can't be associated with the ordinary behavior of stars and interstellar dust, whether it be matter and energy ejected from the nucleus to luminosities of truly astronomical proportions. While all these objects were regarded as puzzles, it was really the quasars that could not be explained by any astronomical processes at all. Of course they do exist, and astronomers rushed to find explanations for them. It was in this storm of hypotheses that the idea of a super-massive black hole lost it's exotic nature and became the most reasonable explanation. In fact, many of the other realistic explanations also support this idea, for they could evolve into a super-massive black hole . If there are a lot of star-star collisions occurring, the stars will lose enough energy such that they become bound in a binary which fairly rapidly decays, if they do not coalesce directly with each other. Such models of AGN could have two natural results without invoking black holes: supernova explosions, or clusters of pulsars.[/FONT]
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[FONT="]The spectrum of the nova, its various emission lines and other spectroscopic details, also does not resemble a classical nova in the same stages, but instead resembles that of the black hole candidates Cen X-4 and V616 Mon. As it is not a classical nova, the distance to Muscae 1991 must be estimated from a known linear relation of the width of the NaD line to distance. This gives a result of ~1.4 kpc (kiloparsecs), which returns some typical values for low mass X-ray binaries and justifies confidence in its validity. Using this distance and the spectral features of the binary, the companion star seems to be a late main sequence star, which is in agreement with current theories of low-mass X-ray binaries. What this all boils down to is that the binary X-ray nova Muscae 1991 behaves very similarly to other black hole candidates in the galaxy, and gives a picture of the nova as a burst of gravitational potential energy released as matter from the disk accreted onto the compact object. The large amounts of energy released in the nova as X-rays indicates the companion is at least a neutron star and possibly a black hole, but no obvious conclusions can be made as to Muscae 1991's containing a black hole. Cygnus X-1 is accepted as a black hole by most astronomers, there is still nothing about it that demands unequivocally to be accepted as such. Cygnus X-1 is the best X-ray astronomy can give us. But X-rays and visible light are not the only ways of probing the sky. Radio astronomy was also discovered accidentally. In the 1930's, a technician trying to clear up intercontinental phone calls discovered radio waves coming from the Milky Way. Curiously enough, nobody really seemed to care very much; an amateur built the world's first radio telescope. A modest 9 meters in size, it had extremely poor resolution, and the larger dishes that were to slowly follow did not fare much better. As in X-ray astronomy, the astronomers couldn't do anything really useful with cosmic radio waves until they could identify an optical counterpart. Since radio waves are on the order of meters long, diffraction effects would require unreasonably large dishes to acquire any decent resolution. To counter this, astronomers came up with radio interferometry. At first the bodies that shone most brightly in the sky could not be associated with an optical counterpart. As radio telescopes improved, the error boxes for these sources shrank until, in 1953, a team at Cambridge had a sufficiently accurate estimate that other astronomers at the Palomar 5-meter optical telescope could identify the radio source Cyngus A with an optical source. This source turned out to be a galaxy, and once it's redshift, and hence distance, were measured, it was found that this galaxy's radio luminosity was millions of times brighter than that of an ordinary galaxy. The first radio galaxy had been found. Now that the technology was in place, more and more of these galaxies were discovered and they began to be studied in great detail. The results troubled astronomers; radio galaxies had two lobes of radio emissions with the dim optical galaxy in the center. These lobes stretched out millions of light-years, indicating a stable source of emission, and conservative estimates of the energy involved in their production was on the order of 10^61 ergs, as much energy as would be released in ten billion supernovas. Radio galaxies were among the first in what are today classified as AGN - active galactic nuclei. Other types of AGN include Seyfert galaxies, N galaxies, BL Lacertae objects, and quasars. They all demonstrate violent behavior that can't be associated with the ordinary behavior of stars and interstellar dust, whether it be matter and energy ejected from the nucleus to luminosities of truly astronomical proportions. While all these objects were regarded as puzzles, it was really the quasars that could not be explained by any astronomical processes at all. Of course they do exist, and astronomers rushed to find explanations for them. It was in this storm of hypotheses that the idea of a super-massive black hole lost it's exotic nature and became the most reasonable explanation. In fact, many of the other realistic explanations also support this idea, for they could evolve into a super-massive black hole . If there are a lot of star-star collisions occurring, the stars will lose enough energy such that they become bound in a binary which fairly rapidly decays, if they do not coalesce directly with each other. Such models of AGN could have two natural results without invoking black holes: supernova explosions, or clusters of pulsars.[/FONT]
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