Quasars Essays (1876 words) - Radio Astronomy, Physical Cosmology

Quasars Since their discovery, the nature of quasars has been one of the most intriguing and baffling problems as evidenced by the following quotations: the problem of understanding quasi-stellar objects? is one of the most important and fascinating tasks in all physics - G.Burbidge and Hoyle. The quasar continues to rank both as one of the most baffling objects in the universe and one most capable of inspiring heated argument - Morrison. The redshift problem is one of the most critical problems in astronomy today - G. Burbidge. Quasars still remain the profoundest mystery in the heavens - Hazard and Mitton. The conventional interpretation of the spectral lines observed in quasars is based on the redshift hypothesis. Three hypotheses have been advanced to account for the supposed redshifts: 1. Cosmological hypothesis; the redshifts are due to the expansion of the universe, 2. Gravitational hypothesis, 3 Local-Doppler hypothesis; in this hypothesis the redshifts are due to the Doppler effect, but the quasars are relatively nearby and have nothing to do with the expansion of the universe. Of these hypotheses, the first one is the most publicized one. One is led to attribute to quasars very many mysterious properties if one assumes the redshift hypothesis to be correct. A patient analysis of the data on quasars over the years has led to the conclusion that the real source of the trouble is in the assumption that the spectra of quasars have redshifts. In the early 1960's quasars were known as 'radio stars' because the method used to discover the first quasars was based on coincidences between a strong radio source and a point-like optical source. Since each radio source was associated with a star it was originally thought that quasars were objects within the galaxy hence the term 'radio stars'. Quasars or quasi-stellar radio source, from the method by which they where originally discovered: as stellar optical counterparts to small regions of strong radio emission. With increasing spatial resolution of radio telescopes the strong radio emission often seemed to come from a pair of lobes surrounding many of these faint star-like emission line objects. The initial method of selection was strong radio emission, and then later any object with blue or ultraviolet excess was considered a good quasar candidate. Very recent evidence from the near infrared portion of the spectrum indicates that a large fraction of quasars may in fact be brighter in the infrared than in other wavelength bands. Answering these basic questions may summarize much of the information regarding Quasar. What is the definition of a quasar? When radio telescopes were first turned on the heavens, point sources of radio waves were discovered (along with spread-out regions of emission along our Milky Way). Astronomers using ordinary visible-light telescopes turned toward these radio points and looked to see what was there. In some cases a supernova remnant was found, in others, a large star-birth region, in others a distant galaxy. But in some places where point sources of radio waves were found, no visible source other than a stellar-looking object was found (it looked like a point of a star). These objects were called the quasi-stellar radio sources or quasars for short. Later, it was found these sources could not be stars in our galaxy, but must be very far away as far as any of the distant galaxies seen. We now think these objects are the very bright centers of some distant galaxies, where some sort of energetic action is occurring, most probably due to the presence of a supermassive black hole at the center of that galaxy. (Supermassive - made up from a mass of about a billion solar masses.) What do quasars have to do with black holes? It is thought the infall of matter into the Supermassive black hole can result in very hot regions where huge energies are released, powering the quasar. How big are quasars compared to galaxies? Well, the region of intense visible emission is quite small compared to the rest of the galaxy that it is imbedded in. The visible emission only occurs very near the center of the galaxy. On the other hand, huge regions of radio emission, produced by the quasar, can stretch out to large distances outside the