Nov. NASA History Series, Siddiqi, Asif A.: The Soviet Space Race with Apollo. Erschienen bei der NASA im Jahre Strategitscheskije. Juli Dampfwolke um einen fernen Quasar enthält Billionen Mal so viel allen Meerwassers der Erde, teilt die US-Weltraumbehörde Nasa mit. Juli Diese Illustration zeigt einen Quasar. Ähnlich soll das von Wasser umgebene Schwarze Loch aussehen. Quelle: NASA/ESA. Im Weltall haben.
Naming them didn't help determine what these objects were. It took years of study to realize that these distant specks, which seemed to indicate stars, are created by particles accelerated at velocities approaching the speed of light.
Scientists now suspect that the tiny, point-like glimmers are actually signals from galactic nuclei outshining their host galaxies.
Quasars live only in galaxies with supermassive black holes — black holes that contain billions of times the mass of the sun. Although light cannot escape from the black hole itself, some signals can break free around its edges.
While some dust and gas fall into the black hole , other particles are accelerated away from it at near the speed of light.
The particles stream away from the black hole in jets above and below it, transported by one of the most powerful particle accelerators in the universe.
Most quasars have been found billions of light-years away. Because it takes light time to travel, studying objects in space functions much like a time machine; we see the object as it was when light left it, billions of years ago.
Thus, the farther away scientists look, the farther back in time they can see. Most of the more than 2, known quasars existed in the early life of the galaxy.
Galaxies like the Milky Way may once have hosted a quasar that has long been silent. In December , the most distant quasar was found sitting more than 13 billion light-years from Earth.
Quasars this young can reveal information about how galaxies evolved over time. Quasars emit energies of millions, billions, or even trillions of electron volts.
The strange spectral lines in their radiation, and the speed of change seen in some quasars, also suggested to many astronomers and cosmologists that the objects were comparatively small and therefore perhaps bright, massive and not far away; accordingly that their redshifts were not due to distance or velocity, and must be due to some other reason or an unknown process, meaning that the quasars were not really powerful objects nor at extreme distances, as their redshifted light implied.
A common alternative explanation was that the redshifts were caused by extreme mass gravitational redshifting explained by general relativity and not by extreme velocity explained by special relativity.
Various explanations were proposed during the s and s, each with their own problems. It was suggested that quasars were nearby objects, and that their redshift was not due to the expansion of space general relativity but rather to light escaping a deep gravitational well special relativity.
This would require a massive object, which would also explain the high luminosities. However a star of sufficient mass to produce the measured redshift would be unstable and in excess of the Hayashi limit.
One strong argument against them was that they implied energies that were far in excess of known energy conversion processes, including nuclear fusion.
There were some suggestions that quasars were made of some hitherto unknown form of stable antimatter regions and that this might account for their brightness.
The uncertainty was such that even as late as , it was stated that "one of the few statements [about Active Galactic Nuclei] to command general agreement has been that the power supply is primarily gravitational",  with the cosmological origin of the redshift being taken as given.
Eventually, starting from about the s, many lines of evidence including the first X-Ray space observatories , knowledge of black holes and modern models of cosmology gradually demonstrated that the quasar redshifts are genuine, and due to the expansion of space , that quasars are in fact as powerful and as distant as Schmidt and some other astronomers had suggested, and that their energy source is matter from an accretion disc falling onto a supermassive black hole.
This model also fits well with other observations that suggest many or even most galaxies have a massive central black hole.
It would also explain why quasars are more common in the early universe: The accretion disc energy-production mechanism was finally modeled in the s, and black holes were also directly detected including evidence showing that supermassive black holes could be found at the centers of our own and many other galaxies , which resolved the concern that quasars were too luminous to be a result of very distant objects or that a suitable mechanism could not be confirmed to exist in nature.
By it was "well accepted" that this was the correct explanation for quasars,  and the cosmological distance and energy output of quasars was accepted by almost all researchers.
Hence the name 'QSO' quasi-stellar object is used in addition to "quasar" to refer to these objects, including the 'radio-loud' and the 'radio-quiet' classes.
The discovery of the quasar had large implications for the field of astronomy in the s, including drawing physics and astronomy closer together.
It is now known that quasars are distant but extremely luminous objects, so any light which reaches the Earth is redshifted due to the metric expansion of space.
Quasars inhabit the center of active galaxies, and are among the most luminous, powerful, and energetic objects known in the universe, emitting up to a thousand times the energy output of the Milky Way , which contains — billion stars.
This radiation is emitted across the electromagnetic spectrum, almost uniformly, from X-rays to the far-infrared with a peak in the ultraviolet-optical bands, with some quasars also being strong sources of radio emission and of gamma-rays.
With high-resolution imaging from ground-based telescopes and the Hubble Space Telescope , the "host galaxies" surrounding the quasars have been detected in some cases.
Most quasars, with the exception of 3C whose average apparent magnitude is Quasars are believed - and in many cases confirmed - to be powered by accretion of material into supermassive black holes in the nuclei of distant galaxies, as suggested in by Edwin Salpeter and Yakov Zel'dovich .
Light and other radiation cannot escape from within the event horizon of a black hole, but the energy produced by a quasar is generated outside the black hole, by gravitational stresses and immense friction within the material nearest to the black hole, as it orbits and falls inward.
Central masses of 10 5 to 10 9 solar masses have been measured in quasars by using reverberation mapping. Several dozen nearby large galaxies, including our own Milky Way galaxy, that do not have an active center and do not show any activity similar to a quasar, are confirmed to contain a similar supermassive black hole in their nuclei galactic center.
Thus it is now thought that all large galaxies have a black hole of this kind, but only a small fraction have sufficient matter in the right kind of orbit at their center to become active and power radiation in such a way to be seen as quasars.
This also explains why quasars were more common in the early universe, as this energy production ends when the supermassive black hole consumes all of the gas and dust near it.
This means that it is possible that most galaxies, including the Milky Way, have gone through an active stage, appearing as a quasar or some other class of active galaxy that depended on the black hole mass and the accretion rate, and are now quiescent because they lack a supply of matter to feed into their central black holes to generate radiation.
The matter accreting onto the black hole is unlikely to fall directly in, but will have some angular momentum around the black hole that will cause the matter to collect into an accretion disc.
Quasars may also be ignited or re-ignited when normal galaxies merge and the black hole is infused with a fresh source of matter.
In fact, it has been suggested that a quasar could form when the Andromeda Galaxy collides with our own Milky Way galaxy in approximately 3—5 billion years.
In the s, unified models were developed in which quasars were classified as a particular kind of active galaxy , and a consensus emerged that in many cases it is simply the viewing angle that distinguishes them from other active galaxies, such as blazars and radio galaxies.
More than , quasars are known, most from the Sloan Digital Sky Survey. All observed quasar spectra have redshifts between 0.
Applying Hubble's law to these redshifts, it can be shown that they are between million  and Because of the great distances to the farthest quasars and the finite velocity of light, they and their surrounding space appear as they existed in the very early universe.
The power of quasars originates from supermassive black holes that are believed to exist at the core of most galaxies. The Doppler shifts of stars near the cores of galaxies indicate that they are rotating around tremendous masses with very steep gravity gradients, suggesting black holes.
Although quasars appear faint when viewed from Earth, they are visible from extreme distances, being the most luminous objects in the known universe.
It has an average apparent magnitude of In a universe containing hundreds of billions of galaxies, most of which had active nuclei billions of years ago but only seen today, it is statistically certain that thousands of energy jets should be pointed toward the Earth, some more directly than others.
In many cases it is likely that the brighter the quasar, the more directly its jet is aimed at the Earth. Such quasars are called blazars.
Quasars were much more common in the early universe than they are today. This discovery by Maarten Schmidt in was early strong evidence against Steady State cosmology and in favor of the Big Bang cosmology.
Quasars show the locations where massive black holes are growing rapidly via accretion. These black holes grow in step with the mass of stars in their host galaxy in a way not understood at present.
One idea is that jets, radiation and winds created by the quasars, shut down the formation of new stars in the host galaxy, a process called 'feedback'.
The jets that produce strong radio emission in some quasars at the centers of clusters of galaxies are known to have enough power to prevent the hot gas in those clusters from cooling and falling onto the central galaxy.
Quasars' luminosities are variable, with time scales that range from months to hours. This means that quasars generate and emit their energy from a very small region, since each part of the quasar would have to be in contact with other parts on such a time scale as to allow the coordination of the luminosity variations.
This would mean that a quasar varying on a time scale of a few weeks cannot be larger than a few light-weeks across. The emission of large amounts of power from a small region requires a power source far more efficient than the nuclear fusion that powers stars.
Stellar explosions such as supernovas and gamma-ray bursts , and direct matter - antimatter annihilation, can also produce very high power output, but supernovae only last for days, and the universe does not appear to have had large amounts of antimatter at the relevant times.
Since quasars exhibit all the properties common to other active galaxies such as Seyfert galaxies , the emission from quasars can be readily compared to those of smaller active galaxies powered by smaller supermassive black holes.
The brightest known quasars devour solar masses of material every year. The largest known is estimated to consume matter equivalent to Earths per minute.
Quasar luminosities can vary considerably over time, depending on their surroundings. Since it is difficult to fuel quasars for many billions of years, after a quasar finishes accreting the surrounding gas and dust, it becomes an ordinary galaxy.
Radiation from quasars is partially 'nonthermal' i. Extremely high energies might be explained by several mechanisms see Fermi acceleration and Centrifugal mechanism of acceleration.
Quasars can be detected over the entire observable electromagnetic spectrum including radio , infrared , visible light , ultraviolet , X-ray and even gamma rays.
Most quasars are brightest in their rest-frame near-ultraviolet wavelength of A minority of quasars show strong radio emission, which is generated by jets of matter moving close to the speed of light.
This can be done with relatively high efficiency by photographing large areas of the sky through two or three different-coloured filters.
The photographs are then compared to locate the unusually blue objects, whose nature is verified through subsequent spectroscopy. This remains the primary technique for finding quasars, although it has evolved over the years with the replacement of film by electronic charge-coupled devices CCD s , the extension of the surveys to longer wavelengths in the infrared , and the addition of multiple filters that, in various combinations, are effective at isolating quasars at different redshifts.
Quasars have also been discovered through other techniques, including searches for starlike sources whose brightness varies irregularly and X-ray surveys from space; indeed, a high level of X-ray emission is regarded by astronomers as a sure indicator of an accreting black-hole system.
Supermassive black holes reside at the centres of many large galaxies. There is a maximum rate set by the Eddington limit at which a black hole can accrete matter before the heating of the infalling gas results in so much outward pressure from radiation that the accretion stops.
In addition to black holes and accretion disks, quasars have other remarkable features. Just beyond the accretion disk are clouds of gas that move at high velocities around the inner structure, absorbing high-energy radiation from the accretion disk and reprocessing it into the broad emission lines of hydrogen and ion s of other atoms that are the signatures of quasar spectra.
Farther from the black hole but still largely in the accretion disk plane are dust-laden gas clouds that can obscure the quasar itself.
Some quasars are also observed to have radio jet s, which are highly collimated beams of plasma propelled out along the rotation axis of the accretion disk at speeds often approaching that of light.
These jets emit beams of radiation that can be observed at X-ray and radio wavelengths and less often at optical wavelengths.
Depending on this angle, different quasar components—the accretion disk, emission-line clouds, jets—appear to be more or less prominent.
This results in a wide variety of observed phenomena from what are, in reality, physically similar sources. Because of the finite speed of light , when quasars are observed at great distances, they are observed as they were in the distant past.
Thus, the increasing density of quasars with distance means that they were more common in the past than they are now.
At earlier ages, the number density of quasars decreases sharply, corresponding to an era when the quasar population was still building up.
The most distant, and thus earliest, quasars known were formed less than a billion years after the big bang. Individual quasars appear as their central black holes begin to accrete gas at a high rate, possibly triggered by a merger with another galaxy, building up the mass of the central black hole.
The current best estimate is that quasar activity is episodic, with individual episodes lasting around a million years and the total quasar lifetime lasting around 10 million years.
At some point, quasar activity ceases completely, leaving behind the dormant massive black holes found in most massive galaxies. Indeed, in the current universe the remaining AGN population is made up predominantly of lower-luminosity Seyfert galaxies with relatively small supermassive black holes.
In the present-day universe there is a close relationship between the mass of a black hole and the mass of its host galaxy. This is quite remarkable, since the central black hole accounts for only about 0.
It is believed that the intense radiation, mass outflows, and jets from the black hole during its active quasar phase are responsible.
The radiation, outflows, and jets heat up and can even remove entirely the interstellar medium from the host galaxy.
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