Alright so what is a quasar?

  1. Ive looked on a good number of websites, including the pedia, and i still feel like i didnt get a straight answer. What exactly is a quasar. Is it a black hole in the center of a galaxy? What is the physical makeup of a quasar?
  2. jcsd
  3. Drakkith

    Staff: Mentor

    It is a supermassive black hole, typically in the center of a galaxy that has an active accretion disk surrounding it which puts out a significant amount of EM radiation. The accretion disk is primarily made up of hydrogen and helium gas, and "younger" quasars may also have dust in their accretion disks as well.
  4. perhaps you could elucidate how an accretion disc could be responsible for such enormous output of EM radiation?
  5. Radiation emission is triggered by compression of the materials falling into the center by gravity. The properties of the central object dictate the frequency range of this radiation.
  6. the accretion process is one of the most efficient ways of converting mass into energy (that we know of anyways). suppose a very small amount of matter undergoes a chemical reaction of some sort (combustion, synthesis, acid-base, etc.), and releases several hundred eV (electron volts) of energy. if an identical amount of that same matter were split apart at the atomic level (nuclear fission), it would produce several hundred MeV (several hundred million electron volts). the rate at which nuclear reactions convert mass into energy is several orders of magnitude greater than any chemical reaction. in nuclear fission, approx. 0.04% of the mass involved in the reaction will be converted to energy. nuclear fusion is approx. 8 times as efficient as fission, converting approx. 0.3% of the mass involved in the reaction into energy. but when it comes to converting mass directly into energy, hardly any other method appears to be more efficient than accretion around a highly compact massive object (the only other method i can think of that is theoretically more efficient is matter-antimatter annihilation). while i don't have a source in front of me, i seem to recall reading that an accretion disk can convert up to a whopping ~10% of its mass into energy.

    now, as AbsoluteZer0 mentioned, the radiation emission is triggered by friction within the accretion disk, which is itself caused by the central body's gravitational pull. now while a supermassive BH is much larger than a stellar mass BH (both with respect to mass and EH diameter), it is still confined to what is otherwise an extremely small volume of space in comparison to the size of the galaxy in which it is at the center. using the concept of energy density, one can easily see that the release of such prodigious amounts of energy from within such a confined volume results in an extremely bright this case, it can sometimes be bright enough to be seen from across the visible universe.
  7. So wait a second here.
    Black holes are huge recycling centers for old used up matter?
    That's awesome! I love this universe!!! It totally kicks ***!!!
  8. Drakkith

    Staff: Mentor

    Recycling centers for used matter? Not really. Once the matter passes into the black hole it is gone effectively forever. The time required to "get it back" via hawking radiation is longer than the current age of the universe.
    Also, remember that mass and energy are not the same things.
  9. anorlunda

    anorlunda 575
    Gold Member

    More mature galaxies, including The Milky Way, have supermassive BHs but not such a big accretion disc. It is thought that those galaxies where themselves quasars when they were younger?
  10. So If allowed enough time would a black hole eventually evaporate?
    Would it eventually transform all its mass to energy?

    Or would it eventually stop once its own mass dropped below the minimum mass for a black hole?
    And if so, would that mean that a black hole could become "demoted" back down to regular matter?
  11. Drakkith

    Staff: Mentor

    It's possible.


    There is no minimum mass for a black hole. What is requires is density. Your own body could become a black hole if compressed enough.
  12. I thought a quasar was an actively feeding galactic center? Or no? Anyway I thought that's where the immense amount of energy comes from that can be seen across the Universe: Consuming numerous close-by star systems until all the close ones are gone and then lays dormant, like ours, until stray ones get too close.

    Is that not correct?
  13. Chronos

    Chronos 9,988
    Science Advisor
    Gold Member

    The theoretical lower mass limit for a black hole is believed to be the Planck mass - about 1/100,000 of a gram. This is actually a rather astonishingly large number compared to other Planck units. The reasoning behind this limit is the Compton wavelength of the event horizon should not exceed its Schwarzschild radius, or more simply put, the Schwarzschild radius should not be less than a Planck length. There have been a number of arguments that the realistic minimum mass of a black hole could be considerably larger. This is consistent with the mass range hypothesized for primordial black holes of around 10^14 to 10^22 grams. Primordial black holes of 10^11 g or smaller would currently be evaporating. The resulting gamma ray background from such evaporations would be inconsistent with observation.

    Jack, you are correct. It is currently believed that quasars are powered by supermassive black holes feeding on a supermassive accretion disc. This fuel supply is eventually exhausted and the beast goes into hibernation. These sleeping behemoths could be temporarily reawakened by galactic mergers.
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