About site: Astronomy/Galaxies/Active Galactic Nuclei - Quasars - Frequently Asked Questions About Quasars

About site: http://www.phys.vt.edu/~jhs/faq/quasars.html

Title: Astronomy/Galaxies/Active Galactic Nuclei - Quasars - Frequently Asked Questions About Quasars Part of an FAQ site that evolved from an exchange of e-mail messages with middle school science students.

Laser_Stars_-_Quasars Quasars are not black holes, they are laser stars within our galaxy.

When_Did_the_First_Cosmic_Structure_Form? Contains press release about the impact of quasars within various galaxies. Includes photos. (November 19, 1996)

Acoustic_material_property_tables Specialty Engineering Associates

ISO_9613 attenuation of sound during propagation outdoors.

Seawater_Sound_Speed_Calculator by Simon Richards.

Velocity_of_sound_in_some_Biological_Material Ultrasound physics. Parameters. Differences between X-rays and ultrasound.( Gonzalo Diaz M. M.D.)

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Related sites for http://www.phys.vt.edu/~jhs/faq/quasars.html

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Frequently Asked Questions About Quasars

Frequently Asked Questions AboutQuasars

Compiled byDr. John Simonettiof the Department of Physicsat Virginia Tech.Back to Frequently Asked Astronomy and Physics QuestionsWhat is your definition of a quasar?What do quasars have to do with black holes?How big are quasars compared to galaxies?Why do some quasars give off radio waves?About how long do quasars last?How long does it take for a quasar to form?How do quasars form?How difficult are quasars to study?Can anything develop from quasars after they die?What reasons do you find quasars interesting?Are the radio waves from objects in space any threat to us?How do some Quasars become radio-quiet?Do quasars have anything in common with a regular star?How are BL Lac objects formed compared to the way a quasar is formed?Are quasars related to pulsars? If so how?What is your definition of a quasar?This is best answered with history. When radio telescopes were firstturned on the heavens, point sources of radio waves were discovered (alongwith spread-out regions of emission along our Milky Way). Astronomersusing ordinary visible-light telescopes turned toward these radio pointsand looked to see what was there. In some cases a supernova remnant wasfound, in others, a large star-birth region, in others a distant galaxy.But in some places where point sources of radio waves were found, novisible source other than a stellar-looking object was found (it lookedlike a point of like --- like a star does). These objects were called the"qausi-stellar radio sources", or "quasars" for short. Later, it was foundthese 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 objectsare the very bright centers of some distant galaxies, where some sort ofenergetic action is occurring, most probably due to the presence of asupermassive black hole at the center of that galaxy (supermassive = madeup from a mass of about a billion solar masses).What do quasars have to do with black holes?See the above answer. It is thought the infall of matter into thesupermassive black hole can result in very hot regions where huge energiesare released, powering the quasar (i.e., producing the emitted light,etc.).How big are quasars compared to galaxies?Well, the region of intense visible emission is quite small compared to therest of the galaxy that it is imbedded in. The visible emission only occursvery near the center of the galaxy. On the other hand, huge regions ofradio emission, produced by the quasar, can stretch out to large distancesoutside the galaxy.Why do some quasars give off radio waves?The electrons near the center of the quasar can be accelerated to speedsnear the speed of light. In the presence a magnetic field (which ispresent in these same regions), the electrons move along helical paths(paths that look like a stretched out slinky), and as a result, they emitradio waves (it'scalled synchrotron radiation, since these waves are observed on Earth whenphysicists send high energy electrons around in circles using magneticfields, in particle accelerators call synchrotrons).About how long do quasars last?It appears galaxies may only act as quasars during the early stages oftheir lives, but it would still be for times of billions of years.How long does it take for a quasar to form?Nobody really knows, since we don't know exactly how they form! However,it can't take much longer than something like a billion years (the apparentanswer to all questions about cosmology!).How do quasars form?See the above. But it's thought the process begins as gas collects nearthe center of a galaxy.How difficult are quasars to study?Not all that difficult, if you have a huge telescope! The HubbleTelescope, for example, is quite nice for various studies of quasars.Can anything develop from quasars after they die?Probably, the only thing that would be left is the supermassive black hole.In other words, the gas near it would have been used up, and so the quasarshuts off. But the remaining stars, etc., in the galaxy as a whole (i.e.,not near the very center of the galaxy), would, of course, still be there.What reasons do you find quasars interesting?For one, they are only seen far away. Thus, since the light takesbillions of years to get to us from a quasar, the quasars are all very old.There are no nearby quasars, so there are no young quasars; quasars are notmade during our era of the universe, only during an ancient era.This also implies the universe was different place in thepast (billions of years ago). It also says the galaxies we see around usnow may have been quasars in the distant past; even our Milky Way galaxymay have been a quasar-like galaxy long ago --- now not much material fallsinto the large black hole at the Milky Way's center, so the radiationoutput from the center is not as great as it used to be.Are the radio-waves from objects in space any threat to us?No, they are extremely weak. The total energy collected by radioastronomers over the history of radio astronomy amounts to about the energyrequired for a mosquito to make one "push-up"!! The reason we don't receivelots more radiation is that the sources (e.g., the quasars) are so very faraway.How do some Quasars become radio-quiet?That's not a well understood thing. It my have to do with the nature ofthe environment around the central black hole, the size of the blackhole itself, and/or theorientation of the black hole and its surrounding, radiating disk ofinfalling material, as seen from Earth.Do quasars have anything in common with a regular star?No, except that quasars are in galaxies of stars.How are BL Lac objects formed compared to the way a quasar is formed?No real big difference. There are some "minor" differences having to dowith time scales of intensity variation, and the presence or absence ofcertain features in the spectra of these objects.Are quasars related to pulsars? If so how?A pulsar is a much smaller mass object, much smaller in radius and not ablack hole, but a neutron star (it "failed" to become a black hole duringits birth due to a supernova explosion of some single star). However, theneutron star is nearly as compact as a black hole of that star's mass.Magnetic fields near the blackhole and a pulsar may be similar instructure and have something to do with some energy output of each.middle width=576 height=5 src="http://www.phys.vt.edu//images/rainbow.gif">Back to Frequently Asked Astronomy and Physics Questions

Part	of	an	FAQ
site	that	evolved	from	an
exchange	of	e-mail	messages	with
middle	school	science	students.

http://www.phys.vt.edu/~jhs/faq/quasars.html

Frequently Asked Questions About Quasars 2008 December

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