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[sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9)
Section - H.06 What are QSO's ("quasars")?

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"Quasi-stellar objects" (or QSO's) are defined observationally as
objects that appear star-like on photographic plates but have high
redshifts (and thus appear extragalactic; see above).  The luminosity
(if we accept that the redshift correctly indicates the distance) of a
QSO is much larger than that of a normal galaxy, and many QSO's vary on
time scales as short as days, suggesting that they may be no more than a
few light days in size.  QSO spectra typically contain strong emission
lines, both broad and narrow, so that the redshift can be very well
determined.  In a few cases, a nebulosity reminiscent of stars in a
normal galaxy has been detected around a QSO.  Quasars (a shortened
version of "quasi-stellar radio source") were originally discovered as
the optical counterparts to radio sources, but the vast majority of
QSO's now known are radio-quiet.  Some authors reserve the term "quasar"
for the radio-loud class and use the term "QSO" generically; others
(especially in the popular literature) use "quasar" generically.

In the standard model, QSO's are assumed to lie at the centre of
galaxies, and to form the most extreme example of the class of active
galactic nuclei (AGN); these are compact regions in the centre of
galaxies which emit substantially more radiation in most parts of the
spectrum than would be expected from starlight.  From the energy
output in QSO's, together with some guess at their lifetime (about
10^8 years) the mass of the central engine can be estimated as of
order 10^7 solar masses or more (this is consistent with estimates of
the masses of other, related types of AGN).  A compact, massive object
of this kind is most likely (on our current understanding of physics)
to be a black hole, and most astronomers would accept this as the
standard assumption.  The luminosity ultimately derives from matter
falling into the black hole and gravitational potential energy being
converted to other forms, but the details are unexplained and very
much an active research topic.

User Contributions:

1
Keith Phemister
Sep 13, 2024 @ 11:23 pm
Copied from above: If the Universe were infinitely old, infinite in extent, and filled
with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun.
Why would anyone assume this? Certainly, we have directions where we look that are dark because something that does not emit light (is not a star) is between us and the light. A close example is in our own solar system. When we look at the Sun (a star) during a solar eclipse the Moon blocks the light. When we look at the inner planets of our solar system (Mercury and Venus) as they pass between us and the Sun, do we not get the same effect, i.e. in the direction of the planet we see no light from the Sun? Those planets simply look like dark spots on the Sun.
Olbers' paradox seems to assume that only stars exist in the universe, but what about the planets? Aren't there more planets than stars, thus more obstructions to light than sources of light?
What may be more interesting is why can we see certain stars seemingly continuously. Are there no planets or other obstructions between them and us? Or is the twinkle in stars just caused by the movement of obstructions across the path of light between the stars and us? I was always told the twinkle defines a star while the steady light reflected by our planets defines a planet. Is that because the planets of our solar system don't have the obstructions between Earth and them to cause a twinkle effect?
9-14-2024 KP

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