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
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 galaxy.
Why do some quasars give off radio waves? The