Galaxy and Quasars

Topics: Galaxy, Milky Way, Supermassive black hole Pages: 10 (1885 words) Published: October 8, 1999
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...
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