Essay/Term paper: Quasars

Essay, term paper, research paper:  Miscellaneous

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

redshifts.

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 electrons near the center of

the quasar can be accelerated to speeds near the speed of light. In the

presence a magnetic field (which is present in these same regions), the

electrons move along helical paths (paths that look like a stretched out

slinky). As a result, they emit radio waves (it's called synchrotron

radiation, since these waves are observed on Earth when physicists send high

energy electrons around in circles using magnetic fields, in particle

accelerators call synchrotrons).

How long do quasars last? It appears galaxies may only act, as quasars

during the early stages of their 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 exactly know how they form. However, it can't take much longer than

something like a billion years.

Can anything develop from quasars after they die? Probably, the only thing

that would be left is the Suppressive black hole. In other words, the gas

near it would have been used up and so the quasar shuts off. But the

remaining stars in the galaxy, as a whole would, of course, still be there.

Why are quasars so interesting to study? They are only seen far away. Since

the light from quasars were emitted billions of years ago and we are just

seeing them today we realize that they are all very old. There are no nearby

quasars, so there are no young quasars; quasars are not made during our era

of the universe, only during an ancient era.

This also implies the universe was a different place in the past. It also

says the galaxies we see around us now may have been quasars in the distant

past; even our Milky Way galaxy may have been a quasar-like galaxy long ago,

now not much material falls into the large black hole at the Milky Way's

center, so the radiation output from the center is not as great as it used to

be. How difficult are quasars to study? Not all that difficult, if you have

a huge telescope such as the Hubble Space Telescope.

The Hubble Space Telescope has shown that quasars live in a remarkable

variety of galaxies, many of which are violently colliding. This complicated

picture suggests there may be a variety of mechanisms some quite subtle for

turning on quasars, the universes most energetic objects. Hubble researchers

are also intrigued by the fact that the quasars studied do not appear to have

obviously damaged the galaxies in which they live. This could mean that

quasars are relatively short lived phenomena which many galaxies, including

the Milky Way, experienced long ago. John Bahcall of the Institute for

Advanced Study, Princeton, NJ, emphasizes that Hubble's clarity opens a

complicated picture. The basic assumption was that there was only one kind of

host galaxy, or catastrophic event, which feeds a quasar. In reality we do

not have a simple picture, we have a mess. Mike Disney, University of Wales

College, Cardiff, U.K., who is a leader of the European team, says, "People

had suspected that collisions might be an important mechanism for feeding

black holes and generating the vast amounts of energy emitted by quasars.

Now we know they are and we didn't know that before Hubble. Although a number

of images obtained through the use of the Hubble Telescope show collisions

between pairs of galaxies, which could trigger the birth of quasars, some

pictures reveal apparently normal, undisturbed galaxies possessing quasars.

The beauty and clarity of the Hubble images, as well as the diversity of

quasar environments amazed investigators.

Discovered only 33 years ago, quasars are among the most baffling objects in

the universe because of their small size and prodigious energy output.

Quasars are not much bigger than Earth's solar system but pour out 100 to

1,000 times as much light as an entire galaxy containing a hundred billion

stars. A super massive black hole, gobbling up stars, gas and dust, is

theorized to be the engine powering a quasar. Most astronomers agree an

active black hole is the only credible possibility that explains how quasars

can be so compact, variable and powerful. Nevertheless, conclusive evidence

has been elusive because quasars are so bright they mask any details of the

environment where they live.

These problems could not be solved without the Hubble Telescope. Observations

by the European team, using the Wide Field Planetary Camera (WFPC2) in high

resolution mode, reveal that quasars appear to be born in environments where

two galaxies are interacting violently and probably colliding. This had long

been suspected as a mechanism for igniting a quasar, but no one knew whether

the idea was really right before the Hubble. In nearly every quasar we look

at we clearly see one galaxy apparently swallowing another . Disney selected

three quasars known to be strong infrared emitters, suggesting that they

might be in spiral galaxies, which typically contain an enormous amount of

gas and dust. When we image them with Hubble we see the most colossal

smashups, where two giant spiral galaxies like our own Milky Way have crashed

head on into one another and flung off pieces violently in all directions.

Some of those bits seem to have finished up in the nucleus of one of the

spirals where there is probably a giant black hole feeding on it. Both teams

agree that Hubble images do show conclusively:

1.That most quasars lie at the cores of luminous galaxies, both spiral and

elliptical.

Though underlying galaxies were suggested in ground-based quasar

observations, astronomers had to wait for Hubble's capabilities to show the

host galaxies clearly enough for astronomers to begin to classify their

shapes.

2. Interactions between galaxies, either through direct collisions or near

encounters, can be important in turning on a quasar, by dumping fuel onto a

black hole. However some quasars look unperturbed, so there may be other,

more subtle mechanisms for feeding the black hole. Some of the galaxies we

observed don't appear to know they have a quasar in their core.

3. Quasars that are radio quiet are often in elliptical galaxies, not always

in spiral galaxies, as previously believed.

Advanced instruments planned for Hubble should also help pin down more

details. The Near Infrared Camera and Multi-Object Spectrometer (NICMOS), to

be installed in 1997, and the Advanced Camera, to be installed in 1999, will

have coronagraphic devices which will block out the glare of a quasar,

allowing astronomers to see closer into a galaxy's nucleus. By viewing

galactic structures in infrared light , the NICMOS should be able to provide

important new details about the host galaxies of quasars. The continued study

of quasars and the information that it will provide us with may help us to

develop a better understanding of space and how we fit in to this great

puzzle.