Quasars
QSO or Quasi-stellar objects, commonly known as quasars are
one of the most magnificent celestial bodies! They have a supermassive black
hole, a solar system sized whirlpool of super-heated plasma that shines
brighter than an entire galaxy and jets of near light-speed particles that fill
the universe with giant radio plumes. It’s no wonder it attracts so many
astronomers and cosmologists!
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| A Quasar |
A Quasar as Wikipedia defines is an active galactic nucleus
of very high luminosity. A Quasar consists of a supermassive black hole
surrounded by an orbiting accretion disk of gas. As gas in the accretion disk
falls towards the black hole, energy is released in the form of electromagnetic
radiation.
This must be a complex definition to understand at this
stage. So, here I come.
To understand it more, we have to take a look at
its history first.
In the 1950s, when the first radio telescopes pointed to the
sky they noticed a fat blob of radio signals. But the telescopes back then had a very low spatial resolution. Thus, the source could not be located. It was only
in 1963 that astronomers had a chance. In an event known as a lunar occultation, (If the moon happens to cross the line of
sight to a source, the source is said to be occulted. The drop in intensity of
a radio source as it is blocked by the moon and the rise when the moon has
moved out of its way gives an accurate indication of when it was occulted and,
since the position of the moon in the sky is accurately known, it is possible to
have an accurate estimate of the direction to the radio source.) the moon
passed in front of one of these brightest of radio blobs. The Parkes radio
telescope in Australia was trained for it. The timing of occultation allowed
astronomers to identify a tiny star-like point of bluish light as the source of
radio emission. The astronomers observed this “star” and split the observed light into a spectrum. The spectrum
looked nothing like the spectrum of stars ever observed. One thing to be noted
was that the spectrum was red-shifted, which meant that the wavelength of this
light stretched out, as it travelled through the expanding universe. That meant
the quasar discovered (named 3C 273) was
very far away, about two billion light-years, to gain the red-shift observed.
Yet, to be as bright as it was observed after travelling so long a distance meant the object that the light came from was emitting many galaxies worth of light
from a seemingly impossible small region of space.
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| QUASAR 3C 273 |
One more factor that contributed to the estimation of a quasar’s size was the fact that many quasars exhibit rapid variation in their light and radio output. This fact places a limit on their physical size; for, if an object exhibits variability on a characteristic time scale T, its size must be limited by ~ cT, where c is the speed of light. This limitation arises from the special relativistic result that no physical disturbance can propagate with a speed greater than c.
This made quasars very compact. A quasar may emit comparable energy to that of a galaxy per unit time from a volume whose linear extent may be only a few light hours!
This huge amount of energy from a seemingly small region in
space invited several hypotheses. Some claimed the light was coming from a
swarm of neutron stars. Some proposed possibility of an advanced alien
civilization harnessing their entire galaxy’s power. But it was only in the 1980s
when the most satisfying explanation came.
For a quasar to form, we need a black hole of mass millions
to billions of times the mass of our sun. And every decent-sized galaxy has one
such black hole at its core including our Milky Way!
So, what happens when
this black hole decides to feed?
A swarm of gases is driven into a galactic core, which may
happen when galaxies merge and grow. So, this swarm of gases approaches the attracting
black hole at incredible speeds. It is swept up into a raging whirlpool around
the black hole which we call an accretion disk. Here, the energy of the motion of
these gases is turned into heat. The heat glow of the accretion disk travels to the
ends of the universe. Some gas is swallowed, causing the black hole to grow.
However, a lot of it does not make it to the event horizon. Some are converted
directly into energy and radiated as light and this same light drives powerful
wind of gas back out into the surrounding galaxy. In some cases, for reasons, we
fully don’t understand some of that gas can also be paralleled and channelled
into jets that erupt from the poles of the quasar. This might be due to the
magnetic field of a rapidly growing black hole.
The exact appearance of this phenomenon depends largely on
our viewing angle. When the jets are perpendicular to our view, we see a radio
galaxy.
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| Radio Galaxy Hercules A |
If they are at an angle, we see a quasar and when staring right down
the barrel of the jet we see a blazer.
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| Blazar |
These are the same objects viewed from three different angles.
Each burst of quasar activity in a given galaxy probably
lasts for only ten million years or so but is enough for making considerable
changes in the entire galaxy.
Our Milky Way Galaxy has such a black hole at its core. It
might have been roaring in the past but is silent now. Perhaps, in a few billion
years, when it will collide with its closest neighbour, the
Andromeda Galaxy and their cores will merge it will roar again, reminding The universe of its existence…
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| Collision scenario for Milky Way and Andromeda Galaxy Encounter |
So, that was all about the enigmatic quasar for now – the brightest and one of the most beautiful phenomena of the Universe. <3
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