The sky that we watch every night is full of thousands stars.
But in reality, what we see with our naked eye is just a tiny part of the Universe.
In reality, the Universe is so big that most of the distant objects are not visible to
us.
However, certain objects are so powerful that despite their immense distance from us, we
are still able to detect them (although only using telescopes).
I’m talking about quasars, or “quasi-stellar radio sources”.
Do you want to know what they are?
Watch this video and I will tell you more!
The discovery of quasars dates back to the 1950s, and it’s interesting to look at the
history of their first observations.
In the 1950s, astronomers started to look at the sky using radio telescopes for the
first time.
Radio telescopes, contrary to normal telescopes, detect radio waves instead of visible light.
When astronomers first started to “watch” the sky with these instruments, they were
surprised to find several objects emitting a large amount of radio waves, but almost
no visible light.
In fact, most of these sources of radio waves did not correspond to any known visible object.
The first quasar ever discovered was 3C 273, a weird name indicating the 273rd object in
the Third Cambridge Catalog of Radio Sources (3C).
It was discovered thanks to a “lunar occultation”, an event that occurs when the Moon passes
between an object and the Earth, hiding the object to our view.
In 1959, a group of astronomers at the Cambridge University identified a radio source in the
sky (3C 273), but they couldn’t find an optical counterpart to it.
Three years later, in 1962, John Bolton and his Caltech radio astronomy group used the
Parkes radio telescope to realize a series of observation of the sky during which the
Moon was passing in front of the radio source discovered three years earlier.
Thanks to these lunar occultations, Bolton and his group were able to calculate the location
of the source with precision.
And more importantly, they were able to associate it to a visible counterpart, a faint stellar
object.
Later on, new quasars were discovered.
Generally, their position in the sky matched with very faint objects, like very distant
stars: that’s why they were called quasars, a contraction of “quasi-stellar” (star-like)
and “radio source”.
However, the chemical composition of these objects observed by looking at their spectral
lines was very different from any known star.
Also, the amount of radiation emitted by them was too large in order to be normal stars.
The observation of these quasars continued throughout the years.
By observing their radiation spectrum, scientists found out that these objects were moving very
fast and away from the Earth.
This could be inferred thanks to a phenomenon known as “redshift”.
What is it?
Every object in the sky emits radiation in a broad range of frequencies – and even
in the visible part of the spectrum, which consists of different colours: violet, blue,
green, yellow, orange and red.
The colour that our eye perceives depends on the frequency of the light that we observe:
violet corresponds to the highest frequency, while red corresponds to the lowest one.
While the light emitted by an object in the sky contains a mix of different frequencies,
it has a peak at a certain colour.
If we look at the sky, in fact, we notice that some stars appear red, while others appear
blue.
However, when the object we are looking at is moving relative to the Earth, the frequency
of the light emitted by the object appears different to our eyes.
In particular, if the object is moving towards us, the frequency of its light appears to
increase (and therefore, it becomes more blueish: this is known as “blue shift”).
Instead, if the object is moving away from us, the frequency of its light appears to
decrease (and therefore, it becomes more reddish: this is known as “red shift”).
So, when scientist observed the radiation spectrum of these quasars, they observed a
large redshift, which means that these objects were moving away from us.
Not only: they also found out that these objects were extremely distant from us.
This information can also be inferred by the amount of redshift.
In fact, in 1931, the famous astronomers Edwin Hubble observed hundreds of galaxies and discovered
that their velocity was proportional to their distance from the Earth.
In other words, the larger the redshift of the light coming from those galaxies, the
faster they were moving away from us, and the more distant they were.
This result is known as the Hubble’s Law, one of the most important results in astronomy.
By applying Hubble’s Law, scientists in the 1960s found out that those quasars they
were observing must have been very distant from the Earth: some of them even 10-20 billion
light years away from us!
This put quasars immediately among the farthest known objects.
Now that we learnt a bit of history of quasar observation, we may wonder: what are quasars,
actually?
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The main problem in the observation of these quasars was to explain how could they generate
such huge amounts of energy: in fact, some of these quasars were visible despite their
enormous distance from the Earth.
There was no known mechanisms to explain that.
So, scientists had no idea of what these quasars were.
The explanation was proposed by Edwin Salpeter and Yakov Zel’dovich in 1964.
According to them, quasars were supermassive black holes surrounded by large amounts of
matter.
But wait… we actually know that light (and any other form of radiation) cannot escape
a black hole: so, how could these objects generated such huge amounts of light and energy?
The reason is, the energy of a quasar is actually produced outside of the black hole.
The supermassive black hole is surrounded by a huge amount of matter in rapid rotation
around it, and falling towards it.
Due to the immense gravitational force exerted by the black hole, this matter undergoes extremely
strong friction, and as a result, it emits a lot of energy as radiation of different
forms (such as radio waves).
This matter in rotation takes the shape of a disc, and it is known as accretion disc.
The amount of energy produced through this mechanism is very high: when this matter falls
towards the black hole, between 6% and 32% of its mass is converted into energy!
As a comparison, less than 1% of the mass is converted into energy in the nuclear reactions
inside our Sun.
Indeed, quasars are among the most powerful and energetic objects in the Universe.
As a comparison, a quasar can emit an amount of energy equal to thousand times the amount
of energy emitted by the whole Milky Way, our galaxy.
Despite this, given their enormous distance from us, quasars are too dim to be observed
from Earth with normal telescopes.
However, if we could put one of these quasars at the same distance as the Sun from the Earth,
it would shine trillion times brighter than our star.
It is interesting to note that despite their extreme brightness, the size of a typical
quasar is relatively small.
In fact, quasars are usually just a few light years of diameter, and some of them can even
be as small as our Solar System.
To give you an idea, the diameter of the Milky Way is about 200,000 light years.
As you can see, it’s a huge difference!
Quasars are part of a larger family of objects known as Active Galactic Nucleus (AGN), which
includes all those galactic cores with anomalous emission of electromagnetic radiation, explained
through the accretion disc mechanism.
AGNs do not emit only radio waves and visible light, but also other forms of radiation,
such as X-rays and gamma rays.
An interesting phenomenon sometimes associated to quasars is the presence of relativistic
jets.
While their origin is not completely clear, it is believed that they are originated by
the interaction between the magnetic field of the black hole and the accretion disc.
In certain conditions, this interaction produces jets of particles travelling almost at the
speed of light.
These jets erupt from both poles of the black holes, so they appear as two beams of extremely
energetic particles travelling in opposite directions.
They are extremely bright (sometimes even more than the quasar itself) and can extend
for million light years.
These quasars having relativistic jets are also known as blazars.
You may wonder, why these quasars are so distant from us?
Are there any quasars close to us?
This is an interesting question, and the answer has to do with the evolution of galaxies.
It is believed that quasars were more common in the early universe.
The reason is that quasars are extremely active objects, so they need a lot of material to
form.
For example, quasars can form as a result of the collision between two galaxies: when
this event occurs, the black holes at the centre of the two galaxies can fuse, creating
a supermassive black holes that can originate a quasar.
The early universe was much smaller in size and more dense than now, therefore, extreme
events such as the collision between two galaxies were much more likely to happen: so, the formation
of a quasar was much more frequently than nowadays.
Given their nature, quasars “burn” a lot of energy very quickly, as matter keeps falling
into the black hole.
This means that the accretion disc does not last forever, and by now, most of the quasars
have “consumed” all the energy available.
We also have to keep in mind that there is a relationship between the distance of a celestial
object and its age.
In fact, the light emitted by an object takes some time to reach us: the farther away the
object is, the longer it takes for light to reach us.
Since the Universe is in continuous expansion, most quasars are now very distant, even 10
or 20 billion light years away, which means that light takes 10 or 20 billion years to
reach us!
The implication of this is that we are actually observing quasars as they were 10 or 20 billion
years ago: it’s like “looking into the past”.
Most likely, these quasars have now exhausted their energy and are no longer that luminous.
This is the reason why we mostly don’t see quasars near to us.
It is also possible that some of the galaxies that we observe nowadays were actually quasars
in the past.
In fact, the galaxies that we observe today also have massive black holes at their centre.
So, it is possible that in their early age, the matter that was very close to the black
hole has undergone the phenomenon that we described before (accretion disc), but we
no longer observe it In this sense, quasars can be though as the cores of young galaxies:
indeed, scientists believe that the Milky Way itself was a quasar in its earliest age.
However, this doesn’t mean that quasars can no longer form: on the contrary!
The Milky Way itself, in fact, may possibly host a new quasar in the future…
In fact, the Milky Way and the Andromeda galaxy are currently moving towards each other, and
it is predicted that they will collide in about 4.5 billion years from.
We have to keep in mind that both galaxies contain a supermassive black hole at their
centre.
Therefore, when this gigantic collision will happen, these two black holes will converge
and will interact in a catastrophic event.
The matter surrounding these two monsters will undergo extreme frictional forces, generating
a quasar, or even a blazar.
So, ultimately, this is the fate of our galaxy!
Fascinating, isn’t it?
Somehow, we can say that quasars represent our past, but also our future…
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