Supermassive black holes are
the largest mass black holes and are found at the centers of galaxies.
Galaxies are large collections of stars ranging from
small galaxies consisting of hundreds of millions of stars,
up to the largest galaxies which can have upwards of 1 trillion stars.
Galaxies fall into several categories developed by astronomers based on their shapes.
Like spiral galaxies and elliptical galaxies.
There are also galaxies that don't fit into the standard classifications.
Our classification system has really become a zoo of galaxy types.
Among the unusual creatures in the galactic zoo,
active galaxies are those which are thought to host
a supermassive black hole at their center. Let's visit the zoo.
The galaxy NGC 7742 is an example of a Seyfert galaxy.
Seyfert galaxies are spiral galaxies,
with unusually bright central regions.
The properties of the light emitted from
the bright cores is not typical of regular star light.
Normally the light from a galaxy is mainly blackbody emission from the stars,
as we learned about earlier.
But the central regions of Seyfert galaxies also have
a bright emission spectrum which indicates extremely hot gas.
We will learn what an emission spectrum is in a later module.
The first Quasar discovered in 1963,
is called 3C 273.
In the first pictures taken of the quasar,
its features could not be resolved and it looked much like a star.
However, the properties of this star seemed very peculiar.
So it was called a quasi-stellar object.
This name was later shortened to QSO or quasar.
Modern images show that the quasar isn't a star but the ultra bright nucleus of a galaxy.
In the left picture,
the quasar is the very bright point of light in the middle of the box.
The spikes that are visible are artifacts of how the light is collected by the telescope.
You might be able to see faint light from the galaxy surrounding the bright quasar.
There is a jet of gas poking down into the right. In order to take a photo on the right,
the Hubble Space telescope placed a shield
over the bright light which allowed it to take a picture of the galaxy.
In 1929, astronomers observed a star-like object that they called
BL Lacertae or BL Lac for short.This thing also looked like a star through a telescope.
But its brightness varied so wildly that it could sometimes
be 15 times brighter than it was during the previous month.
At the time it was classified as a variable star.
But when better telescopes observed BL Lac,
evidence for a galaxy could be seen surrounding the bright point of light.
Other similar galaxies have since been discovered and they've been called BL Lac objects.
Since they seem very similar to the original.
More recently, astronomers have been calling these galaxies BLAZARS.
When Blazars are imaged,
we find them at the center of elliptical galaxies.
Galaxies are made up of lots of stars.
So we expect the light from a galaxy to look like the light from stars.
Stars emit most of their light in
the ultraviolet, visible, and infrared parts of the electromagnetic spectrum.
One thing that stars do not emit significant amounts of are radio waves.
That means that normally we would not detect much radio wave emission from galaxies.
However, a small fraction of galaxies emit lots of radio waves.
We call these galaxies radio galaxies.
Most radio galaxies are also giant elliptical galaxies.
Active galaxies that are part of this zoo,
share many similar properties.
Typically they have a bright central object called an Active Galactic Nucleus or AGN,
which is surrounded by the fainter stars that make up the spiral or elliptical galaxy.
Have a look at this diagram which represents
a unified model for all of these active galaxies.
The unified model for
an active galactic nucleus has a supermassive black hole at its center,
surrounded by a disc of gas that orbits the black hole and emits lots of energy.
In some cases there is a jet of gas shooting out from the central object.
We will learn more about discs and jets in a later module.
The model predicts that when you look directly down the jet you see a blazar.
If you look at the jet from an angle so that you can also see into the accretion disk,
then you see a radio loud quasar.
If you're looking at the edge of the disc
so that the inner part of the accretion disc is blocked,
you will see a radio galaxy.
If there's no jet then there will be
very little radio emission and you'll see a Seyfert galaxy.
Which is also something that could be called a radio quiet quasar.
So, the difference in the names you call
a supermassive black hole really just depend on your point of view.
