We live in an ordinary galaxy and I'm glad we do. Things here are fairly settled. Stars come and go at a normal rate and we travel around our spiral galaxy in a "safe" environment. However, there are some strange galaxies out there and, although you won't be studying them as an amateur astronomer, you should know a little about them because they are so interesting and an active research topic in astronomy. Besides, some are visible with only a little magnification (but what makes them strange is not visible).
What kinds of galaxies are strange?
There are really two types of what I call "strange galaxies" - starburst galaxies and active galaxies. Let's start with starburst galaxies because they are not too strange.
A starburst galaxy is a galaxy undergoing a massive burst
of star formation. They are characterized by having an infrared
luminosity much higher than their optical luminosity. In other
words, starburst galaxies produce more heat than light.
Infrared is the part of the light spectrum "below
the red" in the sense that infrared waves (or particles)
carry less energy than red waves (or particles).
We can see red light but infrared light is invisible to us. Instead, we humans sense infrared as "heat". Our skin is covered with "infrared sensors" but we cannot form an image of the object giving off the infrared because our skin doesn't have the ability to focus the energy into an image. Therefore, we can sense the direction, but not the shape, of an infrared source. [Some animals, such as rattlesnakes, can "see" into the infrared but they don't use their eyes. They use a pair of sensors in the nose to get a good "image" of an infrared object.]
Infrared energy, or simply "heat", is produced by many different things. It's the leftover energy given off by machines (due to friction in their parts), by many animals (as part of their biochemistry) and by just about everything that makes any kind of energy. For example, a light bulb gives off a lot of light in the optical range, including red light, but it also gives off heat - infrared light. Fluorescent lights and most lasers are designed to be much more efficient with their energy and do not waste it so they don't produce infrared. Those special lights are an exception. Most things that give off some light also give off infrared.
Here's a star's spectrum that I showed you several months ago when I taught you about the Maxwell-Boltzmann distribution. The wavelength is along the X-axis (horizontal) and runs from low energy on the left (red) to high energy on the right (violet). The height (Y-axis) represents the amount of energy at that wavelength. This image shows only the optical range but there is a lot of energy given off in the infrared. That's why the surface of a star is hot! Recall that high-energy gamma rays move upward from the center of a star towards its surface. That energy bounces around and gets spread into the lower energy parts of the spectrum. By the time it emerges from the star's photosphere, much of that energy has been distributed into the optical and infrared range. Keep that thought in mind as we go through this lesson. | ![]() |
Very little infrared light passes through our atmosphere. You may be surprised to learn that most of the heat at the Earth's surface is actually produced at the surface - but not from the Earth itself (although a small amount of our heat is from volcanoes and other forms of geothermal energy). Most of our planet's warmth is due to optical energy, which easily passes through our atmosphere, then bounces off the soil and water and is converted to infrared (in a process not unlike the way high energy is distributed down to lower energy as it moves up from the inside of the Sun's surface).
If infrared is invisible and doesn't even get through our atmosphere, how can astronomers "see" it (in a starburst galaxy, for example)?
Technology is the answer. Instruments can sense infrared light and focus it into an image. Rockets can put those instruments above the atmosphere. The first "infrared astronomy" was done using remote telescopes on balloons but in the early 1980s the Infrared Astronomical Satellite (IRAS) surveyed the whole sky from Earth orbit. IRAS found many infrared-bright galaxies (and some other interesting objects, too).
OK, but why are starburst galaxies putting out so much more infrared?
Because they're making stars! New stars are still enveloped in
a dense cloud of gas from the condensing nebula. When these new
stars first form they must shine through all that extra "dust".
The high-energy parts of their spectrum are absorbed by the surrounding
materials and re-emitted at lower energies - mostly infrared energies.
Indeed, up to 50 times as much energy escapes as infrared light
than in the optical (visible) parts of the spectrum.
Note that not ALL of the energy is in the infrared, just most of it, so a starburst galaxy can still be seen at optical wavelengths. M82 is an example of a starburst galaxy. Obviously, it's visible - otherwise Messier would not have seen it and given it a number! | ![]() |
In fact, due to the way the energy is redistributed, during collisions with materials, the final visible color of a starburst galaxy is a bit on the red side - like a red giant. (And for similar reasons. The energy has been "cooled" to a lower energy before being "released" into space.)
So, starburst galaxies are often "dim" in the optical
range but "bright" in the infrared range. That means
the "luminosity" of a starburst galaxies can be deceiving.
Indeed, when total luminosity is calculated - taking into
account ALL the energy in ALL parts of the spectrum (visible and
invisible) - we find that starburst galaxies are one of the most
luminous galaxies known!
When working with
large numbers, scientists like to use scientific notation,
which is a way to represent the number in base ten exponetials.
Using scientific notation, our galaxy has a total luminosity of
2.5 X 1010 Lsun and a starburst galaxy has a total luminosity
of 1014 Lsun.
In a starburst galaxy the star formation occurs in a region a
few thousand light-years in size. This is a much larger area than
you would expect to be involved in "normal" star formation
(and also much larger than the area involved in active galaxies,
which we will discuss shortly). Most importantly, in a starburst
galaxy the stars are formed at a rate that cannot be sustained
for very long. That is, if stars continued to be formed at the
current (fast) rate, starbursting would have used up all the starting
materials in that galaxy long ago. Therefore, starbursting must
be a short-lived phenomenon.
You will recall that star formation involves the contraction of
huge amounts of gas and dust. Eventually enough mass is squeezed
into a small enough space that the heat and pressure cause the
material to undergo nuclear fusion and "a star is born".
Star formation in our Galaxy is a slow steady process - about
one "solar mass equivalent" of interstellar gas and
dust is turned into a star each year in the Milky Way. That is just an average
number and, like most things involving statistics, there is some
variation. It's not like a new star is formed in our Galaxy exactly
each year (like a birthday gift)! All I mean is that it's a slow
and steady process. In a starbursting galaxy, however, there is
a huge amount of star formation - about 100 new stars a year! If
that had been the rate of star formation for the billions of years
that the starburst galaxy had been around, it would have run out
of dust and gas (to make new stars) long ago. Instead, a starburst
galaxy undergoes star formation for a brief period of time - just
a few million years.
So, what causes them to starburst?
We aren't completely sure. Starbursting might occur for different
reasons and by different means. The slow and steady rate of star
formation in our own galaxy is due to the fairly even distribution
of materials, various ages of stars (so recent novas will distribute
materials for new stars) and is helped by the density waves that
go around spiral galaxies.
We see some starbursting when galaxies collide (as you learned
last month) but those are exceptional cases. Most starburst galaxies
are not undergoing collision. Instead, it appears that the burst
in star formation occurs due to an increase in local density of
star forming materials within that galaxy. This might be caused
by local fluctuations in the materials. In spiral galaxies this could
be caused by unusual density waves travelling in the disk. Inside
elliptical galaxies there may be a series of collisions that perpetuate
a chain reaction of stars colliding. When stars collide we would
expect them to produce a new, brighter star and there would be
a cloud of materials around it due to the disruption that occurs
before, during and after the collision.
Sounds like a starburst galaxy is an active galaxy.
Well, yes, but don't confuse them with an "active galaxy".
By definition, an active galaxy emits unusually large amounts
of energy from a very compact source - much more compact then the thousands of light-year radius of star formation that occurs in a starburst galaxy. Also, a starburst galaxy can be emitting the energy from just about any part of its "body" but an active galaxy emits its energy only from its center or nucleus. As a matter
of fact, an alternative name for them is active galactic nucleus
or AGN.
Radio waves are even lower in energy than infrared but don't let
that confuse you. Each photon is of low energy but there are so
many photons that the TOTAL energy output of an AGN is huge!
Take a look at this graph and convince yourself that an AGN produces
large amounts of (invisible) low-energy light as radiowaves.
There are several different kinds of AGNs and we will spend the
rest of this lesson learning about them.
But where does all this energy come from? And how is this
"gas" giving off light and radio waves?
Those are the questions that astronomers are now trying to answer
and they now think they have a good explanation. A massive black
hole seems to be the source of energy! Gas accreting onto the
black hole radiates energy as it falls towards the event horizon.
The black hole at the center of an AGN is more massive than the
black holes you learned about earlier. An ordinary black hole
is formed when a large star collapses into a non-fusing sphere
of more than three solar masses. The super massive black hole at the center
of an active galaxy has a mass between 1 million and ten billion
solar masses! Such black holes would have a Schwartzchild radius
between 3 million and 30 billion kilometers! These are very big
black holes. It would take about 10 seconds for light to travel
3 million kilometers and an entire day to cross 30 billion kilometers!
[Of course, inside the back hole that light would be trapped forever
anyway, but my point is that we are talking about GIGANTIC black
holes here.]
As material, mostly gas, spirals towards the event horizon it
bumps into other materials and they emit X-rays! These X-rays are produced whenever electrons
smash into something and there are plenty of materials to smash into as you approach the event horizon. This type of X-ray production is referred to as bremsstrahlung which is German for "braking radiation".
That's because these X-rays are emitted by the bombarding electrons
as they are suddenly slowed down, braked, upon impact. [Exactly why and how X-rays are produced when electrons brake is explained by a combination of quantum mechanics and relativity, but we won't go into those details.]
X-rays are another form of light but they are very high energy,
so they would be at the opposite end of the spectrum that we've
been talking about. Let's take a look at the high-energy
end of the spectrum.
At the far end of the visible spectrum is
violet - the highest energy wavelength that we can see. Just above
it is the ultraviolet. The ultraviolet has more energy than the
violet so we cannot see it but it's still there and it can do
us damage. Ultraviolet light, or simply "UV", can cause
sunburns and skin cancer. Most of the UV from our Sun is
blocked out by the layer of ozone at the top of the Earth's atmosphere,
but some parts of the UV slip through. Beyond the UV we enter
the part of the spectrum that is home to the X-rays. Obviously,
X-rays are higher in energy than UV. Just to complete the spectrum,
you should know that beyond the X-rays lies the highest energy
part of the spectrum and that is where gamma rays belong. You
may recall that gamma rays are produced by the nuclear fusion
that occurs at the center of our Sun. [Indeed, "gamma
ray bursts" are a sign that something has undergone a burst
of nuclear fusion.] You will also recall that those gamma rays
do not come bolting out from the Sun's center - instead
they interact with materials along the way and in doing so they
lose energy, producing many low-energy photons in the process.
Perhaps you can guess what happens to the X-rays produced around
a black hole.
Ah, collisions cause the X-rays to get changed into lower
energy parts of the spectrum?
Right!
X-rays are reflected off of the surrounding materials and some of
the energy is absorbed and re-emitted at lower energies. The outcome
of all this is similar to what we have been talking about earlier.
A spectrum is produced that includes a variety of wavelengths
including light in the visible range as well as the infrared.
And even further, into the radiowaves! That's why AGNs produce
so much radiowaves. Right?
Ah, wrong. (But that was a good guess.
As the materials fall towards the event horizon, it gets ionized.
(The electrons are stripped away by all the energy.) Ionized materials
carry a charge (by definition) and, therefore, can be moved around
by magnetic fields. A black hole has a huge magnetic field and
it accelerates the ions to nearly the speed of light! The magnetic
field pulls and twists the paths of these high-speed ions. All
this tugging of ions causes them to emit radio waves. [The details
of exactly how that happens are tied up in some complicated physics
involving quantum mechanics, relativity and nuclear energy but
we won't go into that.] Light, including radio waves, emitted
by charged particles moving near the speed of light is called
synchrotron radiation.
This is different from the kind of energy discussed earlier (involving
heat). Unlike infrared radiation (like that produced in abundance
by starburst galaxies), radio waves easily penetrate our atmosphere
and can be detected by Earth-based radio receivers. AGNs produce
huge amounts of radiowaves by synchrotron emission.
The details of AGNs are still being worked out but we can divide
AGNs into different types based upon some of the observed details.
OK. What types of active galaxies (AGNs) exist?
Most radio galaxies are giant elliptical galaxies and display
variations in light output on a scale of a few days.
Contrast radio galaxies with Seyfert galaxies - named after Carl
Seyfert who first described them in 1943. Seyfert galaxies
are another kind of AGN but they are spiral galaxies with very
bright, compact centers. Some have jets and a few (about 10%)
emit radio waves but most of the energy emitted from Seyfert galaxies
is in the infrared! That means the high-energy X-rays (produced
by bremsstrahlung radiation as materials smash into each other
as they spiral towards the event horizon) get bounced
around by a lot of materials before escaping into intergalactic
space. That makes sense because we expect to have a lot of dust
in a spiral galaxy. However, Seyfert galaxies also emit a lot
of X-rays and ultraviolet so clearly some of that energy is getting
through.
About 2% of large spiral galaxies are Seyfert galaxies and it
is suspected that Seyfert galaxies represent a transitory (temporary)
period in the life of a spiral galaxy. If that is true then all
spiral galaxies, including our own, spend 2% of their time as
Seyferts. The nucleus of a Seyfert galaxy, which may be only a
few thousand Astronomical Units wide, is 0.1 to 10 times as luminous
as our entire Milky Way. The luminosity of a Seyfert galaxy can
vary over the course of weeks.
One kind of AGN displays a combination of traits of the previous
two types. BL Lacertae galaxies are a type of elliptical
galaxy emitting large amount of radio waves by synchrotron radiation
(like a radio galaxy) but they also have a very bright nucleus
and a continuous spectrum produced by bremsstrahlung
radiation that has "cooled" to lower energies by collisions (like a Seyfert galaxy).
They show short periods of variability but they differ from radio
or Seyfert galaxies in that BL Lacertae galaxies are highly red
shifted. That means they are moving away from us (or we are moving
away from them) at a very fast speed!
Does that make them the fastest moving objects in the universe?
No. However, that brings us to the first
kind of AGN ever identified. Quasars are the fastest galaxies
in the universe and they are very important in astronomy research
right now.
Quasars were first identified as "radio stars" - stars
emitting lots of radio waves. We now know what is causing that
emission (and so do you if you've understood this lesson). The
optical spectrum of these objects is very "strange"
and at first no one could identify the absorption lines. Some
people were not convinced these things were stars at all, so they
were called "quasi-stellar" objects. In 1960 Marten
Schmidt explained that the strange absorption spectrum was simply
a highly-shifted spectrum. These "quasi-stellar" objects
are moving away from us very fast. We now call them "quasars"
and they are very important in our understanding of the universe.
(Next month we will return to quasars as I teach you some cosmology
- the study of the size, shape, origin and fate of the universe!)
Quasars have a total luminosity between a hundred and a thousand
times that of a normal galaxy like our own. The total luminosity
of the Milky Way Galaxy is 25,000,000,000 Lsun or 2.5 X1010Lsun
so a quasar puts out somewhere between 2,500,000,000,000 Lsun
and 25,000,000,000,000 Lsun energy! That's 2.5 X 1012Lsun or 2.5
X 1013Lsun. Pretty impressive but remember that a starburst galaxy
can have a total luminosity of 100,000,000,000,000 Lsun or 1 X 1014Lsun!
So starburst galaxies, which are NOT AGNs, are a wee bit more
energetic than a big quasar. However, starburst galaxies are very
short-lived, but there is no reason to suspect that a quasar can
last only a few million years. That's because the energy is produced
in a totally different way in an AGN. There is no star formation
going on in an AGN and little, if any, bremsstrahlung or synchrotron
radiation is produced by a starburst galaxy.
This is confusing! Why do you make these comparisons?
Because YOU SHOULD!
Science isn't about memorizing a lot of facts. It's about understanding
how things differ and what makes them different. (Science is a
lot of other things too, but it's definitely not memorizing lots
of details.) If you have been taking notes you will understand
that AGNs and starburst galaxies are very energetic but they are
also very different. Different physics is involved and different
observations are made
I said that we believe that quasars are not a short-lived phenomenon
(unlike starburst galaxies) but that doesn't mean they are constant
in their energy output. They can dim slightly or brighten slightly.
A quasar can vary its energy output in very short periods - days
or a couple weeks. Some quasars have jets of materials and some
of those jets appear to be moving away from the quasar's core
at speeds greater than the speed of light!
But that's impossible!
Right!
Notice I said they "appear to be". Careful observations
and calculations of the jets' motion, along with a little help
from geometry, prove that the superluminal ("faster
than light") motion of these jets is an illusion.
[Einstein's special theory of relativity is NOT violated.]
Quasars are very strange galaxies.
Yes they are, but the Hubble telescope shows that quasars appear
to have normal galactic structure. (In a twisted sort of way -
that makes quasars even more strange!
We are not sure why quasars produce more energy than any other
type of AGN. Some astrophysicists suggest that there is some kind
of very violent core activity going on in a quasar and they propose
that quasars represent a very early stage in a galaxy's evolution.
Perhaps our own Milky Way Galaxy was once a quasar.
Next month I will tell you more about quasars, especially about
their red shift. You will discover that quasars are very distant
and very old. And they are an important clue to understanding
our universe. So, please understand this important lesson (especially about quasars).
Our own Milky Way Galaxy has a total luminosity equal to 25 billion
Suns. Astronomers often use the abbreviation Lsun
to represent the luminosity of our Sun so the total luminosity
of the Milky Way Galaxy is 25,000,000,000 Lsun. A starburst galaxy
can have a total luminosity of 100,000,000,000,000 Lsun. That's
4,000 times more luminous as the Milky Way!
Most of the light in a normal galaxy comes from
the stars and that light is distributed throughout the galaxy,
with most of the light in the visible part of the spectrum. By
contrast, most of the light in an active galaxy comes from gas at the center
and most of that light is in the radio wavelengths of the spectrum.
) The radiowaves produced
by AGNs are created by synchrotron emission. Here's how that works.
Most of the energy of radio galaxies is emitted as radio
waves. (So they are well named. ) Radio galaxies are said to have
a very high "radio to visible" ratio. But that does
NOT mean they are ONLY emitting in the radio wave part of the
spectrum. They are often visible (M87 is an example) and many
of them have jets of gas extending from the nucleus. These gas
jets are produced by the powerful magnetic field hurling gas away
just before it reaches the event horizon. The amount of energy
released from a radio galaxy, as radio waves, can be as much as
ten times the total energy output of the Milky Way.
[Recall that
the total luminosity of the Milky Way Galaxy is 25,000,000,000
Lsun so a radio galaxy has a total luminosity of 250,000,000,000
Lsun but most of that is in the radio wavelengths.]
Apparently
synchrotron radiation is responsible for most of the output of
a radio galaxy and that is why they are so "radio bright".
)
This work was created by Dr Jamie Love and
licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.