THE DIFFERENT TYPES OF NEBULAE Have you ever seen those photos
of incredible clouds with the strangest shapes and color? Do you know what they are?
They’re called nebulae. How can we take these fantastic images?
Why are nebulae important? Do you know that they’re the place where stars usually are born? If
you want to know more about these questions stick with me and I’ll tell you everything in a moment.
The name of these wispy objects is probably the most appropriate one in astronomy,
as nebula (which comes from Latin) means literally cloud or fog. As a matter of fact,
they are agglomerates of interstellar dust, hydrogen, helium and other ionized gases.
When where they first discovered? The first time that this term was used,
probably without a rigorous classification was in the 150 AD by Ptolemy that used it to describe a
group of five stars. Later on, in 1610 the Orion Nebula, one of the brightest ones in the sky,
was the first discovered by using a telescope. Successively, Edmond Halley, the one linked
with the world-wide famous comet, firstly published a list of six nebulae in 1715.
After that, the number of observations grew quickly and was gathered in a catalogue
of nebulae and star clusters by William Herschel and his sister Catherin Herschel.
This definition was incorrectly used to describe any possible kind of cloud and a famous mistake
was made regarding the Andromeda galaxy which was known as Andromeda Nebula until the early
20 th century. More precisely, the so-called Great Debate took place in 1920.
What was it about? It revolved around the nature of a group of “spiral nebulae”. Two astronomers,
called Harlow Shapley and Heber Curtis had two different views about the distance of this objects
and the scientific community was struggling to take a solid position in the discussion.
The first one thought that these clouds laid just at the outskirts of our Milky Way, while the other
was sure that in fact they weren’t nebulae, but independent galaxies separated from our. Thanks
to the important contribution of Edwin Hubble that was able to measure the distance of the “Andromeda
Nebula” scientists understood that Curtis was right and that Andromeda was a separate galaxy.
What can you see from a photo of a nebula? It surely appears to be really dense and thick like
a cloud of dust. But here is a surprising result: even if they are denser than the space around
them, they are so far less dense than the usual world we live in that any vacuum created on Earth
is surely denser than them. A nebula with the size of our planet would weigh just a few kilograms!
How are all nebulae formed? They can be created both by interstellar medium, by stars or by the
rest of a supernova. In fact, there is a wide list of different types of nebulae, depending on the
formation and the characteristics of the cloud. Do you know some of them? If not, don’t worry.
In this video we’ll see the five most important types of nebulae.
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Let’s start from the classification of the Orion Nebula that was previously
mentioned. It is known as one of the most famous “HII Regions”. What are they?
They are mainly characterized by the presence of a 90% of ionized hydrogen and by their shining
red colour. These two things are strictly linked as any kind of ionized hot gas
usually emits light at precise wavelengths. In physics the density of energy emission per unit
of frequency or wavelength is called “spectrum”, that can be considered as a catalogue of the
frequencies of the radiation. The one of ionized gas is called “emission spectrum” and it is mainly
composed of some lines typical of each element. Let’s see if you have followed my thoughts:
H-alpha emission line of Hydrogen is nearly around the value of 656.3 nm,
do you know to what colour does it correspond? Yes, exactly red. HII appear red due to the
presence of its ionized hydrogen. But what is the cause of this ionization? In these regions
there’s a huge abundance of short-lived blue stars that lighten the gas producing the emission lines.
But where are these types of nebulae usually located? Have you ever seen those beautiful
images of spiral galaxies like Andromeda or our Milky Way? HII regions are mostly contained in
the typical branches of these structures. Although these nebulae can give birth to
thousands of blue stars in a time span of million years, they’re usually dispersed by
supernova explosion, an event that occurs at the end of the life of a really massive star.
The second type we are going to describe is the planetary nebula. Don’t let the name confuse you,
it doesn’t have any link with planets. Given by William Herschel, this definition derives from
its spherical shape. In fact, in 1979 the French astronomer Antoine Darquier de Pellepoix described
in his observations of the Ring Nebula, "very dim but perfectly outlined; it is as large as
Jupiter and resembles a fading planet". These particular nebulae are formed by
the remnants of a supernova of intermediate stars (between one and eight solar masses).
These last ones, like the Sun, spend major part of their life converting hydrogen into
helium (period also known as main sequence), which is then turned into carbon and oxygen.
This second phase of the nuclear fusion is called as the asymptotic giant branch phase.
In both cases the reaction in the core of the star generates an outward pressure that balances the
gravitational contraction. During the giant branch period the star loses masses due to stellar wind.
These materials emitted in the surroundings of the star can be enlightened by the burning
stellar shells of hydrogen and helium that can reach around 30000K (an appropriate
temperature to emit ionizing ultraviolet rays). How do planetary nebulae die? When the core of
the star consumes all the burning helium, it doesn’t have enough energy to convert
oxygen and so it stops the nuclear fusion. As a result of that, no more UV radiation is produced
and the nebula fades away becoming invisible to us, because the gases aren’t ionized.
Did you know that planetary nebulae are very important? Why? In these clouds there’s a huge
abundance of metals created by the star: these elements are not so common in the Universe,
which is usually composed of hydrogen and helium. Moreover, planetary nebulae are surely the most
various and spectacular one, with a wide range of shiny colors and crazy shapes.
Do you want an example? Search for the Cat’s Eye nebula or the Ring nebula.
To resume, these last two types can be gathered in a single category of emission nebulae.
They are composed by ionized gas that emits in different wavelengths
depending on the elements of the cloud. Moving on, let’s examine the third type: the
reflection nebula. As opposed to emission nebulae, these ones don’t emit radiation by themselves.
As the name suggests, they just reflect the light from an external star. Do you want a simple
comparison? When you see a marble cave you’re blinded by the light that comes from the rock,
but that radiation is not produced by the marble, instead it is just the sunlight
reflected and directed towards our eyes. The most famous reflection nebulae are located
around the brightest stars of the Pleiades cluster, one of the most visible ones.
This type of scattering always produces a blue light, since this wavelength results
more efficient in the reflection. This process is also called Rayleigh scattering, named after
the nineteenth-century British physicist Lord Rayleigh, and often occurs when we have a particle
much smaller than the wavelength of the radiation. It is possible to find that the power of light
diffusion is proportional to the inverse of (λ4). As a consequence of that, blue (which
has a smaller wavelength of 500-520 nanometers) is reflected much more strongly than red (which
has a greater wavelength of 700 nm). This is the main reason why our sky happens to be blue as
the particles in our atmosphere interact with sun light according to Rayleigh scattering. The same
exact phenomenon happens in reflection Nebulae. As opposed to these last ones, we can describe
Dark Nebulae: black and grey clouds which rarely reflect the light of other stars. They can also
be called as absorption nebulae, a name that surely suggests their main characteristic.
As a matter of fact, they don’t produce or scatter any type of visible light. Composed of molecular
hydrogen and helium, they can only be detected observing other frequencies such as microwave or
infrared. Usually they obscure the view of a backward emission or reflection nebula.
These nebulae are probably the densest ones in the entire universe,
and Giant molecular clouds, which are the biggest existing dark nebulae, can surprisingly reach the
ridiculous length of 150 light years. This means that in order to go through the entire cloud,
moving hypothetically at the speed of light, you would take nearly 150 years.
Compared to our own solar system which is around two light years this distance is totally crazy!
And it doesn’t end here! Do you want to know something more exciting about
dark nebulae? They are one of the coldest objects in the universe, reaching the very
low temperature of 10K, nearly -263 oC. Moreover, it has been studied that the
center of this nebulae might be a region of stellar formation.
The shape and the structure of a dark nebula can be surprisingly fascinating. Have you
ever seen that shocking photo of dusty clouds resembling a horse? That is Horse Head nebulae,
a dark one in the Orion constellation. Finally, we can conclude our journey through
nebulae with the last (but not the least) type: supernova remnants.
What are they? They are produced by a supernova explosion. Two different paths
can bring us to a such energetic phenomenon: black holes and neutron stars. the first ones
are probably the most famous astronomical object, being an ultra-dense collection of
matter that has such a strong gravitational force that neither light can escape from its attraction.
On the other hand, neutron stars are visible and are really dense too, consisting of a central core
of degenerated neutrons. What does this adjective mean? The particles are so near each other
that a strange quantum phenomenon takes place: the nucleus of the star generates an outward pressure
called degeneration pressure which balances the extreme gravitational contraction that would
make the star collapse. Both these objects are the result of the collapse of massive stars (more than
8 solar masses) that have finished the nuclear fusion in the core. The collapse of the star
generates a shock wave with expulsion of material which is the supernova. The remnants of this
tremendous event are accelerated to a incredible velocity of 30000km/s, 10% the velocity of light!
With time the materials slow down to the speed of sound and they form this unique type of nebula,
which is considered the main source of cosmic rays. Do you know what they are? They are
particle beams that reach the earth from different positions. If you want to know more about that,
please check another video in this channel in which we go more deeply into cosmic rays.
Probably the most famous supernova remnant is the Crab Nebula,
a stunning cloud in which we can easily see the materials expelled by the supernova.
To sum up, we have seen a wide variety of nebulae, different in their origins and
compositions. What seemed just a dense cloud in space revealed to hide some important features for
astronomic research such as star formation! "This video ends here! Thanks for watching
everyone! Did you find this topic exciting? Are you amazed by the variety of nebulae
we can observe? Did you already know that lots of stars are born in nebulae?
Have you ever seen one of them? What is your favourite kind of nebula? Let me know in the
comments below, be sure to subscribe, and I'll see you next time on the channel!"