Earlier in this astronomy series, we learned all about star and galaxy formation, and how
the period of time between 150 million years and one billion years after the big bang was
the period in which hydrogen and helium accumulated to form the first generation of stars all
over the universe.
This process is ongoing, and any cloud of gas and dust, also called a nebula, can produce
stars today, which is why large nebulae are referred to as stellar nurseries.
To be clear, when we say gas, we are referring to hydrogen and helium, which is most of the
matter in the universe, since it came about during big bang nucleosynthesis, prior to
the existence of any stars.
By contrast, when we say dust, we are referring to all the heavier elements, the ones that
must be fused inside stars, which were first introduced to interstellar space when the
first stars began to go nova and supernova, spewing their contents into the cosmos.
When gas and dust collect by gravitational collapse to form a new star, this will be
accompanied by a protoplanetary disk, and this is the material from which planetary
systems form.
Nebulae and protoplanetary disks are fascinating objects that we can study with telescopes,
and a number of nebulae are very well-known objects, so let’s take a look at some of
these now.
One of the most famous of these structures is called the Pillars of Creation, which reside
in the Eagle nebula, around 7,000 light years from Earth.
It is quite important to get a sense of the immensity of this object, as well as a more
three-dimensional impression for clarity.
To establish scale, this leftmost column is about four light years long, the distance
from the sun to the nearest other star, and these little wisps along the edges are larger
than our entire solar system.
These structures are hotbeds of star birth, hence their name, and there are many others
in the galaxy that perform the same function.
Now let’s go and find another nebula.
This time we will zoom in on a familiar constellation, Orion, the hunter, one of the best-known and
most recognizable constellations.
If we head towards the middle star in Orion’s sword, we find the Orion nebula.
At only a bit over 1,300 light years away, it is visible to the naked eye, and the closest
region of active star formation to our own system.
Because it is so close, it is a great source of information regarding star formation.
Within the Orion nebula, we can see protoplanetary disks, and gather knowledge regarding system
formation.
We can also look here to better understand how nebulae get their interesting shapes,
since new stars generate lots of radiation upon their birth, which we call stellar wind,
and this stellar wind can sculpt nearby gas and dust, as well as compact certain regions
to aid in the formation of other stars.
This is particularly noticeable with another structure near Orion which is called the Horsehead
Nebula.
Many nebulae are given creative names in this manner, in addition to their NGC, or New General
Catalog number, like this structure, called the Cat’s Paw nebula, another star forming
region around five to six thousand light years away.
This nebula is what is referred to as an emission nebula, so let’s now look at the types of
nebulae so we can know what this means.
Emission nebulae are ones that glow brightly.
This is due to the radiation from newborn stars, which not only heats up the gas and
dust, but also ionizes it, and all of this hot, ionized gas emits lots of visible light.
By contrast, a reflection nebula is one that does not emit a lot of light, but rather reflects
light from nearby stars.
Emission nebulae and reflection nebulae are both types of diffuse nebulae, meaning they
don’t have a well-defined shape, they are just amorphous clouds of gas and dust.
A third type of diffuse nebula is called dark nebulae.
These do not emit or reflect much light, like the other two types, but instead become noticeable
because of the way they block light from luminous objects behind them, which makes them appear
as dark opaque clouds.
The Horsehead nebula is an example of a dark nebula.
So that covers diffuse nebulae, which can be very large, many of which act as stellar
nurseries.
But there are also much smaller nebulae that form as the result of a specific event involving
the lifetime of a star.
One such type of nebula would be a supernova remnant.
When very high mass stars run out of fuel, they explode in a supernova, as we learned
in a previous tutorial.
The expanding shell of gas and dust forms a beautiful expanding structure, such as this
one, which is called the Crab nebula.
This object is the result of a supernova that was visible in the sky in the year 1054, and
remained visible for about two years, as is documented by Chinese and Arabic civilizations.
Now all that is left is this nebula, and the neutron star at the center, remnants of the
star that once was.
By contrast, when low mass stars die they will leave behind a planetary nebulae.
This happens when stars like our own sun reach the red giant phase, and eventually eject
their outer layers to leave behind a white dwarf.
This is nowhere near as explosive as a supernova, but it leaves behind a beautiful nebula all
the same.
Some examples of plantetary nebulae include the cat’s eye nebula, the oyster nebula,
the helix nebula, and the ring nebula.
Finally there are even protoplanetary nebulae, which are short-lived, and form during a specific
stage in the stellar life cycle, between the late asymptotic giant branch phase and the
planetary nebula phase.
At this stage, matter is released and ionized, which forms these structures, such as the
Westbrook nebula, and Gomez’s hamburger.
Nebulae have fascinated astronomers since antiquity.
Only in the past few centuries have we begun to understand what they are, and only in the
past century have we begun to understand the stellar life cycle so as to appreciate the
origin and function of these incredible structures.
Our growing understanding only adds to their beauty, which we will continue to magnify
in the coming decades of research.