Have you ever wondered what it would be like
for our solar system to reside in a nebula
like this one?
Glorious colours, with huge structures stretching
across the sky.
It must be a pretty cool sight.
Well, depending on how you define what a nebula
is, it may surprise you to find out that we
are actually inside of one, called the Local
Interstellar Cloud.
How can this be?
Well, let’s understand first of all what
a nebula actually is.
The term nebula is a blanket term for a variety
of different things.
The word is a relic from early astronomy,
where everything that looked a bit milky,
foggy or cloudlike was called a nebula, including
what we now know are galaxies and star clusters.
However, these days, we do have a bit more
of a handle on what we classify as a nebula.
The most famous type of nebula is the HII
nebula, an Emission Nebula like Orion, which
consists of ionised particles, predominately
hydrogen.
They are Emission Nebulae because they emit
their own light, as ionising radiation from
the young stars found in the middle of them
excite the particles to higher energies, much
like a neon lamp.
On the other hand, there are also Dark Nebula,
or Molecular Clouds where there are no nearby
stars, so ionisation is not taking place.
They often consist of dust and molecular hydrogen.
Sometimes you can have a mix of the two nebulae,
a Dark Nebula found in the middle of a HII
region.
In such a situation, you can often see the
outer particles becoming ionised, with the
cold particles still found towards the centre.
It is especially in regions like these, where
Dark Nebulae become eroded by ionising radiation
that stars can start to form.
In another situation, you can have light from
stars shining on a Dark Nebula, but the energy
isn’t enough to ionise the particles within
it.
In this case, you have a Reflection Nebula,
a nebula that simply reflects the light shining
on it.
The next two types of nebulae are also related,
Planetary Nebula, where a low to medium mass
star has begun to shed its atmosphere and
outer layers during its stellar evolution,
usually before it becomes a white dwarf.
The second type is a Supernova Remnant, usually
the remains of a high mass star after it went
supernova, leaving behind a black hole or
a neutron star in the centre.
These two types of nebula are so colourful
and pretty because they also consist of ionised
particles, charged from the respective events
that caused them, so they are a type of Emission
Nebula.
Eventually, they will dim and disperse.
Now, as you can see, all these nebulae can
be categorised into two groups, emission nebula,
and cold dark nebula.
And these two categories have one big similarity,
they are all Diffuse Nebulae.
They extend for sometimes up to thousands
of light years, and the particles within them
are extremely spread out.
Some of these nebulae only have a few particles
per cubic centimetre, although some can have
hundreds.
But that still means that an Earth sized nebula
would only weigh a few kilograms.
To give you some real perspective, sea level
on Earth has 15 quintillion particles per
cubic centimetre, and 450km up, which is slightly
higher than the altitude the ISS orbits, there
are still 50 million particles per cubic centimetre.
So, when we see a nebula, we have the advantage
of squeezing very diffuse objects into quite
a narrow field of view, which means for objects
like Orion, we probably have a better view
of its structure than what any locals would
have from the inside, because for them, it’s
a lot more spread out.
Chances are that if you were on an Earth like
body inside of a typical nebula, you wouldn’t
be able to tell you are in one with your naked
eyes.
That’s not to say that all nebulae are equal
though.
The Tarantula Nebula, found in the Large Magellanic
Cloud, is said to be such a bright HII emission
nebula that if it were to be where the Orion
Nebula is, it would cast shadows at night
for us and be visible during the day.
A nebula this bright probably could be visible
if you were inside it.
As for our own Local Interstellar Cloud, it
too is an emission type nebula, likely a very
disperse supernova remnant nebula, mixed in
with some cold neutral atoms.
The ionised particles within it are exceptionally
hot, around 6,730°c, with some particles
even approaching millions of degrees Celsius.
However, its heat specific capacity is very
low because the density of particles is so
sparse, at only 0.3 particles per cubic centimetre,
meaning being out in the middle of it would
barely heat you up at all.
Unsurprisingly, this extreme sparsity of particles
means our Local Interstellar Cloud is not
very bright, in fact it is very hard to see
at all, even with the most powerful of scientific
instruments.
That’s because it really is on the boundary
of what you might call a nebula, although
we are starting to find out that like the
spectacular images of nebulae we know already,
it too has a structure.
The cloud is about 30 light years across,
and interestingly, we seem to be moving through
it, and will likely leave it altogether in
about 20,000 years.
That’s because the Sun seems to be taking
us in this direction, while the Local Interstellar
Cloud is heading this way.
So, if you’ve ever seen images of the solar
system interacting with interstellar wind,
it’s not just the particles that are blowing
around our solar system, but it’s also a
combination of us moving through it.
The magnetic field of the Sun pushes ionised
particles around the entire solar system,
leaving behind a magnetotail as it travels,
like a wake following a boat.
While we don’t have a real visual image
of that for our own Sun, as again, it’s
hard to see from the inside, we have seen
it elsewhere in the galaxy.
However, it’s only ionised particles that
interact with our Sun’s magnetic field.
Non-ionised neutral particles pass right through
the magnetic field, going right through our
solar system.
This is how we’ve been able to tell which
way the interstellar wind comes from; we’ve
had spacecraft in orbit detecting the direction
of these neutral particles as they pass through.
Even though the Local Interstellar Cloud is
not dense compared to other nebulae, it is
actually one of the densest regions for hundreds
of light years in any direction.
This is because it is contained in a region
known as the Local Bubble, where the average
density is even lower than the Local Interstellar
Cloud at 0.05 particles per cubic centimetre.
Interestingly, this bubble connects to other
bubbles, and it’s in the regions where these
bubbles meet where it’s believed that clouds
like the Local Interstellar Cloud form.
Let me explain.
When we talk of nebula of any kind of density,
we are actually talking about the Interstellar
Medium.
The Interstellar Medium, simply put, is all
the gas and dust found in the galaxy, in whatever
form that may be.
It isn’t evenly distributed, and the thickest
sections of it are the different types of
nebula we have already talked about.
However, if you look anywhere in the sky,
and look through the right wavelengths, you
will see a faint glow in space.
These are ionised particles emitting electromagnetic
radiation, in a variety of frequencies.
So, the Local Interstellar Cloud is part of
the Interstellar Medium.
But as I mentioned, it isn’t evenly distributed.
Within our Local Bubble, the density of the
Interstellar Medium is far below the average
of the whole Milky Way.
This is because these bubbles are thought
to be the result of supernovae explosions
millions of years ago.
The supernovae not only ionised the particles,
it also pushed most of them outward, leaving
behind the bubbles, or what is also known
as a supershells.
When supernovae shockwaves meet, the particles
suddenly have nowhere to go, so they instead
clump up.
It is believed that our own Local Interstellar
Cloud comes from where our own Local Bubble
and another bubble met.
There are plenty of other bubbles out there,
and the resulting local map of our galaxy’s
Interstellar Medium looks quite messy.
You may wonder though, why any of this is
important?
What’s the point of scientists trying to
figure this out?
Well, it is important to understand our local
space environment, because it helps us better
define the universe around us.
When we are observing something far away,
we can remove contamination from any readings
caused by the Local Interstellar Cloud surrounding
us, because we now know what we can expect
from it.
Also, better understanding the Interstellar
Wind coming into our Solar System means we
can better protect astronauts against it.
So, there we have it, while we may not be
in a nebula as spectacular as something like
Orion, we are right in the middle of our own
Local Interstellar Cloud, which has a structure
and that emits its own light.
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