Stars dot the night sky, giving an
impressive clue to the vastness of the
galaxy we live in. I remember being in
South Africa on a clear night, far away
from any city and its accompanying light
pollution. I simply can't describe how
spectacular the heavens were. No photo I
could show here could possibly give
justice to be in that situation in
person and looking into the seemingly
infinite.
Some of the dots we see in the sky are
stars all in varying stages of their
evolution and are many light years away
from us. But apart from being giant
balls of light and energy, how much do
we actually know about stars? Where do
they come from? Are there different
types? And what happens to them
eventually? I'm Alex Mccoan and you're
watching Astramm. And in this small
series of videos, I will go through the
lifetime of various types of stars,
starting in this episode with how stars
are formed.
When we think of interstellar space, we
often think of a cold vacuum of nothing.
And by our earthly standards, it might
as well be nothing. But this is not
strictly true. Between the stars of our
galaxy is what is known as the
interstellar medium or in other words
the matter and radiation that exist
between stars. It is comprised mainly of
hydrogen followed by a small amount of
helium and trace amounts of heavier
elements. It can't be thought of
something like an atmosphere. Even in
the densest parts there are only 1
million molecules per cm. That may seem
like a lot, but in the same space at sea
level on Earth, there are 10 quintilion
air molecules. And even in a laboratory
vacuum, there are still 10 billion
molecules per cubic cm. In the more
sparse reaches of space, the
interstellar medium's density can be
less than one molecule in a cubic cm.
But what does this mean in figures that
we can wrap our heads around? Well,
here's an interesting comparison for
you. If you're sitting on a chair, look
directly down at the ground. If you were
to have a cylinder, the diameter of your
eye drawn from your eye to the ground,
there are more molecules in that
cylinder than if you were to be sitting
at the edge of the solar system and have
a cylinder drawn from your eye to the
center of the galaxy over 27,000 lighty
years away. Although very sparse, the
interstellar medium is thought to make
up roughly 15% of the visible mass of
the Milky Way. Much of the molecules in
the interstellar medium are ionized. And
there is a special scientific instrument
called WHAM that can see the densities
of ionized gas in space from our
perspective on Earth. And this is what
it looks like. This gives you a
perspective that there is a lot more
matter in the galaxy than you might
initially think.
The interstellar medium, particularly
dust, has a very real effect on us, too.
Over vast distances, the dust in the
interstellar medium acts like a fog,
either blocking the view of stars
thousands of light years away in the
visible spectrum of light
or giving stars a reddish appearance.
If there wasn't an interstellar medium
between the stars, you would be able to
see the entire disc of the Milky Way in
the night sky.
As I mentioned earlier, the density of
the interstellar medium varies greatly
around the Milky Way. If you've been
watching my other videos, you may have
heard me speak about nebula and H2
regions. Nebula are denser regions of
the interstellar medium. Some of these
regions appear as holes in the night
sky. In actuality, they are massive
interstellar clouds of gas and dust. And
when I say massive, I'm talking many
light years across, some many million
times the mass of our sun.
You may have seen this famous Hubble
image before, called the Pillars of
Creation. What you're looking at is an
example of these gas and dust clouds.
What you can see of the left pillar in
this image is four light years long.
Just to give you some perspective, one
pixel might just about cover the
distance of Neptune's orbit if our solar
system was placed right next to the
pillar. Incredibly, however, this image
is in fact just a small segment of this
H2 region called the Eagle Nebula. The
nebula is just one of many stellar
nurseries where stars are formed in our
galaxy. In the background, you can see
lighter colors of greens, blues, and
reds. These bright colors represent
different ionized molecules and are
known as the H2 region of a nebula. In
this image, greens are hydrogen, red for
sulfur, and blue for oxygen. The
electrons freed by the ionization
process are continually absorbed and
remitted, producing the different colors
for the different atoms. This is a very
similar process to what happens in a
neon light. The particles are excited to
a higher energy state and eventually
release that energy in a wave of light.
These molecules are extremely hot,
heated by nearby stars to temperatures
over 8,000°.
They are also reasonably dense for the
interstellar medium from 100 up to
10,000 molecules per cubic cm.
The pillars themselves are known as
molecular clouds, the densest regions of
interstellar medium from around 100 to
about 1 million molecules per cubic cm.
These clouds are cold and dark at around
-260°
and they don't allow light to travel
through very easily. In fact, they would
be invisible if it wasn't for the fact
that some are silhouetted against
brighter H2 regions. Because of the
density of the molecular cloud, the
inner molecules don't interact so much
with the UV light of local stars,
meaning the molecules stay cool and
dark. Incredibly, it is these clouds of
molecules and dust that eventually turn
into the same type of stars that dot our
galaxy and universe. But how? What
exactly is the process? Generally
speaking, these clouds are stable and
would exist for billions of years doing
nothing. Look at this example of how gas
behaves in a molecular cloud at - 260°.
Even at this temperature with very
little gravity, the molecules spread out
in the cloud. The shape of the cloud
supported by a balance of its own
gravity and internal pressure.
So, in order for these clouds to become
stars, there has to be a trigger. And
there are a few different types of
triggers that are thought to kickstart
the star making process. Going back to
the pillars, you can see one of these
triggers in action in this very photo.
Along the outside of the pillars, you
can see a sort of aurora of light around
them. This is because UV light from hot
and bright local stars are blasting the
outermost molecules, exciting them to
higher energies. has a weathering effect
on these otherwise stable clouds as the
excited molecules either escape away
from the clouds joining the H2 region or
they push against the colder molecules
further in compressing them. This
pressure can also be created by a
supernova shock wave. A supernova is
caused by the death of a massive star, a
topic we'll come to later on. It's
interesting to me though that the death
of a star can trigger the birth of
others.
You may also remember me talking about
density waves in previous videos, and
it's thought that molecular clouds
passing through the Milky Way's density
waves can also be the trigger for these
clouds to collapse. On a very grand
scale, galaxies colliding can cause what
is known as starburst within the
galaxies. Starburst is when a galaxy
produces stars at an extremely fast rate
and it can be seen in these areas of
extremely hot blue stars. All these
triggers destroy the equilibrium of the
cloud and the internal pressure starts
to give in to the gravity of the cloud.
This bumping and shoving of molecules
within the dense cloud cause the
molecules to clump together under their
own gravity, gradually growing bigger
and bigger as the gravity increases and
more molecules are pulled in. Depending
on the size of the cloud, there can be
hundreds to millions of these clumps
that get formed within the cloud. As the
density of molecules increases from the
initial trigger, interaction between the
molecules also increases. The
temperature starts to increase. The
higher density causes stronger gravity.
More molecules from the cloud get pulled
in by the increasing gravity.
Temperatures rise and gravity continues
to increase. The domino effect
continues.
These clumps in the molecular cloud are
the beginning of stars called
protostars.
The protostars keep getting bigger as
long as the molecular cloud surrounding
them keeps feeding them material. And to
become a true star, a protoar needs to
be at least 0.08 solar masses. Should
the molecular cloud disperse too
quickly, the protoar will become a
failed star or what is known as a brown
dwarf. The difference between a failed
star and a main sequence star is if the
temperature and pressure in the core of
the star are hot enough for nuclear
reactions to begin. Protoars create
energy too, but their energy comes from
the impact of material entering the
star. A star will hit the main sequence
or the next stage of its life if the
core begins to fuse hydrogen to helium.
A brown dwarf doesn't have the mass to
do this. As a result, it isn't as hot as
a star and will keep cooling off. They
are not very bright in the visible light
spectrum and would actually appear
magenta, red, or orange. They can have
planets, but the chance of life on those
planets is slim as the habitability zone
would be quite small and would move
closer and close to the star as it
cools. But what happens if there is
enough mass to form a main sequence
star? As material gets sucked into the
protoar, the star begins to rotate and
material begins to spiral inwards. There
are few objects NASA have photographed
that are thought to be protoars where
this can be seen. These two arms going
into the protostar impacting at
tremendous temperatures. This in turn
causes the protostar to rotate faster.
The material surrounding the protoar
flattens and turns into what is known as
a planetary disc. Clumps start to form
in the disc too, building up under the
same gravity that caused the star.
Smaller clumps may build up to become
the stars planets. Bigger clumps may
begin to form their own planetary disc
and may become another star. In fact,
this is the most common thing to happen
in star formation. Our sun being quite
unusual in that respect. Most stars will
be in a binary star system. Some stars
will have many stars in the same system,
all orbiting each other. Had Jupiter
been fed more mass during its birth, it
too might have become a star. It would
have needed a lot more mass though,
about 70 times more than it currently
has. We should consider for a moment
size scales. A typical interstellar
cloud is 100 trillion km across or
roughly 10,000 times larger than the
size of the solar system. A cloud
fragment which forms
one or a few protostars is 1 trillion km
or roughly 100 times larger than the
size of the solar system. By the time
the core of the fragment has become a
protoar, its size will be approximately
10 billion km and its temperature of
order 10,000°.
Interestingly, as material is fed into
these protostars, they can unleash shock
waves into the rest of the molecular
cloud they are still surrounded in. This
in turn can trigger further
gravitational collapse and more
protostars can be formed.
Eventually, the molecular cloud's
material will be exhausted. And
depending on the size of the cloud, what
is left is a star, several stars,
a few hundred stars,
thousands of stars, or a whole
supercluster of millions of stars. Some
of these clusters will eventually
disperse and the stars will simply join
the rotation of the galaxy, much like
our sun.
Although I've talked about all this
matterof factly, this is still only a
theory of how stars are formed as
actually seeing the formation of a star
is very difficult due to the time scales
involved and the fact that these
protostars are surrounded by dust that
obscures the view. This is one of the
huge advantages of the James Webb Space
Telescope due to be launched in 2019.
Its ability to see in the infrared gives
it a big advantage over Hubble in that
it will be able to see through the
thickest of dust clouds and hopefully
see proto stars in action.
Hi, I'm Alex Mcolen and you're watching
Astramm. And today I'm starting
something a bit different. Hubble has
released a zip file on their website
containing the top 100 pictures Hubble
has ever taken. What I will do over the
course of 10 episodes is go through
these pictures one by one and explain
what it is you're looking at. And
believe me, some of these pictures
require an explanation.
Number one, this spectacular collection
of stars is the NGC1850
double star cluster found in the large
magnanic cloud, a satellite galaxy to
our own Milky Way.
NGC1850
consists of a main globular cluster in
the center and a younger smaller cluster
seen below and to the right. The main
cluster is about 50 million years old.
The smaller cluster is only 4 million
years old. It's composed of extremely
hot blue OB stars and fainter red tur
stars. Tori stars are younger stars that
are still forming. so young in fact that
they may not have even started
converting hydrogen to helium which is
how our sun produces its energy. Instead
they radiate energy released by their
own gravitational contraction. You see
when a star cools the cooling causes the
pressure to drop and the star shrinks as
a result. This compression in turn heats
up the core of the star. OB stars, on
the other hand, are some of the
brightest and most massive stars out
there. In this image, you can also see
the remnants of stars that have gone
supernova, leaving behind this super
bubble of diffused gas known as N103,
which looks similar to the well-known
supernova remnant Signis Loop in our own
Milky Way. It is believed that the birth
of new stars can be triggered by the
enormous forces in the shock fronts
where the supernova blast waves hit and
compress the gas. Hence why you find
these very young stars in these
clusters.
Number two, the Red Spider Nebula, also
known as NGC6537.
It's a planetary nebula found near the
heart of the Milky Way. What produces
nebula is often when a red star is
dying, the outer layers of the star
shoot off into space by strong stellar
winds. Once the atmosphere has
dissipated, the hot and bright core of
the star emits UV radiation, which
ionizes the ejected outer layers. The
absorbed UV radiation energizes the gas
of the planetary nebula, which produces
all sorts of different colors.
This two-lobed symmetric planetary
nebula definitely looks like a spider
and houses one of the hottest white
dwarfs ever observed, probably as part
of a binary star system. The star itself
is not visible in the image because it's
so hot. Most of the light it radiates is
in the ultraviolet.
Internal winds emanating from the
central stars have been measured in
excess of 1,000 km/ second. These winds
expand the nebula, flow along the
nebula's walls, and cause waves of hot
gas and dust to collide.
Number three, this planetary nebula,
NGC2080,
or the Ghost Head Nebula, is another
member of the large maganic cloud
satellite galaxy. It's called the Ghost
Head Nebula because of the two distinct
white patches it possesses, which look
like ghost size. The western patch
called A1 has a bubble in the center
which was created by the young massive
star it contains. The eastern patch
called A2 has several young stars in a
newly formed cluster, but they are still
obscured by their originating dust
cloud. Because the dust clouds are still
around the two sets of stars,
astronomers believe these stars are not
more than 10,000 years old. The nebula
is 50 light years across. And if you
look to the left of the picture, you
will see a lot of green. This is due to
ionized oxygen atoms. Whereas with the
rest of the nebula, you find ionized
hydrogen atoms producing this reddish
color.
Number four, the Tadpole Galaxy. Now,
I've already covered that galaxy in my
top 10 most beautiful galaxies video.
So, if you want to find out more about
this remarkable galaxy, I'll leave a
link in the description. So, check it
out.
Number five, NGC4676
or the mice galaxies.
Nicknamed so because of the long tail of
stars and gas emanating from each of the
spiral galaxies. They're both very
irregularly shaped as they in the middle
of colliding with each other. Although
it is thought that they will eventually
form one single spiral galaxy. They are
a massive 290
million lighty years away.
Interestingly, in the zip file of the
top 100 Hubble images, the image of the
mice galaxies appears to have another
galaxy just above it. After some
research and with the help of Reddit, it
seems this galaxy is there by accident.
I couldn't find anything else on it. So,
it seems someone in the Hubble team was
just a little trigger happy with the
clone tool, as this galaxy is not in any
other picture I found of it.
Number six, this is the cone nebula,
part of the bigger NGC2264
or the Christmas tree cluster. If we
rotate the image, you can see why it
definitely has a Christmas tree shape.
Plus, the star clusters could be seen as
Bbles or Christmas lights. This star at
the trunk of the Christmas tree is a
massive Otype star.
Looking at the infrared makes the cone
nebula stand out very clearly.
The nebula is about 2,700 lighty years
away from us. And this section of the
nebula is about 7 light years long. The
structure and color of the nebula comes
from ionized hydrogen. The UV radiation
coming from the clusters young stars.
Number seven. This is the Hubble Ultra
Deep Field. Another picture we have
already looked at in another video.
Since then though, I have found a very
cool animation giving us a 3D view of
what the ultra deep field would look
like. Each one of these dots is a
galaxy. Each one containing millions
upon billions of stars. In fact,
astronomers have counted around 6,000
galaxies in this one image alone. And
what is even more interesting to me is
that we are actually looking at
different times right now. The closer
galaxies we see come towards us first in
the animation are maybe only millions of
light years away from us. Whereas at the
back of this image are galaxies which
are billions of light years away. They
may not even exist now. It's just that
all the light from these galaxies hit
the telescope's lens at the same time.
But in actual fact, the further back the
galaxy, the further back in time we are
looking.
Number eight, and we're visiting the
large magnanic cloud again. This star
forming nebula is part of a region
within this galaxy called N11 and is one
of the most active star formation
regions in the nearby universe. Zooming
out a bit, you can see why there are so
many densely packed star clusters full
of young stars made up of the dust and
gas of this giant nebula. Zoom out again
and we can see where the original
picture fits in. It's hard to think that
we can resolve the individual stars in
another galaxy, but I'm glad we can
because the end result is breathtaking.
The colors of the different pictures are
because they were taken with different
telescopes which pick up different light
wavelengths, for example, infrared or
ultraviolet.
Number nine, V838 Monoserotus.
This is a red variable star 20,000
lighty years away from us. In February
of 2002, it underwent a huge eruption,
increasing in brightness massively
before dimming again, as is expected
with these kind of eruptions. But then
in early March, it increased in
brightness again before dimming once
more. And again in April, it increased
in brightness before dimming again to
its previous level before the eruption.
It is unlike anything that has ever been
witnessed before. At its peak, it was
one of the brightest and biggest stars
in the Milky Way galaxy at over 1
million times more luminous than our
sun. We don't know what caused the
eruption, but theories abound, ranging
from nova outbursts to two stars
colliding or even the star swallowing
one of its giant planets.
The structure you see around the star is
its light echo.
Because light to us seems very
instantaneous, it's quite hard for us to
wrap our heads around a light echo. But
it is very much like a sound echo. And
uh the best way I can show you is
through this. This marble represents
Earth and the ripple represents the
pulse of light which shot out from this
star. Unobstructed, the ripple would
look like this. But because there was a
lot of dust around this star, when the
light shot out in all directions, it
bounced off the dust and created a
second ripple which then reached the
Earth. Now, this is still in the process
of happening, which is why the structure
looks like it's expanding.
Interestingly, when the star first
increased in apparent magnitude, it was
so bright that it was shining in blue,
as you can see from the outside of the
light echo.
Number 10, M51, the Whirlpool Galaxy.
This was also featured in the Galaxy's
video, so I'll leave a link in the
description if you want to know more
about that, too. Look up into a clear
night sky, and you'll see thousands of
different stars dot in your view.
While they all may look like pimprics of
light from our viewpoint, to some of you
keen observers, you'll notice that some
of them look slightly different from
others.
Some are bright,
some are barely visible.
Some are white,
and others are reddish or even blue.
How can that be? Why are stars so
different from each other?
I'm Alex Mccoan and you're watching
Astramm. and together we will learn all
about the various types of stars and
find out what makes them all so
interesting.
Let's start off by having a recap of
where stars come from.
Although space is a vacuum, it's not a
true vacuum in that there are still tens
to millions of particles per cubic cm
like hydrogen or oxygen.
These particles make up what we call the
interstellar medium.
The interstellar medium is surprisingly
massive, making up 15% of the mass of
our galaxy, the Milky Way.
This is a lot considering that the mass
of the interstellar medium is compared
to the hundreds of billions of stars
found in the Milky Way.
The interstellar medium exists
throughout space, but it's not evenly
distributed.
The densest parts are called nebula,
which can either be colorful regions of
ionized gas
or cold regions called molecular clouds.
Generally, these clouds are supported by
their own internal pressure pushing out
against the inward pull of gravity.
But every so often these clouds collapse
because of external sources.
When the internal pressure of the cloud
is overcome, this causes the particles
within them to clump together under
their own gravity.
As the clump begins to build up, its
density and thus gravity increases,
causing more nearby particles to get
drawn in. These particles begin
spiraling inwards and around the center
of mass until they eventually impact the
central object.
With the combination of the force of
these impacts and the increase in
pressure inside the object, it begins to
warm up and spin. This is what we call a
protoar or if you want to imagine it
this way, a star in the baby stage of
its development.
Every star has these humble beginnings,
but what happens next decides the fate
of the protoar.
The molecular cloud feeding into the
protoar will eventually run out of
material or the protoar will be ejected
away early in its protostar phase. If
this happens before the protoar reaches
0.08 solar masses or 0.08 08 times the
mass of our sun, the protoar will become
a brown dwarf or a failed star. Should
the molecular cloud feed enough material
into the protoar that its mass amounts
to over 0.08 solar masses, the protoar
will eventually become a main sequence
star or in other words, it will reach
the adulthood phase of being a star.
Why this 0.08 08 solar mass cutoff.
It's because stars with a mass above
0.08 solar masses are big enough for
nuclear fusion to take place in the
stars core where the pressure there will
convert hydrogen to helium as a power
source. At this mass, as long as there
is hydrogen to convert, it is self-
sustaining, needing no external source
to keep it hot.
Brown dwarfs don't reach the mass and
pressure at the core needed for nuclear
fusion to take place, so they will
eventually cool off.
Their cool and dim nature means that
brown dwarfs are pretty hard to see.
There are certainly none in our sky
visible to the naked eye. Their dim
nature means it's hard to put an
estimate on how many there are. There
could be a lot more in existence than we
currently know about.
Zooming in on Hubble's picture of the
Orion Nebula
shows quite a few brown dwarfs visible
against the backdrop of the nebula they
formed in.
The ones we know about are really
interesting, but brown dwarfs could have
a whole video to themselves, so maybe
I'll save it for another time.
So, our protoar has reached 0.08 solar
masses. What happens next determines the
type of main sequence star it will
become.
When the molecular cloud eventually
stops feeding material into the
protostar,
the protoar begins to cool and contract.
This increases the pressure at the core
until the pressure is so great that
nuclear fusion occurs. the reactions of
which create an internal pressure which
counteracts any further contracting or
in other words the forces within the
core of the star are in equilibrium.
At that point the protostar hits the
main sequence but what type of star will
it become? Let's have a look at the
spectral class chart. I want to draw
your attention to this band by here.
This band indicates that these stars are
on the main sequence. As you can see,
stars generally belong within this
curve. The lowest mass stars are found
to the bottom right of the curve where
the molecular cloud ran out of material
early on in the stars protostar phase.
These stars are small and cool in a
category of stars called red dwarfs.
As we go up the curve, it indicates that
more mass was fed into the protoar as it
developed. And the more mass there is,
the hotter the star. As the temperature
of the star increases, the spectral
class shifts, moving from red to orange
to yellow to white to blue.
Not only does the spectral class change,
but the brightness of the star also
increases.
But if this is the case, why is this
band so thick? Surely it should be a
much narrower band if the mass is always
equal to the temperature and luminosity.
Well, that would probably be the case if
all stars were the same age. But
throughout a star's adult phase, it is
still changing slightly.
As a star's hydrogen gets diluted by all
the helium it has now generated, it is
more difficult for the hydrogen to fuse
together, the equilibrium within the
star shifts slightly to a smaller,
denser, hotter core to maintain the
fusion of hydrogen. This means over time
the luminosity of the star increases
slightly. A good example of this is our
sun as it used to be only 70% of the
brightness it is now.
Going back to the chart, a star that may
start here may shift to over here over
its lifetime.
But as is often the case with anything,
there are some outliers and exceptions
to the rule. On this diagram, you'll
notice this band here. This is known as
the instability strip and is full of
variable stars. A variable star is a
star whose brightness and diameter
increases and decreases in regular
intervals, giving them a pulsating
appearance.
Some take only hours to pulsate,
some take weeks.
A breathtaking example of a variable
star seen by the Hubble Space Telescope
is RS Puppus. It varies in brightness by
a factor of five once every 40 or so
days.
Because this star is surrounded by a
beautiful dust structure, we can see the
variable nature of this star through a
light echo. Light travels extremely
fast, but it still takes a long time to
get across astronomical distances. This
means we can see the procession of light
traveling away from the star with some
rings being dark as the star was dimmer
and some rings being brighter as the
star brightened. Hubble imaged RS
puppets over a 5we period and you can
actually see this light echo pulsing.
They exist along this strip here because
at this mass the internal structure of
the star is more prone to instability
due to the ionization of helium in the
stars convective zone. Again, perhaps a
topic for another video as it gets
pretty complicated.
Another variable to main sequence stars
is what material was actually fed into
them during the protostar phase. The
majority of the interstellar medium is
hydrogen, but there are other atoms out
there like helium, oxygen, and even
metals like iron and gold.
If a star has an abundance of metals,
which in the case of stellar structure
is anything above an atomic number of
two, it will be redder on the spectrum
and metal poor stars will be bluer.
So as you can see, there are lots of
factors for main sequence stars to be
different from each other. These are
their mass or how much material was fed
into the star during its protoar phase.
their age. Have they only just become
main sequence stars or have they had
time to increase in brightness?
And their composition. Are they rich or
poor in metal?
When a massive star comes to the end of
its life, it doesn't go out with a
whimper. Rather, it explodes in majestic
fashion in what we call a supernova.
This is one of the most energetic events
that we know of.
At its peak brightness, a powerful
supernova can be as luminous as an
entire galaxy.
This luminosity doesn't last long as the
energy is expended in a short space of
time, perhaps lasting only a couple of
days at most. On an astronomical time
scale, that is just a heartbeat.
So, how often does supernova occur?
There's certainly never been one
mentioned in the media recently, right?
Well, let's wait together for the next
supernova to occur somewhere in the
universe. Start the clock now and stop.
Only 1 second.
As you can see, supernova are pretty
common, but they aren't all that easy to
spot. For one, they have to occur
somewhere in the observable universe.
and be close enough that we can see them
and notice them. But in just the
astronomically short space of time that
humans have been able to observe other
galaxies, we have seen tens of supernova
directly.
This image of galaxy NGC4526
captured by Hubble in 1994 shows a
supernova on the outskirts during its
peak brightness. Astonishingly, it is so
bright in this image, it appears to be a
star in our galaxy. But being in another
galaxy means even though it was this
bright, only strong telescopes were able
to observe it.
But what about naked eye supernova
events? How often do they occur and what
would they look like? Well, it depends
on the distance to us.
The most recent naked eye supernova
event was SN1987A,
a supernova that occurred in the large
melanic cloud in 1987.
This is the only supernova that's been
observed up close in our local group of
galaxies. As the large melanic cloud is
a satellite galaxy to our own Milky Way
at only 160,000 lighty years away.
At this supernova's peak brightness, it
reached an apparent magnitude of three,
meaning that although it was visible to
the naked eye, it would have only
appeared like another star in the night
sky.
However, this event was of particular
interest to scientists as they had had
the large melanic cloud mapped out,
meaning they could pinpoint the exact
star that exploded and view the before
and after shots. And the aftershots are
spectacular.
This is the remnants of the star.
This ring around it is the stellar
material ejected by the star 20,000
years before the explosion.
And as the shock wave from the supernova
expands, you can see it impact the ring,
lighting it up through ionization.
But with an apparent magnitude of three,
it's not super impressive for those of
us wanting to look up and see something
fantastic in the night sky.
The previous naked eye supernova
occurred in6004,
known as Kepler's supernova, occurring
only 20,000 lighty years away. It had an
apparent magnitude of minus2.5,
much brighter than any other star in the
night sky and even visible during the
day. This brightness lasted a couple of
weeks.
We can now observe the remnants of this
supernova using modern-day telescopes.
But perhaps the brightest supernova in
recorded history was in 1006 with an
apparent magnitude of -7.5,
meaning it was 16 times brighter than
Venus, clearly visible during the day.
Thanks to the historical records of
early astronomers, we can pinpoint this
supernova's remnant, too. But even
supernova 1006, the brightest supernova
on record, was still 7,500 lighty years
away. Are there any stars even closer
than that that might erupt in a
supernova soon? And if so, what would it
be like?
As it happens, there are 12 stars that
are approaching the end of their lives
that would erupt in a supernova less
than 3,000 lighty years away from us.
This is the list of them. One of them,
one of the most famous stars in our
night sky, is Beetlejuice.
Found in the Orion constellation on
Orion shoulder, this star is
distinctively reddish in color.
This is because it is a red super giant
which is really near the end of its
life.
We don't know for sure when any
supernova will occur as we haven't
observed any close enough to look out
for telltale signs. But it is expected
that Beetlejuice could go at any moment
really although it could also still take
tens of thousands of years.
It is only a few hundred lighty years
away from us and it is estimated that it
would appear brighter than a full moon
when it finally erupts in a supernova.
But don't fret. Even at this distance,
it is expected that no real harm would
come to life on Earth. A supernova would
have to occur within 50 light years of
us for any harm to befall us. And we
just don't see any eligible candidates
for that right now.
Although it should be noted that a
certain type of supernova can also occur
with white dwarfs. If a lot of mass gets
fed into these tiny dense stars by other
nearby stars, this could trigger a
supernova.
Because of their tiny and dim nature, we
don't have a good understanding of how
many there are out there. It is thought
that there could be a few hundred of
them within 50 light years from us that
we don't know about.
We do seem to be overdue for a supernova
somewhere in the galaxy, though.
Supernova are believed to occur every 50
years on average in the Milky Way. Yet,
we haven't seen one with modern
telescopes yet.
And you can be sure that when it finally
does happen that everything about it
will be observed for years to come.
Number 11, the spire.
The object is actually a tower of cold
gas and dust, part of the Eagle Nebula,
Stella Nursery. The spire is a massive
9.5 light years high.
The spire is actually being eroded by
thousands of hot young stars found in a
cluster just off this image. This haze
around the structure is hydrogen gas
being boiled off by ultraviolet light.
Whereas the black regions are denser,
more resistant gas. This process may
well be producing more stars too, as
seen here where the bright heated gas
pushes against the dark colder gas. This
pressure may well cause stars to form.
Stars are also formed within the tower
where dense gas collapses under gravity.
These bumps may appear small, but they
are actually the size of our entire
solar system. The stars will continue to
grow as long as there is gas around to
feed it.
The background of the image is more
distant illuminated gas. Blue is
ultraviolet light interacting with
oxygen and red is hydrogen.
12. NGC 346.
We're now traveling 210,000 lighty years
away to a satellite galaxy of ours, the
small maganic cloud.
NGC 346 is one of the most dynamic and
intricately detailed star forming
regions in known space. In the center of
this region is an intense and brilliant
cluster of stars surrounded by a jagged
arch of dark cool gas.
As in the last image, the UV light from
these stars is blasting against this gas
structure. You can see the wind trail
left behind these denser globules.
This star cluster also houses the
brightest star in the small magleionic
cloud.
Number 13, the Crab Nebula.
This nebula is the remnant of a star
going supernova.
Interestingly, it is documented that
ancient Japanese and Chinese astronomers
saw this explosion in 1054.
What you're looking at here is the
remains of that star being mostly
hydrogen gas. Barely visible at the
center of this picture is a neutron
star, the ultra dense core of the
exploded star.
It is only 30 km across and rotates an
amazing 30 times per second. As it
rotates, it shoots off two streams of
highowered X-ray beams into space.
This is why when observing the star, it
looks like it's pulsating.
This star is the cause of the bluish
glow of the nebula.
Number 14, the Orion Nebula. I must say,
this is one of my favorite pictures
Hubble has ever released. The colors are
gorgeous. It looks like it should be in
an art gallery. Plus, there's so much
going on.
The Orion Nebula houses about 3,000
stars, some of which weren't discovered
until this picture was taken. In the
center of the bright patch are the four
biggest stars in the nebula, arranged
into a trapezoid, and they're aptly
named the trapezium.
Around these stars are extremely young
stars that likely still have their
protolanetary discs, as you can see
here. These discs of condensing gas
could well form into planets in the
distant future. At the top left here, we
have one star illuminating the
surrounding dust, a region called M43.
We have more dense gas resisting erosion
by UV light
and gas reacting to it.
You can visibly see the bow shock from
some stars as they resist the stellar
wind of the biggest stars. On the left
is a cavity wall of the nebula. And
interestingly, the dim red stars you see
here, these tiny pixels really are brown
dwarfs, sometimes referred to as failed
stars. Brown dwarfs are stars that
cannot sustain nuclear fusion in their
cores like our sun does. As a result,
they're comparably cool, less massive,
and not as bright.
And finally, the Orion Nebula is the
closest star forming region to us, being
over 1,300 lighty years away. Number 15,
the Pinwheel Galaxy. We're having a
little break from Nebula now and looking
at something a lot bigger. The Pinwheel,
also known as M101, is what is known as
a grand design spiral galaxy. The image
itself is massive, and we can take a
detailed journey from one side to the
other. It is a reasonably close neighbor
galaxy to our own being only 21 million
lighty years away and is also a similar
size to the Milky Way. It has very
distinct arms and an unusually high
ratio of star forming regions for a
galaxy seen where the colors are
strongest. This means it's a very active
galaxy. The reason for this is because
of exploded stars, superheated gas and
material falling towards black holes.
Number 16, the Cigar Galaxy, also known
as M82. This magnificent starburst
galaxy is remarkable for its ejected
flamelike red hydrogen gas. Young stars
are being born 10 times faster in this
galaxy than they are inside our own
Milky Way. This is thought to be because
of interactions with its close neighbor
M81. They are only 300,000 lightyears
apart and because of their immense
gravity they have these tidal effects on
each other.
M82 houses the brightest known X-ray
pulsar called M82X2
which is the big pink dot in the middle
of this picture. It's left astronomers
scratching their heads as it's pumping
out 100 times more X-ray radiation than
something of its mass should be able to
do. As a comparison, X2's binary system
companion M82X1,
which is the pink dot to the bottom
right of X2,
is a black hole, but surprisingly it
emits less X-ray radiation.
Number 17, Cassiopia A.
This is the colorful aftermath of a
supernova explosion of a star. It is the
youngest remnant of a supernova
explosion in our galaxy and is only
11,000 lighty years away. The light
hitting Earth from this explosion would
have happened about 300 years ago,
although there is no record of any
sightings. This could be because the
outer layers of the star were already
ejected and so absorbed the light from
the explosion, but it's not really known
why. Incredibly, the explosion shell is
still incredibly hot, about 30 million°
centigrade,
and is still expanding at 6,000 km per
second. This shell is about 10 lighty
years across by now.
Number 18, the Lobster Nebula. This is
another beautiful nebula found in the
Milky Way around 11,000 lighty years
away from us. It also contains a lot of
protostars found with dark discs and
cocoons of gas obscuring the stars view.
This star cluster is known as Pismus 24.
Pismus 241 was thought to be the most
massive star on record at 300 solar
masses. It turns out though this is
actually a multi-star system and is in
fact at least three stars each about 100
solar masses. Even with this reduction
in size though, they are still some of
the most massive stars that we know
about.
Number 19, NGC602.
This picture is not just beautiful for
its shapes, blues, and oranges, but also
because of all the galaxies in the
background. The reason these galaxies
are so clear is because NGC 602 is found
away from the center of the small
maganic cloud, meaning there's not so
many stars to distort the view. This
also makes the nebula a lot easier to
study. In the center is a cluster of
young bright stars eroding the dust
walls away.
Finally, number 20, NGC1672.
This is the last image for this episode.
And what we have here is a bar spiral
galaxy. It has these two arms which come
away from the rest of the galaxy. The
one on the left being a lot more
prominent than the other. This galaxy
actually has four arms. The final two is
not so defined and they're tucked away
inside the galaxy. And interestingly,
these arms do not join at the nucleus,
but rather at the end of this straight
bar of stars. This is actually why it's
referred to as a bar spiral galaxy.
Around the bar, the galaxy's
experiencing starburst, an extreme
amount of star formation.
NGC1672
is an impressive 60 million lighty years
away.
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