Explosions in space are so astronomical that
there is no way that we can’t can’t observe
it, but what if we find ones that are so massive,
yet so elusive to our eyes?
Let’s find out how mergers from neutron
stars and black holes could hide from us!
Ever since we learned to look up and become
amazed with the night time, humankind has
been observing the celestial sphere, finding
new things to fuel up the level of being impressed
that we already have.
And we definitely have come a long way, from
the days of the early Arabic astronomers squinting
their eyes hard to make out something from
their star maps, to devising the very first
telescope, to building huge instruments that
can measure gravitational waves.
It has really been a long way for us in terms
of observing the night sky.
Having all of this firepower, it is practically
impossible for anything to miss our sight,
right?
Well, if the universe is that easy, then we
would have nothing more interesting to learn!
Of course, there are things that are beyond
our observational capacity.
This day, however, we would like to talk about
two of the most powerful, most extreme, and
most hardcore objects we have seen in the
universe: neutron stars and black holes.
These two are so hardcore that they are practically
zombies of dead stars.
I know you’re thinking that’s kind of
an awkward analogy, but bear with me.
I’ll explain how that’s the case.
So, we have known for a long time that just
like us here on Earth, stars also undergo
a certain life cycle: they start from a cloud
of particles, which then accrete and coalesce
to form a baby star, which then eats and eats
more particles to grow into a giant star,
and when it’s large enough, gravity takes
over and collapses it, and then it explodes.
Then, it leaves behind a new product.
For stars with average mass, the final product
is a white dwarf: a star that is about the
size of the Earth, but is about as massive
as the sun.
You can imagine the amount of density on that
one, but believe me when I say, that’s nothing
to what we will talk about later.
Now, if the star grows up to be extremely
massive compared to the average, it fuses
hydrogen and helium at a super fast rate.
More fusion translates to more energy, so,
when the gravity finally takes over and collapses
the star, a supernova so bright that it outshines
the whole galaxy takes place.
From that point, one of two scenarios can
happen.
Firstly, the atoms in the star can collapse
so tightly that it effectively eliminates
the spaces between subatomic particles, squeezes
the protons and neutrons in it to form neutrons.
When the compression resolves to a sphere
with a radius of 10 kilometers, we get the
densest physical object in the universe, the
neutron stars.
It was even believed that this object is so
dense, a part of it about the size of a sugar
cube would weigh about the same as all of
humanity.
All of it!
In a bite sized sugar cube!
Don’t lie, I know you thought neutron stars
were a group of neutrons that clustered together
to form a star, didn’t you?
Although, you somehow hit a point, since neutron
stars are composed exclusively of neutrons,
you can imagine it to be a gigantic atom.
And then, we have another scenario.
Sometimes, the gravitational collapse can
be so strong that it compresses the star to
a volume so infinitesimally small that you
can practically say there is no volume at
all.
At this point, a black hole is born: a region
in space where the gravitational force is
so enormous it practically bends the time
and space around it by a huge magnitude.
How severe is this force?
Well, let’s just say that even light cannot
escape it and eventually gets eaten by it.
Everything that gets close enough just gets
consumed by it.
So I hope at this point, you see how the zombie
analogy from earlier works.
Isn’t it fun?
But here’s where it gets more interesting.
Just as when two zombies meeting one another
can cause a lot of chaos, so does when pairs
of neutron stars and black holes.
Let’s explore first what happens when two
neutron stars are close to one another.
Do you have an inkling on what would happen?
Let us know in the comment section down below!
We discussed earlier how neutron stars are
the densest objects in the universe, and being
that, it tells us that they exert a great
amount of gravitational force as well.
Now, imagine what happens when two of those
come close to one another.
As they approach a certain distance, the two
neutron stars will begin to rotate about a
common center of gravity.
As time goes by, the gravitational force becomes
stronger and stronger and the two stars begin
to spiral towards one another, until finally,
they merge into either a larger neutron star
or a black hole, depending on what scientists
call the Tolman–Oppenheimer–Volkoff limit.
We won’t dig deep into that as this is completely
another topic, but let’s just say it’s
some kind of threshold measurement.
This merger results in a huge outburst of
energy, and it’s just not in one form.
As Einstein’s theory of general relativity
predicts, objects with gravitational force
as strong as the neutron stars are bound to
distort the fabric of spacetime.
This distortion in the fabric of reality is
measurable through quantities which we call
gravitational radiation of gravitational waves.
Besides that, another product that results
from a neutron star merger is electromagnetic
radiation, in the form of light and gamma
radiation, which we call the kilonovae.
What’s so special about this is that scientists
have long been puzzled about where heavy elements
come from, and one hypothesis they formed
is that they could have been from neutron
stars, but it hasn’t been verified through
observation until the first merger of neutron
stars were observed.
I can only imagine how ecstatic the scientists
were when they finally achieved this verification!
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Okay, so we’ve covered one type of gigantic
merger, now we move on to the next.
The merger of one black hole to another.
After emerging from the corpse of two highly
massive binary stars, like the scenario where
we had two neutron stars, they will spin around
a common barycenter, until they finally come
close to one another.
Now, we have mentioned earlier that a black
hole’s favorite activity is to practically
just devour anything that comes near enough,
remember?
Well, this is exactly what happens when two
black holes reach a certain distance.
They consume one another until they merge
into something that we call a supermassive
black hole: a black hole having a mass tens
to hundreds of billions of our Sun’s.
Just imagine the amount of gravitational force
on that one.
And as objects have extremely large gravitational
forces, they cause ripples in the spacetime
which we detect here on Earth through our
GW (gravitational wave) observatories.
It’s easy to think that this is a once in
a lifetime event, but apparently, if we look
up GraceDB, the reports database for the LIGO
observatory, we would see that collisions
like this happen as often as once per week.Thank
goodness we aren’t positioned near one,
right?
All of humanity and all we have ever known
will be gone faster than the blink of an eye!
So, at this point, it is easy to figure out
what we are going to talk about next: we have
covered mergers between two neutron stars
and two black holes.
How about a merger between a neutron star
and a black hole?
This event is of huge interest to the astro
community as this will shed light about a
lot of things.
For instance, the black hole will most probably
shred the neutron star to pieces, so it would
shed light to what exactly do we expect to
see inside it.
Speaking of shedding light, are we going to
expect a huge burst of light or radiation
when this happens?
All of these will finally be answered upon
a verified observation.
And then just last year in August, the LIGO
and VIRGO detected an event which may finally
give the answers the astronomers have long
been looking for.
Gravitational waves were detected from event
S190814bv, reading with a very high mass gap,
and scientists inferred that this couldn’t
be anything but a black hole-neutron star
merger.
So at that point, have we finally done it?
Have we detected a merger between a neutron
star and a black hole?
Well, it’s almost a yes.
Detecting gravitational waves is a great tool
for detecting massive events, but of course,
our ingenuity doesn’t let us settle with
just one measuring tool.
We have also set up a countermeasure to verify
our observations.
Massive events such as neutron star-black
hole mergers must emit some kind of electromagnetic
signature, whether through x-rays, or light,
or other related quantities.
This is the primary task of the ElectromagNetic
counterparts of GRAvitational wave sources
at the VEry Large Telescope (ENGRAVE) spearheaded
by the ESO or the European Southern Observatory.
The twist in this story is, despite the GW
observatories celebrating with the controversial
amount of gravitational wave from event S190814bv,
ENGRAVE on the other hand failed to detect
any electromagnetic counterpart of the event.
Which is why, as awesome as it could have
been, we’re still yet to discover a neutron
star-black hole merger.
But scientists did not lose hope over this!
They stated that it could be possible that
the event was simply too overwhelming for
it to leave any trace.
They think that what could’ve happened is
that the gravitational force from the black
hole effectively just swallowed the neutron
star entirely without even disintegrating
it.
This might be the reason why there weren’t
any traces of electromagnetic counterparts
to support the findings, but this is still
yet to be proven.
Let that fact sink in, though: how a black
hole could get so massive that it wouldn’t
even leave a trace of what it consumed.
Now that’s one scary zombie!
Just this year, scientists released additional
information with regards to finding black
hole and neutron star mergers, stating that
besides being difficult to detect, they could
also be hiding from us inside dense star clusters.
Trying to be sneaky, aren’t we?
When we say dense star clusters, we are talking
about a group of stars that are collected
together.
On average, a star cluster can contain up
to just 0.4 stars per cubic parsec, but in
dense ones, it could get as crazy as 100 to
1000 per cubic parsec.
That’s an extremely huge margin.
With the definition, it’s easy to understand
how a neutron star and a black hole merging
can hide easily.
A dense star group implies an very intense
electromagnetic signal source, so any reading
that will be done in this place can be easily
mistaken for noise.
Moreover, due to the number of stars in these
types of groups, it is almost impossible to
pinpoint a gravitational wave source at this
point.
Now, okay, why not look anywhere else for
the merger that we intend to find?
Well, one thing we haven’t pointed out earlier
is that the mergers that we have already observed
are from the corpses of the binary stars,
even event S190814bv.
We are yet to observe a dynamically interacting
black hole and neutron star, that is a black
hole paired with another star meeting a neutron
star paired with another star.
Scientists thought that the best place to
find this is inside a star group where almost
all scenarios of stellar collisions might
most likely take place, which is in dense
clusters.
Think of it this way, if we are looking to
find a pet cat, would we go to a liquor store?
Of course, there’s the possibility of finding
a stray cat to take home, but if you want
to be able to choose what breed of cat you
want, how fat you want it to be, or whether
you want a boy cat or a girl cat, your best
option is a pet store.
In the same analogy, a lot of cosmic scenarios
are available in a dense star cluster, so
we would understand why astronomers are focusing
a lot of energy in searching these star groups.
But all hope is not gone, as there are new
expeditions being launched into space to study
exactly how stars in dense clusters behave.
In 2034, European Space Agency's Laser Interferometer
Space Antenna (LISA) mission is scheduled
to launch, and scientists believe that this
would give us a better understanding of stellar
behavior in dense star clusters, which could
finally bring us closer to finally seeing
a neutron star-black hole merger in this type
of group.
It’s a long way into the future, but until
we come to that, we are continuing the search
for verifiable mergers between neutron stars
and black holes in places where there’s
less noise.
I don’t know about you, but I feel like
every day, we become closer to that goal.
That’s what I think but I want to know what
you think?
Do you think S190814bv is actually a black
hole merging with a neutron star?
Or do you think it’s something else?
Let us know what you think in the comment
section down below!
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