Normally we think of black holes as stellar
black holes, black holes formed from the collapse
of a sufficiently sized star.
And rightly so!
They make up most of the black holes in the
universe.
We’re also familiar with supermassive black
holes which are generally located in the centre
of most galaxies, getting bigger and bigger
every day by eating the matter that spirals
into them in their accretion disk.
And there’s also a class of black holes
called intermediate mass black holes, black
holes with a mass of 100 to one million solar
masses, being more massive than your typical
stellar black hole but less massive than supermassive
black holes.
Another type of black hole, although theoretical,
is a primordial black hole, a black hole formed
at the very early stages of the universe,
immediately after the Big Bang.
While we have all these different types or
classifications of black holes, they all have
one particular thing in common: they can all
collide and merge.
When this happens, it is one of the most extraordinary
events in the universe.
There are two possible paths where two black
holes can collide and merge.
Firstly, in a binary star system.
These stars have formed together at the same
time in a stellar nebula, and are much more
massive than our sun, having masses several
hundred times that of the mass of the sun.
Because of their exceptional mass, the two
of them only have a lifetime of a few million
years.
After orbiting each other for a few million
years, one of the stars explodes in a supernova
and eventually becomes a black hole.
Now we have still a massive star but orbiting
a black hole.
Soon enough the other star detonates and becomes
a black hole.
Now what’s interesting is that all stars
have a finite lifetime because eventually
they run out of fuel however black holes do
not.
Well in truth, black holes most likely will
die but the process takes exceptionally long.
It’s thought that black holes evaporate
and shrink due to Hawking radiation.
Anyways, this binary black hole system, like
everything else in the universe, will emit
gravitational waves in every direction at
the speed of light.
The emission of gravitational waves decays
the orbit of the black holes and decreases
the angular momentum of each black hole, meaning
that over time the black holes will spiral
closer and closer into each other.
The beginning of this inevitable inward spiral
takes the longest, as the gravitational wave
emission is very low since the black holes
aren’t very close.
As the orbit shrinks the speed increases and
the emission of gravitational waves increases.
Eventually the orbit becomes so small that
the two black holes collide and coalesce into
one black hole with a mass of about the sum
of the two previous black holes.
The mass of the end product black hole will
be less than the sum of the two because some
of the mass will have been converted into
energy in the form of gravitational waves.
With sizeable black holes, this tends to be
a few solar masses.
The third detection of gravitational waves,
for instance, involved the merger of two black
holes, one with a mass of 31 solar masses
and the other with 19.
The final black hole had a mass of 49 solar
masses.
When that final black hole is created the
emission of gravitational waves peaks.
The other possibility where a pair of black
holes can collide is if they happen to run
into each other.
This event includes two black holes forming
completely independently of each other then
coming close enough to each other that they
become locked in orbit, and then following
the same path as we described earlier in the
inward spiral and the eventual merge.
I’d say that this event is a lot less common
than the other possibility we discussed since
the chances of two black holes coming across
each other aren’t high.
My guess would make sense since only one of
the gravitational wave detections has black
holes that has evidence that they did not
originate together.
This event also has a lot more variations
in how it can occur.
As far as we can tell most galaxies have a
supermassive black hole in the centre.
So two galaxies merging involves the supermassive
black holes in the centre of each galaxy colliding
and merging as well.
It’s thought that the Milky Way galaxy and
Andromeda galaxy will collide in about 4 billion
years and take a few billion years to actually
coalesce.
By then the Sun won’t have begun its helium
burning process, so it won’t be a red giant
yet however Earth will become uninhabitable
by then as the Sun will increase in enough
luminosity to wipe all life out on Earth.
Although the two supermassive black holes
will merge, it’s very unlikely that there
will be any star collisions because of the
large distances between each star system.
The supermassive black hole in the centre
of our galaxy, Sagittarius A*, and the supermassive
black hole in the centre of Andromeda will
begin to radiate very strong gravitational
waves once they’re only a light year apart.
It’s estimated that in about 150 billion
years from now every other galaxy in the Local
Group will have merged into this galaxy.
Comment down below your thoughts on black
holes colliding and merging.
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