The Moon orbits around the Earth.
The Earth orbits around the Sun.
And out in the distant Universe, astronomers
have found a system that takes this logical
progression to its most extreme.
There’s a system where a supermassive black
hole with millions of times the mass of the
Sun orbits another black hole with billions
of times the mass of the Sun.
How astronomers discovered this incredible
interaction took careful observations, imagination,
and the hard work of the Spitzer Space Telescope,
taken during its final years of operation.
A regular stellar-mass black hole already
boggles the imagination.
An object with several times the mass of the
Sun compacted down into a volume as small
as a city.
But astronomers also know of the supermassive
black holes that lurk in the hearts of most
galaxies.
Our own Milky Way has a black hole with 4.1
million times the mass of the Sun.
Again, just let the scale wash over you.
4.1 million solar masses compacted into a
region that’s smaller than the orbit of
Mercury.
But our galaxy’s black hole is actually
pretty small in the grand scheme of things.
In the distant galaxy OJ 287, astronomers
have found a supermassive black hole that
weighs in at 18 billion times the mass of
the Sun.
And what’s happening at the heart of this
galaxy is absolutely fascinating.
Orbiting around this monster black hole is
another supermassive black hole with merely
150 million times the mass of the Sun.
Here’s how astronomers figured this out.
Galaxy OJ 287 is located 3.5 billion light-years
away in the constellation of Cancer.
Astronomers have known for a while that it
contains one of the most massive black holes
ever seen in the Universe, weighing in at
18.35 billion times the mass of the Sun.
OJ 287 is an object known as a blazar.
Astronomers have been aware of a series of
objects which are very distant and surprisingly
bright - known as “active galaxies”.
You might be familiar with the term “quasar”
which is the bright center of a galaxy which
has a supermassive black hole that’s feeding
on material.
Blazars are essentially the same thing, but
it all depends on the angle at which we see
them.
If we’re seeing the galaxy edge-on, it’s
known as a “radio galaxy”.
If we’re seeing it at an angle and can see
more of the central accretion disk, it’s
a “quasar”.
And if we’re looking right down the barrel
into the galaxy, face-on, that’s a “blazar”.
OJ 287’s supermassive black hole is surrounded
by a huge disk of gas and dust; the accretion
disk of material that’s spiraling around
the black hole, waiting for its turn to disappear
beyond the event horizon and join the singularity.
Astronomers studying OJ 287 noticed a flash
of light of radiation coming from the galactic
center on a semi-regular basis.
Every 12 years or so, there would be a double-flash
of brightness, where the system would brighten
up by a factor of 4 over the course of 48
hours.
Sometimes the flashes would happen within
a year of each other and sometimes they’d
happen 10 years apart.
Astronomers struggled to figure out what was
causing the flares and tried to predict when
the next one would happen.
Finally, in 2018, a group of scientists led
by Lankeswar Dey, a graduate student at the
Tata Institute of Fundamental Research in
Mumbai, India figured out the orbital parameters,
published a study that predicted exactly when
the next flare would happen within a couple
of weeks of accuracy.
According to Dey and his team, the more massive
18-billion solar mass black hole is being
orbited by a 150-million solar mass black
hole on an irregular orbit.
Each time that the smaller black hole plunges
through the accretion disk of the bigger one,
it generates a flash of light brighter than
the entire Milky Way.
Like a trillion stars shining together at
once, fading away in just a couple of days.
But there was a big problem.
They predicted that the flare was going to
happen in late July 2019.
As luck would have it, that part of the Universe
was inconveniently located behind the Sun,
invisible to any observations from Earth,
and would remain so until September 2019,
long after the flare was due to fade away.
Fortunately, astronomers had an instrument
that would still be able to see that region
of the Universe, even though the view was
blocked from Earth.
And we’ll get to that in a second, but first
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Patrick Carpenter
Don Novak
Ben Slutsker
Anton Sigal
Dark Water
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As you’ve probably heard, NASA’s long-lived
Spitzer Space Telescope was retired in January
2020 after 16-years of observations of the
Universe in infrared.
It fundamentally changed our view of the Universe,
making the first direct observations of atmospheres
of exoplanets, seeing through gas and dust
to see newly forming stars and planets, and
helping astronomers see through the core of
the Milky Way to core structures normally
blocked by the Zone of Avoidance.
Spitzer was on a path that kept it slowly
drifting away from the Earth as it orbited
the Sun.
By 2019, Spitzer was 254 million kilometers
from Earth, allowing it to see the Universe
from a completely different vantage point.
And galaxy OJ 287 wasn’t obscured by the
Sun from its vantage point.
Engineers at NASA directed Spitzer to watch
OJ 287 for that telltale flash, and they were
surprised to see it on July 31, exactly when
the team had predicted it would happen.
And then, just a few months later, Spitzer
was out of fuel and retired forever, shutting
off one of astronomy’s most important views
into the cosmos.
I’m sure you’ve played enough Kerbal Space
Program to have an intuitive sense that things
orbit around other things.
So why was it so difficult to predict the
movements of one supermassive black hole orbiting
another?
In order to make predictions about the movements
of black holes orbiting each other, astronomers
have to account for gravitational waves, ripples
in spacetime caused by the movement of massive
objects.
The more massive an object is, the faster
it’s moving, the more gravitational waves
that it emanates.
And the OJ 287 system takes this to the extreme.
This has always been a rough calculation,
but thanks to the detection of gravitational
waves from colliding black holes that were
detected by LIGO, physicists could calculate
black hole moments with much higher accuracy.
Ley and his team were also able to incorporate
modern theories about the shape of a black
hole’s event horizon.
The “no-hair” theorem proposed in the
1960s by Stephen Hawking and others, said
that a black hole’s event horizon should
perfectly smooth and symmetrical.
Less dense objects, like planets and even
neutron stars, can be lumpy and bumpy, with
uneven bulges on either side.
Kip Thorne predicted that black holes orbiting
each other would interact with each other
in different ways depending on whether their
surfaces were smooth or bumpy, and you could
detect though how the gravitational waves
distorted the space around them.
And that’s why, catching the flare within
just a couple of hours of their prediction
provided an enormous amount of evidence to
the “no hair” theorem of black holes.
Evidence is mounting that black holes have
smooth, featureless surfaces, as predicted
by Hawking in the 1960s.
In my opinion, this story really has it all.
It deals with objects of incompressible size
and mass orbiting around each other, releasing
trillions of times the radiation of the Sun
every few years.
It allowed astronomers to further validate
a theory that’s been half a century in the
making, taking advantage of the first direct
observations of gravitational waves.
It used an aging space telescope, called in
for one final critical mission before it could
pass on to its inevitable retirement.
Thanks for everything Spitzer, we’re going
to miss you.
What do you think?
Let me know your thoughts in the comments.
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Thanks to Advanced LIGO, astronomers are detecting
gravitational waves every week or so now,
and things are only going to get better.
We did a whole video on the new gravitational
wave observatories in the works, and what
the future holds for this science.
And you can watch that now.
