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Forget TMZ - Here on Space Time we have all the latest details on the dysfunctional, explosive
relationships between the stars.
Let me tell you a tale of a pair of star-crossed … well, stars.
When our galaxy was a little younger there were two ordinary stars - perhaps not unlike
our sun, and they danced together in binary orbit.
Romantic, right?
But any good romance is also a tragedy.
After a billion or so years, one star died.
It had burned brighter and faster, until it’s heart of fusing hydrogen shriveled into a
dead core of carbon and oxygen.
Ejecting its outer layers, it became a searing hot, planet-sized orb of incredible density
- a white dwarf.
And this is where the romantic tragedy turns into a horror story.
As the zombified stellar core and still-living companion spiraled closer together.
A stream dull, red gas now connected the two - the outer envelope of the star falling
into the intense gravitational embrace of its old companion.
There, in the extreme surface gravity of the ultradense white dwarf, a layer of hydrogen
built up.
At a critical point, that surface reached the temperature and pressure of a stellar
core.
A storm of fusion ripped around the planet-sized white dwarf, spraying its atmosphere into
space and for a couple of weeks shining 10s of thousands of times brighter.
Centuries later, on March 11, 1437, the light from that explosion swept past the Earth.
There, the royal astronomers of King Sejong’s court in Korea recorded a new point of light
in the constellation of Wei, in what we call Scorpius.
They named it a guest star.
We now call this phenomenon a nova, from stella nova, or new star.
Romantic names, even if the stellar partnership is a disaster.
Our galaxy is full of these sorts of dysfunctional stellar relationships.
With more than half of all stars existing in binary orbits, it’s inevitable that many
stellar remnants will end up in these parasitic spirals with their partners.
Today we’re going to look at the worst of these - from the novae produced by white dwarfs,
to X-ray binaries created by neutron stars and black holes - and much weirder things
besides.
These days, if you point one of our newfangled giant telescopes at the same spot where the
royal Korean astronomers saw their guest star … you see nothing.
But if you pan a bit you find a puff of gas - a beautiful nebula, all that remains of
that explosion.
Mysteriously, the ill-fated binary isn’t in the center of inside the nebula - it’s
wandered a bit since 1437.
But it can be found if you look a little off center for a spot of light that
flares erratically from visible to X-ray wavelengths.
It was Mike Shara Astrophysics Curator at the American Museum of Natural History, who figured all
of this out.
After discovering the nebula from the 1437 nova back in the 80s, he spent decades tracking
down the culprit system.
He finally identified a nearby flaring white dwarf binary - a so-called cataclysmic variable
- and realized that it was the same object as a dwarf nova that he found on multiple
old photographic places as far back as 1923.
A dwarf nova is what you think - it's like a regular nova, but much weaker.
They result when denser streams of matter hit the white dwarf and flare due to heat,
but do not produce the storm of fusion of the classical nova.
But as Dr Shara discovered, it turns out that dwarf novae are just what classical nova do
between those bigger explosions.
With observations of this dwarf nova spanning the last century, Shara could extrapolate
its path back another half-millenium.
That placed it exactly where those royal astronomers saw their classical nova.
So cataclysmic variables must slowly build up their hydrogen layer, sputtering and flaring
as they do so, until a critical temperature and pressure sends them over the edge.
After which they start the whole process all over again.
The system responsible for the 1437 nova is by no means unusual.
50 or so classical novae go off in our galaxy every year.
Cataclysmic variables do come in some variety - for example we have polars.
If the white dwarf has a strong magnetic field, the flow of gas from its companion is channeled
by that field.
As charged particles spiral along the magnetic field lines they emit synchrotron radiation,
and bright X-ray light is emitted as the gas hits the polar regions of the white dwarf
- like a particularly violent auroras.
Cataclysmic variables are somewhat impressive, but for a real cataclysm it’s hard to go
past an X-ray binary.
Just replace the white dwarf with a neutron star or black hole.
Those are what you get when the most massive stars die.
The remnant core now contracts to the point that atomic nuclei are no longer distinct
- instead they meld together, protons and electrons combine to become neutrons, and
you’re left with a ball of hyperdense matter the size of a city.
And its mass is high enough it sucks itself into a black hole.
This is all stuff we’ve talked about before - be we haven’t seen the effect on a hapless
companion star of having one of these as its binary partner.
Once again, if the two are close enough, gas is syphoned from the star onto the black hole
or neutron star.
If forms an accretion disk - and in X-ray binaries, it’s the accretion disk itself
that glows bright.
That’s because the gravitational field of the compact object is so strong, falling gas
reaches incredible speeds - which means incredible friction - which means heat and light.
They glow X-ray hot.
And like cataclysmic variables, the flow is uneven so the X-rays fluctuate.
In the case of neutron star X-ray binaries, that fluctuation includes powerful flares,
resulting from denser clumps of material hitting the rapidly rotating surface of the neutron
star.
Sometimes we also see the neutron star as a pulsar.
Its powerful magnetic field channels high energy particles into a jet that traces a
circle across the sky - and often sweeping past the earth to produce metronome-precise
pulses - most brightly in radio light, but potentially at all wavelengths.
Black hole x-ray binaries seem a bit more boring by comparison, because the black hole has
no surface for the gas to fall onto - so no x-ray flares.
The nearest such system is the famous Cynus X1 X-ray binary, where a black hole the mass
of 15 Suns is busy gorging on a blue giant star.
As with cataclysmic variables, X-ray binaries are relatively common - we know of 100s in
the Milky Way.
But there are some much rarer, and, frankly, more awful manifestations of this phenomenon.
Take the black widow.
This is almost as cool a detective story as the 1437 nova.
To start, you need to know that when you look at our galaxy in gamma rays - the highest
energy light there is - the brightest points you see are pulsars, and those gamma ray spots
are pretty much always accompanied by the classic metronome-precise pulses of radio
light.
Except of course when they’re not.
And there are a handful of mysteriously pulse-free gamma ray sources that otherwise look like
they should be pulsars.
It was Roger Romani of Stanford who figured this one out.
He observed these objects using visible wavelength of light - and found one object was indeed
pulsing.
But the pulses were far too slow - it brightened and dimmed avery … hours, while pulsars
flash on the scale of seconds, or even microseconds.
The source also become bluer as it brightened, redder as it faded.
Well it turns out this object is a pulsar, and it’s in orbit around a companion star - in
this case a brown dwarf, which is a star not quite massive enough to generate its own energy
by nuclear fusion.
In this case the companion didn’t start out as a brown dwarf - it became one after
losing most of its mass to its ravenous partner.
That brown dwarf orbits perilously close to the neutron star.
The neutron star’s jets sweep it hundreds of times per second, slowly blasting away
its gas.
That gas forms an enveloping ring around the whole system, which then falls onto the neutron
star.
The same gas blocks any radio light, but allows the more penetrating gamma ray light to pass
through.
And the pulsing of visible light?
Well that’s when the super-heated “daytime” side of the brown dwarf comes into our view,
while the red, dim phase is when we are looking at its night side.
This object became known as the black widow, and with the discovery of several similar
systems, black widow is now the name of the object class.
Sticking with the deadly spider motif, if that second star is a red dwarf we have a red back.
That second star is doomed to an ignominious end.
First its whittled away until its not a star any more, and eventually we expect it to become
to become more and more Jupiter-like and then just an icy core.
Finally that core is expected to break up in the neutron star’s tidal field and be
scattered into the void.
On the other hand cataclysmic variables - like the one that produced the 1437 nova - have a more impressive
end to look forward to.
The white dwarf in these systems builds up mass until releasing it as a nova.
But in that explosion it only ejects maybe 5% of the accreted material.
The rest stays with the white dwarf, which slowly grows in mass.
Eventually, the core of the white dwarf reaches a temperature of hundred of millions
of Kelvin, and the star’s carbon and oxygen can begin to fuse.
A runaway fusion reaction rips through the star, which explodes as a Type 1 supernova.
Those supernovae are visible not just across the
galaxy, but in galaxies across the universe.
To them, they were the explosives ends to long and fiery relationships, but to us they
seem a little petulant. Like the final slamming of doors from distant parts of spacetime.
As you know, at the start of the pandemic we all had to quarantine on Earth to avoid contaminating
space with the virus.
Well, after several months shooting in my apartment we finally managed to develop protocols
to protect the universe from Earthly lurgies and so here I am, floating in the void once
again.
So for this comment response we’re going back a few weeks to our episode on this strange
new observation by LIGO: gravitational waves from the merger of a black hole with ... something
else.
Something that seemed to straddle the mass between black holes and neutron stars, and
which will change the way we think about whichever of those it turns out to be.
Let’s get to the questions.
Zack Hamburg asks how we know that a black hole isn’t just a neutron star behind an
event horizon.
Why should the star have crushed down into a point-like singularity at all?
To give everyone some context: When a massive star dies, its core becomes a neutron star
- but if that core is above a certain mass it shrinks so that the escape velocity at
its surface is greater than the speed of light.
That’s when it becomes a black hole.
So what happens to the neutron star after it collapses enough to form an event horizon?
Well, below that event horizon, we can think of space flowing downwards faster than the
speed of light.
That means the neutron star has no choice but to contract until no more contraction
is possible - when it has zero size.
This conclusion is unavoidable if you’re only using general relativity.
We think a theory of quantum gravity probably prevents the singularity from really forming
- but quantum gravity effects would not kick in soon enough to save the neutron star.
Catinboots81 and Vivallamannen asks whether the strange smaller object in the merger might
have been a primordial black hole.
Good insight there.
Dead stars aren’t the only way to make black holes.
Some black holes may have formed from the extremely dense matter of the early universe,
and these would have different mass restrictions than stellar black holes.
In fact, people have considered primordial black holes as an explanation for other LIGO
mergers - which often involve black holes MORE massive than was thought normal for stellar
remnants could create.
But it’s also possible that primordial black holes could be less massive than black holes
that come from stars, so might explain this weird teensy possible black hole.
Interestingly, this should be testable.
If primordial black holes exist in some abundance at these masses, then the universe should
be very faintly humming with a gravitational wave background from the countless mergers
than happened in the earlier universe.
LIGO hasn’t seen that background yet - which actually limits how many such primordial black
holes there might be.
If LIGO continues to fail to detect this background then it’ll become less and less likely that
primordial black holes are responsible for any LIGO events.
Some of you also asked why the less massive object can't just be a regular star.
That's an easy one - in order to generate detectible gravitational waves, both objects
need to be extremely compact.
The waves get generated when extreme masses spiral together at very small distances.
Compared to a black hole or neutron star, regular stars are giant puffed up balls.
They are ripped apart before getting close enough to generate gravitational waves.
Frank and Jim asked how the event horizons of merging black holes change just before
they combine.
You guys have it right - they do deform into a sort of 8 or hourglass shape - in the sense
that the event horizons sort of reach out to each other, connect, before coming together
into an ovoid and then finally a sphere, or flattened sphere for a rotating black hole.
Note that this doesn’t say anything about the shape of the stuff inside the black hole
- we’re just the event horizon - the surface below which there’s a faster-than-light
flow of space.
Here’s a simulation from the SXS - simulating extreme spacetimes group at ... that shows
how the event horizons merge.
Military Archive says finally here is an episode I can kind of understand. Well Mr. Archive, maybe the reason episodes
get easier is that you’re getting smarter with all that Space Time you’re watching.
Or maybe I’m just getting better at explaining stuff.
Can we ever know?
Laura Chapple follows up by saying this episode was worryingly comprehensible and wonders
if we’ve run out of physics.
Well, as Einstein said: The eternal mystery of the world is its comprehensibility.
So if you’re comprehending Space Time does that make you Einstein?
We can test that - we have many ideas for episodes that are currently totally incomprehensible,
at least to me.
Let’s see how far we can go.