One of my favorite fictional astrophysics catastrophe scenarios is the sun being consumed by a black hole.
Fortunately, the chance of a black hole randomly wandering into our solar system is pretty tiny. That’s
good news. But what if it’s already here, hiding in the core of the Sun and slowly eating it from
the inside out?
Once upon a time we really did think there just might be a black hole in the heart of
the Sun. And by “we” I mean a small handful of scientists that, at least for a minute,
included Stephen Hawking. In a 1971 paper, Hawking pointed out that we wouldn’t even
know if the Sun was hiding a black hole with a mass of up to 10^14 kg. Sounds unlikely
given that black holes are supposed to have masses at least 20 trillion times larger,
or a few times larger than our Sun. But in that same 1971 paper Hawking also alerted
us to the possibility that countless much smaller black holes might have been created
in the very early universe—what we now call primordial black holes—which we’ve
discussed on multiple occasions. So what if one of those somehow ended up inside the Sun?
Today we’re going to talk about whether or not our Sun, or indeed any star,
might harbor a captured primordial black hole—what this would do to the star, how we
might figure out its presence, and why this could tell us the nature of dark matter, of all things.
First up, let’s talk about why Hawking’s idle speculation captured the imagination
of a number of astrophysicists. It’s because a black hole in the sun’s core
offered a solution to a major conundrum about our star’s behavior. By the the middle of the last centruy we thought
we understood the fusion reactions that power the Sun very well—under the temperatures and
pressures that we knew must exist in the Sun’s core, hydrogen should fuse into helium at just
the right rate to produce the amount of energy needed to power the Sun. Those reactions must also spray a
ludicrous number of neutrinos out into space, and checking that the Sun was blasting out
the right number of neutrinos was to be our nail in the coffin confirmation of our model of the Sun’s interior.
The first neutrino detectors came online in the mid 60s,
but the number of neutrinos came up short—by a lot. Only about a third of the expected number
of neutrinos were observed. This was the solar neutrino problem. One proposed solution was
that the Sun does not generate all its energy through fusion—perhaps some of its radiation
came from a non-neutrino-producing source. Then a few years later came Hawking’s idea
of black holes inside stars, and with it a potential new power source for the Sun.
Now you might think that a black hole would suck energy out of a star rather than provide energy,
but that’s not the case. Black holes are actually at the hearts of the most
efficient engines in the entire universe. While matter and energy that crosses a black
hole event horizon is lost forever, matter approaching the black hole can reach speeds
close to the speed of light. This results in crazy friction between infalling streams
and correspondingly crazy temperatures. So a feeding black hole glows bright. For example,
quasars—powered by truly vast black holes in a feeding frenzy—these are among the brightest objects
in the universe. A black hole in the heart of the Sun would gobble up some of the matter, but plenty
more would be blasted outwards as radiation. That way the Sun wouldn’t need to be fusing nearly as
much hydrogen to reach its energy output, and so also wouldn’t produce as many neutrinos.
Cool idea, except then we figured out the real reason for the solar neutrino
problem. It turns out the Sun is producing exactly as many neutrinos as we predicted—we
just weren’t detecting all of them. Neutrinos oscillate between their three different types,
and our detectors were only sensitive to one of those types. With neutrinos accounted for,
the black hole engine was no longer needed.
OK, that sounds like good news. But there’s still the nagging issue that
if primordial black holes exist they could still end up inside stars and
we would never know it. And in the decades following Hawking’s paper,
we only found more and more motivations for primordial black holes really existing.
Let me talk about those motivations real quick before we get to the meat of black holes inside
stars. The first is another big conundrum like the solar neutrino problem—except this one hasn’t been
solved yet. It’s dark matter. Most of the mass in the universe is something completely invisible,
and while the mainstream idea is that dark matter is some kind of undiscovered particle,
it’s also possible that it’s lots and lots of primordial black holes. This isn’t a new idea,
and in fact we’ve managed to rule out most of the mass ranges at which these PBHs could account
for dark matter. But t here is a mass window remaining—PHBs with masses of 10^14-10^20 kg,
or a medium to largish asteroid, aren’t ruled out by observations,
and so might account for dark matter. And if dark matter is made of PBHs then the black holes would need to
exist in such enormous numbers that it’s not crazy that some would end up inside stars.
The other motivating factor is gravitational waves. We’ve now observed 100s mergers of
black holes by the tiny ripples in the fabric of spacetime generated in the last second of their
inspiral. These mergers were probably of black holes created in the deaths of massive stars,
but there are some niggling inconsistencies with the sort of black holes we expect from
stellar deaths that might be explained if some of these mergers are instead primordial black holes.
OK, so there’s some frankly pretty tenuous reasons to think there might be lots of primordial black
holes out there. But the implications of one of them getting into the Sun are pretty fun,
so let’s dig into it. In fact, two papers appeared on the arXiv this week that do the digging for us.
They’re by the same team, with the first led by Earl Bellinger at Yale and the Max Planck
institute and the second by Matt Caplan at Illinois State. Now full disclosure—Matt
co-wrote this episode with me—all the good parts probably. He’s worked with us a bunch
in the past on a range of topics, and I really wanted to shout out his team’s awesome idea.
So both papers contain new state of the art simulations of Sun-like
stars that capture primordial black holes during formation,
and follow their evolution all the way through some pretty unique life phases until they’re
ultimately swallowed by their black holes. They refer to these black-hole-harboring stars
as Hawking stars after the guy who first set us down this crazy line of reasoning.
The evolution of stars without black holes has been well understood for many decades.
A giant cloud of gas collapses under its own gravity until the core reaches a high
enough pressure and temperature to fuse hydrogen into helium. The star is born,
and that core self-regulates to fuse somewhat consistently for billions of years. Once the fuel
for that fusion is used up the star goes through a series of death-throes. For a Sun-like star that
means puffing up to red giant then entering a helium-fusing stage then ripping itself to
pieces by overdoing the fusion energy production to leave behind its exposed core as a white dwarf.
Let’s now add a black hole into the mix. The papers in question simulate
what happens if a primordial black hole with the mass of a medium-sized asteroid
happens to be in the cloud of gas so the star forms around it. This stuff
also works for a star that captures a black hole after its formation.
A black hole with the mass in question is miniscule—the size of an atom,
and its gravitational reach is limited. As a result it has no effect on the formation and
early life of the star. It sinks to the center of the star and begins slowly feeding. Energy
produced by the superheated infalling material radiates outwards. This alters the state of a
tiny pocket in the center of the core that’s around 1% of the star’s radius—roughly the size
of the Earth. Within this region the normally relatively static material of the core begins
churning wildly—it becomes convective. This seething ball grows as the black hole grows,
and will eventually subsume the entire star—but not for a long time.
You might think that with the endless supply of matter might cause the black hole to grow fast,
but that’s not the case. The energy radiated from near the black hole
actually pushes back against new infalling material, severely limiting the growth rate
of the black hole. The tiny size of the black hole also makes it a choke-point.
There’s a limit to how fast you can cram matter into an atom-sized black hole.
That starting rate is something like a hundred tonnes per second,
which may sound like a lot but it’s basically nothing. For some perspective,
the sun loses almost a million tonnes a second to the solar wind.
But as the black hole gains mass its gravitational pull increases,
as does its size. The choke-point widens and matter flows in more easily. It also blasts
out more energy, providing some limit to its growing appetite—but the rate of star-guzzling
nonetheless increases. At the same time, the rate of fusion reactions through the rest of
the core stay roughly constant. That means the energy provided by the black
hole represents an increasing fraction of the star’s total energy production.
Over a few billion years, that atom-sized black hole grows to around 10 cm across,
with a mass around that of the planet Uranus. At that point an important transition happens.
The energy output from the infalling matter now roughly equals the output from fusion across the
rest of the core. As we approach and pass this point, the effects of our Hawking star’s dark
secret become very obvious. The additional energy disrupts the delicate balance between gravity and
outward energy flow, causing the star’s outer layers to bloat. It would look like the star
had entered its giant phase billions of years too early. In a regular giant-phase, the star’s core
remains compact. But in this phase for a Hawking star, even the core expands. The core density
drops so much that fusion cuts off altogether. Now the star is running entirely on black hole power.
Although the black hole would still fit in the room with you, the bubble of churning plasma
encompasses the entire star. During this phase the Hawking star looks like a regular dying star,
at least to start. Our Sun will ultimately expand to maybe 100 times or so its current radius
and be 1000 times it brightness. But a Hawking star stalls before getting
that big. The initial expansion actually makes it harder for the central black hole to feed,
limiting the growth of both the black hole and the surrounding star. It reaches a maximum size
of 4 or 5 times the current size of the Sun, glowing about 10 times as bright. It can spend
as much as ten billion years looking like a relatively rare type of star - a ‘sub-subgiant’
or a ‘red straggler’ - before the black hole finally finishes consuming the star.
So I guess there’s at least one upside to the Sun containing a secret central black hole.
While it gets less time as a normal-looking star dominated by fusion, when it finally does expand
it won’t actually swallow the Earth—which will probably be the case during the Sun’s regular
red giant phase. We would still fry, but let’s try to find silver linings where we can.
So those are the simulations. To take this out of the realm of pure speculation,
let’s see what we can say about potential black holes inside our Sun and the prospects
of detecting Hawking stars. Well, in general if there’s one in the Sun then it has to be less
massive than the transition-point —less than the mass of Uranus—or one-10,000th
the mass of the Sun. Any more than that and the Sun would be expanding already.
But what about a smaller secret black hole? Well, it would need to be quite a bit less
massive than Uranus or we’d have noticed signs that the Sun’s behavior is not consistent with
all of its energy being produced by fusion. For example we now know that the Sun is producing
basically the right amount of neutrinos if all of its energy is coming from fusion. But
there’s some wiggle-room in the uncertainty of our neutrino measurements. Up to 1/1000th of the
Sun’s energy could come from a non-fusion source. An embedded black hole would also alter the core’s
temperature and pressure and so its fusion rate, which would in turn affect the neutrino
production. Add these together and the team finds that any black hole in the Sun would have
to have a mass less than Mercury’s or we would have noticed its effect on its neutrino output.
There’s another way to look for Hawking stars that isn’t even restricted to the Sun,
and that’s by watching how they vibrate. Powered by the vast currents of convective plasma sloshing
around inside, all stars vibrate. Different types of waves bounce around inside, setting
up these global oscillations like in a musical instrument. The vibrational
modes that an instrument can sustain define the characteristic sound of the instrument,
and depend on its materials and its structure. Same with a star—different internal composition
means different global oscillations. Now the field of asteroseismology, which of course
we’ve discussed before, studies how seismic waves can teach us about stellar interiors.
In the case of the Sun, its global oscillations suggest that there’s relatively little movement
of material—relatively little sloshing—in the core and surrounding regions, while in regions
closer to the surface there are great currents of plasma carrying heat outwards. But a Hawking
star that otherwise looked just like the Sun would also have this region deep inside the core where
material was in motion. Seismic waves traveling through that region would propagate differently,
leading to a shift in the harmonics that the star could sustain—just as a drum would sound
different if you replaced even a small patch on the drum skin with a different material.
Current asteroseismology studies of the Sun don’t show any evidence of unexpected
regions of convection in the interior. However those studies also don’t probe
to the deepest regions of the core so we wouldn’t have noticed the effect of
a black hole about a millionth the mass of the sun based on current observations.
However the real power of asteroseismology in this context is that it might enable us to
detect Hawking stars out there in the galaxy. The chance that our Sun contains a black hole is very,
very small. But as long as primordial black holes actually exist in the right mass range
then there’s a fair chance that at least some of the 100s of billions of stars in
the Milky Way may have caught one. If so, some of those stars should be in
the bloated phase of a Hawking star’s life. From Earth they would be indistinguishable
from certain rare types of giant-phase stars, at least from their colours and
sizes. But these bloated Hawking stars would vibrate differently to their lookalikes.
At that stage in their lives, a Hawking star is just a big ball of churning plasma with no
core. Its vibrational modes would be much simpler than other visually similar stars,
whose layered interiors lead to complex mixtures of vibrational modes. Now we
haven’t found candidates for this yet, but the data that might contain such signals
already exists in the latest release by ESO’s Gaia satellite. The team promised
to get right onto that as soon as its asteroseismology analysis is released.
Whether or not they end up finding Hawking stars we will learn something. If primordial
black holes exist some should end up in stars, so if Hawking stars are not detected then it gives us
another limit on the number of primordial black holes that might be out there. Perhaps we’ll be
able to rule these things out as the culprit behind dark matter. Or if a detection is made,
rule them in. And this is actually the stated point of this work—less about worrying if the Sun
harbors a black hole, and more about using stars as detectors of primordial black hole dark matter.
I would bet my life that the sun doesn’t contain a tiny black hole,
even if it's impossible to prove otherwise. But I wouldn’t be at all surprised if at
least some stars out there are shining just a bit brighter than they should be,
eaten from the inside out by a growing engine of warped space time.
Hey Everyone. We wanted to thank you for the amazing year we've had here at Space Time
and to remind you that we still have 15% off at our the merch store until the end of the December.
Just click the link in the description and use the code: PBS at checkout.
Have a great holiday season and new episodes will return on January 10th.