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.