Stars are some of the most beautiful and awe-inspiring objects in the universe.
Through violent thermonuclear reactions, they emit enormous amounts of light,
illuminating the night sky and heating up any nearby planets in the process.
Through mergers and powerful explosions, stars are also responsible for producing the heavy elements
that make up much of the matter we see around us. Astronomers and astrophysicists have come a
long way toward understanding how starsoperate, to the point where we can even predict how long
it will be before our Sun runs out of fuel and transforms into a Red Giant. But today,
we’re not just going to talk about the types of stars we’re familiar with. Instead,
we’re going to contemplate stars we’ve never seen before. Ones that astrophysicists say
could hypothetically exist— including ones that are quite literally made of vacuum.
I’m Alex McColgan, and you’re watching Astrum. Join me today as we dive into
the world of exotic stars. Some of these stars can mimic black holes and resemble dark energy,
which is why they have been the subject of many recent studies and even some sensational drama
in the world of physics. In this video, we’re going to explore all of it—from the
physical theories at the core of these ideas to the wildest speculations surrounding them.
First, we need to lay some groundwork and understand how stars work in general—starting
with the most important star of all: the Sun. Since the Sun is made of gas and not solid matter,
you couldn’t stand on its surface like you would on the Earth. The Sun’s gravity would just pull
you down toward the centre. So, why doesn’t the gas at the surface get pulled inwards too?
It turns out that there is a pressure pushing outward from the Sun’s core
that counterbalances this pull of gravity. The main source of this pressure is the radiation
created by fusion reactions in the core—the same radiation responsible for carrying light
and heat from the Sun to the Earth. Whether you think of radiation as individual photons
or waves in the electromagnetic field, its path from the core toward the outer layers
of the Sun will cause an outward push in all directions. This radiation pressure
wouldn’t be enough to stop you from falling into the Sun, but it’s just enough to stop
the Sun’s own gas (which is relatively light) from collapsing in on itself.
This balancing act between inward-pulling gravity and outward-pushing pressure is a
feature of pretty much all known types of stars. What differentiates different types of stars is
where that pressure comes from. For our Sun, the pressure comes from fusing hydrogen into
helium. When the Sun runs out of hydrogen in about 5 billion years, it will become a red giant star,
whose pressure comes from fusing helium into even heavier nuclei. Eventually, the Sun will also run
out of helium and any other light elements that could be used as fuel for nuclear fusion. At
that point, it will collapse into a small star known as a white dwarf. Since no more fusion is
occurring anymore, white dwarfs are sometimes called stellar remnants instead of stars.
But if there’s no more fusion, where does the pressure to support these
stellar remnants come from? It turns out that gravity pulls the gas of a
white dwarf inward until it’s so dense that a new type of pressure emerges,
known as electron degeneracy pressure. This is a purely quantum-mechanical effect, and it’s the
same effect that’s responsible for you not being able to pass your hand through solid objects:
electrons simply cannot get too close to each other without an extraordinary amount of energy.
But some stellar remnants are so massive that even electron degeneracy pressure isn’t strong
enough to counteract the pull of gravity. This is called the Chandrasekhar Limit:
it’s absolutely impossible to have a stable white dwarf any heavier than 1.4 times the mass of the
Sun. If a star has burned through all its nuclear fuel and its core is still heavier than the
Chandrasekhar Limit, that core will collapse in on itself until it becomes so dense that an even
stronger type of quantum pressure emerges, known as neutron degeneracy pressure, which - as you
may have guessed - prevents neutrons from getting too close to each other. A star that reaches this
state is called a neutron star, the smallest and densest known class of stellar objects.
So now you might be wondering—is there a maximum mass that a neutron star can have while still
being supported by neutron degeneracy pressure? And there is! This time, it’s about 2.2 times the
mass of the Sun. Any heavier than that, and the neutron star would keep collapsing under
the influence of its own gravity, until… well, actually, not until anything. As far as we know,
neutron degeneracy pressure is the strongest possible pressure that ordinary matter can exert,
so there’s nothing left to prevent a sufficiently heavy stellar remnant from
collapsing in on itself forever. This is the fate that might await some of the most
massive stars we’ve observed in the universe. From the perspective of an outside observer,
these stars would get so dense that all we’d be able to see is a black hole. But then scientists
started to wonder—what would happen to a star that wasn’t made of ordinary matter?
If exotic forms of matter could hypothetically produce even larger
pressures than neutron degeneracy pressure, then what new kinds of stars could exist?
You might think that we could just continue the pattern that we were starting to see earlier:
stars could get even smaller and even denser than neutron stars,
as long as they were supported by some even greater pressure. You might think that the
only limit on how small these stars could get would be the point of no return where
they would turn into black holes. That point of no return is when a star becomes smaller
than its own Schwarzschild Radius. This is almost true, but not quite. Instead,
Physicist Hans Adolf Buchdahl showed in 1959 that a star made of any “realistic” form of matter,
with positive energy density and pressure, can’t be smaller than 9/8 times its Schwarzschild
Radius. It might not sound like a big difference, but that factor of 9/8 means that any “realistic”
star has to be noticeably larger than a black hole, making it much easier to tell them apart.
That’s when physicists began asking about stars that Buchdahl would call…
less realistic. For example, what if a star was made of matter that allows for pressure
to be negative in some regions and positive in others? Then Buchdahl’s limit wouldn’t apply,
and that star could be less than 9/8 times its Schwarzschild Radius—in fact,
it could be arbitrarily close to the size of a black hole with the same mass! As a side note,
theoretical stars like these that could mimic the appearance of black holes came to be quite
a nuisance for astrophysicists, since now every time they discovered something they
thought was a black hole, they couldn’t just say they discovered a black hole;
they had to qualify that it’s either a black hole or an extremely compact object mimicking
a black hole, even though in their hearts most of them are quite convinced that it’s a black hole.
But to the main point—what would these exotic stars with negative pressures even look like?
The answer turns out to be even weirder than you might have guessed. Tolman, Oppenheimer,
and Volkov derived an equation in the 1930’s that describes how the
pressure of a stable star changes as you go outward from its centre. These days,
it’s just called the TOV equation for short. If the centre of a star has a significant
negative pressure, which would exert some sort of attractive pull instead of a push,
the TOV equation says that this star can only be stable and in balance with its own gravity,
if its pressure does something really wonky. First, the pressure has to blow up to negative
infinity somewhere in the star. Then, it has to flip sign to positive infinity and decrease
outwards from there until it reaches zero pressure, marking the star’s outer surface.
The more negative the central pressure of the star, the sooner it will go to infinity.
On the other hand, the less negative the central pressure, the farther out that infinite-pressure
surface gets pushed. It turns out that there’s an interesting limit where the pressure stays
finite right up until the very edge of the star. That happens when the pressure at the centre
is exactly equal and opposite to the star’s energy density there. In this special case,
the TOV equations say that the pressure and energy density remain constant, equal,
and opposite throughout the entire volume of the star, right up until its surface.
If you’ve seen some of our other episodes, this might ring a bell or two. A constant positive
energy density, paired with a constant but negative pressure—that doesn’t sound
like any star we’ve seen before, but it’s starting to sound more like a mysterious
component of our universe: Dark Energy. And since the pressure and energy density
in a gravastar are equal in magnitude, it’s not just any kind of dark energy:
those are the exact properties of vacuum energy. That’s why this special exotic star
came to be known as a “gravitational vacuum star,” or gravastar for short.
Inside of a gravastar, there is precisely zero gravitational force,
because the positive energy and negative pressure respectively exert a gravitational
pull and push that cancel each other out. And since the pressure in gravastars is constant,
there is no pressure gradient to exert any sort of push in one direction or another,
either. This means that if you found yourself inside of a gravastar, you would neither be pulled
in nor pushed out: you could float freely, just as though you were in a real vacuum.
Another curious feature of stars with negative pressures is that the closer you get to the
special limit of a gravastar,which if you recall is when the infinite-pressure surface
gets pushed out to the edge of the star, the more their size approaches that of a
black hole with the same mass. This raises two key questions that have recently gotten a lot
of attention from physicists: First, how can we tell whether the extremely compact
objects astronomers have observed in merger events and at the centres of
large galaxies are black holes or just exotic stars with nearly the same size ? And second,
if gravastars are real and scattered throughout the entire universe, could they be the dark
energy that’s been making the universe accelerate for the past five billion years?
To answer the first question, physicists rely on the main distinction between black holes and
other compact objects: the existence of an event horizon. You see, the defining feature of a black
hole is that nothing that falls inside or even touches the surface of a black hole can ever come
back out. On the other hand, if an exotic star is even slightly larger than its Schwarzschild
Radius, it would not have an event horizon, and it would potentially be able to emit information
outwards in the form of light. Not only that, but objects without event horizons could have
different signatures in their gravitational wave emissions than those of black holes during a
merger. This would allow us to tell them apart by measuring and analyzing those gravitational
waves (using observatories like LIGO). But even with this battery of tests, it turns out that
sufficiently small and dense stars can still be difficult to distinguish from black holes. So far,
the best we can do is to say that the radius of the black hole candidates we see in the
universe is at least approximately equal to their Schwarzschild Radius—in some cases, that’s just a
rough estimate, but in others, we’ve calculated that radius to within 40 decimal places.
As for the second question about dark energy, that’s where things get really heated. In early
2023, an analysis led by physicists at the University of Hawaii found that astronomical
observations of supermassive black holes showed them to be growing in mass over time, at exactly
the same rate that dark energy would be expected to grow over time due to the expansion of the
universe. They came to the inevitable conclusion that black holes must be made of dark energy,
and that they are really more like gravastars than ordinary black holes with infinite-density
singularities at their centers. This conclusion was fiercely contested! Astronomers came out
arguing that the analysis of the observational data wasn’t quite right, and theoretical
physicists came out arguing that the expansion of the universe should have nothing to do with
the growth of black holes in any case. So while the idea that dark energy could exist in the form
of gravastars sounds exciting, it looks like the scientific community isn’t going to be convinced
unless it’s simultaneously supported by better data and more compelling theoretical models.
But where do you fall in this debate? If it walks like a black hole and talks like a black hole,
should we just call it a black hole? Or should we consider the possibility that
black holes could really be secret troves of dark energy? Let us know
your thoughts and questions in the comments below.
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