astrophysics's biggest mysteries, the formation of super massive black holes.
Since the James Webb Space Telescope
launched on the 25th of December, 2021,
it has completed over 32,000 hours of
observation and sent back more than 600
terab of data. That's enough to fill the
memory of more than 2,000 phones.
And within these tens of thousands of
images, one type of object keeps
cropping up. These dots were first seen
in the Aigga and Fresco surveys,
observational studies hunting for
galaxies at extreme distances. Their
purpose was to examine the very
beginnings of our universe some 500
million years to 1 billion years
following the Big Bang. As James Webb
scanned the skies, nearly every photo it
sent back of this era revealed the same
strange object. little red dots
scattered across the early cosmos. By
600 million years after the Big Bang, it
seems our universe was filled with them.
But 1.5 billion years later, they
disappeared completely.
This instantly made them one of the
biggest mysteries in the universe. But
what do we know about them so far? First
up, they are very compact, usually no
more than 500 light-years across, which
is about 200 times smaller than our own
galaxy. They burn unusually bright for
their size, and are very red, which is
why they've evaded detection for so
long. Until now, we literally had no way
to see them. Hubble was calibrated for
shorter wavelengths than we are seeing
from these objects and other telescopes
didn't have the power needed to look
that far back in time. James Web is the
only telescope specifically designed to
see the mid infrared wavelengths
characteristic of these little red dots.
By combining data from its near infrared
camera and mid-infrared instrument,
astronomers caught a glimpse of these
ancient objects for the first time.
Though their properties wouldn't be
described and categorized until March
2024 in a breakthrough paper written by
Yorit Mati from the Institute of Science
and Technology, Austria.
Now, as many of you will already know,
the older the object we are looking at,
the redder it appears to us. It's
classic red shift in action. Since the
universe is expanding, the fabric of
space and all the light passing through
it gets stretched too. The wavelengths
of light get physically elongated on
their journey and the longer they
travel, the more they get stretched, so
the redder they look. But when
astronomers broke the little red dots
light apart into a spectrum, they found
the redness runs deeper than that. The
red shift alone couldn't account for
just how red they appeared. The
researchers first instinct was that this
may be caused by dust. Dust particles
are much better at scattering shorter,
bluer wavelengths of light than longer,
redder ones, making the object appear
red. The same thing that happens to
sunlight hitting Earth's atmosphere at
sunset, which is why the sun looks red
in the evening. But to be sure, the
scientists enlisted the help of another
instrument, the near infrared
spectrograph, and what they saw just
confused them even more. The near spec
aboard the James Web takes the light
from a distant object and splits it into
its component wavelengths. Different
elements absorb light at specific
wavelengths, leaving gaps in the
spectrum like a fingerprint. So if there
is something like a dust cloud between
the telescope and the light source, not
all the light will make it to the
instrument. By looking at which
wavelengths are missing from the
reading, we can figure out what the dust
cloud is made of. Researchers noticed
that these little red dots were emitting
a lot of blue and UV light. Lots of red
and infrared light, but not much in the
middle. The result is a highly unusual
V-shaped spectral energy distribution.
Nothing we knew of in the distant
universe was associated with such a
spectral signature.
But we did get one clue. At 364.6
6 nanome. The spectrum shoots up
sharply. For some reason, light with
wavelengths shorter than this was being
readily absorbed and longer wavelength
light was just passing through
unobstructed. This is a well-known
signature and it even has its own name,
a bomber break. And when we see this in
a galaxy, there's only one explanation
for it. Young hot stars. Lots of them.
Scientists thought they had it. Little
red dots must be early starburst
galaxies. It certainly would have been a
very elegant solution, but that's rarely
how these things go.
So, theory number one is starburst
galaxies. Now, this starburst galaxy
theory had a lot going for it. Hot young
stars are rich in hydrogen. We'll come
back to the chemistry later, but this
could explain the strong bomber break. A
dense stellar population would explain
the brightness. And if these galaxies
were packed with dust, as starburst
galaxies often are, that would explain
the redness, too. It was almost all said
and done, except for one annoying
detail. The maths just didn't work.
Researchers soon realized they were
running up against two major problems.
The first was how much dust they would
need. Dust in galaxies comes from one
main source, stars. And there's a very
wellestablished relationship between how
many stars a galaxy contains and how
much dust these stars can produce. In
late 2024, Caitlyn Casey and her team at
the University of Texas in Austin
calculated their expected stellar
population based on the optical light
detected from the little red dots. But
once they applied the star to dust
ratio, their stomachs dropped. Their
calculations only yielded 1% of the dust
needed to explain the redness they were
seeing. And now a shortfall of two
orders of magnitude could only mean one
of two things. Either the
wellestablished dust formation models
are wrong or the main light source of
these little red dots wasn't stars.
The second problem was the brightness.
To explain the overall brightness of
little red dots using just stars, you'd
need an impossibly high density of stars
in a very compact region of space.
They'd only be a few astronomical units
apart, but this close together, stars
would be colliding and merging all the
time, and the gravitational dynamics
would be far too unstable. What's more,
these little red dots don't look like
normal galaxies. Usually, we can see
some kind of internal structure. But
even though James Webb has the highest
resolution of any telescope we've ever
built, the little red dots just appear
as points of light. Single dots in the
night sky, like stars appear to the
naked eye. So either the little red dots
are really tiny or their light is so
dominated by a single central source
that it drowns out any other surrounding
signature that might exist. Which brings
us to the second theory. What if little
red dots aren't star-filled galaxies at
all? What else could explain their
properties?
spotted by the James Web Space Telescope? Well, at the center of most
galaxies, you can find a super massive
black hole surrounded by an accretion
disc of gas and dust. As it spirals
inward, the gas is compressed and heated
to over 12 million°,
and it emits a powerful glow. Active
black holes are some of the brightest
objects in the universe. Could they be
behind little red dots?
Surrounding the accretion disc of a
black hole is a donut-shaped ring of gas
called a Taurus. If you're looking at
the Taurus side on, that dust sits
between you and the bright black hole,
absorbing shorter wavelengths and
letting longer red and infrared ones
through. Sound familiar? little red dots
as black holes made sense. It would also
explain the brightness, point source
morphology, and reddish color. To top it
all off, their emissions show broad
balmer lines, which suggests there is a
lot of gas spinning at thousands of
kilome/s around something central.
Orbiting gas moves both towards and away
from us at extreme speeds. The light
moving away from us is redshifted while
the light moving towards us is
blueshifted.
This exaggerates the spectral lines in
each direction broadening their profile.
The faster the orbital velocity of the
gas, the wider that broadening becomes.
And this is exactly what we would expect
from a massive black hole feeding on
lots of gas. So far, things were looking
good for the black hole theory. That was
until researchers noticed something
vital was missing. You see, as far as we
know, active black holes always emit
X-rays from their accretion discs. But
when astronomers measured the little red
dots, they found no trace of X-rays at
all. This was a devastating blow to a
promising theory. As if that weren't
enough, there was one more problem. The
black holes seemed too big for the
galaxies they were in.
In the local universe, black holes are
usually around 0.1% of the mass of the
galaxy around them. This is such a
consistently observed relationship. We
think galaxies and their black holes
must co-evolve together using some kind
of feedback loop mechanism that keeps
them in tight check with each other. But
when researchers started looking at
little red dots, they noticed they had
much higher black hole to galaxy mass
ratios than we're used to, closer to
10%. It was almost as though the black
hole had somehow grown to full size
before the surrounding galaxy had the
chance to catch up. Both the star-based
galaxy and active black hole theories
had their merits, but neither could
fully explain what was going on with
these little red dots. Astrophysicist
Fabio Pakuki at the Harvard Smithsonian
Center for Astrophysics explained the
dilemma perfectly. If they little red
dots contain black holes, those black
holes are enormous for such small
galaxies. But if they only contain
stars, the galaxies are too compact to
contain all of them, reaching stellar
densities that are unthinkable.
So if little red dots are not young
galaxies of hot stars, and they're not
black holes, what could they be? Is it
possible we've stumbled upon an entirely
new type of cosmic object, one unlike
anything we've ever seen before?
This was exactly the question. Anna
Degraphth and her team set out to
answer. Degraph is an astrophysicist at
the Center for Astrophysics Harvard and
Smithsonian with a particular interest
in the most extreme objects in the early
universe, the things that sit furthest
outside the expected distributions. So,
she designed a survey to go after them
directly. The red unknowns bright
infrared extragalactic or rubies survey
was a deliberate hunt for the reddest,
brightest, rarest objects in the sky.
During 2024, the graph's team spent
nearly 60 hours of web time obtaining
spectra for 300 red sources, of which
they think 30 to 50 were the mysterious
little red dots. This made it the
largest ever spectroscopic sample of
these dots at its time of publication in
March 2025. The objects ranged in age
from 650 million to 1.5 billion years
after the Big Bang. But while she hoped
her survey would bring calm clarity and
definitive answers to the field, it did
of course the exact opposite.
Rubies revealed that these bright red
objects are far more diverse than anyone
initially imagined. Even though they
seemed similar when imaged,
spectroscopic data showed they were
actually several different types of
objects, such as dusty galaxies still
forming stars, other galaxies that may
have stopped forming stars surprisingly
early, and active black holes powering
galactic nuclei. It seemed no matter
which side of the starburst galaxy
versus black hole debate you were on,
you were harshly right. But within that
sample, there was a subset that didn't
fit any of these categories. Objects
with the V-shaped spectra, the broad
emission lines, the point source
morphology, all the signatures that had
resisted explanation from the beginning.
And then in July 2024, the team stumbled
upon a little red dot 11.9 billion
lighty years away with a spectrum so
extreme it stopped them in their tracks.
They called it the cliff. To understand
why the cliff was so important, you need
to understand what the bomber breaks we
saw earlier are really telling you.
Think of electrons around an atom like
fixed rungs on a ladder. They can only
exist at specific energy levels. When a
photon comes along carrying just the
right amount of energy to push an
electron up a rung, that photon gets
absorbed and the electron gets excited.
Every element has a unique electron
configuration. So each element absorbs
photons at unique wavelengths. This is
why spectroscopy works as such a precise
fingerprinting tool. For example, for
hydrogen, 364.6
nanome is the critical wavelength. It
corresponds to the exact energy required
to liberate an electron from the second
energy level completely. Longer
wavelengths are lower energy. Shorter
wavelengths are higher energy. So any
photon with a wavelength shorter or
equal to 364.6
nanome has enough energy to ionize the
hydrogen from that level and will be
absorbed. Any photon with a longer
wavelength isn't energetic enough to
bring about this change. That's why the
presence of a bomber break tells you
that there's hydrogen between a light
source and your telescope.
Usually a messy complex object like a
galaxy exhibits a smeared gradual break
because stars of different temperatures
and densities are sending their lights
through gases of varying densities. The
sharper the break, the more pure the
hydrogen atmosphere you're dealing with.
But a near vertical break like the cliff
is twice as strong as that of any
ancient cosmic body previously observed.
which is why it's so intriguing.
The extreme properties of the cliff
forced us to go back to the drawing
board and come up with entirely new
models. The graph admitted, "It seems
this little red dot was sending mixed
signals. On one hand, spectra analysis
appeared to suggest the object behind
the cliff was a super massive black
hole. On the other hand, the dramatic
balmer break indicated a huge amount of
relatively homogeneous hydrogen gas
around it like you'd expect of a very
young star. Could this be an entirely
new type of object? One that was somehow
both black hole and star at once?
Degraphth thought so. She dubbed the
object a black hole star.
A black hole star. unsurprisingly has
characteristics of both black holes and
stars. They're shrouded in giant spheres
of hot, dense gas, which makes them look
like the atmospheres of traditional
fusionpowered stars. But instead of
running on fusion, they're powered by
super massive black holes, accreating
matter at breakneck speeds, converting
it to energy and releasing light as a
result.
But where does all that hydrogen come
from? And if there really was a black
hole at the center, why were there no
X-rays? Although perhaps the biggest
question is how are they made?
The universe seems to be made up of an
invisible infrastructure of dark matter,
spinning scaffolds called halos. You can
think of them as being embedded in an
interconnected web, one that looks
surprisingly similar to the networks of
neurons in your brain. We can't see
these scaffolds, but we can map them
with gravitational lensing, and they
seem to indicate the parts of the
universe where galaxies form. Most dark
matter halos spin relatively fast, which
causes the gas and matter inside them to
spread outward, like the swings on a
carnival ride, stretching further out
the faster they spin. But a very small
fraction of halos spin extremely slowly.
And in those low spin halos, the gas
doesn't spread. It stays dense, compact,
and concentrated. We think newborn black
hole stars could form in places like
these where enough hydrogen is still
present to give us the properties we see
in little red dots. As for the missing
X-rays, they're not missing at all. They
are trapped. In a black hole star, the
surrounding hydrogen envelope would be
so dense that even high energy X-rays
couldn't punch through it. they would
get absorbed by the gas and remitted as
thermal energy and the red optical light
we see instead.
Okay, but what about all the other
unanswered questions? The strange
V-shaped spectral energy distributions
or the missing dust and the black holes
too big for their own galaxies. How does
the black hole star tackle these
challenges?
Lucky for us, astronomer Rohan Naidu of
MIT and his team have some ideas.
In 2025, they were studying an object
they also referred to as a black hole
star and found it to have a thick
hydrogen gas envelope about 40
astronomical units wide plus one of the
strongest bomber breaks ever witnessed
at any red shift. Naidu argued the black
hole star is a two component object. A
black hole star which produces broad
emission lines, a strong balma break,
point source compactness of light and
strong red emissions and a star forming
host galaxy that produces narrow
emission lines in the UV spectrum. Taken
together, this compact object would
produce a V-shaped spectral energy
distribution just like what we see from
little red dots. The low energy red
emissions from the black hole star and
high energy UV emissions from the
surrounding host galaxy superimpose to
create one unusual spectrum. The object
Naidu and his team were studying wasn't
a little red dot itself. But by
examining it they concluded that black
hole stars when embedded in brighter
host galaxies produce the little red dot
properties we've seen. The remaining
issues of dust and overly massive black
holes go hand in hand. Under the black
hole star model, little red dots
wouldn't have any dust at all. They're
red due to the opacity and the
surrounding dense gas. Since there's no
dust to worry about, there's no shortage
to account for. And the over massive
black hole problem is also connected to
this dense cloud. The standard way to
calculate a black hole's mass is to
measure how broad its emission lines
are. Broader lines mean faster moving
gas, which means a more massive black
hole. But in a black hole star, photons
don't travel cleanly outward. They
bounce repeatedly through the thick gas
envelope. With every collision, their
wavelength shifts slightly. The
cumulative effect produces emission
lines that appear broader than they
should be, leading scientists to
overestimate their mass. When
researchers corrected for this, the
estimated black hole masses dropped by
orders of magnitude, bringing them more
in line with the expected 0.1% galaxy
mass we see in our cosmic neighborhood.
Black hole stars sound like something
out of science fiction, but they
elegantly solve the biggest hurdles
we've encountered with the little red
dots. And the craziest part of all this
is that they were predicted 16 years ago
by theoretical astrophysicist Mitchell
Begelman from the University of
Colorado. In a 2008 paper, Begelman
proposed something called quasy stars.
His model described a black hole forming
from a stellar remnant or small seed
inside a dense gas cloud. Then rather
than the surrounding envelope
dispersing, it would stay bound. The
black hole sits at the core feeding
while the outer envelope glows not from
nuclear fusion like a normal star but
purely from the energy of the black hole
consuming gas at its center. From the
outside it would look like a single
enormous star but inside it's actually a
super massive black hole. Hence black
hole star or as he called it a quasi
star. Begelman's model predicted that a
black hole in this configuration could
grow at extraordinary rates, reaching
thousands of solar masses in just a few
million years. While the envelope slowly
cools, eventually it hits a temperature
floor of around 4,000 Kelvin. And at
that point, radiation pressure wins. The
envelope gets blown away. The quasi star
phase ends, and what remains is a naked
black hole. Begelman argued that rather
than a type of object, quasi stars could
be a brief phase early in a black hole's
life lasting only a few million years.
He even suggested that if a black hole
were later to encounter another episode
of extremely high gas inflow, similar
conditions could theoretically arise
again, raising the intriguing
possibility that some black holes might
pass through quasi starlike phases more
than once across their lifetimes. This
could explain why the little red dots
vanish from sight 1.5 billion years
after the Big Bang. Perhaps the perfect
conditions for high gas inflow are just
not satisfied this late in the
universe's lifetime. Could we be
confirming observationally what
Beagelman has already known for close to
two decades?
It probably goes without saying, but
despite the black hole star
breakthrough, there is still a lot of
uncertainty in this field. papers are
being published faster than researchers
can read them with hundreds coming out
in just the last 2 years alone. For
right now, the black hole star
interpretation seems like the best fit
to the data we currently have, but it
hasn't been confirmed yet. There are
still plenty of open questions that the
theory doesn't cleanly answer. For
instance, how common is the black hole
star phase? How long does it typically
last? What triggers it? We don't know
exactly how the envelope forms or what
determines when it disperses. The life
cycle of a black hole star, if that's
what these objects are, is still
unmapped territory. But what makes
little red dots genuinely significant
beyond the immediate mystery is what
they might represent in the larger story
of how the universe came to look the way
it does. One of the deepest unsolved
mysteries in astrophysics is the origin
of super massive black holes. We know
they exist at the center of almost every
large galaxy, including our own. We know
some of them were already billions of
solar masses just a few hundred million
years after the Big Bang, which is just
too fast to seem possible. The
conventional pathway to black hole
formation predicts objects should be
about 10 to 100 solar masses by that
point in time. The black hole star model
offers a potential way out of this
paradox. And it works on two fronts.
Firstly, as we discussed, the broadening
we see in the emission lines could be
exaggerated due to photon scattering,
making the black hole seem more massive
than it is by a factor of about 100 to
300. And secondly, super massive black
holes might not be restricted by the
speed limit we once thought. See, black
holes have a natural ceiling on how fast
they can feed, called the Edington
limit. It's set by the pressure of their
own radiation pushing back against
infalling gas. But dense enough
surroundings can push that limit higher
than initial predictions suggest. The
fact that black hole stars are
surrounded by dense hydrogen atmospheres
makes them exactly the kind of
environment that could allow black holes
to bypass their Endington limit
entirely. Since the limit shifts from
being calculated from the black hole
mass alone to being calculated on the
entire envelope's mass, which is much
larger, the findings surrounding black
hole stars indicate super massive black
holes might be smaller than expected and
able to grow faster than we thought. A
possibility that excites scientists
working with their growth models since
it seems to ease the unresolved tension
slightly. And an even bigger question is
where did these black holes come from so
early in the universe? So far, the only
pathway we've directly observed is
through stellar collapse, which takes
billions of years. But these little red
dots already existed 600 million years
in and are far too massive to have grown
from stellar mass seeds in that amount
of time. So what happened? Well, in
theory, massive clouds of primordial gas
in the early universe under the right
conditions could collapse directly into
a black hole without ever forming a star
at all. These direct collapse black
holes are a wellestablished theoretical
concept, but we've never actually seen
observational evidence of it. Could the
little red dots be the first sightings
of such a phenomenon? Only time will
tell. For now, the focus of little red
dot research is on building out the
basics. Measuring the temperatures,
luminosities, and surface
characteristics of little red dots with
enough precision to construct something
like the life cycle chart we have for
ordinary stars. We've never had one for
black holes, and little red dots might
finally make it possible. Ultimately,
science is the pursuit of truth. If one
new discovery calls old models into
question, it's worth throwing the old
playbook out, no matter the mountain of
work it creates, if it means getting
closer to understanding the true nature
of our universe.
clicks. Thanks so much for considering
it. I'll see you next time.