turbulent past and predict what violence might be on the cards in its future.
We've been observing Andromeda for more
than a thousand years. It was first
spotted in 964 AD by Persian astronomer
Abdul Rayman, who described what he saw
as a nebulous smear. We didn't really
get a better look than that until nearly
700 years later when on the 15th of
December 1612, Simon Marius became the
first ever person to observe the
Andromeda galaxy through a telescope.
Slowly but surely, more details of this
seemingly fuzzy nebula emerged. Charles
Messia officially classified it M31 in
1764.
But one of the most important
breakthroughs came 100 years later. The
third Earl of Ross, William Parsons, was
a keen yet eccentric amateur astronomer.
He had the massive 72in Leviathan of
Parson's town telescope built in the
garden of his Burr Castle in Ireland. It
remained the largest telescope in the
world for some 70 years. Parsons used
this beast of a scope to spot the spiral
structure of a nebula named M51
or as it's known to most of us the
Whirlpool Galaxy. It was the first time
a spiral structure had ever been seen in
a nebula and the discovery sparked a
flurry of interest from astronomers of
the time. As they turned their attention
to other nebula, it resulted in the
first ever photograph of the Andromeda
galaxy taken by amateur British
astronomer Isaac Roberts in 1888.
And that photo, as you can see, shows a
clear spiral structure.
Even with this photographic evidence,
scientists weren't sure what Andromeda
was. But they classified it alongside
thousands of other spiral nebula that
all had something else in common. They
didn't move like the rest of the stars
in the night sky do. This realization
inspired one of the major debates among
astronomers in the late 19th century.
Were spiral nebula part of our own Milky
Way galaxy? or could there possibly be
other island universes beyond?
In 1925, Edwin Hubble resolved the
issue. Hubble identified two sephied
variables in the Andromeda Nebula known
as standard candles. These variable
stars can be used to measure galactic
distances thanks to the correlation
between their period and luminosity.
Hubble calculated that these seafoods
were about 1 million lighty years away.
Far too far for them to lie within the
bounds of the Milky Way, which at the
time was estimated to be no wider than
300,000 light-years across. With that
single realization, Hubble proved
Andromeda was a galaxy in its own right,
and the universe suddenly became a lot
bigger than anyone had previously
thought. It must have been annoying to
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Since Hubble's discovery that the
universe extends beyond the Milky Way,
we've discovered countless galaxies, but
we still share a special relationship
with the one that opened our eyes to the
vastness of space. our cosmological
nextoor neighbor. Andromeda has arguably
taught us more about how galaxies work
than even our own. You see, it's tricky
to study galactic structure in the Milky
Way because we're inside it. There's
quite a lot of stuff to get in the way.
Interstellar dust can scatter light so
that objects look redder than they
should. It's tricky to measure accurate
distances and sometimes details are
blocked by noise in the foreground.
Andromeda or M31, however, doesn't have
many of these issues. At around 2.5
million lighty years away, it's a place
where we can study galaxy formation and
evolution relatively unimpeded.
But to make the best use of this
laboratory, we need clear images.
Experts and amateurs, groundbased and
space telescopes have all taken a look
at Andromeda from a number of different
angles, resolutions, and wavelengths.
But on the 16th of January, 2025, the
Hubble Space Telescope released an image
that put all our previous efforts to
shame. After 10 painstaking years of
observing Andromeda, it has created the
largest photo mosaic ever assembled.
Taken over 1,000 Hubble orbits, it
encompasses more than 600 snapshots and
2.5 billion pixels. The result is the
most in-depth and complete view of
Andromeda we have ever seen. So without
further ado, let's take a look.
This image shows Andromeda nearly edge
on, tilted 77° relative to our view from
Earth. In it, you can see the northern
region to the right and the southern
region to the left. Astronomers predict
that Andromeda could be home to as many
as 1 trillion stars, but most are too
dim to be detected even by a powerful
telescope like Hubble. Nevertheless, it
was able to resolve a whopping 200
million stars, making it the most
detailed image ever taken of any galaxy
anywhere in the universe. With such a
stunning census of our closest neighbor,
scientists have been able to reveal more
about its history, how it compares to
our own cosmic home, and how it may
evolve in the future.
We know Andromeda is a spiral galaxy
measuring around 150,000 lightyears
across with arms that extend out from
its nucleus across the sky. Despite this
classification, many of its features are
pretty unusual. And thanks to this new
image, we are starting to understand
why.
Created in 2015 with the conclusion of
the FAT program or pancromatic Hubble
Andromeda Treasury, this image is highly
zoomable, and so it should be. Hubble
spent more than three years
photographing a third of Andromeda's
star forming disc. These images were
taken across six spectral bands ranging
from near ultraviolet to near infrared.
If you take a look at the outer region,
you can see an elliptical ring of star
formation found roughly 10 kiloparex
from Andromeda's nucleus. These newborn
stars are bluer and therefore hotter
than most other stars in this region.
They are also metalrich, meaning they
contain elements heavier than helium or
hydrogen. The more you zoom in, the more
of these individual stars appear. There
are so many. The Hubble team say they
look like grains of sand on a beach. The
resolution is truly that good.
This ring of star formation puzzled
astronomers. And it wasn't the only
thing. Using images taken by the
infrared array camera on the Spitzer
Space Telescope in 2006, researchers
found a second inner dust ring situated
about half a kiloparc from Andromeda's
center. This one was hiding in plain
sight. Initially, scientists had assumed
it was a mini spiral related to dense
regions of overlapping stellar orbits.
Two rings meant that what scientists
were seeing couldn't just be an anomaly.
But where did they come from? These
rings were about to unlock Andromeda's
past.
For decades, scientists had looked at
the galaxy and found little evidence to
suggest a violent history. But these
rings showed a different story. As
astronomer and author of the 2006 study,
David Block puts it, "These dust rings
are like ripples in a pond." And where
do ripples come from? A point of impact.
That's right. Scientists concluded the
rings were the remnants of a collision
between Andromeda and one of its
companion galaxies around 210 million
years ago. Just like a stone being
dropped into a pond, the impact between
Andromeda and this other galaxy produced
shock waves of dust that radiated
through the disc, leaving behind two
ghostly trails we can still observe
today.
Block and his colleagues blame dwarf
galaxy Messia 32 for this impact. Today
it appears as a bright fuzzy haze of
stars found just 5 kiloparex from
Andromeda's center. When they ran
numerical simulations to recreate the
impact, the authors found that a head-on
collision was able to reproduce the
rings we see today. There is a
conspicuous hole in the outer ring that
was formed when it interlaced with
Andromeda's spiral. And the simulations
were even able to recreate this feature.
Since that discovery in 2006, many
thought that we had revealed the extent
of our galactic neighbors trauma. But
thanks to Hubble, Andromeda was about to
give up more of its secrets.
You see, the 2025 image of Andromeda was
made from the combined efforts of not
only the fat survey, but also the fast
survey. This extended the original
program to photograph Andromeda's
southern region and ran from 2021 to
2024. Fast added a further 90 million
stars to the census and expanded its
scope to cover 2/3 of the galaxy's star
forming disc. The need for this southern
focus survey was born out of the
realization that Andromeda's polar
regions are almost nothing alike. The
southern region appears to be far more
disturbed than the north and therefore
carries more evidence of Andromeda's
violent history, and there's plenty of
battle scars to find. You can't really
make it out in the Hubble image because
it's too dim, but the giant southern
stream is a mammoth feature of Andromeda
that spans around 150 kiloparex from the
galaxy's southern disc into the halo.
You can think of it as a stream of
debris from a galaxy that was stripped
of its stars by Andromeda's gravity. The
GSS is dissipationless,
meaning the stars in it are cold and
hold no significant amount of gas. Their
paths are still dictated by
gravitational forces, but they don't
radiate heat or collide with other stars
in the stream. It's like a group of
marbles rolling down separate
frictionless tracks. The GSS along with
M32 and the inner halo are three of the
most prominent metalrich features found
in Andromeda. For some astronomers,
their shared metallicity suggests they
are all products of another major
collision event in Andromeda's history,
even larger than the one that produced
the rings. But this event has a familiar
antagonist.
A study in 2018 sought to investigate
this bigger collision and put the
spotlight back on M32.
Well, M32's ancestor. Cosmological
simulations named Andromeda's most
likely victim as an earlier version of
M32 named M32P.
In its heyday, this ancient metalrich
galaxy had a mass of 25 billion suns and
merged with Andromeda between five and
two billion years ago. Its gas would
have been vacuumed up by the greater
force of Andromeda's gravity, triggering
a burst of star formation, which created
about a fifth of the stars we see in our
galactic neighbor today. And what's
more, we think we know which ones they
are. Andromeda contains several billion
stars that are of intermediate age. And
astronomers had been struggling for
years to explain why. They expected
Andromeda's stellar population to look
more like that of our Milky Way.
Distinct regions of much older stars in
the bulge and halo and younger stars in
the thin disc. However, if these
middle-aged stars formed during the
merger with M32P,
then we finally have an answer for where
Andromeda's second wind of star
formation came from. For more proof, we
just have to look at M32 in Hubble's new
image. If we go with the theory, then
what we are seeing is the stripped core
of M32P.
And that description seems to fit. M32
has a central black hole with a
relatively low mass between 1.8 and 5
million solar masses. And astronomers
can use this to work out how big its
central bulge might have been before it
merged with Andromeda. These
calculations show that M32P's central
bulge was small, which explains why the
galaxy did not sink completely into the
center of Andromeda during the merger.
Instead, the stripped core was able to
survive on the outskirts of the galaxy,
where it remains to this day. As I said,
this is still a theory, but it appears
to answer a lot of questions about
Andromeda's troubled past and unites
several of its most unusual features
together. But things might still be more
complicated than that.
Andromeda has 36 different dwarf
galaxies surrounding it. And any one of
these galaxies may have interacted with
it in the past. It's only now that we
have such a highresolution view of our
neighbor, thanks Hubble, that we can
start to look for signs of smallcale
interactions with its satellites. Who
knows what we'll find?
But it's not all about the past. This
spectacular image can also tell us about
the future. our future.
As the two biggest gravitationally bound
members of the local group, Andromeda
and the Milky Way are often thought to
be the most likely candidates for a
collision. It was estimated in 2012 that
Andromeda is falling towards our home
galaxy at a rate of 110 km/s
and will hit in about 4 billion years.
Of course, this is probably not a threat
that the human race will ever have to
worry about. But it's fascinating to
think about how these two beloved
galaxies could change if a collision
ever comes to light. But before you
start imagining what Milkometer would
look like, it's important to highlight
that this is still a debate. Thanks to
the new image from Hubble, we have
reason to believe that our galaxy could
live to fight another day. In a paper
published this year, Til Savala from the
University of Helsinki and his
colleagues used the Hubble data on
Andromeda to refine the models of the
local group. Specifically, the FAT
survey measured distances to 55 seiids
within Andromeda. These combined with
the orbit of other local group galaxies
like the large melanic cloud showed that
the fate of our galaxy was much more
uncertain with only a 50% chance of a
merger with Andromeda in the next 10
billion years. Predicting what will
happen to our galactic neighborhood in
billions of years is no mean feat, but
it will be much easier now that we can
see its current state so clearly.
The 10-year endeavor of the fat and fast
programs has now come to an end. But the
work doesn't stop there. With enough
detail to cover 75 8K ultra
highdefinition screens, scientists are
nowhere near done finding the secrets
hidden within this image. And it's just
one of a seemingly neverending stream of
clues. Hubble may have turned its
attention somewhere else for now, but in
June this year, more images were
released from the Shandra X-ray
Observatory, showing Andromeda's super
massive black hole in unprecedented
detail. With each observation adding
ever more detail, the story of Andromeda
will become even more complete over the
next few years. And so far, it's shaping
up to be a good one. I can't wait to see
what Hubble's scientists piece together
next. But in the meantime, I hope this
video has brought you closer to
Andromeda. After all, we are neighbors
and we could be getting a lot closer in
a few billion years.
The link is below. Thanks so much for
considering it. I'll see you next time.