﻿We do not know the Milky Way Galaxy.
Yes, even though we are in it.
Before you think that I’m talking about niche details about stellar formation,
how galaxies evolve,
or things like that – I’m not.
Even some of the basics about the Milky Way are mysteries to us,
like; how many stars are here?
What is its basic shape?
Specifically, does its spiral have 2 arms or 4?
Although those might seem like simple details, we don’t actually know them.
“Number of stars” estimates range from 100 billion to as high as 400 billion,
depending on what estimates are being used.
Scientists have gone back and forth between 2 or 4 arms over the last two decades.
(Currently, we’re leaning towards 4).
This makes sense. It’s naturally harder to look at a thing that you’re inside –
it’s easier to look out your window at the house across the street than it is
to look at your own house from that same position.
But here’s a fun one for you:
How many galaxies are in the Milky Way galaxy?
According to ESA’s Gaia space telescope,
which has just finished its 10 year mission,
the answer is not just 1.
Gaia has been mapping the stars of the Milky Way,
performing 3 trillion observations.
And now, as it takes its final look at our galaxy, the data is being evaluated.
30,000 stars are moving a little strangely.
They are going the wrong way.
They are originally not from around here.
I’m Alex McColgan, and you are watching Astrum.
Join me today on a deep-dive into Gaia’s data to answer:
exactly how many Galaxies did the Milky Way eat?
As I already alluded to, looking at our own galaxy is surprisingly hard. I’ve mentioned
before in a previous video how all of the stars, stellar clouds and gas can actually get in the
way of what we’re trying to look at. This gets particularly bad towards the centre of the Milky
Way’s plane where the most matter accumulates, creating an area astronomers refer to as the
Zone of Avoidance. Visible light cannot easily penetrate this zone, leading scientists to resort
to other mediums like x-rays to scrutinise what lies beyond. To carry on our house analogy from
earlier, we are sitting in our bedroom, trying to figure out the layout of the rest of the house,
but the walls right in front of us are preventing us from seeing any of the other walls and rooms.
Further compounding this is the difficulty in telling how far away a star is from Earth. To
make an accurate 3D map of the Milky Way Galaxy, you need to know if the stars you can see are dim,
small, close ones, or large, bright, distant ones, that only look dim and small because
of their distance. Not all stars make this easy. While some types of stars such as Cepheid Variable
stars have a predictable enough luminosity that their distance can be calculated if you know a
few things about them, not all are quite as accommodating. So, for unpredictable stars,
how can you tell how far away they are? The answer is parallax. And that’s
what Gaia has been using over the last 10 years for its galaxy map.
Parallax is the principle that, depending on your viewpoint, things can appear to be in
different places. You can see an example of this right now, by holding up your finger in front of
your face. Close one eye, then instead close the other - now alternate quickly
between the two. See? Your finger moves quite significantly in relation to its background.
The closer an object is, the greater the effect of parallax. Simple trigonometry takes care of
the rest, allowing you to calculate how far away an object is. While this effect was very
noticeable based on a finger in front of your face, you’ll see much less parallax on a tree
on the other side of a field, which means it’s further from you. The further away something is,
the harder it is to use parallax to tell you its distance – things can get a little imprecise.
But how then can we use this method to calculate the distance to a star? The secret is to widen
your base viewpoint. Just as the distance between your eyes can influence the apparent parallax of
an object, Gaia viewed stars not just once, but multiple times over the course of a decade. In
that time, the face of our night sky will have changed, as we can see in time lapses like this
one (with an exaggeration of 10,000 times): Rather than using the 6cm between your eyes
as the base, every half a year we can make the solar system our base instead. Our Earth
moves from one side of the Sun to the other, which causes parallax effects to be visible
in every star we see - note how in this animation stars are taking little looping patterns? That’s
not really how they’re moving, but how we are; that’s the effect of our own loops around our Sun.
This can help with measuring stars’ distance, but there’s actually an even larger scale we
can use - the proper motion of stars. We can ignore the loops for this one,
as an even larger base is coming into play. Rather than just use the motion of our planet on either
side of the solar system, we can make use of the fact that our own solar system - along with the
other stars in the galaxy - are also moving. So, we can use that motion as an even larger base for
establishing parallax. The same principle applies - stars with larger proper motions
against the backdrop of space are closer, while stars with less proper motion are further away.
In this way, Gaia has been building a precise 3D map of 2 billion stars in our Milky Way Galaxy
and beyond, over the course of 3 trillion observations. But Gaia has not just been looking
at their motion and distance. ESA’s mission for this telescope was to study the luminosity,
temperature, mass and chemical composition of these stars too. It houses two powerful
telescopes that channel light from two different directions into a camera with nearly a billion
pixels – the most precise camera in space. Its radial velocity spectrometer can measure
doppler shift to give us clues about its velocity. It also has photometric instruments
to reveal spectral information about each star’s properties like composition and temperature. So
although 2 billion stars is only a drop in the ocean of the 100 billion stars thought to inhabit
the Milky Way, this still allows us to create an incredibly detailed map of our galaxy over time.
And it’s that “over time” bit that allows us to see the Milky Way’s eating habits.
Since 2013, when Gaia was launched, it has been collecting data which ESA scientists have
been processing and releasing in large data packets for the general scientific community
to study. The larger the gap in time, the more comprehensive and precise the data – more time
equals more parallax. 3 such data releases have taken place so far (in 2016, 2018,
and a two-parter that released a simplified version in 2020 and then the full version
in 2022), and scientists have already had the chance to evaluate what Gaia has seen.
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And as they witnessed the motions and locations of stars in our galaxy over time,
it became possible to model the whole galaxy – and even wind it backwards. Take a look.
It’s impossible to photograph our own galaxy, as no camera equipped spacecraft has travelled
out that far. But here is a representation, created using Gaia’s data, of what it looks like,
to an unprecedented degree of accuracy. Note its wobbly edges – these indicated
to scientists that the Milky Way crashed into another galaxy in the distant past.
Using the second data release, researchers from the Netherlands looked at 7 million
stars and found that 30,000 of them were moving strangely. While most stars, including our own,
move around the Milky Way in one direction, this odd collection was moving the opposite way.
Their colour and brightness were also strangely distinct, indicating there was something special
about this particular group. The researchers ran the numbers, and found that a model that fitted
their observations was what would happen if another galaxy collided with our own.
The collision must have happened 8-11 billion years ago, and involved a small dwarf galaxy
that the researchers named Gaia-Enceladus, or sometimes the Gaia-Enceladus Sausage. See how its
stars – indicated here in red – actually managed to retain a fairly discrete form for a time,
before eventually spreading out across the Milky Way. This merger may have been
an important part of how our Milky Way came to have some of its unique characteristics,
such as its bar in the centre – something that only 2/3rds of spiral galaxies have.
The Milky Way didn’t go in for a little late-night sausage-eating just once,
though. Another dwarf galaxy known as Arjuna/Sequoia/I’itoi was also found to
have merged with the Milky Way, and perhaps was in a binary pairing with Gaia-Enceladus.
Then, in a later study, another dwarf galaxy’s remnants were found. More than one,
in fact. According to this study, our Milky Way may have eaten as many as six
other galaxies. Gaia data reveals this hasn’t stopped – the nearby dwarf galaxy Sagittarius
has collided with our own 3 times already in the past 4-5 billion years, and is set to do
so again. This collision may even have been the spark that lit the flame of our very own sun,
which came into being around that same time. Eventually, Sagittarius may well become galaxy
number 7 that the Milky Way has consumed; all of its stars being slowly ripped away,
and brought into our own galaxy. These revelations are just some of
the incredible discoveries made possible through the Gaia telescope’s data. Which is why it is sad
that Gaia’s time has come to a close. The Gaia telescope maintained its position in the 2nd
Lagrange point using judiciously applied cold gas propellant – a dozen grams a day. However,
that cold gas has finally run out. On the 15th of January, Gaia took its last observation,
and has begun the process of doing a few last calibrations, before finally powering
down on the 27th March 2025. But its story is not over
yet. 2 more data packets await release. The first of the two – Data Release 4 – will
include 500TB of information, and will be released in 2026. It will only cover the first 5.5 years of
Gaia’s mission, but will do so to an incredible level of detail – its precise measurements of
stars’ wobbling motions might even highlight possible locations of exoplanets, and will create
the largest catalogue of binary stars to date. It’s also worth recognising that the discoveries
Gaia brought us don’t end with just prior or ongoing galaxy impacts. It’s possible to
fast forward the data to model the motion of our local stars millions of years into the future,
allowing us to see incredible models like this one. Note how although the stars near
us initially spread out across the Milky Way, they later reconvene due to what is thought to
be density waves. Seeing this play out helps explain the spiral arm shape of our galaxy.
Gaia data has helped improve our picture of events much closer to home, too. Gaia
revealed the position and direction of travel of over 150,000 asteroids in our solar system,
and thanks to its ability to track those movements over time, Gaia was able to
demonstrate that 352 of them probably have moons - as shown by slight wobbles in those asteroids'
trajectories. This nearly doubles the number of such binary-system asteroids that we are aware of.
Gaia has been called a treasure trove, and it’s hard to overstate just how much information can be
found in these data releases. Gaia has provided us with the largest ever 3D map of quasars across the
wider universe - identifying 1.3 million of them, with the earliest being spotted at the edges of
the universe, with light that started travelling when the universe was only 1.5 billion years old.
Gaia has found a previously unknown black hole in our own Milky Way, 33 times the mass of the Sun,
and only 2000 light years away from us. This type of black hole had never been seen in our galaxy
before - most local black holes are only 10 times the mass of the Sun, making this new find truly
massive. Many more such discoveries can be found thanks to Gaia data. It’s no surprise that ESA is
already considering plans for a Gaia replacement - GaiaNIR, a space telescope similar to Gaia but
able to see much further into the infrared to peer through the gas clouds that obscured Gaia’s
vision. But don’t hold your breath on this one. If it goes ahead, GaiaNIR will look to launch in
2045, to provide an appropriate gap between that mission and Gaia, to allow ESA to take even more
advantage of parallax effects - GaiaNIR will be able to see how much further Gaia’s observed
stars have travelled in the intervening time. When it comes out, Data release 5 will be the
big one. Tentatively set to be released by the end of the decade, this data release
will include everything – all 10.5 years of Gaia’s mission, to the highest level of detail
imaginable. It will truly be a treasure trove of information. Just think what discoveries can be
made with so much raw data to sift through! Who knows what else we will learn about our galaxy?
So while Gaia’s mission coming to a close is sad, the wealth of data it has collected is
also very exciting! Our Milky Way Galaxy might be surprisingly hard to learn about. But Gaia’s data
will give us the best crack at it to date. Did you find this dive into our Milky Way
interesting? Would you like to know more? Let me know in the comments below, as there’s plenty more
we can talk about. But for now, this has been Astrum, and I’ll see you next time.