When I was a young graduate student I got to use
one of the giant telescopes at the Las Campanas observatory in the Atacama Desert in Chile. I was
traveling with a much more experienced astronomer from Europe, but it was also his first time in
the southern hemisphere observatory. He went outside for a weather check and came back looking
annoyed to report two gigantic clouds in the sky. I went to look - it was the most
crystal clear night I’d ever seen. Was my colleague suffering altitude
sickness? Then I realized - there were two clouds on the sky - two hazy blobs just off
the dust-streaked center of the Milky Way band. I laughed out loud - this wasn’t water vapour
- this was lunch - the Milky Way’s lunch. Find a dark night in the southern hemisphere
and you’ll see too: the several billion stars of the large and small magellanic clouds in
their slow death spiral towards the Milky Way. My colleague needn’t have worried - we did some
great observing that night. But astronomers watching the resulting collision in around 2
billion years might have more cause for concern. When we scan the heavens with giant
telescopes like those on Las Campanas, we see galactic cannibalism everywhere. We see
moments that appear frozen on the human timescale, but are really snapshots of the incredibly
violent process of galaxy formation. This is how all galaxies are made. We can
piece together a pretty good understanding of this process from countless snapshots. Looking
into the distance means looking into the past, so it’s possible to stitch together a
frankenstein flipbook of galaxy evolution. But what about our home galaxy? It would be nice
if we could learn the Milky Way’s true history and final fate. And, in fact, we can. Since
that night on Las Campanas, enormous surveys have tracked the positions and motions of more
than a billion Milky Way stars. We’re now able to calculate a detailed and dynamical map of the Milky Way. We can predict its future mergers with the Magellanic clouds and Andromeda - but perhaps more astonishing, we can reconstruct its past. But before we get to this, let’s review what
we know about the Milky Way of the present, and of galaxy evolution in the general sense. The Milky Way is a pretty typical barred
spiral galaxy. Our sun is in the disk, on a minor outcropping of one of the spiral
arms. It orbits in the same direction as all the disk stars once every 230 Million years. In the
center we have the bulge envelopes - an elongated spheroid of stars that all orbit randomly at
different angles. All of this is surrounded by the halo - also spheroidal, but twice the diameter of
the 100-thousand light year wide disk. This thing is sprinkled with wayward stars and ancient,
dense mini-galaxies called globular clusters. But mostly the halo is made of dark matter,
which also suffuses the disk and halo and constitutes 80% of the Milky Way’s mass.The entire galaxy is beautiful and intricately structured. Weird to think that it built itself
into this through a life of violence. So let’s look at what we know about galaxy evolution
based on all the other galaxies in the universe. We actually talked about this process recently
in our episode on the galactic habitable zone. The first galaxies collapsed from very slight
over-dense regions in the hot hydrogen and helium gas that filled the universe after the Big Bang.
It’s hard to see the galaxies in the first billion years or so. Galaxies were still growing, and most
are too small to see at those great distances. The most distant galaxy known as of the filming
of this episode was discovered only in April this year. It shines out from a young universe, only 350 million years after the big bang. Back then, galaxies were raging storms of
star formation due to the abundance of gas back then. We only see the brightest of
those first galaxies. But based on the flipbook that we’ve assembled of the following several billion years we do have a clear picture: Galaxies assemble from the bottom-up, which
just means that small clumps form first, and then merge into larger clumps. So
early galaxies were messy and mostly small, and formed stars furiously. These clumps fell
together and spun each other up into whirling disks, and their violent convulsions settled into
the density waves that we see as spiral arms. The new spiral galaxies continued to gobble up
wispy irregular galaxies to grow that disk. So that’s how the Milky Way got to its current
form. But how can we possibly reconstruct the details of such a chaotic process? The stars from
every previous merger are mixed all across the Milky Way disk or through the halo by now. Teasing
out the Milky Way’s history is sometimes called stellar archaeology. Perhaps galactic forensics
comes is a better description - teasing out the evidence of violent encounters from evidence
that in some cases has been carefully buried. Let’s review the evidence. Item 1: Stars that
join the Milky Way at the same time, like during a merger, should have similar properties. For
example, stars that form from the gas of the same galaxy tend to have similar amounts of heavy
elements in them, as the gas that formed them was enriched by the same number of supernovae and other explosions. We can measure the heavy element abundance - also called metallicity - by looking
for the dips and spikes in a star’s spectrum that result from specific elements sucking up
or producing light at specific wavelengths. Stars with similar metallicities could have come from the same merger events, or were perhaps produced in the same burst of star formation triggered by that event. Spectra on their own aren’t really enough to tell if two stars came from the same galactic snack. So Evidence item number 2: If two stars came from the same merger, they should also have similar
orbits. If their orbital speeds are even slightly different, they may have drifted
to opposite sides of the galaxy by now. But there are other orbital properties we can
try to match. For example, how stretched out, or eccentric, are their orbits. And the orientation of their orbits relative to the galactic disk. Let’s see what our forensic investigation has
told us so far. A lot, actually. For example, astronomers have identified the last truly
gigantic merger that happened to the Milky Way around 10 billion years ago. This merger was so
large - around 50 billion Suns worth of matter - that it must have reshaped the galaxy and can be
thought of as the birth of the “modern” Milky Way. The galaxy the Milky Way consumed has been
dubbed ‘Gaia-Enceladus’ - named for the Gaia Space Telescope which was used to discover the
event by identifying star from this devoured galaxy in the Milky Way’s halo. The “Enceladus”
part is for the Greek titan of that name, and has nothing to do with the moon of Saturn. Gaia
is able to identify the stars from this merger because of the satellite’s incredible
ability to pinpoint stellar positions. That enables the telescope to detect tiny motions,
which in turn allows astronomers to reconstruct detailed orbits of Milky Way stars. The stars from
Gaia-Enceladus move in highly elongated orbits in the inner halo of the galaxy, but with a slight
‘backwards’ bend to the orbit, a clear indication that these stars were not born in our galaxy.
We’ve also found a group of 13 globular clusters with matching orbital properties and spectra
that were probably once part of Gaia-Enceladus. We see more evidence for this past violence in
the disk of the Milky Way. That disk has two parts - there’s the thin disk, which is a few
hundred lightyears thick, and is the main star factory of the Milky way. It’s home to the spiral
arms and the big, bright star forming clouds and our sun. We did an episode on why galaxies
become flat disks - but the TLDW is that giant gas clouds tend to collapse into thin gas disks, and then produce stars that share that geometry. Surrounding the thin disk we have
- you guessed it - the thick disk, which extends a few thousand light years
above and below its more slender counterpart. Not all spiral galaxies have a thick disk, which
suggests something special happened to the Milky Way to create it. It’s made of stars, not gas,
and these stars orbit just a little faster on orbits that are more inclined than the
thin disk. That causes them to rise above and drop below the Milky Way disk, leading
to the aforementioned thickness. Those stars are different in other ways - for example, they tend to have fewer heavy elements. That suggests they formed before the thin disk stars - around 9
billion years ago, plus or minus a billion years. We already have a perfectly consistent
explanation for the origin of the thick disk. It may have been formed during
the merger with Gaia-Enceladus. While the stars of Gaia Enceladus got mixed
into the halo, the crazy gravitational pulls of the two galaxies slamming into each other
kicked up the orbits of many of the stars in the Milky Way’s original thin disk to create
the thick disk. Meanwhile, the fresh gas from the merger replenished and reformed the thin
disk, and triggered a new round of star formation. Since its very large and thickening breakfast 10
billion years ago, the Milky Way has only snacked lightly. But we can trace that history also.
When a dwarf galaxy gets too close to the Milky Way it gets pulled thin as tidal forces cause
its near-side to move faster than its rear. It gets drawn out into a lengthening tidal
stream, and ultimately can be wrapped around the galaxy multiple times. In the end
it disperses into the Milky Way’s halo. Our galaxy has dozens of known streams. Some of
the littlest ones are still close together so it’s easy for astronomers to pick out the little stripe
of stars, like GD-1: a globular cluster that’s in the process of being pulled apart. On the other
hand, some of the biggest, like the Helmi stream, contain tens of millions of stars and
wrap in a full loop around the Milky Way. Perhaps the biggest of all the snacks the Milky
Way has had since the Gaia Enceladus breakfast is the Sagittarius Dwarf Spheroidal Galaxy
which first fell in about 5 billion years ago. Since then, it’s wrapped all the way around
the galaxy, going up and over the poles and coming back down to punch through the disk three
times. Most importantly, the massive core is still relatively intact and moving together, so every
billion years or so when it punches through the Milky Way’s disk its gravity hits like a
hammer. Actually, more like a drum stick. As the mass of the galaxy approaches, it pulls
the disk of the Milky Way up, but then when it passes through it pulls it back down. Like
beating a drum or plucking a guitar string, the stars oscillate up and down in the galactic
plane, making a very faint ripple through the disk. As best we can tell, the three passes
that the core of the Sagittarius Dwarf have made through the disk correspond to three
episodes of star formation in the Milky Way, and one of these passes even happens to line up
with the formation of our sun and solar system 4.5 billion years ago. Now, we’re not saying the
Sagittarius Dwarf galaxy is fully responsible for the existence of our solar system, but we're not saying it's not. It’s certainly highly plausible. All together, we know of up
to 7 mergers in the history of the galaxy and at least 42 distinct
streams surrounding the Milky Way, and as we continue to study the data from
the Gaia mission we’re likely to find more. This, finally, brings us to the present, as the
Milky Way prepares to have its lunch and second biggest meal yet. The Milky Way’s two brightest satellite
galaxies, the Large and Small Magellanic Clouds, are currently making their first pass. Already,
we see them being pulled apart- the Magellanic Clouds have massive tails that make a great
loop all across the ‘bottom’ half of the galaxy, a 600,000 lightyear long tail of
gas called the ‘Magellanic Stream.’ And while the Magellanic clouds themselves
are only about 1% the mass of the Milky Way themselves, the entire stream may be 10 billion
solar masses due to the enormous amount of dark matter it contains, making this easily the biggest meal the Milky Way has seen since Gaia Enceladus. When that merger happens the Milky Way will get a fresh infusion of gas, probably triggering another bout of star formation in about
2 billion years. The accompanying supernova waves may not be the best thing for life on Earth, but
we do have 2 billion years to get ready for that. All of the Milky Way’s past mergers have been
‘minor’, meaning that the Milky Way was always significantly more massive than its meal. That’s
going to change with the final merger of our local group of galaxies - when Andromeda and the
Milky Way collide. This is a major merger, because Andromeda is a full-blown spiral galaxy
in its own right. In fact it’s around twice as massive as the Milky Way. We’ve gone into the gory
detail of this collision in a previous episode. Go ahead and watch that one if you want to see
our galaxy get a taste of its own medicine. But for now let’s enjoy these short
few billion years of being the biggest and hungriest kid in the playground, as we gobble up any galaxies foolish enough to stray into the Milky Way’s little patch of space time. Hey Everyone. Before we get to
comments, I wanted to let you know about Search.pbsspacetime.com. This incredible tool
allows you to search the entire Space Time catalog for any word or phrase and get links
to the time code where I mention those words. It’s an incredible resource, built entirely
by one of our fans: Vegard Nossum. Vegard, we hear at Space Time can’t thank you enough for
the creativity and ingenuity of your work. There’s a link in the description, so everyone can start
exploring Search.pbsspacetime.com right away. Today we’re doing comment responses
for the last two episodes: the one on the galactic habitable zone and its implications for the fermi paradox, and the one where we showed why space is not expanding inside gravitationally bound systems like the Milky Way. Let's start with the habitable zone:
In that episode I said that parts of the galaxy with too much heavy element abundance might not
produce life because those systems would produce too many gas giants. Some of you asked why that’s
the case. It’s simply because gas giants form when a large enough rocky or icy core forms to start
holding on to hydrogen and helium atmospheres. That needs to happen before the star turns on and blasts away all of the lighter gases. A few of you pointed out that even a system with
lots of gas giants could have habitable moons. Now that’s true. There’s a good reason to restrict ourselves to Earth-like systems when we do these calculations of the abundance of life-bearing
worlds. It’s because we’re trying to find the most conservative number for possible origins of life
to help explain the Fermi paradox. We want to find out if even the most conservative estimate still
gives us a large number, because that tells us that we need to do work to look to other things to explain the absence of evidence of technological life. Some of you pointed out other
possible explanations for the Fermi paradox and the specialness of Earth. Lucas Nicholson and others remind us that
the Earth’s moon is exceptionally large, and such large moons are probably very
rare even if Earth-mass planets are common. The large moon has been proposed as
an important factor in the appearance of life, which may have first appeared in tidal
pools. The tidal pools hypothesis no longer the clear frontrunner for abiogenesis.
For example, geothermal vents on the ocean floor are looking good option for the origin of life, and they don’t care about the moon. These are also found on gas giant moons - so that boosts our
potential abiogenesis location number quite a bit. John Thatcher told us about another idea that I hadn’t heard of - that the late heavy bombardment may have been needed
to seed the surface of the planet with a high abundance of heavy elements - which was
perhaps necessary for life to get started. The late heavy bombardment was a massive
meteor shower that lasted millions of years, and probably isn’t something experienced by
most terrestrial planets. On the other hand, some scientists doubt that the late heavy
bombardment even happened - all of the evidence is from cratering on the moon,
and biases in the analysis of that data may have led to miscalculations of the bombardment rate. OK, on to the expanding universe stuff. SnufkinMatt asks “If space doesn't expand
inside a gravitational field, then what happens at the boundary between this and where
space is expanding? Would the expanding space try to 'drag' neighbouring space with it and would
you get a kind of tug-of-war between the two?” Yes and no. First, assuming no cosmological
constant and dark energy, the expansion of the universe does not continue to tug on
the space within gravitationally bound regions. However those bound regions fell
together from matter that was initially moving apart in the expanding universe, and
that kinetic energy affects that final form of the bound system — so I would think thank
galaxies that formed in an expanding universe would be more puffed up than those
that formed in a static universe. Nathaniel Cleland responded to that comment to address the
dark energy issue. He rightly points out that the Schwarzschild metric isn’t really valid in a universe with a cosmological constant. There would indeed be an additional effect
due to the tiny vacuum energy inside the galaxy. However the effect is minuscule, and
couldn’t really be characterized as a tug of war. It would result in a very very slightly larger size to the Milky Way than without dark energy. Eric Marsh asks if space inside
gravitationally bound systems actually contracts, rather than simply not
expanding with the rest of the universe. The answer is kind of, yes. You can interpret the
math that way. There is a valid interpretation of general relativity in which we can say that
space is flowing inwards in a gravitational field. You may have heard me say that space
flows across the event horizon of a black hole at the speed of light - that’s the velocity
of an inertial frame that falls from infinite distance towards a black hole. In the same way, an
inertial frame at the cosmological event horizon is flowing away from us at the speed of light. Evelyn always tells people not to imagine intergalactic space as expanding, but rather
that galaxies are shrinking. Jonathan Rose has a similar hypothesis that all matter is
constantly shrinking within a static space. Listen, guys, I understand that as smart,
science-educated people it’s extremely tempting to make up almost-plausible
nonsense to fool the “common folk”. Believe me, I feel the temptation as powerfully
as anyone. But neurological studies have shown that people who accept OR
propagate science misinformation are 50% more likely to suffer degenerative
microcephaly in later years. In other words, everyone’s brain shrinks like those galaxies.
I think I can feel it happening right now.
