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.