In four billion years, anyone left in our solar system
will witness the most spectacular event
to take place in the history of the night sky
as the Andromeda Galaxy plows headlong into our own Milky
Way.
But will that be the very last night
sky our solar system witnesses?
See that fuzzy blob on the sky, the one
just left of the Milky Way center
in the constellation of Andromeda?
That's M31, the Andromeda Galaxy.
It's two and 1/2 million light years away
and host to a trillion stars.
It has a beautiful spiral structure,
spanning its gently rotating disk 220,000 light years
in diameter, and a central bulge that
hides a giant black hole that contains the mass of well
over 100 million suns.
Andromeda is also racing towards our galaxy
at 110 kilometers per second.
That faint blob will slowly grow to around half again
its current size over the next two billion years.
Then its growth will accelerate.
At three billion years, it'll be two and 1/2 times bigger.
At three and 3/4, it'll fill half the sky.
At around four billion years from now,
it'll crash through the Milky Way,
and both galaxies will be utterly disrupted
in the monumental collision.
This much we know for sure.
But what about the sun, the solar system, the Earth?
The Andromeda Galaxy was our first clue
that there existed a universe outside the Milky Way.
We've known about it forever.
On the dark sky, it's visible to the naked eye
as a faint smudge.
But being so far away, you can't see
individual stars in Andromeda without a good size scope.
Because of this, there was originally
no way to know whether Andromeda was
a much smaller cloud of gas, a nebula inside our galaxy,
or whether it was a galaxy in its own right at a much
greater distance.
The distance and fundamental nature of "the Great Andromeda
Nebula" was the subject of long debate,
beginning with a Immanuel Kant.
In the mid 1700s, he hypothesized
that Andromeda was an island universe, a vast sea of stars
distant to our own.
It was a guess, albeit a very good one.
Milky Way philosophers living a few billion years from now
won't have to speculate.
The galactic nature of Andromeda will be clear to the naked eye.
That galactic nature is also clear
when we train modern telescopes on that faint smudge.
The first incontrovertible evidence
came when Edwin Hubble calculated its distance
by watching the pulsation of stars in Andromeda.
He observed Cepheid variables, which
have a pulsation rate that depends on their energy output.
Time the pulsation rate, and you know how luminous the star is.
Those Cepheids appeared extremely faint
in Edwin Hubble's observations due to the galaxy's
great distance.
But knowing their intrinsical luminosity
allowed Hubble to calculate that distance.
It was finally clear that Andromeda
was, after all, an island universe
far outside the Milky Way.
Hubble went on to combine distance measurements
to many galaxies with measurements
of their velocities to discover the expansion of the universe.
Those velocities were found by another astronomer,
Vesto Slipher, by measuring Doppler
shifts of spectral lines.
Almost all of Slipher's galaxies seem to be moving away from us.
Andromeda was a striking exception.
It's close enough that the mutual gravitational attraction
between it and the Milky Way overcomes
the outward expansion, allowing them to fall together.
But Doppler shift measurements only
gives the line of sight velocity, the component
of the galaxy's motion directly towards or away from us.
That doesn't tell us whether Andromeda will actually
hit the Milky Way.
If the galaxy has enough sideways or transverse
velocity, then it could miss us completely.
For a long time, we had no idea about Andromeda's
transverse velocity.
It's actually very hard to measure.
The galaxy is so far away that its motion
relative to background galaxies is almost imperceptible.
Even with a transverse velocity equal to its line of sight
velocity, Andromeda's motion over several years,
in terms of angle on the sky, would be minuscule,
a fraction of a percent of the angular
width of one of the Hubble Space Telescope's tiny pixels.
So how do we measure Andromeda's transverse velocity?
Well, we use the Hubble Space Telescope over several years,
of course, with a heavy dose of being extremely clever.
A team of researchers, led by Roeland van der
Marel of the Space Telescope Science Institute,
did just this.
They mapped the locations of thousands
of stars in Andromeda between 2002 and 2010
and compared them to background galaxies.
Then they averaged the observed motion of all of those stars
and removed the effects due to the rotation of Andromeda
and the motion of the sun.
They calculated a transverse velocity
of 17 kilometers per second.
Even taking uncertainties into account,
Andromeda is racing towards us much faster
than it's moving to the side.
A head-on collision is inevitable.
Van der Marel and team also ran a computer simulation
to study the consequences of this collision.
They used simulations of the gravitational interactions
of millions of particles representing groups
of stars and dark matter.
In other words, they made a little Andromeda
and a little Milky Way in their computer
and watched them smash together.
They also included the Triangulum,
or Pinwheel Galaxy, the third-largest member
of the Local Group.
This is an animated representation
of the predictions of that simulation.
The giant spiral galaxies fall together,
and the little Triangulum Galaxy joins the party.
The first impact in around four billion years
completely disrupts the spiral structure of both galaxies,
creating these amazing tidal tails.
We see these in other distant galaxies,
like the Antennae, which are currently
in the process of collision.
After slamming through the Milky Way,
Andromeda's core travels on for a bit before falling back,
and the two galaxies merge into a vast football-shaped
elliptical galaxy in around six billion years.
Both galaxies contain a supermassive black hole,
which will fall towards the center
of the new merged galaxy.
They do that through a process called dynamical friction.
Gravitational interactions with stars
slingshots those stars into larger orbits
or even completely out of the galaxy.
Meanwhile, the black holes lose angular momentum and fall
towards the center.
When those black holes are around a light year apart,
they'll start losing orbital energy to gravitational waves.
Then they spiral towards each other and merge.
The resulting super supermassive black hole
may briefly power a new quasar as it consumes any gas that
also ended up in the core.
There's also a chance that gas throughout the galaxy
will be shocked into a storm of new star formation.
This isn't completely clear because, in four billion years,
both the Milky Way and Andromeda will
have burned through a lot of their remaining gas reserves.
But what about the sun and the earth?
Well, for one thing, we don't expect
any collisions between stars.
The average distance between stars
is around 100 billion times greater than the average size
of a star.
They'll slide right past each other.
There's a higher chance of another star passing
inside Neptune's orbit, which might cause
some gravitational disruption.
But that chance is still low, at something like one
in 10 million.
No, our planetary system will probably
survive this encounter.
One big question is where we will land when the new uber
galaxy settles.
Van der Marel, et al's, simulation
follows several candidate suns, simulation particles
with similar orbits and masses to our sun,
and they track their final locations.
Most end up in the outer parts of the merged galaxy,
but many have orbits that periodically plunge them
through the central regions.
And some even travel far enough from the center
to make dashes through the Triangulum galaxy
before that galaxy also gets gobbled
by the giant elliptical.
There's also a small chance that the sun
will encounter one of the supermassive black holes
as they descend to the core.
And that could slingshot our solar system
into intergalactic space.
But most likely, we'll remain within the system
with front row seats.
So what will this look like to us?
Well, for around two billion years after the initial impact,
our sky will be full of a galactic train wreck
as the two galaxies settle down.
Finally, the giant orb of Milkdromeda
will fill much of the sky.
At this point, the sun will already
have expanded into a red giant.
And so we best be watching from the warm oceans
of Enceladus or Europa.
Earth will long ago have been roasted by our own brightening
and then expanding sun, which we talked about
in earlier episodes.
I sometimes think how lucky we are
to live in the time before our collision
with Andromeda, a time when we have
such a clear view of our dynamical evolving universe,
when we have a neighbor whose visible stars revealed
its great distance, and whose spiral structure
helped us guess the shape of our own galaxy.
Astronomers in the distant future
will see only a single featureless orb in the sky,
and the next nearest galaxies will
be very far and fast receding.
Will those astronomers ever figure out
that there are countless other island
universes stretching across a much vaster space time?
Last week, we talked about a stunning new result
in astrophysics, the detection of the first stars
to ever form.
And we also gave you the answer to our trebuchet challenge
question.
Exoplanets Channel says, "If they detected this with current
radio telescopes, I cannot imagine what they will discover
with the square kilometer array."
Yeah, SKA is going to blow our minds repeatedly
and in many different ways.
The EDGES experiment integrated for hundreds of days
and added together the radio signal from the whole sky
to measure their signal of the first stars.
SKA should be able to map the signal across the sky
and so create images of the structures in which
those first stars were forming, presumably
some sort of proto-galactic clusters.
Patrick Hogan points out that it's more accurate not to think
about dark matter as a thing.
Rather it's the name we give to the effect,
whereby the gravitational response of the universe
doesn't match the visible matter given
our understanding of that matter and/or the laws of gravity.
That's very fair, Patrick, but I would
say that the evidence is converging on dark matter
being some sort of particle or at least a stuff.
For one thing, there's the consistency
of the dark matter mass measurements
of galaxies and galaxy clusters from gravitational lensing
versus kinematics.
There's also the fact that dark matter
appears to distribute itself differently to regular matter
but still comes together under gravity, for example,
in the Bullet Cluster.
In the case of the result we discussed,
the entire hypothesis that dark matter
was responsible for the cooling of the early universe
relies on it being some sort of stuff that can interact
with either matter or light.
So in that context, it makes sense to refer to it that way.
But in general, I agree.
We should add the caveat that we really don't know for sure.
And finally a huge shout out to Springfield High School's two
AP physics classes, led by the brilliant and dedicated Wesley
Morgan.
These guys submitted videos of their answers
to the trebuchet challenge.
Here they are, Springfield's finest physicists.
Your solutions were exactly correct.
Sorry we didn't end up selecting you as official winners.
That would have taken a lot of T-shirts.
But official or not, you conquered this challenge.
Instead of T-shirts, we're sending some stacks
of Space Time stickers.
When you end up as astronauts or famous physicists
or genius inventors or medieval warlords,
we hope you'll remember us.
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