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. [MUSIC PLAYING]
