Kind: captions
Language: en
a long long time ago our ancestors gazed
up in the night and they saw a bright
band of light stretching across the sky
but what could this luminous phenomenon
have been
was it actually milk as described in the
ancient myths of egypt and greece
was it an obstacle to cross as suggested
by the silver river story from eastern
asia
perhaps it was a map
or a pathway of the birds as northern
europeans called it
or maybe it held the universe together
southern africans referred to it as the
backbone of night
[Applause]
after millennia of studying that strip
of light in the night sky
scientists have now mapped out our milky
way galaxy even creating a
high-resolution three-dimensional model
that shows the locations and movements
of nearly two billion
stars that means we not only have a
detailed picture of what our galaxy
looks like right now but we can fast
forward the movie to see where things
are headed and even rewind it to see how
our galaxy formed mind blowing
today on nova now universe revealed
galaxy formation and design you know the
easy stuff i'm alok patel
[Music]
here on earth people look for fossils
under the ground to try to reflect back
on the history of the ground and history
of earth so i do the same but with stars
rana azadine is an astrophysicist
cosmologist and a galactic historian at
the university of florida i dig for the
oldest stars in the galaxy and in other
galaxies in order to trace them back to
understand the history of our milky way
i do call myself the indiana jones of
the galaxy sometimes but you don't have
to call me that
whoa indiana jones of the galaxies is a
very cool nickname
[Music]
so let's say we want to cook up a milky
way style galaxy from scratch
first we need a list of ingredients
alright chefs start with a generous
portion of gas as much gas as you can
get the more gas there is the stronger
the gravity will be and it's gravity
that enables stars to be born what
happens is that the gas will start
collapsing on itself and when it
collapses it starts rotating so when the
gas and our proto-galaxy starts
collapsing due to gravity it'll start
rotating effectively stirring itself
it's pretty convenient in our
intergalactic kitchen
and so this rotation usually flattens
out the galaxy imagine making a pizza if
you throw that ball of dough straight up
into the air it'll come back down as a
ball of dough
but if you spin the pizza dough as you
throw it it stretches out and flattens
when we look into the night sky we see
this beautiful strip the galactic milky
way disc so this is actually how it came
to form because of the rotation of the
whole ingredients of our galaxy
[Music]
alright chefs to keep the galaxy from
flying apart we need an ingredient
that's invisible astronomers call it
dark matter it's what holds the
structure of our galaxy together dark
matter is the secret agent that kind of
glues everything together we just don't
see it because it's dark we only infer
its presence because there needs to be
much more mass than what we're measuring
from stars if there were no dark matter
the stars toward the outside of a galaxy
would travel at a slower speed than
stars towards the center where the
galaxy's gravitational force is focused
since we observe the outer stars
traveling about as fast as inner stars
we infer there's another source of
gravity we aren't able to see
so it's dark matter that gives our
galaxy its current structure it's there
it's interacting gravitationally with
the baryons or the elements that make up
the universe but we just don't see it
finally chefs to the center of the
galaxy add one supermassive black hole
but and this is important there's no off
the shelf supermassive black hole you've
gotta build it up from tinier black
holes and so what happens is that they
start to collide together or attract
each other toward the center of the
galaxy where most of the mass is located
and this is what forms the supermassive
black hole this giant hoover that is
just eating everything around it
including light so even light cannot
escape it and this is why we call it a
black hole now that things are in motion
set that oven timer for a few hundred
million years
are galaxies ready let's take a peek
we now know of two main types of
galaxies or two main shapes of galaxies
the first are elliptical galaxies which
are blob like but ours looks more like
the second type we call spiral galaxies
which includes the milky way meaning
that it's this galaxy which has a really
big bulge at the center and then it has
these spiral arms that come out of it
and it's surrounded by something we call
the halo i'm curious how we were able to
figure out the shape of the milky way if
we're in the middle of it like i can't
even wrap my head around this
that's a really good question because
before we had such advanced technologies
to look at the nice sky people have
always wondered about where we are and
why do we see this white strip in the
night sky they realize that there must
be something called a disc or there must
be a disc to the milky way where we are
located somewhere inside of this disc
because if instead we were seeing a
spherical distribution of the matter it
would be all around bright
so if our galaxy were shaped like a
sphere
the stars would be scattered evenly
across the whole sky
instead of concentrated in a bright line
but since we do observe the milky way as
a bright band of stars
astronomers deduced we're inside a
spiral flat disc galaxy
if you picture our galaxy as a giant
pizza then our view from earth would be
from a topping like jalapeno my favorite
looking across the pizza to the outer
crust
and today of course with advanced
technologies such as one of the most
important telescopes or spacecrafts
called gaia which for the first time
allowed us to draw the most high
resolution map of our milky way which
confirmed what we have been doing for a
long time confirming that we are in a
spiral galaxy
could you tell us more about gaia it
just seems so cool to me like how it
works and what exactly
it's been allowing us to see
so let me just give you a brief kind of
history about how we observe stars
usually we go to our telescopes and we
observe a single star at a time and then
over the past few decades i would say
particularly in the past decade people
started looking at stars using surveys
meaning having a telescope on the ground
look at the night sky all night long and
collecting as many stars as there is but
for
these observations we have nowhere been
able to really look at all of the stars
all at once we can't view all the stars
in our galaxy from one single point on
earth first the atmosphere gets in the
way plus you can only hope to see about
half of them
if you're in the northern hemisphere
let's say you stand on the north pole
and you look up you see different stars
and if you stood on the south pole
looked up which would be like looking
down through your feet from the north
pole if you could see through the earth
but you obviously can and you see why
this is confusing
anyway to see them all you need to get
off this planet that's what i'm trying
to say
we needed a spacecraft we need to send
it to space and let it collect much
higher resolution data on our stars
the european space agency launched the
gaia mission in december 2013.
and so gaia's mission was to measure the
astrometric properties which is
positions plus velocities of about 1.5
billion stars in our milky way even
though gaia has surpassed its goal and
studied more than 1.5 billion stars
that's still roughly 1
of the stars in our galaxy and yet guys
measurements thus far have already
transformed our understanding of the
milky way
it's kind of like everything was blurry
and now we can see so clearly because
not only do we see these static lights
in our night sky but we can know where
they came from where they're going and
how they're moving so we can create kind
of a moving map of our milky way not
just a static one
gaia not only confirmed that we are
indeed inside a spiral-shaped galaxy it
helps reveal other details about the
milky way structure like the fact that
it's actually divided into two pieces a
thin disc and a thick disc our solar
system is located in the thin disc and
it's one of the youngest parts of the
milky way telling us that our solar
system is a very young one as compared
to for example if we were to observe a
system or a solar system similar to ours
but revolving around a different star
let's say in the halo of the milky way
then this would be a very old one
because the milky way halo is very old
so when we talk about the milky way and
we
very compassionately say it's our home
and then our local group is our
neighborhood
could you use that metaphor and tell us
about the other homes if you will that
are on our block and what other galaxies
in our local group look like
yeah one of my favorite local group
galaxies are the magellanic cloud which
we beautifully see in the night sky if
we were ever in the southern part of the
hemisphere so i used to go observing in
chile a lot and it's so beautiful
because they're as bright as the milky
way disc if you're in the southern
hemisphere you can view both the large
and small magellanic clouds which are
irregularly shaped satellite galaxies
meaning they orbit our milky way now if
we go a little bit farther we know
another very famous and important galaxy
that is in the local group called
andromeda
so andromeda is the closest large-scale
galaxy in our local group and we know
that the milky way and andromeda are
already interacting with each other
gravitationally and actually are meant
to merge at some day together to form a
different type of galaxy called an
elliptical galaxy so spiral galaxies
when they merge they convert and become
an elliptical galaxy
scientists predict the milky way and
andromeda galaxies will start to
interweave in roughly 4 billion years
but this won't be the milky way's first
merger other mergers that have already
happened with the milky way such as one
called gaia enceladus so this was the
dwarf galaxy much smaller than andromeda
this is a galaxy that now with gaia we
are seeing the remnants of it inside the
milky way meaning that the milky way has
already ingested this galaxy and has
many of the stars already accreted onto
our milky way and we can identify these
stars because they look different they
move different and also they have a
different chemistry than our local stars
that have been formed in the milky way
now let's see one of these collisions
happens right now let's say it happens
tomorrow morning
would this affect
life on earth is there any noticeable
difference that we would see
so no and i'm going to say unfortunately
no because what a show that would be
because these things happen over a very
long time scale so the collision is not
really just a collision that all of a
sudden the milky way galaxy is like oh
what is this thing coming toward me it's
going to collide
this happens over millions and billions
of years where the gravity starts
attracting the other galaxy slowly and
ingesting the star slowly
like the fate for our milky way and our
andromeda eventually they're going to
turn off their star formation but
continue in this beautiful gravitational
dance until they merge however what it
will do is that sometimes it induces new
star formation so because the gas in the
galaxy is being heated by this
interaction and more stars are in
falling and more gases and falling from
the other galaxy it starts a new era of
star formation where younger stars all
of a sudden are basically coming from
this merger and they have two parents
they have both the milky way and
whatever galaxy has involved
so no need to freak out our merger with
andromeda is billions of years away and
the stars are so spread out it's
unlikely that any of them will collide
which means we have time for a second
half to this episode
when we return simulating galaxy
formation using supercomputers
[Music]
so i think from just a human perspective
trying to understand what is the origin
of everything it's a very interesting
question
saunak bose is an astrophysicist at the
harvard smithsonian center for
astrophysics and predominantly i do this
work by using large supercomputers to
essentially simulate the formation
process of galaxies over cosmic time now
while using computers to simulate galaxy
formation isn't a new idea it's been
around since the 70s it's gathered
momentum recently as computers have
improved and essentially the procedure
is as follows
we start with
some assumptions of what we think
the initial conditions of the universe
look like so how did the universe look
say 13 billion years ago
it turns out that these initial
conditions
are amongst some of the best measured
data that we have available to us made
by space-based satellites like the
planck satellite
which essentially measures the microwave
background radiation or essentially
photons that have been liberated from
just after the big bang that have been
streaming towards us over billions and
billions of years
which we can measure and then use to
infer exactly what the
structure of the universe was like at
the time when these photons were created
you see
so this measurement actually provides us
a snapshot of what the baby universe
looked like
baby is a relative term that quote baby
universe revealed by the cosmic
microwave background radiation was
already 400 000 years old not so baby so
we have this as a starting point and
what we then need to do is to basically
teach our computers
how to evolve these starting conditions
over time
using the laws of the universe and then
give an end result
remember the ingredients we needed to
create a galaxy dark matter is the
secret agent that kind of glues
everything together formulas for dark
matter and dark energy are crucial for
developing the right computer models for
galaxy formation so sonic has to know a
lot about them to program them into his
simulations so i think the word dark is
in many ways
equally
an apt description of our inability to
actually see what either of these
components are but also an equally apt
description of our lack of understanding
of what either of these are as well so
it's a absence of knowledge in some
sense
dark matter is probably more easily
explained than dark energy is
so it's basically this idea that
the universe that we live in is
dominated by gravity on these large
cosmic scales so even though we can't
see dark matter scientists surmise it's
out there by studying the behaviors and
physical properties of the stuff we can
see one specific example being this idea
of gravitational lensing
so this is an idea that was put forward
by einstein
where he said that as light goes past an
object that creates gravity the path of
light actually gets warped and bent
much like how light bends in the
presence of a magnifying glass or a lens
and if we look at images of distant
galaxies taken for example by the hubble
space telescope we see these
characteristic warps and stretching in
the images of galaxies
created by some distribution of matter
that exists
between us and these distant galaxies
that can't just be explained away by any
kind of
visible material that exists between us
and them
and this other
way of viewing the universe through this
gravitational lensing effect also
becomes
a very powerful additional source of
evidence for dark matter so what happens
when these computers evolve these
starting conditions and ingredients into
a galaxy
this is kind of one of the really odd
things which is that
the stuff that we understand very little
about which is the dark matter that we
can't see and the dark energy that we
know very little about
the physics of those is actually
relatively easy to model
because it is ultimately the physics of
gravity which we understand pretty well
and can represent very well with
equations
but for the ordinary matter that you and
i are made of there is a lot more that
actually goes on with these things so
for example
bits of hydrogen can cool and collapse
and then form stars and these stars can
then grow and radiate energy and the
stars will then undergo supernovae and
all these kinds of complex processes
take place which we don't need to worry
about in the case of dark matter
and putting all of these into a
mathematical form that we can then
pass on to a computer for it then pass
and then feed back into our models
becomes a very very complex task and
that's something we've only really made
progress in in the last 10 or 15 years
or so
and is there a certain
ratio if you will or ratio that's been
seen throughout the universe of dark
matter so when the universe was say 400
000 years old
at that point the amount of dark matter
outweighs the amount of ordinary matter
by around a factor of five or so
so quite a lot more of dark matter than
there is of ordinary matter
however when we actually go around the
universe today and look at galaxies
we don't necessarily find this ratio
being constant so
there are certain galaxies
like dwarf galaxies which are the really
really tiny galaxies that orbit around
the milky way
which are very good laboratories for
studying dark matter because actually
their dark matter dominated
however when you go to other types of
galaxies like the milky way that ratio
is actually
less stark compared to what we find it
in 12 galaxies
for fun let's try simulating a galaxy
without dark matter we'll just use
regular everyday matter
i think what would come out at the end
is a very different realization of
galaxies than anything we think of today
and the reason for this is that stars
and galaxies and so on
the reason they're able to form in the
very first place is because there was
dark matter
the modeling reveals that the universe
not only has a lot more dark matter than
regular matter but the dark stuff is
fundamental for creating galaxies in the
first place
and so what actually happens is these
halos of dark matter which are the
regions where the dark matter has
collected in the highest densities and
gravity has essentially collapsed them
into these big balls if you will of dark
matter
they essentially act as wells
gravitational wells that are able to
draw into them kind of like sinkholes
almost
the hydrogen gas that eventually then
forms the galaxies that gas is what gets
compressed to give birth to stars the
fact that this process actually happened
in the time that it did
was because
dark matter was already present as the
scaffolding upon which this visible
universe was built
so we basically need to assume some
theory of the dark matter and assume
some kinds of properties that it needs
to have which will therefore dictate
what equations we put into the computer
and this is kind of how we play this
game really so we
put in a theory and then we get the
final result which we can then compare
with the data and then see how well that
theory works and then kind of work
iteratively
scientists experiment with these
simulations by inputting different
theories and checking on the resulting
galaxies but what theory explains those
spiral arm galaxies that they sometimes
end up with like the milky way the
concept of spiral arms is
something that people have been
wondering for a while and
there are i think many hypotheses as to
why some galaxies are able to create
spiral arms and others or not but
essentially the idea comes about
with the fact that when these galaxies
are forming there are these disturbances
and perturbations that take place in the
disk where the gas has collapsed and
contracted into a region where stars are
forming
and these galaxies are rotating as well
and as something is collapsing and
rotating at the same time you
essentially have you know different
parts
of the
disk essentially moving at slightly
different rates from one another
there are certain points in this disk
where there will be an overabundance of
gas on the part of the disk where
there's a slightly under abundance of
gas and as this is rotating what
actually happens is in these regions
where there's the overabundance that's
where the new stars are preferentially
forming
and because of the rotation this starts
creating
points where the motions of stars
slightly lag behind other points you see
and this is what creates spiral arms
by creating and testing these models and
simulations scientists can better
understand what goes on in the lives of
galaxies galaxies generally have very
tumultuous lives so our own galaxy for
example
doesn't really you know kind of live in
isolation so all galaxies undergo
a very large number of murder events and
they're constantly bombarded by tiny
galaxies and other dark matter halos
throughout their lifetimes i love that
we just described galaxies as having
tumultuous lives i'm in my mind i'm
picturing this like galaxy soap opera
right now yeah of like how much drama
they go through yeah turns out that
peaceful strip of stars we see in the
night sky is only a brief snapshot in
the turbulent and dramatic life of our
galaxy
by trying out different galactic recipes
and binge watching reruns of these
galactic soap operas scientists will
continue their efforts to predict the
future and season finale of our milky
way
nova now universe revealed is a
production of gbh and prx it's produced
by terence bernardo jenny cataldo r.a
daniel caitlin falls and jocelyn
gonzalez julia court and chris schmidt
are the co-executive producers of nova
sookie bennett is senior digital editor
christina moden is associate researcher
robin kasmer is science editor robert
boyd is digital associate producer shyla
duff is digital video intern and devin
maverick robbins is managing producer of
podcasts at gbh
i'm alok patel
we'll be back next week with the cure
for loneliness the search for alien life
on far away planets
if you love stories about our universe
visit pbs.org nova now podcast and check
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five-part film series airing wednesdays
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