The Milky Way galaxy is a vast, dynamic, and complex structure, far more immense and active than its comforting name suggests, serving as our cosmic home and a testament to the ongoing processes of creation and destruction in the universe.
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in the beginning there was darkness
and then bang giving birth to an endless
expanding existence
of time space and matter now
see further than we've ever imagined
beyond the limits of our existence
it's a crucible of creation and destruction
destruction
the name milky way sounds like something
kind of comforting sweet
but the milky way galaxy is a monster
it's just one galaxy among billions
and we're living on the edge just
recently we've discovered that there are
two small galaxies colliding with the
milky way right now
it's a tapestry of brilliant suns and
blinding dust
it's surprising how little of the light
from our rather luminous milky way
galaxy reaches us
it's a place of extremes where stars can
drift lazily
or be flung out at more than a million
miles per hour
now scientists have pierced the galaxy's
heart of darkness
it's a hundred thousand light years in
diameter has a trillion times more mass
it's our galaxy the milky way
the milky way galaxy is an extremely
active place
it's like a construction project there's
things going on all
the time and you have old stars die and
torn down
and then that material it's used to
in the midst of this work zone lies our
little solar system
and a whole lot more i suppose the best
way to think of the milky way galaxy is
our family of stars
these are the stars that we travel
through the universe with
in a clump all orbiting at common center [Music]
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within its far-reaching spiral arms lie clues
clues
to where we started and how
it will all end
on a clear summer night the stars of the
milky way
unfurl like a shimmering banner across
the sky
ancient egyptians saw this river of
stars as a pathway to the afterlife
but the greeks were the first to name it
the milky way comes from the word galacos
galacos
which is greek for milk
and though we once believed we lived at
the center of the universe
we now know that we don't even live in
the center
when we look up into the night sky and
we see this milky swath of stars that we
call the milky way
what we're actually seeing is a spiral
arm of the galaxy
that's closer to the center of the
galaxy than we are we can't really see
the center of the galaxy from here
but what we can see is one of the spiral
arms that's
a few thousand light years closer to the
as we gaze at the milky way from our
earthbound position
it's like looking at the edge of a coin
we get no sense of the galaxy's real shape
shape
if however you look at a galaxy from the
top down
it's a disk remember and it looks it's
like looking at a frisbee from the top down
down
you can see its full glory
even though it's thin you don't see how
thin it is but you see
its full structure [Music]
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you can make the analogy of the milky
there's a central region there's big buildings
buildings
there's a lot of action in the middle
then you move out to the suburbs where
life is a little bit more comfortable a
little more relaxing it's a better place
this is where we reside our solar system
is among the milky way's spiral arms
26 000 light years from the bustling center
center
our galaxy is so large that it takes
earth more than 200 million years to make
make
one lap the sun is located in what would
be just an
average neighborhood around the city
center but again
if you're standing in the middle of this
neighborhood you don't really know what
the neighborhood on the opposite side of
the city looks like
as we move outward beyond the suburbs
the population becomes more sparse
and yeah you've even got sort of the
boondocks in our galaxy
you've got the halo where you have very
old stars in
sort of wide orbits around the galaxy
the galactic sprawl doesn't stop at the
milky way's loose and undefined halo
it reaches far beyond our neighborhood
and out to a group of galaxies called
the local group if you think of the
milky way galaxy as a city like los
angeles then you can think of these as
counties all within
the state of california and together
this local group
besides our huge milky way and the even
larger andromeda galaxy
the local group consists of close to 50
smaller galaxies
the closest of which are roughly 40 000
light years away
there are two relatively nearby dwarf
galaxies the large
and small clouds of magellan that orbit our
our
milky way galaxy and are easily seen in
the southern hemisphere
they're small little galaxies but our
local group
has several dozen such galaxies sort of
wandering around inside it
the big galaxies are the minority
one advantage of actually being in the
outer edges of our galaxy
we have a clear view of outside of our
milky way we're able to see
the rest of the universe our neighboring
galaxies and galaxy
clusters beyond our own local group of galaxies
inside the milky way's halo lie massive
globular clusters
globular clusters are densely packed
regions of stars
these are like the ethnic neighborhoods
indeed these globular clusters formed
when the galaxy was very young
they are among the first stars to have
formed some globular clusters are 12 or
13 billion years old
they contain a hundred thousand or even
a million stars
globular clusters almost as old as the
universe itself
gave us the first clue to our place in
the galaxy
we saw these globular clusters in the
sky but their center
was somewhere far away from us and that
was one of our first measurements of
where the middle of the galaxy should be
it weren't orbiting around us but around
an area somewhere about twenty seven
we can never get far enough away from
our own galaxy to actually
see it but by looking at other galaxies
and comparing what we know about
our own galaxy we've got a pretty good
the very first time we saw galaxies
through telescopes we didn't even know
they were galaxies
i thought they were just nebulae in our
own galaxy and we were just amazed by
the beautiful spiral shape
so it was sort of natural for
astronomers to classify galaxies
according to their shape what we see
astronomers recognize four basic
galactic shapes
elliptical built of old stars and which
doesn't spin
lenticular consisting of a bulge and a
disc and little or no new star formation
irregular which has no real shape at all
like the magellanic clouds in our own
local group
and spiral which includes our milky way
it's a pinwheel of young and old stars
spinning gracefully through space
a long time ago people thought that
maybe an elliptical galaxy eventually
collapses down into a spiral
or maybe eventually spirals all come
together and form an elliptical
and it was sort of hard to figure out
exactly what the sequence is
one thing we know is that elliptical
galaxies tend to be very large [Music]
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centaurus a a misshapen elliptical
galaxy about 13 million light years away
suggests why elliptical galaxies get so big
there's a lot of evidence that there's a
spiral galaxy in there somewhere
that got absorbed by a larger galaxy so
right now it's possible that these
really big elliptical galaxies we see
may be the mergers of several spirals
our milky way isn't in danger of being
absorbed anytime soon
but the heart of our spiral galaxy has
revealed a secret of its own
only recently we discovered that the
milky way is not a classic spiral but a
barred spiral there's a bar of stars
going through the center
and the spiral arms sort of attach off
from that bar
spanning 27 000 light years
it's the most popular bar in the galaxy
30 million stars gravitate to it
the bar of our galaxy is natural result
of gravity
the mutual gravitational interactions of
the individual stars that form
the disc of our galaxy and the bulge of
our galaxy
causes sometimes the stars to sort of
bunch up into this bar
configuration surrounding the middle of
the milky way
is a huge central bulge it's mostly
composed of stars between 10 and 11
billion years old
the size of the bulge is linked to the
milky way's most gripping feature of all
a super massive black hole
though it would easily fit in the space
between the earth and our sun
it's four million times more massive
than our sun
now that sounds like a lot but other
galaxies have central black holes which
extend up to a billion times the mass of
our sun
so whereas it's a we call our
black hole at the center of our milky
way a supermassive black hole
among supermassive black holes it's kind
of a runt
black holes can't be seen directly
because light
can't escape them astronomers have
located galactic ground zero through a
radio source in the constellation sagittarius
sagittarius
known as sagittarius a star
it's creating quite a stir
the black hole in the middle of our
galaxy is spinning and it appears to be spinning
spinning
at a rate of about one spin per 11 minutes
minutes
as it spins central region stars caught
in its gravity
gets swept along for the ride orbiting it
it
the black hole affects the central
region the most
but we can't feel its tug on earth since
we orbit the galaxy
far away from the center the colossal
forces at the galaxy's heart
are negated by the milky way's
unimaginable size
to me the name milky way sounds like
something kind of comforting
sweet you know a candy bar that sort of
thing but the milky way galaxy is a monster
monster
it is incredibly huge so when you think
about how vast our solar system is the
fact that it takes
years for the fastest spacecraft to get
out to saturn or jupiter
if the entire solar system were the size
of a cd the earth would be comparable to
the milky way
that's huge that's absolutely
mind-blowing and it never ceases to
amaze me
and the amazement goes on and on
the galaxy's bustle and commotion may be
concentrated in the center
but the spectacular spiral arms have
action of their own
it's here that stellar neighborhoods are
being built
this is our galaxy it's a lot to take in
and we're just beginning to probe its depths
depths
our suburban location makes it difficult
to get the big picture
and the hazy clouds of cosmic dust only
block our view
even the most powerful optical telescope
can't pierce the darkness
so if we want to learn more about the
milky way we need to look beyond what
for all its vastness and empty space the
milky way
is tremendously active and populated
with some astonishing phenomena
star clusters nebulas
blazing invaders from other galaxies
technology is making us rethink old beliefs
beliefs
and is showing us things we've never
plus 15 seconds sending the new horizon spacecraft
spacecraft
on its way to the very edge of our solar system
system
we live on a dusty planet in a dusty galaxy
galaxy
in a dusty universe empty space
isn't so empty optical telescopes can
see only as far as the nearest
dust cloud which isn't far at all
most of our galaxy is invisible to us
however and that's because
the galaxy is full of dust dust clouds
if you will
and these dust clouds block the light
from most of the stars in our galaxy
it's surprising how little of the light
from our
rather luminous milky way galaxy reaches us
us
the reason for that is simply because
the dust blocks it
but visible light is just a small sliver
of the energy spectrum
and radio waves rush in where visible
light beams
fear to tread on a foggy day
you might not be able to see very far at
visible wavelengths which your eye can see
see
but you can still listen to your radio
or watch your tv
the ability of radio waves to penetrate
space dust
is crucial to the study of the stars
in 1933 karl jansky an engineer at bell
labs in new jersey
built an antenna to track down the
source of static
on transatlantic telephone lines he was
surprised to discover
the interference was raining down from
the center of our galaxy
the constellation sagittarius it took
several decades for scientists to
realize that jansky was onto something
celestial bodies emit electromagnetic radiation
radiation
and thus radio astronomy was born
but radio waves were just the beginning
because the human eye can't see all the
light that's available we have to resort
to technology
and one of the best ways of seeing the
universe in a very different way
is with an infrared camera an infrared
camera sees only the wavelengths
generated by heat
then converts it to something the human
eye can see
there actually is no visible light at
all that passes through this lens this
lens only lets heat light through
and then you can see what everything
looks like in infrared light
three two main engines start in 2003
the spitzer space telescope equipped
with a battery of infrared cameras
its mission is to explore some of the
youngest stars and farthest galaxies
in the universe the instruments that
spitzer has on it are actually
many times millions of times more
sensitive than these cameras
but they're basically the same thing
this camera doesn't peer into the heavens
heavens
but is used by firefighters to save
the camera can see through a
smoke-filled room to read the body heat
from an unconscious or immobilized victim
the same technology has enabled
astronomer susan stolovy
to see 26 000 light years away
visualizing the center of the galaxy as
never before
this high resolution mosaic was
assembled from
roughly 12 000 individual images taken
from the spitzer space telescope
even though that sounds like a lot of
data and it is it only took about 16
hours of telescope time
this particular region of the galactic
center spans an area of the sky
that's equivalent to four full moons in
one direction and three full wounds in
the other
that's the equivalent of 900 light years across
across
by 700 light years high
a small fraction of what's out there to
see but a phenomenal achievement nonetheless
nonetheless
just a few decades ago the galactic
center was not a subject of study because
because
you couldn't see it visually nothing
gets through only one light wave and a trillion
trillion
can penetrate the dust but if you
go into the infrared or use radio
astronomy or x-ray astronomy
you can see what's going on there
different wavelengths of the
electromagnetic spectrum can reveal different
different
aspects of the galaxy because they penetrate
penetrate
celestial objects differently [Music]
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radio waves have the lowest energy or
longest wavelength
but most celestial objects emit them
then comes infrared visible light
ultraviolet high energy x-rays
with wavelengths about the size of an
atom are emitted
by black holes and supernovas the
highest energy gamma rays come from the
collision or decay of subatomic particles
particles
like when stars explode at billions of degrees
degrees
together these waves give astronomers a
more complete picture of the activity
and shape of our galaxy
many if not all of the wavelengths are
needed to study the cosmos
the various wavelengths of the
electromagnetic spectrum
in music many wavelengths of sound are
used to communicate a musical idea
this piece has a very large range from a
if we were to restrict ourselves to the
visible light spectrum
it's almost as if we were only to hear
two notes in the middle of that piece
using alternate wavelengths the more we look
look
just recently we've discovered that
there are two small galaxies colliding
with the milky way right now
and the only reason we didn't know they
were there is because there was so much
dust in the disk of our galaxy we
couldn't see them
we're living inside this cloud and it's
something we're not aware of but with
infrared light
you can cut through that dust and the
minute we turned infrared telescopes to
the sky
we saw these little galaxies up there
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from our cockeyed position in the milky
way it's difficult to gauge what our
galaxy really looks like
radio and optical astronomy give us a
glimpse of its features
but to get the big picture we need to
look outward
so the way we gain some understanding of
our own galaxy is first of all by
looking at other galaxies and seeing
what they look like
and seeing things in other galaxies that correspond
correspond
to things that we see in our own galaxy
like clouds of gas that are kind of
lined up along what looks like a spiral arm
arm
you know we can see parts of spiral arms
in our own galaxy and we figure that
they're all connected
kind of like the spiral arms of other
galaxies that we can see from the outside
outside
we know that the milky way's four main
spiral arms
swing out from the downtown center like
wide streets
from the inside out they're named norma
scudum crux sagittarius
if the arms are the galaxy's suburbs
then our solar system lives on a quiet
dead-end street
between sagittarius and perseus on
what's called
the orion spur
all the stars in the milky way add up to
a community of about
200 to 400 billion and they're on the move
move
we are orbiting around the galaxy we
change our position
so far we think the sun has always been
about the same distance away from the center
center
but we've been in and out of pretty much
the spirals are called density waves
areas where the stars and gas get pushed together
together
as the density waves spiral around the
billions of stars
ride over and through them
when you think about watching the tour
de france and you see all of these
bicycles they're all moving forward
sometimes they kind of clump up around
one bicyclist and sometimes
they're stretched out that's sort of
what the spiral arms are like
that the stars are going around like the
bicyclist sometimes in clumpy areas and
sometimes in more
spread out areas but they keep going
stars don't usually travel alone
while giant globular clusters populate
the galaxy's halo
the galactic disk has open or galactic clusters
clusters
these bundles of young stars are barely
held together
by their mutual gravity now open cluster
implies that the stars are actually free
to go it is open
so this is a cluster usually of many
stars that have formed together
all from one of these giant clouds of
dust and gas
over time they're going to move away
from each other distribute themselves
around the galaxy
astronomers have counted about 20 000
open clusters in the galaxy
the pleiades is the one found nearest to
the earth
it formed a hundred million years ago
and will be around at least
twice that long before the galaxy's
spiral arms
closer to home our own sun orbiting in solitude
solitude
may have once been part of an open
cluster star
that struck out on its own the sun
the star clusters our own planet in fact
the entire galaxy
and the universe beyond are built of dust
dust
and gases these particles
that now block our view are what got us
here in the first place
cranking
nothing beats the spectacle of a glorious
glorious
sunset but we owe it all
to dust and gas
the setting sun appears yellow orange or
even red
for two reasons first of all the
molecules of air in the atmosphere are
scattering the violet blue and green light
light
out of our line of sight leaving the
yellows oranges and reds
to reach our eyes and second of all
particles like dust or smoke or smog
in the atmosphere absorb blue light more
even the grandeur of a blue sky is really
really
an optical illusion why is the sky blue
there's nothing blue about the gases of
our atmosphere
but as sunlight comes through our
atmosphere the shorter wavelengths the
blue light gets scattered
more than the longer wavelengths do so
if you look at any particular part of
the sky you're more likely to see blue
space is dark because there aren't
enough gas or dust particles to reflect
the light
of a billion stars
and though space may be a vacuum it's
not perfect
the galactic disk the largest portion of
the milky way
owes about 15 of its mass to dust and gas
gas
gas clouds can span hundreds or even
thousands of light years
providing the raw material that fires
the galaxy
these regions of cosmic dust and gas are
called nebulae
and they produce effects rivaling
anything seen
a good example is the orion nebula in
the constellation orion
this region is active with stellar
formation which makes the gas around the stars
stars
glow it's literally fluorescing
in response to the light coming out of
the massive stars that are near the nebula
nebula
and this nebula literally glows and can
be seen you can see it with your naked eye
eye
when you see the orion nebula in a real
way those are baby pictures for us
five billion years ago we were in a
glowing hot nebula
and the sun and the planets were forming
orion which contains hot stars ionizing
its gases with ultraviolet light
is called a diffuse or emission nebula
astronomers classify two other
categories of nebulae
some nebulae are what are called
reflection nebulae they're simply
the dust in these nebulas simply
reflecting the starlight from the bright
the witch head nebula is an example of a
reflection nebula
borrowing light from the star rigel
reflection nebulae appear blue for the
same reason
our sky does blue light is more easily
reflected than red
and last there are dark nebulae like the
when low-mass stars like our sun die
they form
another kind of nebula called a
these dim short-lived nebulae like the
cat's eye nebula
spew elements back into the galaxy
these may become raw materials for new suns
suns
just as dying stars spew out clouds of
dust and gas
dust and gas can come together to form stars
at a construction site you have old
buildings being torn down
and new buildings going up and it's very
much the same in the milky way galaxy
you have old stars explode and they cast
out new material
raw material gas and dust that can be
used to form
new stars [Music]
nebulae are the galaxy's recycling centers
centers
where old becomes new again
recycling is not just a good idea here
on earth
it's a natural cosmic law
in fact our own bodies are made out of
recycled material
from earlier generations of stars that
had dispersed material into the
interstellar medium before our own solar
system formed
each generation of stars creates heavier elements
elements
which become the ingredients for
everything in the universe
most of the galaxy's hot young stars get
built in the milky way's spiral arms
as gas clouds orbit the center of the
galaxy like the stars do
they get squeezed as they go through a
spiral arm
remember a spiral arm is simply a wave
in the pattern of stars and because
stars are denser
there the gas clouds that orbit through
it tend to get compressed
that compression allows gravity to get a hold
hold
of that gas and cause it to collapse to
form stars more readily there than
stars often die in the spiral arms
because they are formed here more frequently
frequently
victims of their own enormous mass
the more massive stars are extremely
powerful extremely luminous
and to be that luminous they have to use
up their
their energy source very quickly so
massive stars
live only short lives and they are thus
found in or near their birthplaces
the spiral arms because they simply
don't have enough time
to wander away from the places where
although we can anticipate the future of
some stars
it's often difficult to learn their history
history
but new techniques are revealing new
secrets about a well-studied star in the
milky way
called myra a myra has actually been a
favorite star of astronomers for 400
years it's a very easily visible star in
the night sky
recently the galax spacecraft the galaxy
evolution explorer
photographed myra in invisible
ultraviolet light
and revealed that it's leaving a trail
13 light years long
behind it we think that's actually
caused by the fact that as the star
plows through
the gas the gas heats up in a bow shot
very much like waves breaking up against
a boat and then that streams out into a
wake of hot material
you're actually looking at myra acting
very much like a boat plowing through
the water
when you look at how fast myra is going
right now about 291 000 miles an hour
and you do the calculations that long
tail is its path
the last 30 000 years we can't predict
exactly where the sun will go
in its orbit around the milky way
there's all kinds of things that could
interact with
but here we have the history of one star
we know this is the path it took
and that'll help us model how the galaxy
works and how all the stars move around
the middle of the galaxy
within the milky way's suburban spiral arms
arms
young stars enjoy plenty of space to
move around
as we move into the galactic bulge
conditions get much more crowded
and urban closest star to the sun is a
little more than four light years away
and when we look up into the night sky
even on a perfectly clear night
with no lights around you can't see more
but if our planet was down in the middle
of the galaxy
there would be a million stars in the
night sky
as bright as the brightest star that
we've ever seen in our sky
and it would be so bright that in fact
it wouldn't be nighttime it would be
life as we know it would be completely different
different
so what do we owe our position to
scientists believe that gravity has a
gravity is the power that drives the galaxy
galaxy
and at the galaxy's center churns the engine
engine
it feeds galaxies are
like a city in that they are ever changing
changing
and you're constantly being rebuilt and reinvigorated
reinvigorated
there's no question that the milky way a
few billion years ago looked a lot
different than it does today
it probably was smaller it probably
didn't have the beautiful spiral shape
that we see
today as things collapse under gravity
you tend to naturally form a disc you'll
notice there are discs everywhere
our solar system is a disc our galaxy is
a disc
so the stars all start rotating in the
just as in a city not all the stars in
the galaxy are natives
some stars born beyond the milky way
settle here and begin to make their mark
but then if another galaxy comes by the
gravity affects the way the stars move
and this may initiate the spiral arms
so in fact our spiral shape may be some evidence
evidence
that the milky way is composed of more
than one small galaxies that came together
together
historically the center of the galaxy
has been an
impenetrable mystery until we developed
x-ray vision one of the first ways we
really identified where the exact
center of the galaxy was was with an
x-ray telescope
well the x-rays were able to pass
through all of the dust and gas
in the disk of our galaxy and so even
though we can't really see
this bright center to the galaxy in
x-rays there's this giant glowing hot
source right in the middle
the x-ray emitter sagittarius a star
is associated with the super massive
black hole
in the milky way's center
by definition a black hole doesn't allow
light or even
x-rays to escape the radiation comes
from gas
caught in its gravity spun and heated
to millions of degrees
they're moving at extremely high velocities
velocities
for example the more extreme cases that
we've been able to observe
the stars are moving as they pass by the
black hole
at a speed of 10 000 kilometers per second
second
that's like going around the world in
four seconds
and we're talking about a whole star
when we watch stars orbiting the very
center of our galaxy it's obvious that
there's some sort of invisible monster there
they're orbiting around a giant mass and
the orbits of the stars imply that
there's about three to four million
times the mass of the sun
in the very center of our galaxy we're
not sure which came first the galaxy or
the black hole
but we know that it's there and it's tremendous
tremendous
for all its power and weirdness the
supermassive black hole is pretty typical
typical
for a galaxy the size of the milky way
other spiral galaxies and big elliptical galaxies
galaxies
also seem to have supermassive black
holes in their middle
ranging from a million times the mass of
our sun
up to several billion times the mass of
the stars surrounding the black hole are ancient
ancient
many are red giants hundreds of times
bigger than our sun
the galactic center is crowded with them
like an
urban downtown crowded with people
and stars like people can be pushy
most stars in the gladiator center
simply just keep orbiting the central
black hole
but in the galactic center there's so
many stars packed so close together
that stars are constantly nudging each
other a little bit gravitationally
and their orbits are being perturbed
changed a little bit
a bump star can get stripped of its atmosphere
atmosphere
leaving just its orbiting core or rarely
it can tumble into the black hole and vanish
but something else is happening around
the black hole
this turbulent dangerous neighborhood is also
also
a stellar nursery the stars that we're
observing moving fastest around the
black hole are the young stars that
uh have very recently formed and
it's something we call the paradox of
youth because it's
it's hard to imagine how to form these
massive young stars
in the presence of a black hole and yet
there they are
recently astronomers discovered that not
all stars caught in the black hole's grip
grip
are doomed to stay there a few
managed to break away and see the universe
barreling through the universe at one
and a half million miles per hour
hyper velocity stars are the escapees of
the galaxy
and what's interesting about high
velocity stars is the only way to
explain their extreme velocities
is that they're rejected by a
supermassive black hole
for a star to go ballistic takes a very
specific set of circumstances
and in fact it requires two stars
most of the stars you see in the sky are
not single stars
they orbit around each other linked by
gravity's embrace
but a star pair in the galactic center
might get jostled by surrounding stars
and stray too close to the black hole
when that happens the moment that the
gravitational pull of the
black hole exceeds the gravity that's
binding the two stars together
the pair of stars is broken apart one of
the stars will be captured by the black hole
hole
usually into a very tight orbit around
the black hole and the other star will
then gain all the energy of that system
and be ejected with this incredible velocity
velocity
if the galaxy were a city where most of
the stars would be cars or pedestrians
a high velocity star would be more like
an airplane
or a high-speed train rushing out of the country
country
if you're on high flossie star the ride
would be
quite amazing the sky would be covered
with stars as bright as the full moon in
every direction
but that view would quickly change
because the hypoplasty star moved so
quickly out of the galaxy
the stars appear fewer and fewer in the
night sky
the galaxy is constantly in motion like
a giant wheel
in the heart of town the supermassive
black hole's gravity
whips stars around in an orbit of around
where the earth sits two-thirds of the
way out on a spiral arm
we traverse the milky way once every 250
million years
our solar system has been around the
block only 18 times
since it formed the milky way's
incomprehensible size makes it easy to forget
forget
it's just one small part of an expanding universe
universe
when people hear about the expanding
universe a common misconception is that
everything is expanding
and in fact i'm not expanding right now
my atoms are the same size my cells are
the same size
the earth is not getting farther away
from the sun
the expansion of the universe only
applies to celestial objects that aren't
bound together by gravity
since the planets within the milky way
have stronger gravitational pulls than
the expanding forces outside
our galaxy the expansion of the universe
doesn't affect our solar system
our own milky way a spiral galaxy is on
a collision course with another spiral galaxy
galaxy
the largest spiral near us and that is
the andromeda galaxy
we think that in maybe three or four
billion years
our two galaxies may merge together
it'll be very interesting to see what happens
happens
what probably won't happen is a
collision of stars
even though both galaxies contain
billions of stars
the space between them is enormous
they will gravitationally interact
changing their direction
and motion eventually the merged spirals
will settle down
to become an elliptical galaxy
essentially all of the several dozen
galaxies in our local group
will be part of one super galaxy
and then gradually that super galaxy
will start losing stars because of
gravitational interactions
among the stars within that galaxy some
will get
flung away into intergalactic space
when it first formed the milky way built
stars at a rapid pace
using raw materials that were ejected in
space from the big bang
as the galaxy aged the star production
slowed down
from a few hundred a year to about four
to six new stars each year
today over time the milky way galaxy has
changed dramatically and we don't know
exactly what it looked like
long ago but probably early on there was
a lot more gas and dust and probably
fewer stars
and you had a lot of very large very
massive stars that would have formed
early on
and then these early stars exploded fantastically
fantastically
and spat out new material heavier metals
that could be used to form
smaller second generation and then third
generation stars [Music]
[Music]
some younger galaxies are still enjoying
that kind of building boom
we can see some galaxies where the rate
of star formation is very high
compared to our galaxy those are called
starburst galaxies
the rate of star formation there can be
anywhere from 10 to 100 times what it is
now in our galaxy
with every generation star production
slows down
and the milky way has been in business
for 13 billion years
one of the reasons that the rate of star
formation in our galaxy has changed over time
time
going from a very high rate to the
current modest rate of star formation is
because the
gas is being used up gas is used up to
form stars
and we're running out of gas literally
eventually over trillions of years star
formation will stop
completely the great galactic
construction project will shut down
and one by one the twinkling stars
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