[MUSIC]
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
IN A PLACE WE CALL--
"THE UNIVERSE." IT'S A CRUCIBLE
OF CREATION AND DESTRUCTION. THE NAME "MILKY WAY"
SOUNDS LIKE SOMETHING,
KIND OF, COMFORTING AND 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
TO FIND OUR WAY THROUGH THE MILKY WAY. [MUSIC]
IT'S A HUNDRED THOUSAND
LIGHT YEARS IN DIAMETER, HAS A TRILLION TIMES
MORE MASS THAN OUR SUN. IT BEGAN
ABOUT 13 BILLION YEARS AGO,
AND IS STILL UNDER CONSTRUCTION. 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. YOU HAVE OLD STARS DYING
AND TORN DOWN,
AND THEN THAT MATERIAL GETS USED TO BUILD BRAND NEW STARS. 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 A COMMON CENTER. WITHIN ITS
FAR-REACHING SPIRAL ARMS
LIE CLUES TO WHERE WE STARTED AND HOW IT ALL WILL 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 "GALAXIAS,"
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 OF OUR OWN GALAXY. 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 CENTER THAN WE ARE. 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. IF, HOWEVER,
YOU LOOK AT A GALAXY
FROM THE TOP DOWN, IT'S A DISC, REMEMBER, AND IT'S LIKE LOOKING
AT A FRISBEE FROM THE TOP 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. YOU CAN MAKE
THE ANALOGY OF THE MILKY WAY
AS BEING VERY MUCH LIKE A CITY. THERE'S A CENTRAL REGION,
THERE'S BIG BUILDINGS,
THERE'S A LOT OF ACTION IN THE MIDDLE,
AND THAT'S CERTAINLY TRUE
OF OUR GALAXY. THEN YOU MOVE OUT TO THE SUBURBS
WHERE LIFE IS
A LITTLE BIT MORE COMFORTABLE, A LITTLE MORE RELAXING. IT'S A BETTER PLACE
TO RAISE A FAMILY. THIS IS WHERE WE RESIDE. OUR SOLAR SYSTEM
IS AMONG THE MILKY WAY'S
SPIRAL ARMS, 26,000 LIGHT YEARS FROM THE BUSTLING CENTER. OUR GALAXY IS SO LARGE
THAT IT TAKES EARTH
MORE THAN 200 MILLION YEARS TO 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
BECAUSE YOU CAN'T SEE IT
DIRECTLY. 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 GOT THE HALO,
WHERE YOU HAVE VERY OLD STARS
IN THAT, 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
MAKES UP THE ENTIRE STATE. 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 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 THAT ARE ALL OF SIMILAR COMPOSITION. THESE ARE
LIKE THE ETHNIC NEIGHBORHOODS
OF THE MILKY WAY GALAXY. 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. -- WEREN'T ORBITING AROUND US,
BUT AROUND AN AREA SOMEWHERE
ABOUT 27,000 LIGHT YEARS AWAY. 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 PICTURE
OF THE MILKY WAY. THE VERY FIRST TIME
WE SAW GALAXIES
THROUGH TELESCOPES, WE DIDN'T EVEN KNOW
THEY WERE GALAXIES. WE 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. 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
AND OTHER TYPES OF GALAXIES. OUR MILKY WAY ISN'T IN DANGER
OF BEING ABSORBED ANY TIME 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 A 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. IT CAUSES SOMETIMES THE STARS
TO, SORT OF, BUNCH UP
INTO THIS BAR OF 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 SUPER MASSIVE BLACK HOLE,
AMONG SUPER MASSIVE 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
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
AT A RATE OF ABOUT ONE SPIN
PER 11 MINUTES. AS IT SPINS,
CENTRAL REGION STARS
CAUGHT IN ITS GRAVITY GETS SWEPT ALONG FOR THE RIDE,
ORBITING IT
AT ABOUT THREE MILLION MILES PER HOUR. 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. 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 AND STARS ARE BEING BORN. THIS IS OUR GALAXY. IT'S A LOT TO TAKE IN,
AND WE'RE JUST BEGINNING
TO PROBE ITS 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 OUR EYES CAN SEE. [MUSIC]
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
AND IS SHOWING US THINGS WE'VE NEVER EVEN CONSIDERED
BEFORE. WE HAVE IGNITION. AND LIFT OFF. TWO PLUS 15 SECONDS. WE'RE SENDING
THE NEW HORIZON SPACECRAFT
ON ITS WAY TO THE VERY EDGE OF OUR SOLAR SYSTEM. WE LIVE ON A DUSTY PLANET
IN A DUSTY 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. 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,
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,
BUT THAT USE
WAS DISCOVERED BY ACCIDENT. IN 1933, KARL JANSKY,
AN ENGINEER AT BELL LABS
IN NEW JERSEY, BUILT AN ANTENNA TO TRACK DOWN
THE SOURCE OF STATIC
ON TRANS-ATLANTIC 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 ON TO SOMETHING. CELESTIAL BODIES
EMIT ELECTROMAGNETIC 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 ENGINE START--
IN 2003, THE SPITZER SPACE TELESCOPE,
EQUIPPED WITH A BATTERY
OF INFRARED CAMERAS, WAS LAUNCHED INTO SPACE. --AND THE EVOLUTION
OF OUR UNIVERSE. > 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,
BUT IS USED BY FIREFIGHTERS TO SAVE LIVES
RIGHT HERE ON EARTH. THE CAMERA CAN SEE
THROUGH A SMOKE-FILLED ROOM
TO READ THE BODY HEAT FROM AN UNCONSCIOUS
OR IMMOBILIZED VICTIM. THIS 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 MOONS
IN THE OTHER. THAT'S THE EQUIVALENT
OF 900 LIGHT-YEARS ACROSS
BY 700 LIGHT-YEARS HIGH, A SMALL FRACTION
OF WHAT'S OUT THERE TO SEE,
BUT A PHENOMENAL ACHIEVEMENT NONETHELESS. JUST A FEW DECADES AGO,
THE GALACTIC CENTER
WAS NOT A SUBJECT OF STUDY BECAUSE YOU COULDN'T SEE IT. VISUALLY, NOTHING GETS THROUGH,
ONLY ONE LIGHT WAVE
IN A 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 ASPECTS OF THE GALAXY
BECAUSE THEY PENETRATE
CELESTIAL OBSTACLES DIFFERENTLY. 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, LIKE WHEN STARS EXPLODE
AT BILLIONS OF 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. VARIOUS WAVELENGTHS
OF THE ELECTROMAGNETIC SPECTRUM
ARE SIMILAR TO THE STRINGS ON A VIOLIN. IN MUSIC,
MANY WAVELENGTHS OF SOUND
ARE USED TO COMMUNICATE A MUSICAL IDEA. THIS PIECE
HAS A VERY LARGE RANGE,
FROM A LOW NOTE TO A HIGH NOTE. 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,
THE MORE WE'RE DISCOVERING. 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 WERE
SO MUCH DUST IN THE DISC 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, COMING RIGHT AT US. [MUSIC]
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 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. 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. WE KNOW THAT THE MILKY WAY'S
FOUR SPIRAL ARMS SWING OUT
FROM THE DOWNTOWN CENTER LIKE WIDE STREETS. FROM THE INSIDE OUT,
THEY'RE NAMED NORMA,
SCUTUM-CRUX, SAGITTARIUS, AND PERSEUS. 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. 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,
BUT WE'VE BEEN IN AND OUT
OF PRETTY MUCH EVERY SPIRAL ARM THAT THERE IS. [MUSIC]
THE SPIRALS
ARE CALLED DENSITY WAVES, AREAS WHERE THE STARS AND GAS
GET PUSHED TOGETHER. AS THE DENSITY WAVES
SPIRAL AROUND,
THE BILLIONS OF STARS RIDE OVER AND THROUGH THEM. WE 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 BICYCLISTS,
SOMETIMES IN CLUMPY AREAS
AND SOMETIMES IN MORE SPREAD-OUT AREAS,
BUT THEY KEEP GOING
AROUND THE CENTER. STARS DON'T USUALLY
TRAVEL ALONE. WHILE GIANT GLOBULAR CLUSTERS
POPULATE THE GALAXY'S HALO,
THE GALACTIC DISC HAS OPEN OR GALACTIC 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
TEAR IT APART. CLOSER TO HOME, OUR OWN SUN,
ORBITING IN SOLITUDE,
MAY HAVE ONCE BEEN 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 AND GASES. THESE PARTICLES
THAT NOW BLOCK OUR VIEW
ARE WHAT GOT US HERE IN THE FIRST PLACE,
AND THE STAR-MAKING MACHINERY
IS STILL CRANKING. [MUSIC]
[MUSIC]
NOTHING BEATS THE SPECTACLE OF A 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
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 THAN THEY DO RED LIGHT. EVEN THE GRANDEUR
OF A BLUE SKY
IS 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 LIGHT BEING SCATTERED
TOWARDS YOUR EYE. [MUSIC] 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 DISC,
THE LARGEST PORTION
OF THE MILKY WAY, OWES ABOUT 15 PERCENT
OF ITS MASS TO DUST AND 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 ON EARTH. 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 GLOW. IT'S LITERALLY FLUORESCING
IN RESPONSE TO THE LIGHT
COMING OUT OF THE MASSIVE STARS THAT ARE NEAR THE NEBULA,
AND THIS NEBULA LITERALLY GLOWS
AND CAN BE SEEN. YOU COULD SEE IT
WITH YOUR NAKED 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 TOGETHER
UNDER THE INFLUENCE OF GRAVITY. 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
IS SIMPLY REFLECTING THE STARLIGHT
FROM THE BRIGHT STAR NEARBY. 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 HORSEHEAD. [MUSIC]
WHEN LOW MASS STARS
LIKE OUR SUN DIE, THEY FORM ANOTHER KIND OF NEBULA
CALLED A PLANETARY NEBULA. 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 AND NEW PLANETS. 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, 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,
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 OF THAT GAS
AND CAUSE IT TO COLLAPSE TO FORM STARS MORE READILY THERE
THAN ANYWHERE ELSE. STARS OFTEN DIE
IN THE SPIRAL ARMS
BECAUSE THEY ARE FORMED HERE MORE 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 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 THEY WERE BORN. ALTHOUGH WE CAN ANTICIPATE
THE FUTURE OF SOME STARS,
IT'S OFTEN DIFFICULT TO LEARN THEIR HISTORY. BUT NEW TECHNIQUES
ARE REVEALING NEW SECRETS
ABOUT A WELL-STUDIED STAR IN THE MILKY WAY CALLED MIRA A. MIRA HAS ACTUALLY BEEN
A FAVORITE STAR OF ASTRONOMERS
FOR 400 YEARS. IT'S A VERY EASILY VISIBLE STAR
IN THE NIGHT SKY. RECENTLY,
THE GALEX SPACECRAFT,
THE GALAXY EVOLUTION EXPLORER, PHOTOGRAPHED MIRA
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 BOWSHOCK 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 MIRA
ACTING VERY MUCH LIKE A BOAT
PLOWING THROUGH THE WATER. WHEN YOU LOOK AT HOW FAST
MIRA'S 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
IT 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,
YOUNG STARS ENJOY PLENTY OF SPACE TO MOVE AROUND. AS WE MOVE
INTO THE GALACTIC BULGE,
CONDITIONS GET MUCH MORE CROWDED AND URBAN. THE 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 THAN ABOUT TWO OR 3,000 STARS. 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 DAYTIME
ALL THE TIME. LIGHT, AS WE KNOW IT,
WOULD BE COMPLETELY DIFFERENT. SO,
WHAT DO WE OWE OUR POSITION TO? SCIENTISTS BELIEVE THAT GRAVITY
HAS A LOT TO DO WITH IT. [MUSIC]
GRAVITY IS THE POWER
THAT DRIVES THE GALAXY, AND AT THE GALAXY'S CENTER,
CHURNS THE ENGINE IT FEEDS. GALAXIES ARE LIKE A CITY
IN THAT THEY ARE EVER-CHANGING
AND YOU'RE CONSTANTLY BEING REBUILT AND 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 SAME DIRECTION. JUST AS IN A CITY,
NOT ALL OF 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
THAT THE MILKY WAY IS COMPOSED OF MORE THAN ONE SMALL GALAXIES
THAT CAME TOGETHER
A LONG TIME AGO. 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 DUSTS AND GAS IN THE DISC 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 SUPERMASSIVE 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. 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. THAT'S LIKE
GOING AROUND THE WORLD
IN FOUR SECONDS. AND WE'RE TALKING
ABOUT A WHOLE STAR
MOVING AT THAT SPEED. 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. FOR ALL ITS POWER
AND WEIRDNESS,
THE SUPERMASSIVE BLACK HOLE IS PRETTY TYPICAL FOR A GALAXY
THE SIZE OF THE MILKY WAY. OTHER SPIRAL GALAXIES
AND BIG ELLIPTICAL 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 OUR SUN. THE STARS
SURROUNDING THE BLACK HOLE
ARE 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 GALACTIC CENTER
SIMPLY JUST KEEP ORBITING A CENTRAL BLACK HOLE. BUT IN THE GALACTIC CENTER,
THERE ARE 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 BUMPED STAR
CAN GET STRIPPED
OF ITS 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 A STELLAR NURSERY. THE STARS
THAT WE'RE OBSERVING
MOVING FASTEST AROUND THE BLACK HOLE
ARE THE YOUNG STARS
THAT HAVE VERY RECENTLY FORMED. AND IT'S SOMETHING WE CALL
THE PARADOX OF YOUTH
BECAUSE 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
ARE DOOMED TO STAY THERE. A FEW MANAGE TO BREAK AWAY
AND SEE THE UNIVERSE. [MUSIC]
BARRELING THROUGH THE UNIVERSE
AT ONE AND A HALF MILLION MILES PER HOUR, HYPERVELOCITY 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 EJECTED
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, BUT PAIRS OR BINARY 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 GRAVITATION 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 USUALLY INTO A VERY TIGHT ORBIT
AROUND THE BLACK HOLE,
AND THE OTHER STAR WILL THEN GAIN ALL THE ENERGY
OF THAT SYSTEM
AND WILL BE EJECTED WITH THIS INCREDIBLE 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. IF YOU'RE
ON A HIGH-VELOCITY 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
WILL QUICKLY CHANGE
'CAUSE THE HIGH-VELOCITY STAR MOVES 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 OR A SPRAWLING METROPOLIS. IN THE HEART OF TOWN,
THE SUPERMASSIVE BLACK HOLE'S
GRAVITY, WHIP STARS AROUND IN AN ORBIT OF ABOUT 11 MINUTES. WHERE THE EARTH SITS,
2/3 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 IT'S JUST ONE SMALL PART
OF AN EXPANDING 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,
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 WILL BE VERY INTERESTING
TO SEE WHAT 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'VE FORMED EARLY ON. AND THEN, THESE EARLY STARS
EXPLODED FANTASTICALLY
AND SPAT OUT NEW MATERIAL, HEAVIER METALS
THAT COULD BE USED TO FORM
SMALLER SECOND GENERATION AND THEN THIRD GENERATION STARS. 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 A HUNDRED TIMES TO 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
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
WILL FADE AWAY.
