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  • From a distance, our galaxy would look something like this.

  • A flat spiral, some 100,000 light years across, with pockets of gas, clouds of dust, and about

  • 400 billion stars rotating around the galaxy's center.

  • That center - bulging up and out of the galactic disk - is tightly packed with stars.

  • Thick dust and blinding starlight have long obscured our vision into the mysterious inner

  • regions of this so-called "bulge."

  • And yet, the clues have been piling up, that something important...something strange...

  • is going on in there.

  • The first to take notice was the physicist Karl Jansky back in the 1930s.

  • He was asked by his employer, Bell Telephone Labs, to investigate sources of static that

  • might interfere with what it saw as the killer app of its time... radio voice transmissions.

  • Using this ungainly radio receiver... Jansky methodically scanned the airwaves. He documented

  • thunderstorms, near and far... and another signal he could not explain.

  • It sounded like steam - a hiss of radio noise. Jansky narrowed it to a spot in the constellation

  • of Sagittarius, in the direction of the center of the galaxy.

  • Located within a larger pattern of radio emissions... ... Jansky's sighting would become known

  • as Sagittarius A*.

  • The word of Jansky's finding got out. He assured the public that it was not aliens seeking

  • contact.

  • But that's just about all anyone could say... for over three decades.

  • Then Erik Becklin got on the case.

  • Becklin is one of those rare researchers whose curiosity and determination push our understanding

  • to a whole new level.

  • It was the 1960's and astronomy, like society, was in a period of ferment. Startling new

  • observations were being made... and new interpretations were in the air.

  • Quasars had just been discovered... extremely bright beacons of light from deep space. Were

  • they coming from the centers of distant galaxies? And what powerful objects were generating

  • them?

  • To study an event at the center of a galaxy, you have locate it. Young Becklin first took

  • aim at our neighboring galaxy, Andromeda.

  • In ultraviolet light, you can see a dense glow in the middle. Becklin found the point

  • where the light reaches peak intensity... and marked it as the Center.

  • From our orientation in space, all of the Andromeda galaxy is in full view.

  • But our galaxy is a different story. We live inside it, of course. Becklin had to find

  • a way to see through all the dust and gas that obscure our line of sight into the center.

  • So he went to a military contractor...

  • ...and obtained a device that reads infrared light... whose wavelengths are similar to

  • the distances between particles in a dust cloud, allowing them to move right through.

  • Becklin began measuring the brightness of the light as it rose to a peak... marking

  • the location of the galactic center.

  • Pinpointing this site would now allow astronomers to begin probing for details with a new generation

  • of powerful telescopes... to peer into the bright lights... the forbidden zones... deep

  • in the heart of the Milky Way.

  • Becklin wasn't the only astronomer interested in the galactic center.

  • Reinhardt Genzel, and a team based at the Max Planck Institute for Extraterrestrial

  • Physics in Germany, began a similar campaign in 1990... from the New Technology Telescope

  • in the mountains of Chile.

  • A few years later, in 1993, high atop Hawaii's Mauna Kea volcano...

  • Eric Becklin and colleagues, including Andrea Ghez, began using the newly christened Keck

  • Telescope. The American and German groups shared the same goal... to pinpoint the precise

  • location of Sagittarius A*, and find out what it is.

  • Because the object is too small to see... at 26,000 light years away... they would study

  • it by tracking the orbits of stars around it.

  • Even seeing them would take the sensitivity of Keck's wide aperture; an instrument powerful

  • enough to detect a single candle flame at the distance of the moon...

  • Meanwhile, using a similar technique, astronomers had focused the new Hubble Space Telescope

  • on a different galaxy... a giant elliptical cloud of nearly a billion stars, lying some

  • 50 million light years away called M87.

  • They tracked gas whipping around its center, figuring its speed at three million miles

  • per hour.... which led them to calculate the mass of whatever occupied M87's center...

  • at some 4 billion times that of our Sun.

  • Their measurement - first-ever of its kind - pointed to the presence of a black hole...

  • of truly supermassive proportions.... But it didn't conclusively prove its existence.

  • Back on Earth, the German and American teams each hoped that the proximity of the Milky

  • Way's center would allow them to...

  • ...look through the curtains of swirling gas clouds...

  • ...into the monster's lair...

  • ...to conclusively prove, for the first time, the existence of supermassive black holes.

  • This search was part of a larger effort to unravel the complex terrain of the galactic

  • center, in search of clues to the origins and evolution of our galaxy.

  • Recently, using Hubble, astronomers documented vast arcs of gas heated up by ferocious winds

  • from large stars.

  • Capturing infrared light, the Spitzer Space Telescope, picked up the pervasive swirling

  • heat signatures of all these stars.

  • The Chandra X-ray space observatory recorded high-energy radiation mostly likely given

  • off by ultra-dense neutron stars and small black holes.

  • Based on Chandra data, scientists estimate that a swarm of 20,000 black holes inhabits

  • the inner three light years of the galactic center.

  • If there is a supermassive black hole in the center of it all, the teams would have to

  • show that it's confined to a very small volume... and that it has enough gravity to whip the

  • stars orbiting it to high speeds.

  • The light of these stars travels 26,000 light years to reach us, only to be blurred in the

  • last few miles as it hits the Earth's atmosphere. So both teams turned to a method designed

  • to sharpen it back up.

  • The idea is to snap thousands of pictures in a short time. Because the atmosphere is

  • in motion, a star's apparent position may shift from image to image. To hone in on the

  • star's true location, a computer averages the positions, and looks for correlations

  • in the wavelength of the stars' light.

  • Here are the stars they began tracking... clustered around the center of the galaxy.

  • The first few years' data allowed the teams to calculate the speeds of the stars... and

  • their rough trajectories around the center.

  • That allowed them to pinpoint the position of their target...

  • ...as well as its gravitational pull. And that gave them its mass: roughly 3 million

  • times that of our Sun.

  • Because no other single object is known to weigh that much, it's strong evidence of a

  • black hole...

  • ...but it's still not iron-clad proof.

  • These data, for example, don't rule out a dense concentration of stars packed into the

  • center... held there by their mutual gravity.

  • The proof the teams sought would have to wait for an extraordinary event.

  • In the early years of the new century, large telescopes around the world began to install

  • upgrades.

  • Most large new telescope mirrors these days are thin... designed to be mounted on metal

  • scaffolding.

  • Behind the mirrors, engineers install pistons and motors to subtly correct the shape of

  • the glass as changing temperatures deform it... or as atmospheric turbulence blurs the

  • incoming light.

  • Some have added lasers... designed to project an artificial star onto the upper atmosphere.

  • As turbulence causes its light to distort, a computer can use it to subtract the net

  • effect of that turbulence from the light of the real stars, bringing them back into focus.

  • This is a Keck image of the galactic center... without adaptive optics applied....

  • And with them. With this increase in sharpness...

  • ...the teams were ready for what happened in 2002.

  • The German team had begun making observations at the new Very Large Telescope Array at the

  • Paranal Observatory in Northern Chile.

  • In the spring of that year, one of the stars they had been following, known as S2, made

  • a dramatic move.

  • S2 suddenly swooped around the center, accelerating to around 3 million kilometers per hour.

  • The American team saw it too.

  • It had come incredibly close to the suspected black hole ... about three times the distance

  • between the Sun and Pluto. If there had been a cluster of stars in there, S2's path and

  • its light would have wobbled. It did not!

  • This was the evidence the teams had sought. It showed that Sagittarius A* is a single

  • object... without doubt... a black hole.

  • You can argue whether that's definitive proof... but it's nothing short of spectacular.

  • This observation came at a time when astronomers had begun to believe that black holes play

  • an active role in the evolution of the universe.

  • They had found that giant black holes occupy the centers of nearly every large galaxy.

  • In fact, the larger the galaxy, the larger the black hole. That suggests that the two

  • must have evolved hand in hand, each shaping the life story of the other.

  • As matter flows into a black hole, it heats up to millions of degrees. Despite the black

  • hole's intense gravity, much of the inflowing matter blows off in fierce winds ... and powerful

  • jets roaring out of its poles.

  • The more matter that rushes in... the more the black hole pushes back out.

  • The force... and the heat... from active black hole outbursts can have the effect of limiting

  • a galaxy's growth ... by putting an end to starbirth ...and also pushing loose gas out

  • of its central region.

  • This has been going on since the earliest days of galaxy formation.

  • One result... a strict relationship has developed between the size of the black hole... and

  • the size of the galactic bulge that surrounds it.

  • Here in the Milky Way galaxy, is our own supermassive black hole still growing... and still shaping

  • its galactic surroundings?

  • Just as the black hole, Sagittarius A*, finally revealed its existence... it would now show

  • its true colors.

  • The year, 2001: scientists were working to commission the newly launched Chandra X-ray

  • space telescope.

  • They pointed the telescope at Sagittarius A*... and, by chance, at that moment, it erupted!

  • The teams on the ground began focusing on it for longer periods, hoping to see it happen

  • again.

  • And so they did... They saw what's now thought to be flares; outbursts that erupt when matter

  • builds up near the event horizon, before falling in.

  • A group of astronomers is now making plans to get an even closer look at these flares...

  • and for the first time ever, to directly glimpse a black hole.

  • To date, no single telescope on Earth has enough resolution to see something so small...

  • so far away.

  • Radio astronomers think they have a way. By linking observatories around the world, they

  • can create what amounts to an Earth-sized radio-telescope.

  • This simulation shows what they expect to see... just a few years from now. A supermassive

  • black hole in silhouette... framed by eruptions on its surface that travel around the monster

  • as it spins.

  • Perhaps images like these will shed light on a particular mystery: the flares appear

  • to be very weak...

  • ...considering the amount of matter swirling around the galactic center.

  • What these flares seem to be showing us is that our black hole... and our galaxy... have

  • settled into a period of "semi-retirement."

  • But they are bound to become active again.

  • Observing with the Very Large Telescope in Chile, astronomers recently picked out a straggler

  • from the intergalactic wars of old.

  • They spotted, deep in the recesses of the galactic center, what's left of a smaller

  • galaxy that had been torn apart by the gravity of the Milky Way.

  • This group of stars is so densely packed that it's able to survive the tidal forces around

  • it. Eventually it will find its way into the galactic center... where doom awaits.

  • Most likely it will be swept into the torrent of gas and dust and stars that are destined

  • for the mouth of the monster.

  • Working in the cold, clear air of the Antarctica, one group of radio-astronomers has tried to

  • find out when the Milky Way's central black hole will begin feasting again.

  • Data from their South Pole Telescope has delivered signs of a disaster in the making.

  • A huge ring of gas looms beyond the galactic center. When it accumulates some 300 million

  • sun's worth of matter, it will reach a tipping point.

  • The cloud will begin to funnel into a second ring that orbits close to the center.

  • This inner ring will condense... then erupt with star formation...

  • ... before spiraling down toward the ravenous black hole.

  • As the cloud falls into it, the black hole too will erupt in a blaze of glory visible

  • across much of the universe.

  • Based on these findings, the thinking is that outbursts like this repeat every 400 million

  • years or so.

  • Meanwhile, on one rocky outpost... a safe 25,000 light years from the turmoil at the

  • center of the galaxy... curiosity continues to reign.

  • We have found ways to track the patterns of change over billions of years that have shaped

  • our universe.

  • And yet sometimes... it's the small events that feed our sense of wonder.

  • Take the star S2, speeding around Sagittarius A*. In 2008 it made its way back to the exact

  • same spot where astronomers had begun tracking it.

  • That was the first time any object has been seen making a complete orbit around the center

  • of the Milky Way.

  • What will happen over the course of its next orbit?

  • No doubt, we'll answer many of the questions raised by this star and the supermassive companion

  • that whips it around.

  • Along the way, we're sure to see things we don't yet understand...

  • ...that raise questions we cannot yet answer.

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From a distance, our galaxy would look something like this.

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天の川銀河の超巨大ブラックホール (Supermassive Black Hole in the Milky Way Galaxy)

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    Wonderful に公開 2021 年 01 月 14 日
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