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Cosmic Journeys: Supermassive Black Hole in the Milky Way Galaxy
Poziom:
Temat: Nauka i technologia
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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