stars are being born.

Yet for the dying stars that set this process in motion, the consequences are grim.

Supernovae leave in their wake a range of bizarre objects. Among them, a tiny dense

ball of neutrons.

You form an object that we call a neutron star, which is something that has the mass

of our sun, but a radius of only about six miles or so. That is a very compact object.

Every centimeter cubed of that mass has a mass of a hundred-million tons.

Because of its intense gravity, a neutron star is a perfect sphere, with a surface like

polished metal.

It has the mass of the star, but it only has the size of a city. And the structure of that

matter is that it's like ordinary matter, where there are electrons and so on orbiting

around, ah, ah, nuclei, but with all the electrons sort of squashed into the nuclei and the particles

up close together.

So something that we think of as incredibly dense, like lead, is really mostly empty space.

And if you squish all the empty space out of it and get the nuclei right up next to

each other, that's something that has the density of a neutron star.

The existence of neutron stars was first proposed in the 1930s. It wasn't until the '60s that

hard evidence was finally detected.

Within the remnants of some supernovae, astronomers detected a presence that pulsed with radio

signals.

When these were discovered by a graduate student in England in the 1960s, the supervisor thought

that they should not announce the discovery for a few months, partly in order to check

the -- that it was a genuine astronomic object.

But also because of the possibility that this might be an alien signal. And in fact, the

code name for the object in the early days was LGM, to stand for Little Green Men.

Aliens it was not. Strange it was.

The Crab Nebula is the shell of the same supernova that caught the attention of so many cultures

in the year 1054.

Deep within, astronomers found a pulsar, a neutron star that spins rapidly, emitting

radio waves.

Now scientists are using the Hubble Space Telescope to zero in on the pulsar -- The

star on the left. It is spewing waves of radiation that have etched circular patterns in the

surrounding gas.

Yet, some dying stars meet a fate that is stranger still. Nature, it seems, has contrived

a monster.

Early in this century, Albert Einstein speculated about a star with such intense gravity that

absolutely nothing, not even light, escaped its grasp. He at once dismissed this prospect

as impossible.

The notion that you could squeeze something without limits, you know, right down to a

zero size, was considered basically absurd and offensive.

And so there was a sort of temperamental reaction against this idea of total gravitational collapse

right through until the 1950s.

What once seemed beyond reason now defines the frontiers of science. Astronomers believe

that when a large star explodes, enough matter can collapse into its core that it literally

exits the known universe.

If the mass of that core is large, is larger than about three times the mass of our sun,

nothing at the end can stop the final collapse. Gravity wins the final battle and the thing

collapses to form a black hole.

From our vantage on earth, we define our universe by familiar criteria. But black holes defy

discovery. What after all can we detect of objects that emit no light?

The fact is we don't have enough cases where we really know what's going on to be able

to study it in detail.

We'd like to know, for example, if the black holes are a little different from the theory

and at the moment we're just at the stage of finding out whether there are good candidates

for black holes, things that we think can't be neutron stars, can't be white dwarfs, can't

be anything else.

The usual argument for a black hole is somebody shrugging and saying, "Well, what else can

it be?"

In 1991, astronauts placed one of the milestones of modern science into orbit. The Compton

Gamma Ray Observatory monitors high-energy radiation that pummels our upper atmosphere.

In 1994, it picked up a sudden eruption in the Constellation Scorpius.

Astronomers around the world zeroed in on the source. A network of radio telescopes

stretching from the Caribbean to Hawaii recorded a rare specter.

This is the object that came into view. Matter rushing into it is sending out jets of intense

radiation.

This week marks the culmination of a year-long effort to study the object. Teams in eight

countries are simultaneously training their most advanced technology on it, to discover

its true nature.

From a telescope in the Andes Mountains, in the heart of Chile, the astronomer Charles

Bailyn was the first to pinpoint its location.

Now he's venturing back to the observatory at Cerro Tololo -- to prove once and for all

it's a black hole.

It's the strange objects that always tell you the most about, ah -- about the universe

and about science. So, if you want to increase your knowledge of how gravity works, for example,

you don't look at things falling on the Earth, which we understand.

You look at the really, really strong gravitational fields that happen near black holes. Ah, and

those are the things where our current theories might possibly break down.

No one has ever proven conclusively the existence of a black hole. If ever there was an opportunity,

this may be it.

Looming within the dense star fields of our Milky Way, 10,000 light years away, this is

the brightest and most spectacular object of its kind. What's more, it's not alone.

A normal star circles the object in a dance of death. Gas from the star flows into the

object through a disk, while jets of radiation shoot from its poles.

So powerful is the object's gravity, that the companion star's shape is being distorted

and squashed, causing its appearance to dim, then brighten as it makes its orbit.

As it goes around, ah, the black hole, we're gonna start seeing the edge-on, rather than

side-on, so you can see less of it.

So it's gradually going to get fainter because in the time we're watching it tonight it's

going to go, uh, from almost side-on to almost end-on.

Though Bailyn and his team can't view the object directly, they can study its companion.

If you remember in Alice in Wonderland there is a Cheshire Cat. The cheshire cat disappears

from view and only leaves its smile behind to be seen. Black holes have this property.

They disappear from the eye, but they leave their smile behind. They leave their gravitational

force behind and it is by that that we then see them and are able to infer their properties.

To prove that his subject is a black hole, Bailyn must measure its mass at at least three

times our sun. The only way he can do that is by studying the effect of its gravity on

the motion of its companion.

It takes two-and-a-half days to make it all the way around, and in order to get that far,

ah, in that amount of time, it's got to go at several hundred kilometers a second, ah,

and so during the time we're going to be observing it tonight, ah, that's about 15,000 seconds.

So it will have gone, ah, over a million kilometers, ah, travel through space in the time -- in

the five hours we watch it tonight...

With its every movement, the star reveals the nature of the object that shadows it.

Well the first night, first night, we were here. Ah and ah, getting brighter. And then,

ah, on the second night, we're down here at the minimum.

And sure enough there was a minimum and it started to turn around right at the end. And,

ah, sure enough the first point comes in just were it's supposed to.

Bailyn has studied the companion star before, but only during a blinding outburst of radiation.

Now, the flares have died down -- leaving it exposed in stark relief.

Okay, I see it. It's this. So here's the star, right where the cross is. And, ah, that's,

that star's different from all these other stars. All these other stars are perfectly

ordinary stars just like the sun.

And this thing here is a double star with a black hole eating its companion. That's

what we're going to be watching for the whole rest of the night here.

A lot of people have trouble trying to visualize the nature of a black hole. They know it's

something that's, ah, greedily sucking in material, it's something that if you fall

into you can't get out of again...

And the question is, how can we make sense of all this? How can we, ah, human beings,

with our limited imagination, ah, try to -- to get some sort of common sense handle

on the nature of the black hole?

I think the answer is, ah, you can't. That these are circumstances where space and time

is -- is so warped, ah, that, ah, it's entirely outside of any sort of human experience.

So the only way we can come to understand objects like black holes is, ah, through mathematical

exploration.

A handful of numbers. Some morsels of data. In a scientist's deliberations, one of the

strangest phenomena in nature plays out.

A

black hole contains the mass of at least three suns collapsed to a point too small to measure.

This is a realm gravity has severed from the rest of the universe.

For five painstaking nights, Bailyn has kept vigil.

He has found that the object he's studying is massive indeed. Weighing in at seven solar

masses, it is, with little doubt, a black hole.

I like to see these numbers come out on the page and, ah, it's a sort of a combination

of doing puzzles with, ah, doing a kind of profound philosophical thing...

"What happens inside the event horizon of a black hole?" is essentially unanswerable

in scientific terms because we are talking about matter falling out of the universe.

And so, ah, you come right up against the edge of what science can tell you about the

universe and what it can't.

And I think that that boundary, ah, is where philosophy and religion and, ah, ah, all,

ah, these other kinds of thoughts ah, come into contact with science. And that's an exciting

thing for me.

The black holes Bailyn studies may number in the millions in our galaxy alone. But they

are by no means the ultimate showcase for gravity's power.

In the heart of the M87 galaxy, 50 million light years from earth, gas swirls into a

truly massive black hole, and glows as it heats up.

Astronomers clocked the speed of the gas as it circles around, and measured the black

hole's mass at more than two billion times that of our Sun.

The ancestors of these giant black holes inhabit regions at the limits of our vision: a class

of objects known as "quasars."

They are powerful beacons that take us back to a time when gravity began to draw gas into

the first galaxies.

A quasar is the product of a massive black hole in the center of a galaxy that is swallowing

huge volumes of gas and stars. A flood of blinding radiation erupts.

Not only is this an interesting process, the notion that we could have a black hole of

a billion solar masses shining more brightly than a thousand galaxies.

That's pretty wild. But this was -- these are sign posts. These were, ah, a phase in

the lifetime of infant galaxies and we can see them to great distances.

So they are beacons out there at the edge of the universe showing us when galaxies first

began to form.

In time, as they consumed their fuel, quasars like these grew dim. But the black holes that

powered them remained. Today, they are thought to loom at the heart of many a galaxy, including

our Milky Way.

There are two reasons why black holes are important.

One is because it's increasingly obvious that they play, ah, a major role in shaping the

universe, ah, both at the centers of galaxies and, ah, as the remnants of burnt out stars.

We still have, ah, yet to learn a lot about them. Ah, but there's a deeper reason, I think,

why they're important.

The black hole conceals something which is very profound, which is often given the word

singularity.

This is like an edge or a boundary to space-time. It's a point where space and time, so to speak,

come to an end.

As we look out into space, we probe a universe we can see -- one whose matter emits light

strong enough for our telescopes to record.

But there is another side to our universe, one that has evaded our detection. In fact,

fully ninety percent of the universe is unseen, and unidentified. Astronomers call it, simply,

"Dark Matter."

Mysterious particles, countless burned-out stars or black holes. Whatever it is, "Dark

Matter" is out there.

There's probably more dark matter than there is visible matter, but we really haven't,

ah, much of an idea as to what this dark matter is.

A whole lot of candidates, maybe 30 different types of things that the dark matter might

be and of course it could be more than one of them.

And I think that is the most urgent question that we must, ah, try to answer. What is the

dark matter?

Like a black hole, dark matter can be detected by the influence of its gravity.

Astronomers have, for example, measured the speed galaxies move within clusters, and found

that the gravity pulling them along is simply too great for the amount of matter we can

see.

So too, spiral galaxies, such as our own, seem to be enveloped within vast, dark halos