Our Universe 3D Page #5

Altogether, the Milky Way has a diameter

of about 100,000 light years.

So even if we could travel as fast as light,

almost 700 million miles an hour,

it would take over 100,000 years

to get from one end to the other.

This begs the question how a formation

of this size can be held together at all.

The spiral shapes and bright centres

of most galaxies

give a hint toward the explanation.

All these individual solar systems

revolve around a central point of gravity.

But what force in the universe could

have such an immense gravity?

The answer would have shocked

astronomers just 50 years ago.

It is a black hole.

They were thought to be science fiction,

but these behemoths really do exist.

Basically, they are stars that have imploded

from the pressure of their own gravity.

This caused a chain reaction,

that draws more and more matter

to the centre of the black hole.

In effect, it becomes denser and denser.

Accordingly,

the mass and gravity keep increasing,

so the black hole grows

more and more powerful.

The stars in the bright

centre of most galaxies

are flung around the black hole like yo-yos,

travelling at speeds

of millions of miles an hour.

Black holes seem gruesome in one way,

because they relentlessly obliterate

anything that comes their way.

But most of the suns, planets and moons

are a safe distance away,

so they are not in any danger

of being sucked in.

Instead, the black holes are

the central focus for all of them,

determining the shape and size

of the different galaxies.

A formation like this wouldn't exist

without a black hole,

and we would miss out on some of the

most exciting examples of natural beauty.

Like the Messier 74 galaxy, for example.

It's a classical spiral-shaped galaxy,

not unlike our own.

However, the arms of M74 are

decorated with bright, pink areas.

Those are clouds of gases, lit up

by the ultraviolet light from young stars.

Much more asymmetrical is

the appearance of Messier 66,

the biggest galaxy of the Leo Triplets.

Its displaced looks are

due to the gravitational forces

of its two nearby siblings.

Galaxies are drifting through space,

which allows for exceptional compositions.

For example,

this pair of galaxies known as Arp 273.

Scientists assume that the smaller galaxy

has fully passed through the bigger one,

and as a result,

created a form that reminds us of a rose.

In a couple billion years, our own galaxy

may collide with our neighbour, Andromeda.

Something similar has happened

with the Antennae galaxies.

These two galaxies merged

when they crashed into each other,

and the resulting forces have

spawned billions of new suns.

And who knows?

In one of those young solar systems,

life could emerge.

Or could it already exist

somewhere out there?

Maybe even much closer to us.

Granted, our local planets

don't have the best conditions.

Mercury doesn't have an atmosphere.

It's way too hot on one side,

and way too cold on the other.

Venus has no water on the surface,

because it evaporates in the heat

of the crushing atmosphere.

Mars' atmosphere, on the other hand,

is too thin,

it can store neither warmth nor water.

And the four gas giants Jupiter, Saturn,

Uranus and Neptune,

they do not even have a surface.

But there are not just planets

that could host life,

there are also moons.

Our main planets have

over 100 of them in total,

and some of these are bigger

than the planet Mercury.

An impressive example is lo.

It orbits Jupiter at a

distance of 250,000 miles

and is about the same size as our own moon.

On getting a closer look at lo,

you might describe it as a hell of a moon.

After all, the most distinctive structures

on its surface are volcanoes

and lava pools of gigantic dimensions.

There are seas of liquefied sulphur,

an element covering the whole planet

in various aggregate states,

from gaseous to liquid to solid.

This gives lo its colourful appearance.

Of all bodies in our solar system,

lo has the most active volcanoes.

The constant eruptions hurl scorching lava

up to 180 miles high.

Aside from the huge lava pools

that are up to 250 miles wide,

there are also rivers of lava,

that flow hundreds of miles.

Next to the burning heat of the lava,

the temperature instantly drops

to minus 200 degrees.

Extreme conditions like these can't exactly

be called life-friendly environments.

Our next stop in the vicinity appears

to be even colder.

We're closing in on Europa.

This moon is covered

with an ice crust 6 miles deep.

Europa has a very thin atmosphere,

mostly made up of oxygen.

The surface is very bright

and is among the smoothest

and youngest of all moons.

It features a network of chaotic ridges

and trenches which is visually striking.

These so-called Linea are strongly

reminiscent of ice fields on Earth.

Since the positions and alignments

of the Linea

cannot be explained by geological processes,

they are a clue to one of the biggest

secrets of our solar system.

Under Europa's icy crust lies

a huge ocean of liquid water.

This water interacts with the surface,

constantly renewing it,

and the Linea are created in the process.

Scientists are already

thinking about a probe

that can drill its

way through the ice

and dive into the water,

the element that spawned

the first life on Earth.

Two other important ingredients

are oxygen and warmth.

On the surface of Europa, the average

temperature is minus 240 degrees.

It is too far away from the sun

for any significant warmth to arrive here.

But the gravitation from Jupiter creates

tidal forces that heat up Europa's inside

and keep the water from freezing.

And there is oxygen, too,

released from the ice by cosmic radiation.

According to scientific estimates,

the waters here could be more oxygen-rich

than our earthly oceans.

So, in spite of all the cold on the surface,

Europa is indeed a hot contender

for alien life.

Still, this icy world seems

very different from our own.

There is one that is much more Earth-like,

namely the biggest of Saturn's moons.

Its size alone is impressive. In fact,

it's even bigger than the planet Mercury,

so it has been aptly named Titan.

Titan is the only known moon to

have a really dense atmosphere.

Consisting mainly of nitrogen

and rich in methane.

On the surface, temperatures are as low

as minus 275 degrees Fahrenheit.

Still, Titan has landscapes

that look a lot like those on Earth.

Along the equator lies a

region called Xanadu,

which is about the size of Australia.

Here we find mountains of up to a mile high.

They are made of frozen water,

washed out by methane rain.

Due to the low temperatures,

this ice is as hard as rock.

There is no liquid water due to the cold,

instead, pools and even seas of methane

define the landscapes.

In contrast to the methane seas are

desert-like areas dominated by dunes.

These dunes are several hundred miles long

and up to 500 feet high.

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