How Gravity Shapes the Universe The Sky at Night

The Universe is filled with beautiful objects -

from shining stars, to amazing clusters of galaxies,

to clouds of gas and dust -

and one force has created them all.

Gravity.

This month, we're looking at how the wonderful shapes

and spectacular structures we see in the Universe have been created.

Welcome to The Sky At Night.

MUSIC: "Pelleas and Melisande: At the Castle Gate" by Sibelius

Spring is finally here and the nights are warmer, at least in theory,

and we've escaped the bright lights of the city to come and join

this rowdy bunch of amateur astronomers

at the Brecon Beacons AstroCamp.

Keen astronomers have come from hundreds of miles away

and they're setting up camp

in the hope of using the coming darkness

to see deep into the night sky.

Of course, being Britain, the weather isn't on our side

and it's pretty cloudy but we're hoping

there'll be some breaks in the cloud later on.

Whatever the weather,

we're going to use the time to explore the extraordinary ways

in which gravity shapes the objects in the night sky.

Coming up, I'll be finding out

how gravity makes stars and planets round.

And why, despite the power of this force,

they're often not as round as they seem.

Chris North and Jon Culshaw are here

and they'll be taking us on an intergalactic tour

to show us how gravity sculpts each and every galaxy.

And we'll be seeing gravity in action right now,

as it creates an extraordinary drama in Saturn's rings.

This is the first time

that we've ever seen anything like this.

Plus Pete Lawrence and some campers will be showing us

simple tricks to capture star trails.

Oh, that's amazing!

As darkness sets in,

we're still waiting for the clouds to properly break.

But some bright stars and planets, like Mars, are shining through.

So while we wait and hope, here's Paul Abel

with a guide to what it's possible to see

if we embrace the dark and if the skies are clear.

Well, here we are in Wales in a lovely dark site.

And viewing the night sky from a place like this

is really very different from viewing it from a town centre.

The main reason for that is there's very little light pollution here.

Another significant reason is that our eyes can become

very dark adapted in sites like this.

The reason we have dark adaption is

because our eyes manufacture a chemical called rhodopsin

which allows us to see in the dark.

Unfortunately, that chemical

is completely destroyed by bright, white light.

You can experience all of this for yourself.

The first thing you'll notice when you step out into a dark sky

are the bright objects in the sky like the moon or the planets -

Mars and Jupiter we have around at the moment.

Later on, you'll notice the bright stars,

like Arcturus, shining away up there.

After 10 or 15 minutes,

you'll notice the fainter stars that make out the main constellations.

Like the faint stars that mark out the handle of the Plough.

After 25 to 30 minutes,

you become totally dark adapted.

And in a dark sky like this, the sky is literally

ablaze with stars.

But perhaps

the most impressive thing is the sight of the Milky Way running down

through the constellations.

You can use exactly the same process for getting the most out of your

telescope because the more you look, the better you'll be able to see.

If we take Saturn for example,

initially the view is not too impressive.

We notice it's a planet surrounded by a ring system

but after five minutes,

the subtleties of Saturn start to come out.

We notice the delicate cloud bands, the pastel hues, the brighter zones,

and the rings themselves take on more of a 3-D effect.

So the moral of the story is, in order to get the best views

out of your telescope, spend as much time as you can looking.

Frustratingly we are still not getting

any decent holes in the clouds.

So let's take a look at the awesome creative power of gravity.

Now whatever you look at in the night sky, whether it's a star or a planet,

you're looking at an object that's basically a sphere.

The sphere is the most common shape in the Universe.

But when it comes to the cosmos, a sphere isn't always what it seems.

Perfect spheres are actually surprisingly rare in space.

To understand how gravity forms spheres,

and why they're often not perfect, you have to start with

how objects like stars and planets form out of stardust.

It's gravity that caused the planets to form in the first place.

But gravity is also the key to their spherical shape.

Let's imagine that these sugar cubes are lumps of rock

in the early Solar System

orbiting a young sun some 4.6 billion years ago.

Gravity pulls them towards each other

but gravity has no preferred direction.

So they come in from here, from here, from here,

from all over.

The shape you end up with

is the only shape that looks the same from every direction.

A sphere.

Gravity created the blue planet we call home.

But although it may look flawless, our planet is not perfect.

Imagine I start at the equator

and I walk all the way round the Earth.

By the time I get back to the beginning,

I would have walked 40,000km.

But let's say I do the same thing, but this time pole to pole.

By the time I get back to the beginning,

I would actually have walked around 130km less.

That's because the Earth isn't a perfect sphere.

It's actually a bit fatter in the middle.

And it's not just Earth.

Most of the planets in our solar system

also have a bulge around the equator.

The reason some planets grow a bit fat around the middle

is because of the way they rotate on their axes.

It's because of the phenomenon we can experience here on Earth.

Spinning something causes it to be thrown outwards,

away from the centre.

These chains are the only things that are keeping me

and this chair from flying off into the distance.

The same thing happens to a planet when it spins.

Gravity acts like the chains, pulling everything inwards.

But the speed of rotation pushes everything outwards.

Just like the chairs on this ride,

as a planet rotates on its axis it grows wider around the middle.

And, of course, the faster we go, the greater the effect.

In our Solar System, Jupiter spins the fastest

taking just ten hours to complete one rotation

and therefore it has an enormous bulge.

Its circumference is 29,000km greater

when measured around the equator rather than the poles.

Venus spins the slowest. A day on Venus is 243 Earth days.

As a result, Venus has no bulge at all

and is close to a perfect sphere.

In general, the faster the spin, the bigger the bulge.

But there is one rather big exception to the rule

that we still don't fully understand.

Our sun is very large and it rotates at nearly 7,000km/hr.

That's incredibly fast

so you'd expect it to have a bulge but it doesn't.

It's an almost perfect sphere.

This astonishing discovery was only made in 2012 as a result of

the most detailed measurements of the sun that have ever been taken.

Dr Chris Scott is a solar expert and a space scientist.

He's been trying to understand

this intriguing mystery on our astronomical doorstep.

So why did it take so long to get this measurement?

It's a very difficult measurement to make. When you get up into space,

you can see the sun and its atmosphere

and it's not smooth.

There are eruptions of material off the surface of the sun

all of the time.

The new spacecraft that's enabled these measurements is called

the Solar Dynamics Observatory.

It's a solar telescope that's in orbit around the Earth,

so it's sending back something like 15,000 images a day.

These images it's taking are ten times HD resolution.

We've been able to work out that the sun is much,

much rounder than it has any right to be.

So what do you think is causing this result?

Well, it's another force of some sort

that's got to be stopping it from bulging.

So the theories have been perhaps that the poles

would be slightly hotter and so maybe would expand out

a little bit more so that would even up the difference.

It seems unlikely that it's just the right temperature

-to make it spherical.

-Indeed. There's no evidence that there is

this temperature difference. It could be a magnetic field.

We know that the sun has a strong magnetic field,

it comes out of the sun at the north and south poles.

Could there be some concentration of magnetic field

that is stopping the material,

like an electromagnet on the fairground ride pulling the cars in?

Is there some force that's stopping the material from bulging out?

And again, there doesn't seem to be much evidence

that there's that strong a force.

So perhaps it's something in the solar interior.

Perhaps some part of the solar interior

isn't rotating as fast as we thought it was.

Or perhaps there's some kind of stresses going on

between the different layers to distribute the mass of the sun

in a different way from how we thought.

In a previous programme,

we looked at the sounds travelling through the sun.

-Do we need something like that to try and solve this mystery?

-Yes.

It's a really cunning technique called helioseismology.

And it's looking at the interior of the sun by using these

shock waves from the explosions in the sun's atmosphere.

And just like we can use earthquakes on Earth to study

the interior of the Earth, you can do the same thing on the sun

by looking to see how long it takes shockwaves to propagate through

and that tells you something about the solar interior.

We've been using this technique for some time now and clearly

the answer is in there somewhere but we didn't know to look before.

-Now we know to look.

-Thanks very much, Chris.

So we'll have to wait to resolve this particular mystery.

Although gravity is the master sculptor shaping the planets

and the stars, there are other factors at work,

some of which we don't yet fully understand.

But what fascinates me is that something as simple

as an object's shape can reveal so much about it.

Back at AstroCamp,

we can just see some stars poking through the odd holes in the clouds.

If it were a little clearer we could train our telescopes

on some objects shaped in different ways by gravity.

There are two very special objects

to see in our night skies this month.

The dwarf planet Ceres and the minor planet Vesta.

These two unsung heroes of our solar system lie between Mars and Jupiter.

But what makes them fascinating is their relative sizes,

which means they sit either side of a very significant divide.

At about 1,000km across, Ceres has enough mass

for gravity to have carved it into a smooth sphere.

That makes it what is known as a dwarf planet.

But that's not the case for Vesta.

Because it is smaller, its gravity is too weak to form a sphere,

leaving it with a more irregular shape.

It fits into a lesser category, a minor planet.

To become a major planet like Earth or Jupiter,

a body needs to be spherical, which rules out Vesta.

But it also needs to have cleared out all

the material in its orbit, which is where Ceres is wanting.

The orbits of both Ceres and Vesta

lie deep within the main asteroid belt.

But the planets themselves aren't big enough

for their gravitational fields to have cleared out

the nearby asteroids, which is why

they're not major planets but they're still great to see.

Coming up, Pete's guide to the highlights of what to view

in this month's night sky.

We can't see much at the minute but here's Pete with some tips

on how to take dramatic astronomical photographs when the clouds clear.

It's really easy to take some

fabulous photographs of the night sky

using nothing more than just a camera and a tripod.

This month I'm going to show you two different techniques

which can be used to take some really great shots.

First up, a wide shot of the sky

that captures a panorama of the stars.

The best type of camera to use is a digital SLR camera.

Now you need to fit a fairly wide-angle lens,

say 50mm or shorter focal length,

the camera needs to be set into a manual mode

and you need to focus that lens as accurately as possible.

You must use a tripod to keep the camera steady,

otherwise the stars will appear blurred.

To get the best results it's useful to have the aperture,

or the opening on the lens, as wide as possible

because that allows all that delicate starlight to come into it.

So use the lowest f-number you can on your camera.

You want a fairly high ISO, between 400 and 1600.

And experiment with an exposure time of around 30 seconds.

Sadly the clouds are stopping us imaging tonight

but you should end up with a photograph like this.

When you start taking longer exposures of the night sky,

if you look at each individual star carefully,

you'll see they are no longer pinpricks of light

but they start to elongate into little lines.

We can use that effect creatively to take star trail photos

and that's our second type of astrophotography.

A star trail is a long exposure photograph that captures

the apparent motion of the stars as the Earth rotates.

The trick here is to do the opposite to the wide shot method.

To get those lovely, long streaks of brilliant light,

you need a long exposure time.

15 minutes will do but you can easily push it to 30

if you're confident of clear skies.

'Remarkably some of the campers managed to get some images

'last night during a momentary break in the weather.'

That's amazing!

So you've got those lovely green lasers pointing up there

and look at that star field behind there.

It was just a gap in the clouds

and everybody got so excited.

I was just trying to capture the excitement of the astronomers

as well as the star field so it was like combining the two.

-Did somebody hold a red light?

-I kind of annoyed everybody

flashing my red light.

I said, "I'm just going to do it for a couple of seconds."

A bit of red light painting.

You painted everybody, yeah, in red light.

But that is really effective and it contrasts beautifully with

the green of the laser. It's really effective.

That's come out so well. You've got Cassiopeia down there - the W.

It's great you've got a bit of a horizon in there as well.

I think it just adds to that,

otherwise it just sort of loses itself so it's nice to have a tree

or, in this case, campervan and a few tents on the way.

-Brilliant result.

-Thank you.

While you're taking your long exposures, why not take that time

to explore the night sky in a bit more detail.

So here are my highlights of this month.

Galaxies are plentiful in May.

Below Mars at the moment is as distinctive star shape

known as the Sail -

part of the constellation of Corvus.

Two stars in Corvus point to M104 - the famous sombrero galaxy.

A small telescope shows its distinctive shape well.

The Plough, or Saucepan, sits roughly overhead around midnight.

Close by the star marking the end of the Saucepan's handle,

you'll find the wonderful whirlpool galaxy - M51.

Visible in good binoculars,

a telescope is required to bring out its spiral shape.

Early in the morning of May 24, a short but intense meteor shower

may appear to come from the constellation of Camelopardalis.

If it arrives, the meteors will be due to Earth passing through

the debris of comet 209P/LINEAR.

Finally, Saturn reaches opposition on May 10,

making it bright in the sky.

Look for it due south around 1am.

Through a telescope at opposition

the rings can appear to brighten quite noticeably.

With the cloud set in for the night,

we've retreated inside to look at this month's astro news.

We have to start with the death of LADEE,

the NASA spacecraft that crashed into the moon this month

after completing its mission to look at the moon's atmosphere,

or at least the dust that's kicked up from the lunar surface.

And it sent back just before it died this amazing sequence of pictures.

So you see the lunar horizon there.

As we flick on, what you see is a rather magnificent lunar sunrise,

something we haven't really seen like this since Apollo 17.

It's fabulous. It's great to see it. Actually if you go back a couple,

go back a few minutes, what you can see here

is that glow is dust in the solar system,

what we call the zodiacal light,

but seen from the surface of the moon. It's a beautiful thing.

It is. And illuminated by the sun. Fantastic.

Our next story comes from the Kepler mission,

whose mission in life has been to go out and find exoplanets

and it's done a fantastic job so far.

But of course the Holy Grail is to find an earthlike planet

and it looks as if it's done just that.

This is an artist's impression of an exoplanet called Kepler-186f

and it's going around a red dwarf which is slightly colder than

our sun. We found this on the outskirts of the Goldilocks zone.

So it's an earthlike planet which could have life.

It's slightly bigger than Earth

but it's just what we've been looking for.

Now we've also made another discovery just in our own neighbourhood.

This is the artist's impression

of the star that's been found just seven light years away.

So that makes it the fourth closest star known.

We only found it in the last month or so.

The reason we've only just found it is that it's rather cool.

In fact so cool, that it has

a temperature you'd expect at the Arctic -

-13 degrees centigrade.

To me that's counterintuitive. Stars shouldn't be cold.

You could think of this as a really big planet

but it will also have weather and clouds

just like the giant planets do.

The definition planet, star, where do we lie?

It gets much more interesting.

We've never really decided so let's just say we've found a new object

and it's very exciting.

Now back to how gravity shapes the night sky.

Chris North and Jon Culshaw are looking at how gravity

works on a huge scale to create galaxies of many different shapes.

With most of the campers on their way to bed

and the cloud still thick,

Chris and Jon are having to resort to some well chosen

photographs to guide us around an extraordinary spot in the night sky.

One particular zone we've been looking out for tonight

just above the constellation of Virgo is an area where, many a time,

you wouldn't see too much detail in there.

But get to a dark sky area like the Brecon Beacons

and place a scope on this particular zone

and it comes alive wonderfully, and you can see exactly

why this area is called the realm of the galaxies.

There are dozens, if not hundreds of galaxies to look at

with a small telescope in this area of the sky.

A bigger telescope will obviously show more.

And something that is called the Virgo cluster.

This image here shows us half a dozen bright galaxies

and dozens more fainter ones, all different shapes and sizes.

You can see elliptical galaxies that look like spheres.

We've got a close-up of a galaxy here and it looks like a round,

spherical blob. There's not a lot of structure there.

Rather like a supermassive star.

And that's the combined light of billions of stars all glowing

together, so it's quite a humbling thought when you think of that.

So if we take a look at the very familiar spiral galaxy,

what would be the forces that would cause a galaxy to form

rather like this?

This is one of the galaxies in the Virgo cluster,

this is M100, Messier 100,

and you can really see the characteristic spiral form.

What this doesn't really tell is quite how flat this structure is.

That's very much the way that gravity evolves our own Solar System

around the sun, this familiar flat disc.

Both the Solar System and galaxies form from roughly

spherical-ish blobs, clumps of gas and dust.

They collapse under gravity.

If there is a preferred direction of rotation to that gas and dust,

then that will settle into a disc.

And that's what happens with spiral galaxies like this,

the gas and dust collects into this disc

and new stars form and we see the patterns we see today.

And then in the solar system as well, the gas and the dust

collect into a disc and it's out of that gas and dust that the

planets form and that's why they're all in the same plane.

Essentially the same process.

Just because of this common axis of rotation.

What's behind the formation of the elliptical galaxies?

Well, this is an example of an elliptical galaxy.

What's happened is that elliptical galaxies

have formed from the mergers,

the combinations, the collisions

between other galaxies over billions of years.

And because that's lots of things combining together,

there's no one favoured direction

and you don't get this flat disc that we see in the spiral galaxies.

Isn't it fascinating to think that at

the centre of galaxies, where the stars are much more dense

and much more tightly packed, imagine being on a planet

orbiting one of those stars, what kind of a night sky would you see?

There'd certainly be many, many more stars in the sky.

And one of the reasons we can look at the Virgo cluster

and study it in the detail we can

is because we are looking out of our own Milky Way galaxy.

If we were in the centre of a galaxy

and there were stars all around, we wouldn't be able to do

extragalactic astronomy and look at other clusters.

There might be no evidence we were inside a cluster

or group of galaxies at all.

So we really are in quite a special location here to be able to

look out of our galaxy and see the rest of the Universe.

Next, we're sticking with gravity

but this time how we can see it in action closer to home.

Now one of the most beautiful things

to look at in the night sky must be Saturn.

The sixth planet in the solar system orbiting about nine times

further out from the sun than we do.

And with relatively basic equipment it's possible to get

a beautiful view of its magnificent rings.

And it's to these rings that we turn next because we've seen something

that's never been seen before - a moon forming amongst the rings.

Earlier today,

I talked to Dr Caitriona Jackman about these observations,

and about what they might mean for the early Solar System.

The NASA spacecraft Cassini has been orbiting Saturn

for the last ten years.

And in that time, it sent back incredible images

of the gas giant, of its moons

and of its glorious rings.

Cassini sent back some of the most spectacular images of the last

decade but the one that people are excited about right now is this.

So what are we looking at?

So this is a beautiful image that the Cassini spacecraft cameras

have taken when this spacecraft was looking down on top of the rings

and this is an image of a very bright feature

on the outer edge of the A ring.

So we're actually looking at this blob down here in the corner?

It's a very special blob.

It's actually the formation of a brand-new moon.

So it's material clumping together under its own self gravity

and in doing so dragging material out of the rings with it.

So a very small moon at the centre,

probably less than a kilometre in diameter,

but as it's orbiting around within the rings,

it's dragging local ring material and collecting it onto itself.

So how unusual is this?

This is a one-off.

This is the first time that we've ever seen anything like this.

This is our first time that

we've ever seen a moon being born in real-time.

-Saturn already has 60 moons.

-It does.

-Is it continually producing more?

We don't think so.

And what makes this opportunity so rare is that, as you say,

Saturn has more than 60 moons

and has some very famous icy moons in particular,

like Enceladus, and it is thought that Saturn's rings

used to be a lot bigger and that moons like Enceladus

were formed from the rings.

As those moons were formed by material in the rings

clumping together, they took a lot of material with them

and so the rings that we have today are quite depleted

relative to what they once were.

-So this is 2013 this image, so from last year?

-Yes.

Do we know how our new moon is doing?

-I kind of want it to succeed.

-Yeah, I want it to succeed too

but we're not sure what's happening to one part of it.

It's broken in two.

So object one is moving in through the rings and, as of last week,

it's causing a lot of disturbance locally and it's pulling

and tugging at the ring material near it.

Object two has gone the other way.

So object two has migrated out of the rings

and it's actually too small to be observed directly by Cassini.

-We may catch it again on a further orbit.

-I hope so.

And of course this tells us about the rings

but it's also just telling us about physics.

If we go back five billion years

or so to a Solar System that looks something like this,

this disc of material from which the planets are forming

-looks rather like a ring system.

-It does, yes.

So this is a disc that formed from the solar nebula

and from this disc you had planets and proto-planets forming

and then migrating outwards from the point of formation.

And so observing the formation of a moon like this,

and then its subsequent migration out,

is kind of a window on what might have happened

in the formation of the early Solar System.

And so Cassini has been there ten years

and we are still getting fabulous science from it.

What's next for the mission?

Cassini has got another three years to go.

It is going to finish in September of 2017 with a plunge through

Saturn's atmosphere

where the spacecraft will automatically vaporise.

-That's to get it out of the way.

-Yes.

-It has to end up somewhere

-and we don't want it crashing into anything.

-Absolutely.

Before it vaporises, we're going to make

the best use of the time that we have left.

The final phase of the mission

will take the spacecraft just above the upper

atmosphere of Saturn and between the inner edge of the D ring.

That's a unique vantage point...

-That's between Saturn and the rings.

-Absolutely.

So you're looking outwards at the rings for the first time ever.

That's going to shed light not only on Saturn's ring system

but on discs more generally and on how rings

and moons form more generally in the Solar System.

-And how gravity works wherever it is in the Universe.

-Yes.

I look forward to seeing those images

and to hearing you talking about the results.

Thank you for now. Thanks a lot.

Last month we launched a competition to give one viewer the chance

to take control of HiRISE -

the most powerful camera in Martian orbit -

and choose a location for it to image.

We can announce the winner is John Green from Cambridge.

He's chosen a spot in the canyon Hebes Chasma

that he thinks has an odd black mark.

Hopefully the satellite will take the image in the next few months

and we'll put it on our website as soon as it reaches Earth.

Well, that's it for this programme.

Wonderful star parties are happening all over the country

so check on our website to find out what's happening near you.

When we come back next month, we'll be talking about the awesome power

of impacts - from asteroids in our Solar System to the distant cosmos.

-In the meantime, get outside and get looking up.

-Good night.

MUSIC: "Pelleas and Melisande: At the Castle Gate" by Sibelius