The Sky at Night

This is the first map of the Milky Way

made by William Herschel back in 1785

in a paper he called On The Construction Of The Heavens.

And it's a wonderful thing, but it's not that accurate.

It shows the sun at the centre, which is wrong.

There are no spiral arms.

But he did get one important thing right.

He shows the Milky Way as a disc seen almost side on.

Since then, our knowledge of the Milky Way has greatly improved,

but these images are actually artists' impressions.

We think that the Milky Way looks like this,

but we don't know for certain.

In fact, we know surprisingly little about our home galaxy.

But a revolution in our understanding

of the Milky Way is underway,

and the more we learn about our galaxy,

the more surprising and interesting it becomes.

So tonight, we bring you the bang up-to-date Guide To Our Galaxy

with the most exciting and unusual discoveries.

Welcome to The Sky At Night.

Like all good guides, ours will include

the best maps available,

the most interesting destinations

and, of course, a bit of history.

We'll be joined by guest presenter

Neil deGrasse Tyson for a tour

of his top five strange and spectacular stars.

This white dwarf is effectively flaying

the outer layers of this giant,

consuming it, body and soul.

Pete will show us how to find some of the highlights of our galaxy

in the night sky.

But first, we're going to start

with the most essential part of any guide, the map.

We're here at the Institute of Astronomy in Cambridge,

where the researchers have been making the most detailed

and the most accurate map of the Milky Way ever.

To do that, they're using one of the most impressive space telescopes

ever constructed. If it were here on Earth,

it would be capable of measuring the thickness of a coin on the moon.

In December 2013,

the European Space Agency launched the Gaia space telescope.

Its mission - to map the stars of the Milky Way

in unprecedented detail.

Gaia is actually two telescopes,

each observing 100,000 stars per hour

and capturing the data with a one billion pixel camera,

giving Gaia resolution that is ten times greater

than most ground-based telescopes.

Over its five-year mission,

Gaia will observe each part of the sky 70 times

in order to measure the exact positions of the stars,

their distances and their motions.

This room is where data from Gaia comes pouring in and gets processed.

It's effectively the nerve centre for understanding our galaxy.

You get some idea

of the scale of the data-processing operation in here.

This supercomputer is made up of 1,296 processing cores...

...but this is just the tip of the iceberg.

There are five more centres like this dotted around Europe,

all handling Gaia's data.

To convert raw satellite data from Gaia

into an actual map of the Milky Way

with precise positions, distances and brightnesses is no mean feat.

In fact, for the first batch alone,

they've had to analyse around 120 billion images.

And to achieve the incredible precision

required by the Gaia mission,

the data-processing teams have to try to correct

for every possible source of error.

They even have to account for the gravitational influence

of the other planets in the solar system on the light from the stars.

It's a tremendous task,

so much so that it took a year to process

the first 14 months' worth of data.

But it's been worth all the effort,

because the result is nothing less

than the first truly accurate map of our home galaxy.

And Chris went to meet lead scientist Gerry Gilmore

to see the map for himself.

So, Chris, welcome to our new map of our Milky Way.

It's beautiful.

You can see immediately the sort of big picture structure.

First thing to notice is that this

is a map of the entire sky.

But this stripe across the middle,

this bright stripe, that's the galaxy.

That's the Milky Way galaxy, that's right, yes.

But there is more to the Milky Way than just the main disc.

There are also millions of stars, both above and below the disc.

Everything white that you can see in this image is starlight.

What we need to do is zoom in.

This is the top-layer map, and now we are zooming in.

So this is the inner parts of the Milky Way here,

these big dust clouds and dust lanes,

the dust lane, these are dust lanes,

and you start to see that all the white light

is now breaking up into stars,

into individual stars.

So, as you get further in, as we zoom in,

we see more and more stars individually.

Look at how they suddenly appear

as you go that little bit deeper into space and, ping!

What you thought was just white light is actually starlight.

And how many stars are there in the galaxy?

There's maybe 100 billion, 200 billion, 300 billion.

The number's probably near the high end of that range.

What we know is that there's maybe twice as many

as we previously thought.

There are twice as many stars in the galaxy as we thought?

That's right, yes. It's just in the bit of the Milky Way

that Gaia has measured. There are twice as many stars in there.

How has Gaia doubled...?

That seems surprising to me.

I assumed that you didn't know the position before,

but where have they been hiding?

It's just because the image quality of Gaia,

the fact that Gaia has such high spatial resolution.

Because it can see sharply?

Exactly, see sharply,

and so Gaia can tell the difference

between two or three stars that are very close together,

but, nevertheless, are separate stars,

whereas, previously, from our images from the ground,

blurry sort of things,

this stuff here would have all been

merged into what we thought was one star.

And so, we've been looking at the map as it appears on the sky,

but one of the exciting things about Gaia is

that we have three-dimensional information as well.

We can go beyond this sort of two-dimensional picture.

Exactly, and that's the unique feature of Gaia.

Gaia measures distances as well as all these other things.

And once we get into measuring distances,

then we can measure the three-dimensional structure

of our Milky Way.

So far, we've only just had a taster of that,

so Gaia's just released two million accurate distances.

And so, here is the beginnings of a picture in three dimensions.

This is our sun, and we are about to go and see

the Hyades and the Pleiades cluster.

Both part of the autumn sky.

Oh, look.

There's Betelgeuse and the Hyades there

with Aldebaran in front of them.

The Pleiades in the corner.

There goes Aldebaran. It's closer to us.

And so, here we are, for the first time ever,

seeing a star cluster in three dimensions.

So we can tell the difference

between the front side and the back side of the Hyades.

So we know exactly in 3-D where all these stars are.

And this helps understand the cluster.

We know more about it than we did before we had this map.

And so, we have already discovered

not only how deep the Hyades cluster itself is,

but, actually, it's about twice as big.

It's amazing. What other features should we look for in this 3-D map?

So this dramatic Hyades example,

one very nearby cluster, is just a taster for what's going on.

We've got good distances now for just two million stars.

We are going to have one billion at least,

that will give us a three-dimensional map

of half of our Milky Way.

But, even more interestingly,

because Gaia's continuing to observe over time,

it's telling us how everything's moving,

and that combination of where things are

and how they are moving

allows us to determine how the galaxy works as a machine,

how the Milky Way actually functions, what is a spiral arm,

where is the dark matter and how it's evolving.

Well, it's going to be very exciting.

It's going to be fantastic.

So now we have a map of over a billion stars in our galactic guide,

but the question is, where do we go and visit?

What are the most interesting and exciting stars within our galaxy?

We invited astrophysicist Neil deGrasse Tyson,

who has written about this very subject

in his book, Welcome To The Universe,

to give us a guide

to the weird and wonderful stars of the Milky Way.

The sun.

It's a million times bigger than the Earth.

It's widely regarded as a yellow orb in the sky, but it's actually white.

You know, our sun is not the hottest

or the coolest or the biggest or the smallest.

There's nothing to distinguish it

among the hundreds of billions of other stars in the galaxy.

In fact, if you were an intergalactic alien,

and you came upon the Milky Way,

there'd be nothing about our star system

that would attract your attention.

But there's plenty of other stars in our galaxy worth a look,

and I've made a personal list of the top five.

Coming in at number five

of our exotic star countdown of the Milky Way

is SAO 206462, previously thought to just be an ordinary star.

When you go in and look close,

and you blot away the light of this relatively new star