Hello... And welcome to the University of Cambridge
and to the beautiful courtyard at Trinity College.
This place is home to Astronomer Royal Martin Rees,
and a few centuries ago it was home to Isaac Newton,
who knew a thing or two about gravity.
This month we are talking about some of the universe's most strange and
perplexing objects, objects that would have confounded Newton,
I'm sure - black holes.
Exotic and perplexing they might be,
but black holes have turned out to be rather important.
We know that large ones lurk in the centre of almost all galaxies
and we are still struggling to understand these strange beasts.
But, we are about to witness something for the first time
that will help our understanding.
A cloud of gas and dust has come dangerously close to the black hole
in the centre of our galaxy, the Milky Way,
and it's currently being ripped apart as it orbits around it and
astronomers have a bird's-eye view of this gory encounter.
But before all of that,
here's our guide to the September night sky with Pete Lawrence.
The warm September nights can be a delight for observing
and it is getting darker earlier, too. After sunset,
you still have a chance to enjoy the Summer Triangle.
Face south and look slightly to the west or right to see
the triangle formed from the three bright stars,
Vega, Deneb and Altair.
If you have clear, dark skies,
you will see the Milky Way running down the triangle.
Now to find our nearest big galaxy, the Andromeda Galaxy or M31.
As the sky is darkening,
look to the east to find the Great Square of Pegasus.
A wedge of stars can be seen heading away from the upper-left star
in the square and this represents the constellation of Andromeda.
Sitting near the star Mirach is the galaxy,
and it looks like an elongated smudge of light.
If you spot it, the light reaching your eyes
left the galaxy around 2.5 million years ago.
All the charts to find these objects are on our website at...
Cambridge University is over 800 years old,
but by the late 18th century,
astronomers wanted darker skies for their new telescopes
so they could glean information about the universe around them.
The Cambridge Observatory was built in 1823 on the site
known as Gravel Hill.
The Directors and their assistants lived in this building,
and on the roof is a very pretty dome.
'Getting there requires a bit of negotiation!'
It's a bit like Alice in Wonderland in here.
Everything keeps getting smaller and smaller!
This is one of the original domes of the Observatory and in the 1930s,
there would have been a four inch equatorial telescope
placed right here.
Now, unfortunately, the observation stopped in the 1960s and in fact,
they have since lost track of where this telescope went.
Physicist Arthur Eddington was the Observatory Director
from 1914 to 1944.
Eddington was fascinated
with Einstein's Theory of General Relativity.
This photo of them was taken outside the Observatory in 1930.
The Institute of Astronomy was created in 1972
and is based on the site.
Today, around 180 astronomers work here on objects from stars to
distant galaxies and, of course, black holes!
Chris, Paul Abe and I took a look at how Einstein viewed gravity
and how that relates to black holes.
So, this is a story about gravity,
and to understand gravity, we need to understand space.
And I have got some space here.
This is an empty universe and so, space is flat and undistorted.
So, let's put something small into our universe, like the moon.
And if we pop it in...
..as you can see, not much happens.
To have a bigger impact, we need to put an object in that's more massive,
so this represents the sun and when I put this into our universe,
you can see immediately that it changes the shape of the fabric,
the shape of the universe,
and that is felt through the effect of gravity.
Gravitational field, the distortion around the sun,
allows things like planetary orbits to occur.
So, there is a planet, happily orbiting the sun
because of the curvature of space.
Now, we could scale this up and put in the place of the sun,
an object that is incredibly massive, yet incredibly small
and that's the extreme case that forms a black hole.
Now the distortion is so great that nothing can escape
if it gets too close - not even light.
Black holes are notorious, with reputations that would suit
the most destructive objects anywhere in the universe.
Theorists with their relativistic equations proved that black holes
could exist, but showing they really did was another matter.
And astronomers needed wavelengths other than visible light to see
black holes in all of their glory.
This is the Andromeda Galaxy - 2.5 million light years away -
and a whole bonanza of black holes
have been discovered amongst its stars.
These bright X-ray sources are the result of a 13-year study
by NASA's X-ray telescope, Chandra.
This is the galaxy Centuros A, 12 million light years away.
In the optical invisible light,
it is one of our prettiest nearby galaxies.
But at its heart is a supermassive black hole
55 million times the mass of the sun.
Nine radio telescopes worked together and revealed this jet
emanating from the black hole, which is circled here.
Those jets are so powerful that they create these giant radio lobes
where they're smashing into the surrounding material.
In the quiet at the Institute of Astronomy, it is
coffee time, where professors and students alike meet to share
ideas, talk about the latest research and have a bit of a gossip.
I spent the summer working here as a student.
I dreaded anyone coming up to me
'and asking me a question and I still feel a bit nervous today.'
Sir Martin Rees is the Astronomer Royal and he spent a career
studying black holes and their weird effects on space and on time.
'Martin and I went up to the old observatory building to the room
'that was once Eddington's drawing room, but which is now the library.'
I thought I might start by asking
why people are fascinated by black holes.
It's an idea people have grabbed on to. Why do you think that is?
Well, it's true they've figured in popular culture
and science fiction more than most other scientific concepts.
That's because, in a way, the idea is quite easy to grasp.
The idea is that an object has collapsed,
become so small or so dense that gravity is too strong even to allow
light to escape and the black hole therefore has a property
that it is possible for things to fall in, but nothing can come out.
So, let's take a typical black hole, something, I don't know, with a few
times the mass of the sun. How would something like that form?
It's thought that it forms as the end point of a supernova explosion.
Supernovae are the way stars that start off with more than
eight or ten times the mass of the sun
and the end point, when the explosion is over,
will be a compact remnant,
either a neutron star, which we see as a pulsar, or perhaps a black hole.
It's thought that the more massive stars, maybe above
20 or 30 solar masses, are the ones most likely to be black holes,
rather than neutron stars.
And we now know black holes have a size which depends on their mass
and if you had a black hole that was
the mass of the sun, its radius would be only about three kilometres.
In that radius that you are talking about, it sounds a bit like
we are talking about the black hole having an edge, but what we're
really talking about is something called the event horizon.
That's right. It's not a hard surface.
It's like a sort of semipermeable membrane.
Things can cross inside it and nothing can come out of it.
It's the point of no return. That's right.
And also space and time gets screwed up in rather complicated ways
near a black hole.
In particular, if something falls straight into a black hole, then if
you are the unwary experiment who is falling in,
then you will cross this surface, so-called,
and you will get to the centre
where gravity becomes infinite
and you reach a so-called singularity.
And you are what some people call spaghettified, torn apart
and distorted.
But if someone is watching you from a safe distance,
then they will actually never see you get past this horizon.
So if we send... we sit here in Cambridge
and we send a student into a black hole, we will watch them recede...
Watch them recede, but however long we watch them for,
we will never see them get inside this horizon.
As they get nearer the horizon during the fall,
then any clock they are carrying runs slower and slower, so even if you
watch for an infinite time,
we will only see a finite part of their future.
And we will therefore never observe the exciting or devastating
part of their future when they get across, so
if you want to know the answer, it's no good sending a student!
As they fall in, they'd be able to look out at the outside world,
they can see us,
they may not even notice they've crossed the event horizon, I guess.
Right, and they could in principle see a speeded up version of part
of the future of the external world in their final moments.
But not be able to communicate them back. Indeed, right.
In the 1960s,
astronomers noticed what they call quasars, extremely luminous
objects at vast distances, shining out across the universe.
At Cambridge, they did much work to catalogue these enigmatic beasts.
We now know that these quasars are galaxies powered by supermassive
black holes, which are gobbling stars and gas at a ferocious rate.
As the matter is shredded on its way in,
enormous amounts of energy are released in jets,
like shrieks across space.
We now believe that supermassive black holes
lie at the heart of all galaxies.
There's a mystery about how these very big black holes form.
One thing we have learnt is that even
when the universe was only one or two billion years old, I mean,
10% of its present age, there were already some big galaxies which
hosted in their centres, black holes weighing as much as a billion suns.
And this is a bit of a conundrum,
how these big black holes can form so quickly.
And I think the most likely picture is that
an object of about 10,000 or 100,000 solar masses forms in one go
fairly quickly and then gradually gains extra mass
by gas and stars falling in.
We know that once they are formed,
there are two ways in which they are going to grow further.
One is that the gas which makes them
shine brightly, the quasar phenomena, is going
to fall into the black hole and add to its mass,
so the black hole is gaining mass all the time,
but the other thing that's happening is that as a consequence
of this hierarchical build-up of galaxies, there'll often be
situations when two galaxies,
each with a black hole in its centre, merge.
I mean, we see that happening all the time.
We see that happening and when that happens,
we would expect that the black holes will eventually settle towards
the centre of the merged system and spiral together
and make one big black hole,
adding together the masses of the two.
This colourful picture shows the centre of our galaxy,
and at its heart
is a black hole which has more than 30 stars orbiting it.
This is the result of a 16-year study
by the European Southern Observatory in Chile which shows the stars
caught in a dangerous gravitational dance.
The stars are out of danger, they are safe,
but it is a tango which is about to go disastrously wrong
for a cloud of gas which has got too close to the black hole.
Astronomers have been predicting what might happen as the cloud
gets pulled in and then shredded by the black hole.
We've got a ringside seat
for what seems to be an incredibly rare event.
So, this is what we are hoping to see? We hope so, yes.
Lovely, look at that.
The real observations are not as pretty as the movie,
but they do show the cloud being stretched by the black hole.
Paul Abel caught up with Dan Wilkins,
who's watching the event with keen interest.
So, Dan, what do we hope to learn about our own supermassive
black holes and galaxies from this? What do you think it will tell us?
It'll really put Einstein's theory of gravity,
that's general relativity, to the test, making sure the orbits
it predicts are correct.
Because there's already material in the centre of our own galaxy,
this gas cloud as it comes in will interact with it,
collide with it, so it will give us a probe of how the gas is arranged
in our black hole and will give a view for the first time
of a black hole actually in the process of swallowing material.
In these active galactic nuclei,
enormous amounts of energy are released,
but we really don't have any idea exactly how gas falls into
a black hole and how the energy gets converted into the radiation we see.
And what personally, if you could control events,
what would you like to happen?
So, a black hole physicist, really wanting to understand how
these bright objects work
and of course just wanting to see a big, bright light in the sky,
what I want to see is this gas cloud get shredded to pieces
by the gravity around the black hole and just have it all,
all fall in or get swallowed up.
That's very, very violent but I think I'd like to see that too.
Dan, thank you very much. Thank you.
The Cambridge site still has many old telescopes left
now used for the public.
But there is still one large working professional telescope here.
Roger Griffin is one of astronomy's valiant soldiers,
still battling on with the English climate.
For the past 56 years, Roger has been looking at stars
and their orbits on average about 140 nights per year.
Roger is also interested in how stars move relative to us,
known as their radial velocity, and also binary stars,
where two stars orbit around each other.
His favourite cluster is the Hyades,
and he is proud of the fact that he's catalogued 52 new star orbits.
Hello, Professor Griffin. Hello. This is a magnificent telescope!
Can you tell me a bit about it?
What's your typical observing night like here?
Well, I work the whole night if the conditions permit.
In the summertime, that's not difficult
and that's sort of 10pm until 4am.
In the wintertime, it can be a long night.
I can start observing about 4.30pm in the afternoon in December
and work through until 7.30am in the morning.
Some people might be surprised to have a 36-inch
telescope in this country for research. Yes, it's not popular.
People like to go to overseas telescopes,
but of course they don't get as much observing time as there is here.
Now, you've been observing here since 1957,
so you must feel very familiar with these stars you've been studying.
Yes, many of them I have observed 50, and some of them 100 times.
One that I have observed most often, which is a star which changes its
velocity but in an irregular way, is called our R Coronae Borealis,
and I have observed it on more than 1,000 nights.
And I see them as friends,
but I don't think my friendship is reciprocated!
In the early days of astronomy,
finding out how a star was moving was a long
and tedious process and involved taking photographic plates
and analysing the star's light by hand.
What Roger did was to find an elegant
and simple solution that drastically sped up the whole process.
Starlight can be spread out into its rainbow of colours or spectrum.
In this are many dark lines that are like the star's fingerprint,
showing us what it is made of. Dark lines also tell us something else.
By observing the spectrum on a number of nights, you can
tell how fast the star is moving from the position of the dark lines.
When Roger first started, astronomers recorded their spectra
on photographic plates, each one needing considerable time to analyse.
What he did was to develop a new method which meant
the astronomer looked only at the information they needed.
It automated the work, making it more efficient and more accurate.
How many stars are you able to measure each night using this
technique? In the summertime, it may be 50, and in the wintertime, 100.
50? Yes. So you went from having a couple per day
to potentially 100? Yes.
It must have been so exciting to have that transition.
Well, it was exciting because for some years, actually, I made
more and better radial velocity observations here in this dome
than the rest of the astronomers in the world put together!
The autumn nights are closing in.
Let's hope Roger gets clear skies to continue his tireless star catalogue.
Go to our website at...
..for Pete's charts for what to see in September.
We also have a film of the team's astronomy adventures at last year's
Peak Star Party, and there is an extended version of the interview
with the Astronomer Royal about black holes.
Don't forget, there is still plenty of time to take part
in our Moore Moon Marathon throughout September and early October,
so do visit our website for more information and to get those maps.
Thank you to everyone at the Institute of Astronomy
and the University of Cambridge for hosting us this month.
Next month, we'll be at the Brecon Beacons Star Camp,
where we are expecting some of the darkest skies in Britain!
Let's hope it is clear.
We'll also be at an international meeting of planetary scientists
in London and so, until next month...
BOTH: Good night.
Subtitles by Red Bee Media Ltd