BBC The Sky at Night (1957) s25e10 Episode Script
Light Echoes
Good evening.
And welcome to my garden, with one of my telescopes.
It's been a lovely, hot, glorious day, but in Selsey, it generally is.
Now, echoes.
We all know about sound echoes, but did you know we can also have light echoes? I can assure you we can.
The story really goes back to the year 1572, and the Danish astronomer Tycho Brahe.
Now Tycho was a remarkable man.
He had a very eventful life, and when he was a student, he fought a duel, had part of his nose sliced off and he made himself a new nose out of gold, silver and wax.
He was passionately keen on astronomy.
Obviously, no telescopes.
They didn't come along until years later.
But Tycho set up an observatory on the island of Ven, and there, he drew up an amazingly accurate catalogue of stars.
It remained the best for a long, long time.
A masterpiece of careful, accurate work.
Come now to 1572, Tycho here puts the starry sky.
Of course, he knew all the stars perfectly.
And he looked at the constellation of Cassiopeia.
Well, you must know Cassiopeia.
The five main stars make up a W.
On the far side of the pole star from the Great Bear.
Tycho looked them and there was something else there.
A very brilliant star, where no star had been before.
Became visible to the naked eye in broad daylight.
And then, as the months passed by, slowly faded away, and was lost.
Until 1952, when the English astronomers Hazard and Hanbury Brown detected a radio source in just the position where Tycho's star had been.
We now know what that star was.
It was a supernova.
The sudden throw up of a very faint star.
Well, what's all this got to do with light echoes? Chris, over to you! Thanks, Patrick, but don't go anywhere, I need you to represent Ti-co Brahe Not Ti-co Brahe - Tycho Brahe! I've got a gold, silver and wax nose to prove it.
I think it suits you, Patrick, and I know better than to argue with somebody who fights duels.
But if you're Tycho down on Earth, that I must be the star, the star that he saw a few hundred years ago.
The year, in fact, is 5500 BC, and I'm about to explode, to go supernova.
Light from the explosion is emitted in all directions, and Pete and Paul are going to represent two nimble light rays heading out into space.
One of them, Pete, happens to be heading directly for Earth.
But Paul is heading into deep space, or at least he is until he collides with a cloud of gas and dust, and is reflected towards the earth.
Time goes by, and as we reach 1572 AD, the first of our light rays, Pete, is arriving at earth, where he is observed by Tycho Brahe - got it, Patrick? I have indeed.
Excellent.
So Tycho has seen the supernova, but it's not until 2008 that the second light ray, Paul, arrives at earth and is observed by the Subaru telescope in Hawaii.
That's all a light echo is.
It's a reflection that allows us to see light through the supernova many hundreds of years since it happened.
We are actually seeing the spectrum of Tycho Brahe's star.
That's right, and the spectrum is important because that's how we know the light came from the supernova.
It has the same spectrum - the same fingerprints as light that we might observe directly.
Light beams, thank you very much.
Well, light echoes aren't common, but there are other examples.
And recently, I talked about them to our two eminent guests.
Professor Mike Bode and Dr Tim O'Brien.
So, light echoes are really important.
I think quite excitingly, in astronomy we're used to being limited to one particular point of view of an object, we're sort of stuck here on the Earth, and we're looking in one direction, so we tend to see things from just one side.
But with a light echo, because the light's travelling in different directions, and bouncing back off these clouds around it, we actually get to have a look round the back of something in space, which is quite a bizarre idea.
It certainly gives us a lot more information about the three-dimensional structure of these astronomical objects.
The story of light echoes really begins with a star that flares up in the constellation of Perseus in 1901, GK Persei.
That's right, GK Per was the first first bright Nova of the 20th century, and it changed the face of the constellation of Perseus.
Now that explosion of GK Persei in 1901 was soon followed observationally by wispy nebulosity seen to surround it.
It was realised that actually those nebulosities seemed to be spreading out faster than the speed of light.
You can't do that.
But you can't do that.
It was known even back in the early 1900s that nothing could travel faster than the speed of light, and the crucial point here is that no thing can travel faster than the speed of light, so what was at first thought to be material ejected in this nova explosion moving at these extreme speeds, we weren't actually seeing material moving.
What we were seeing were different parts of pre-existing material around the site of the nova that were being illuminated by the flash of the explosion itself.
It's just an optical illusion that it's faster than the speed of light.
One picture I like shows the exploding star V838, in the constellation of The Unicorn.
Yes, there's a lovely sequence of images that were taken by the Hubble space telescopes back in 2002, when this star brightened suddenly.
What you see in these images is this gorgeous view of the reflected light scattered back off the material around the star itself.
So when you see what looks to be expansion of material thrown out in an explosion, that's not the case.
All you're seeing is this repeated scattering of light, as it moves through the material around the star, it's really a beautiful sequence.
Other fascinating things too, RS Puppis? Yes, RS Puppis is a well-known Cepheid variable.
Their luminosity, the amount of energy they give out per second is related to the pulsation period.
And RS Puppis is a Cepheid variable with a period of around 41 days, that is surrounded by gas and dust, and that gas and dust is scattering the light from the cepheid, and you can see it illuminating this pre-existing material, waves of illumination spreading out from the centre.
So it looks for all the world like there's these ripples of material expanding out, but in fact they're just ripples of light that come from the pulsing star at the centre.
Again, echoing the light variation of the central star.
Come back for a moment to supernovae.
We haven't had one in our galaxy for a long time now.
But we did have one in 1987, in the large cloud of Magellan, the satellite captured it at 169,000 light years away.
Yes, in fact in the case of 1987 a, we observed light echoes in the form of two rings, and these rings are caused by scattering from dust in planes between us and the supernova.
If you imagine the light from the supernova spreading out in a sort of sphere as it spreads out away from the supernova, and then there's these sheets of dust that sit between us and the supernova - where that sphere of light intersects those planes of dust, you see these big circles on the sky.
Supernovae are unbelievably powerful.
They don't happen often, but there are records of them going back a long way.
Yes, in fact there are historical records going back to several hundred BC.
We have extremely detailed records, particularly from the Far East, of supernovae that went off in 1006 AD, in the constellation of Lupus.
The object is the brightest recorded stellar object ever, it was so bright that it was equivalent to something like the half moon, the first quarter moon, put into a single point.
It cast shadows, it was even said that at midnight you might have been able to read manuscripts by the light of this object.
It was seen in daylight for a long time.
It was seen at night for couple of years or so.
But no records from Britain? No records from the West, except for one, actually, which was from a monastery in Switzerland.
The problem was that the dogma at the time in the West was that nothing above the moon changed, so if you saw something that had changed, like a new star appearing, if you then noted that down, you might have been burnt at the stake.
The same thing happened in 1054, when another supernova was observed, this time in the constellation of Taurus.
It was visible in daylight for 23 days, visible at night for many, many months.
And nobody in the West, as far as we are aware, noted it down.
The remnants are now detectable both by visual and by radio.
The remnant of the supernova that we discussed in 1054 is the Crab Nebula, one of the most beautiful objects in the night sky.
It's 11 light years across, now 10,000 times the size of the planetary solar system.
And material expanding at 1500 kilometres per second.
And very easy to find.
Indeed, I'm very proud of the fact I found it myself with a small telescope from my back garden! One of my major achievements as an astronomer! I've seen it with binoculars in the same field.
Then come to 1572, the Danish astronomer Tycho Brahe.
One evening, he looked up in the constellation of Cassiopeia - a star that hadn't been there before, miraculous.
That was a supernova.
That supernova, again, was extremely bright as a visible object, about as bright as Venus gets in the sky.
And it was seen continuously and noted down in the West this time, particularly by Brahe, over a period of 18 months or so, until it faded from view.
Very recently, a group of astronomers working at the Calar Alto observatory in Spain imaged regions of the sky close to the site of Tycho's supernova, and they spotted these very faint wisps of nebulosity, and in a series of images, they saw this nebulosity moving across the sky, much like the light echoes we saw earlier, and when you point to where they appear to be moving from, it pointed directly towards the site of the explosion.
So this animation shows a God's eye view, if you like, illustrating what light echo of 1572, what the cause was.
So here, the supernova is just about to explode, it brightens, and you will see the sphere of light travelling out from the supernova, some of it goes backwards away from the Earth, but then reflects back off that cloud of gas, in interstellar space.
So you've have now got the first light that arrives directly from the supernova arrives in 1572, when Tycho saw it, and the light that's travelled further and arrives later is just arriving now from the sites of these light echoes.
You might think that we'd missed our chance to observe what was one of the closest type one A supernovas there's been in our galaxy, and we know these things are fundamental to our understanding of cosmology now, but amazingly, this light echo phenomena gives us this chance to see effectively the same light, that Tycho would've seen, but it's been on a longer journey - it travelled away from us, bounced back up a cloud and is only now arriving.
Well, very informative indeed.
Mike, Tim, thank you very much.
Thank you.
Thank you.
Well, I'm back in the garden, it's getting dark now, very interesting.
These light echoes can turn up in all kinds of places, have all kinds of forms.
And there's one very odd case, in that organisation you began yourself, Galaxy Zoo.
Yes, this is my project, galaxyzoo.
org on the Web, which invites people to sort through images of galaxies - mostly we're interested in the galaxies themselves, but in this one particular image, a Dutch schoolteacher found this green blob, looks a little bit like Kermit the Frog, or some people think it looks like a runner.
It's called Hanny's Voorwerp - "voorwerp" was her name for it, we thought it was a technical term, like quasar or pulsar and it turns out to mean thingy.
I'm very proud that Hanny's thingy is now in the literature.
This is actually a light echo, but a light echo on an unimaginable scale.
You remember that's a whole spiral galaxy there, so the blob is tens of thousands of light years across.
What's happened is that galaxy, which goes by the name of IC2497, used to have a very, very bright nucleus, the black hole at the centre was feeding, the material that was dropping in gave out radiation, you would have been able to see it with binoculars, would you believe.
And that light has hit gas, excited the gas which has emitted light, and we see it as an echo of what was happening at the centre of the Galaxy 20, 30, 40,000 years ago.
We've pointed the Hubble space telescope at it.
I hope, in a few months' time, I will show you the most amazing images from Hubble, of this unique object.
Things may turn up in any way, and this certainly has.
Well, Chris, congratulations on that.
You started the entire programme.
That's true, but it's that instinct to go, "What on earth is that weird thing?" It turns out, as so often, to be a light echo.
Chris, thank you very much.
We won't see that tonight, but there's plenty to see.
Pete and Paul are getting ready to look for a comet.
The appearance of a reasonably bright comet in the night sky is always a welcome sight for an astronomer.
And this month, we have comet 103P Hartley, or Hartley 2 as it's known.
Now, Hartley 2 should get to naked-eye brightness about the middle of the month, but to be honest, this could be quite a tricky comet to see with just your eyes, because it is quite spread out.
I would recommend going out with a pair of binoculars to hopefully guarantee you a view.
Now, locating comets can be quite tricky in the night sky but there are a couple of pointers this month to help you find it.
On the night of the 7th, and on the night of the 8th October, Comet Hartley 2 passes very close to the famous Double Cluster in Perseus, the Greek hero.
You can use the constellation of Cassiopeia to locate the Double Cluster using our chart, and this hopefully should help you find it.
Point a pair of binoculars at the Double Cluster, and you should see this faint, fuzzy patch, and that will be Hartley 2.
Now, after this, the comet heads off into the constellation of Auriga and it could become quite difficult to see at this time because the bright moon will interfere with the view.
We are in for a real treat with Hartley 2 at the beginning of November, because on the 4th, the NASA EPOXI mission is said to do a close fly-by of the heart of Hartley 2 - the nucleus of the comet.
Now, we are covering that next month and if everything goes to plan, this should give us our first view of the nucleus of Comet Hartley.
That should be absolutely fantastic.
I've mentioned the constellation of Auriga, the charioteer, and just below that is another familiar constellation known as Taurus the Bull.
And at the start of November, in the first week of November, there is a meteor shower which should send out meteor trails emanating from that constellation.
The shower is known as the Taurids and it's not a particularly active shower - quite a low rate of meteors that you are likely to see.
What makes it really interesting is the fact that you often get mixed in with the normal meteors some bright fireball events.
It's well worth keeping an eye out in the sky, if it's clear, throughout the first week of November.
The moon is new at this time, so it should be right out of the sky, and the sky should be dark.
It should be a fantastic sight.
So Paul, what will you be looking at throughout October? I'm going to be making observations of the planet Uranus.
Oh, right, OK.
It's had a bad press, the planet.
I feel sorry for it in a way.
It's been well south for British observers, now it's moving back up and becoming a viable target.
And it's quite easy to see, as well.
It's very near to the planet Jupiter, so you have got a little guide, and it's one of those planets that's got this bad press as being dull, bland, uninteresting, but that's really just become astronomical hearsay.
Nobody observes it on a regular basis.
It's a long way away, so it presents a very tiny disc.
It does.
It's about the same sort of size as Jupiter.
That's quite small, isn't it? Very, very small.
You will need a larger telescope to get anything out of it.
You will need a large telescope to see the disc or any details on it.
What sort of things could you see? With my eight-inch reflector, I have been able to see this beautiful equatorial band, some cloud structure and a couple of the brighter moons, but that's about it.
If you want to see more, you need a larger telescope, something like this.
I took a picture couple of weeks ago, and it did show a sort of light band through the middle.
I took a picture last night as well, and it doesn't show that any more.
It's very interesting.
The planet has shown complete changes in magnitude over a course of a period of observation.
We don't know much about it.
As it has been sort of neglected by professional astronomers, if we get amateurs to look at it, it might be amateurs that discover it's actually quite dynamic.
But of course, one of the problems that I have when I'm looking at Uranus, is that there is something much more tempting very close by.
Jupiter's right near to it.
What a wonderful world that is.
One of the things I like doing, but it's of no scientific use at all, Pete, but I do love watching the satellites, the shadows.
It's beautiful, and in particular, there's a nice one on October 31st.
If you are up at about 3.
15am, you will see Ganymede and Europa transiting the disc.
Ganymede will be almost finished, Europa will be just starting.
That will be beautiful.
That's a fantastic thing to watch.
That gives the whole system a three-dimensional feel.
It does.
You really do feel like you are looking at a planet and its moons.
Jupiter's not the only interesting thing in the sky, is it? It isn't.
There's a variable star called Mira, a name which means "wonderful", which is coming to maximum in the middle of this month.
It's a beautiful star, a red colour.
It is, and the variable period is 332 days, which is almost a year.
At its dimmest, you need a pair of binoculars to see it, but at its brightest you can see it with the naked eye, so it adds an extra star into the constellation.
It's a very barren constellation.
It is.
Hardly anything there.
So it's well worth going outside to see whether there is an extra star.
The beauty about Mira is you never know how bright it's going to get.
Hopefully, I'll catch it, because most of the time it's been dim.
I've always managed to miss the maximum! Well, middle of the month.
I shall be looking and hoping.
If we return to the solar system, Chris has got some quite exciting news about the planet Mars.
We can't see Mars this month.
It's lost in the evening twilight, but spacecrafts have a good view.
The European Space Agency's Mars Express spacecraft has been taking a look at Phobos, one of Mars' two moons.
It's been trying to work out how Phobos formed.
It was thought it was an asteroid that had been captured by the Red Planet, but Mars Express tells us that its composition isn't like that of any asteroid we know of, nor is it anything like any of the meteorites that we have down here on Earth.
So Phobos could be an unusual asteroid, a whole new class of asteroid, or it could have formed when something hit Mars' surface, throwing material up into orbit which then coalesced to form Phobos, just like our moon formed when something very big hit the Earth.
Talking of Mars, I think we have a solution to a 30-year-old mystery.
When the Viking landers touched down on the surface of the Red Planet in the '70s, they carried experiments that were explicitly designed to look for the signs of life.
One of them involved digging up the soil and then heating it.
The idea was that complicated molecules, the organic compounds, the necessary building blocks to life, if you heat the soil containing them, they will be given off and be detected.
Viking found absolutely no trace of these organic chemicals.
It's not surprising, but it contradicted some other results from the other tests that Viking did.
Fast forward to a few years ago when the Phoenix probe touched down on the Martian Arctic surface.
It dug down into the soil as well, but it had more sophisticated instruments to look at the chemistry of the soil.
And it found some strange compounds called perchlorates.
Doesn't matter what they are, but they are highly reactive.
Now, tests in the laboratory here on Earth have shown that the perchlorates can explain the confusing Viking results.
If you take Martian soil, even if you put in a lot of organic compounds, if you add perchlorates and heat, rather than compounds being released, the perchlorates go to work and they attack the organic compounds and destroy them.
So no matter what else was in the soil, if you add perchlorate down at the Viking lander site, you'd never have found any organic compounds.
Now this doesn't mean that there IS life on Mars.
All it means is that this one test that the Vikings did can't tell us anything about what was in the soil.
We're going to need to go back with more sophisticated instruments, rather like the ones that were on Phoenix.
This telescope of Patrick's is well-loved and well-used, but sometimes you need something newer.
Up the road in Hampshire, they're building a very unusual radio telescope, so I went up to give a hand and take a look.
LOFAR is looking at the sky, not at any one particular part of it, but the whole sky.
And because it's looking in radio waves, it can do this 24 hours a day.
This site is only one small part of LOFAR, just one in a network of similar sites right across Europe.
10 years ago in the Netherlands, some astronomers built a revolutionary new kind of radio telescope, look at very low radio frequencies which haven't been probed before.
It was realised, in order for the project to be a big success, it would be important to have stations, antenna stations, like this one, in countries beyond the Netherlands, and that gives you the ability to make very high resolution pictures on the sky.
Without the international stations, you re-observe the galaxy, with a black hole at the centre, and this was just a kind of fuzzy blob.
Once you bring in the international stations to get the high resolution, you can see structure in the centre of the galaxy.
Over the past few millions of years, there's been intermittent periods of activity from the central black hole, and that in turn ties in to things we know about - galaxy formation.
So you start to see down into the details of things like activity around black holes.
'LOFAR doesn't look anything like a traditional radio telescope.
' My first contribution to LOFAR.
'For starters, it seems to be made from bits and pieces 'that you'd find in a DIY shop, or in your local garden centre.
' What does this do? This is the heart of the telescopes.
This is a radio mirror.
Doesn't look much like a mirror.
It's for reflecting the radio waves.
In an optical telescope, we spend all the money and energy making the mirror.
In this telescope, it's the cheapest, most rudimentary part of the telescope.
It's the electronics.
And the computers.
'Despite its industrial look, LOFAR's still susceptible to all sorts of interference, 'including from an unexpected source.
' The most important part of the whole antenna are these silver bits This is armour to stop the rabbits nibbling through the wires.
DRILLING 'LOFAR UK is now online, busily scanning the entire sky, whatever the weather.
' Scan the cables.
BUZZING 'In conjunction with the other European sites, it will generate an enormous amount of data 'about a range of astronomical objects from the sun to black holes, and back to the early universe.
' When we look at the history of the universe, we don't really know the teenage years of the universe.
We know the beginning and we know the end, but there's missing photos in the collection of the life of the universe.
And that's what we call the Dark Ages, or the Epoch of Reionization.
So we think at that time was just gas, just neutral hydrogen gas.
Before stars.
Before stars, and so it's just sitting around there, and it emits radio waves.
It emits 21cm The wavelength.
Then that gets heavily stretched .
.
the expansion of the universe Because of the expansion of the universe, and that's why it puts that radiation close to the FM band.
So that's why LOFAR is really looking around this region of the radio spectrum, cos we want to look at the universe about a billion years after it was formed.
CHAMPAGNE CORK POPS THEY CHEER Well, the antennas are all in place, a huge amount of work remains to be done, but it won't be long now before there's real science flowing from this amazing site.
I wonder what that's going to tell us, Chris.
Coming back now to the picture competition, we all have our own favourite choice.
Mine is this lovely green Aurora, but I think you don't agree.
They're from the Astrophotographer of the Year competition, in which both of us were judges.
The Aurora was good.
I liked the winner of the Young Astronomer competition.
Yes.
Which is this beautiful image of an annular eclipse, captured from India.
The eclipse is tiny in the centre, with dramatic clouds around it.
They were lucky to see anything at all.
Taken by a 14-year-old, I believe.
Exactly.
A stunning image.
Well, Chris, we're coming up for a landmark.
Not quite yet, but very soon, it will be our 700th Sky At Night programme.
Patrick, we wanted to make the 700th programme special, so we're putting you, the viewers, in charge.
We'll do a question and answer show, and have a set of eminent astronomers there to answer your questions about the universe.
So if you'd like to submit a question, go to our website If you don't have access to the web, you can send in your question by post AND get your Sky At Night newsletter.
Send a self-addressed enveloped to .
.
and we'll do our best to answer as many as possible on our 700th programme next March.
Well, next month when we're back, we're going to talk about comets.
And so, from my darkened garden, good night.
And welcome to my garden, with one of my telescopes.
It's been a lovely, hot, glorious day, but in Selsey, it generally is.
Now, echoes.
We all know about sound echoes, but did you know we can also have light echoes? I can assure you we can.
The story really goes back to the year 1572, and the Danish astronomer Tycho Brahe.
Now Tycho was a remarkable man.
He had a very eventful life, and when he was a student, he fought a duel, had part of his nose sliced off and he made himself a new nose out of gold, silver and wax.
He was passionately keen on astronomy.
Obviously, no telescopes.
They didn't come along until years later.
But Tycho set up an observatory on the island of Ven, and there, he drew up an amazingly accurate catalogue of stars.
It remained the best for a long, long time.
A masterpiece of careful, accurate work.
Come now to 1572, Tycho here puts the starry sky.
Of course, he knew all the stars perfectly.
And he looked at the constellation of Cassiopeia.
Well, you must know Cassiopeia.
The five main stars make up a W.
On the far side of the pole star from the Great Bear.
Tycho looked them and there was something else there.
A very brilliant star, where no star had been before.
Became visible to the naked eye in broad daylight.
And then, as the months passed by, slowly faded away, and was lost.
Until 1952, when the English astronomers Hazard and Hanbury Brown detected a radio source in just the position where Tycho's star had been.
We now know what that star was.
It was a supernova.
The sudden throw up of a very faint star.
Well, what's all this got to do with light echoes? Chris, over to you! Thanks, Patrick, but don't go anywhere, I need you to represent Ti-co Brahe Not Ti-co Brahe - Tycho Brahe! I've got a gold, silver and wax nose to prove it.
I think it suits you, Patrick, and I know better than to argue with somebody who fights duels.
But if you're Tycho down on Earth, that I must be the star, the star that he saw a few hundred years ago.
The year, in fact, is 5500 BC, and I'm about to explode, to go supernova.
Light from the explosion is emitted in all directions, and Pete and Paul are going to represent two nimble light rays heading out into space.
One of them, Pete, happens to be heading directly for Earth.
But Paul is heading into deep space, or at least he is until he collides with a cloud of gas and dust, and is reflected towards the earth.
Time goes by, and as we reach 1572 AD, the first of our light rays, Pete, is arriving at earth, where he is observed by Tycho Brahe - got it, Patrick? I have indeed.
Excellent.
So Tycho has seen the supernova, but it's not until 2008 that the second light ray, Paul, arrives at earth and is observed by the Subaru telescope in Hawaii.
That's all a light echo is.
It's a reflection that allows us to see light through the supernova many hundreds of years since it happened.
We are actually seeing the spectrum of Tycho Brahe's star.
That's right, and the spectrum is important because that's how we know the light came from the supernova.
It has the same spectrum - the same fingerprints as light that we might observe directly.
Light beams, thank you very much.
Well, light echoes aren't common, but there are other examples.
And recently, I talked about them to our two eminent guests.
Professor Mike Bode and Dr Tim O'Brien.
So, light echoes are really important.
I think quite excitingly, in astronomy we're used to being limited to one particular point of view of an object, we're sort of stuck here on the Earth, and we're looking in one direction, so we tend to see things from just one side.
But with a light echo, because the light's travelling in different directions, and bouncing back off these clouds around it, we actually get to have a look round the back of something in space, which is quite a bizarre idea.
It certainly gives us a lot more information about the three-dimensional structure of these astronomical objects.
The story of light echoes really begins with a star that flares up in the constellation of Perseus in 1901, GK Persei.
That's right, GK Per was the first first bright Nova of the 20th century, and it changed the face of the constellation of Perseus.
Now that explosion of GK Persei in 1901 was soon followed observationally by wispy nebulosity seen to surround it.
It was realised that actually those nebulosities seemed to be spreading out faster than the speed of light.
You can't do that.
But you can't do that.
It was known even back in the early 1900s that nothing could travel faster than the speed of light, and the crucial point here is that no thing can travel faster than the speed of light, so what was at first thought to be material ejected in this nova explosion moving at these extreme speeds, we weren't actually seeing material moving.
What we were seeing were different parts of pre-existing material around the site of the nova that were being illuminated by the flash of the explosion itself.
It's just an optical illusion that it's faster than the speed of light.
One picture I like shows the exploding star V838, in the constellation of The Unicorn.
Yes, there's a lovely sequence of images that were taken by the Hubble space telescopes back in 2002, when this star brightened suddenly.
What you see in these images is this gorgeous view of the reflected light scattered back off the material around the star itself.
So when you see what looks to be expansion of material thrown out in an explosion, that's not the case.
All you're seeing is this repeated scattering of light, as it moves through the material around the star, it's really a beautiful sequence.
Other fascinating things too, RS Puppis? Yes, RS Puppis is a well-known Cepheid variable.
Their luminosity, the amount of energy they give out per second is related to the pulsation period.
And RS Puppis is a Cepheid variable with a period of around 41 days, that is surrounded by gas and dust, and that gas and dust is scattering the light from the cepheid, and you can see it illuminating this pre-existing material, waves of illumination spreading out from the centre.
So it looks for all the world like there's these ripples of material expanding out, but in fact they're just ripples of light that come from the pulsing star at the centre.
Again, echoing the light variation of the central star.
Come back for a moment to supernovae.
We haven't had one in our galaxy for a long time now.
But we did have one in 1987, in the large cloud of Magellan, the satellite captured it at 169,000 light years away.
Yes, in fact in the case of 1987 a, we observed light echoes in the form of two rings, and these rings are caused by scattering from dust in planes between us and the supernova.
If you imagine the light from the supernova spreading out in a sort of sphere as it spreads out away from the supernova, and then there's these sheets of dust that sit between us and the supernova - where that sphere of light intersects those planes of dust, you see these big circles on the sky.
Supernovae are unbelievably powerful.
They don't happen often, but there are records of them going back a long way.
Yes, in fact there are historical records going back to several hundred BC.
We have extremely detailed records, particularly from the Far East, of supernovae that went off in 1006 AD, in the constellation of Lupus.
The object is the brightest recorded stellar object ever, it was so bright that it was equivalent to something like the half moon, the first quarter moon, put into a single point.
It cast shadows, it was even said that at midnight you might have been able to read manuscripts by the light of this object.
It was seen in daylight for a long time.
It was seen at night for couple of years or so.
But no records from Britain? No records from the West, except for one, actually, which was from a monastery in Switzerland.
The problem was that the dogma at the time in the West was that nothing above the moon changed, so if you saw something that had changed, like a new star appearing, if you then noted that down, you might have been burnt at the stake.
The same thing happened in 1054, when another supernova was observed, this time in the constellation of Taurus.
It was visible in daylight for 23 days, visible at night for many, many months.
And nobody in the West, as far as we are aware, noted it down.
The remnants are now detectable both by visual and by radio.
The remnant of the supernova that we discussed in 1054 is the Crab Nebula, one of the most beautiful objects in the night sky.
It's 11 light years across, now 10,000 times the size of the planetary solar system.
And material expanding at 1500 kilometres per second.
And very easy to find.
Indeed, I'm very proud of the fact I found it myself with a small telescope from my back garden! One of my major achievements as an astronomer! I've seen it with binoculars in the same field.
Then come to 1572, the Danish astronomer Tycho Brahe.
One evening, he looked up in the constellation of Cassiopeia - a star that hadn't been there before, miraculous.
That was a supernova.
That supernova, again, was extremely bright as a visible object, about as bright as Venus gets in the sky.
And it was seen continuously and noted down in the West this time, particularly by Brahe, over a period of 18 months or so, until it faded from view.
Very recently, a group of astronomers working at the Calar Alto observatory in Spain imaged regions of the sky close to the site of Tycho's supernova, and they spotted these very faint wisps of nebulosity, and in a series of images, they saw this nebulosity moving across the sky, much like the light echoes we saw earlier, and when you point to where they appear to be moving from, it pointed directly towards the site of the explosion.
So this animation shows a God's eye view, if you like, illustrating what light echo of 1572, what the cause was.
So here, the supernova is just about to explode, it brightens, and you will see the sphere of light travelling out from the supernova, some of it goes backwards away from the Earth, but then reflects back off that cloud of gas, in interstellar space.
So you've have now got the first light that arrives directly from the supernova arrives in 1572, when Tycho saw it, and the light that's travelled further and arrives later is just arriving now from the sites of these light echoes.
You might think that we'd missed our chance to observe what was one of the closest type one A supernovas there's been in our galaxy, and we know these things are fundamental to our understanding of cosmology now, but amazingly, this light echo phenomena gives us this chance to see effectively the same light, that Tycho would've seen, but it's been on a longer journey - it travelled away from us, bounced back up a cloud and is only now arriving.
Well, very informative indeed.
Mike, Tim, thank you very much.
Thank you.
Thank you.
Well, I'm back in the garden, it's getting dark now, very interesting.
These light echoes can turn up in all kinds of places, have all kinds of forms.
And there's one very odd case, in that organisation you began yourself, Galaxy Zoo.
Yes, this is my project, galaxyzoo.
org on the Web, which invites people to sort through images of galaxies - mostly we're interested in the galaxies themselves, but in this one particular image, a Dutch schoolteacher found this green blob, looks a little bit like Kermit the Frog, or some people think it looks like a runner.
It's called Hanny's Voorwerp - "voorwerp" was her name for it, we thought it was a technical term, like quasar or pulsar and it turns out to mean thingy.
I'm very proud that Hanny's thingy is now in the literature.
This is actually a light echo, but a light echo on an unimaginable scale.
You remember that's a whole spiral galaxy there, so the blob is tens of thousands of light years across.
What's happened is that galaxy, which goes by the name of IC2497, used to have a very, very bright nucleus, the black hole at the centre was feeding, the material that was dropping in gave out radiation, you would have been able to see it with binoculars, would you believe.
And that light has hit gas, excited the gas which has emitted light, and we see it as an echo of what was happening at the centre of the Galaxy 20, 30, 40,000 years ago.
We've pointed the Hubble space telescope at it.
I hope, in a few months' time, I will show you the most amazing images from Hubble, of this unique object.
Things may turn up in any way, and this certainly has.
Well, Chris, congratulations on that.
You started the entire programme.
That's true, but it's that instinct to go, "What on earth is that weird thing?" It turns out, as so often, to be a light echo.
Chris, thank you very much.
We won't see that tonight, but there's plenty to see.
Pete and Paul are getting ready to look for a comet.
The appearance of a reasonably bright comet in the night sky is always a welcome sight for an astronomer.
And this month, we have comet 103P Hartley, or Hartley 2 as it's known.
Now, Hartley 2 should get to naked-eye brightness about the middle of the month, but to be honest, this could be quite a tricky comet to see with just your eyes, because it is quite spread out.
I would recommend going out with a pair of binoculars to hopefully guarantee you a view.
Now, locating comets can be quite tricky in the night sky but there are a couple of pointers this month to help you find it.
On the night of the 7th, and on the night of the 8th October, Comet Hartley 2 passes very close to the famous Double Cluster in Perseus, the Greek hero.
You can use the constellation of Cassiopeia to locate the Double Cluster using our chart, and this hopefully should help you find it.
Point a pair of binoculars at the Double Cluster, and you should see this faint, fuzzy patch, and that will be Hartley 2.
Now, after this, the comet heads off into the constellation of Auriga and it could become quite difficult to see at this time because the bright moon will interfere with the view.
We are in for a real treat with Hartley 2 at the beginning of November, because on the 4th, the NASA EPOXI mission is said to do a close fly-by of the heart of Hartley 2 - the nucleus of the comet.
Now, we are covering that next month and if everything goes to plan, this should give us our first view of the nucleus of Comet Hartley.
That should be absolutely fantastic.
I've mentioned the constellation of Auriga, the charioteer, and just below that is another familiar constellation known as Taurus the Bull.
And at the start of November, in the first week of November, there is a meteor shower which should send out meteor trails emanating from that constellation.
The shower is known as the Taurids and it's not a particularly active shower - quite a low rate of meteors that you are likely to see.
What makes it really interesting is the fact that you often get mixed in with the normal meteors some bright fireball events.
It's well worth keeping an eye out in the sky, if it's clear, throughout the first week of November.
The moon is new at this time, so it should be right out of the sky, and the sky should be dark.
It should be a fantastic sight.
So Paul, what will you be looking at throughout October? I'm going to be making observations of the planet Uranus.
Oh, right, OK.
It's had a bad press, the planet.
I feel sorry for it in a way.
It's been well south for British observers, now it's moving back up and becoming a viable target.
And it's quite easy to see, as well.
It's very near to the planet Jupiter, so you have got a little guide, and it's one of those planets that's got this bad press as being dull, bland, uninteresting, but that's really just become astronomical hearsay.
Nobody observes it on a regular basis.
It's a long way away, so it presents a very tiny disc.
It does.
It's about the same sort of size as Jupiter.
That's quite small, isn't it? Very, very small.
You will need a larger telescope to get anything out of it.
You will need a large telescope to see the disc or any details on it.
What sort of things could you see? With my eight-inch reflector, I have been able to see this beautiful equatorial band, some cloud structure and a couple of the brighter moons, but that's about it.
If you want to see more, you need a larger telescope, something like this.
I took a picture couple of weeks ago, and it did show a sort of light band through the middle.
I took a picture last night as well, and it doesn't show that any more.
It's very interesting.
The planet has shown complete changes in magnitude over a course of a period of observation.
We don't know much about it.
As it has been sort of neglected by professional astronomers, if we get amateurs to look at it, it might be amateurs that discover it's actually quite dynamic.
But of course, one of the problems that I have when I'm looking at Uranus, is that there is something much more tempting very close by.
Jupiter's right near to it.
What a wonderful world that is.
One of the things I like doing, but it's of no scientific use at all, Pete, but I do love watching the satellites, the shadows.
It's beautiful, and in particular, there's a nice one on October 31st.
If you are up at about 3.
15am, you will see Ganymede and Europa transiting the disc.
Ganymede will be almost finished, Europa will be just starting.
That will be beautiful.
That's a fantastic thing to watch.
That gives the whole system a three-dimensional feel.
It does.
You really do feel like you are looking at a planet and its moons.
Jupiter's not the only interesting thing in the sky, is it? It isn't.
There's a variable star called Mira, a name which means "wonderful", which is coming to maximum in the middle of this month.
It's a beautiful star, a red colour.
It is, and the variable period is 332 days, which is almost a year.
At its dimmest, you need a pair of binoculars to see it, but at its brightest you can see it with the naked eye, so it adds an extra star into the constellation.
It's a very barren constellation.
It is.
Hardly anything there.
So it's well worth going outside to see whether there is an extra star.
The beauty about Mira is you never know how bright it's going to get.
Hopefully, I'll catch it, because most of the time it's been dim.
I've always managed to miss the maximum! Well, middle of the month.
I shall be looking and hoping.
If we return to the solar system, Chris has got some quite exciting news about the planet Mars.
We can't see Mars this month.
It's lost in the evening twilight, but spacecrafts have a good view.
The European Space Agency's Mars Express spacecraft has been taking a look at Phobos, one of Mars' two moons.
It's been trying to work out how Phobos formed.
It was thought it was an asteroid that had been captured by the Red Planet, but Mars Express tells us that its composition isn't like that of any asteroid we know of, nor is it anything like any of the meteorites that we have down here on Earth.
So Phobos could be an unusual asteroid, a whole new class of asteroid, or it could have formed when something hit Mars' surface, throwing material up into orbit which then coalesced to form Phobos, just like our moon formed when something very big hit the Earth.
Talking of Mars, I think we have a solution to a 30-year-old mystery.
When the Viking landers touched down on the surface of the Red Planet in the '70s, they carried experiments that were explicitly designed to look for the signs of life.
One of them involved digging up the soil and then heating it.
The idea was that complicated molecules, the organic compounds, the necessary building blocks to life, if you heat the soil containing them, they will be given off and be detected.
Viking found absolutely no trace of these organic chemicals.
It's not surprising, but it contradicted some other results from the other tests that Viking did.
Fast forward to a few years ago when the Phoenix probe touched down on the Martian Arctic surface.
It dug down into the soil as well, but it had more sophisticated instruments to look at the chemistry of the soil.
And it found some strange compounds called perchlorates.
Doesn't matter what they are, but they are highly reactive.
Now, tests in the laboratory here on Earth have shown that the perchlorates can explain the confusing Viking results.
If you take Martian soil, even if you put in a lot of organic compounds, if you add perchlorates and heat, rather than compounds being released, the perchlorates go to work and they attack the organic compounds and destroy them.
So no matter what else was in the soil, if you add perchlorate down at the Viking lander site, you'd never have found any organic compounds.
Now this doesn't mean that there IS life on Mars.
All it means is that this one test that the Vikings did can't tell us anything about what was in the soil.
We're going to need to go back with more sophisticated instruments, rather like the ones that were on Phoenix.
This telescope of Patrick's is well-loved and well-used, but sometimes you need something newer.
Up the road in Hampshire, they're building a very unusual radio telescope, so I went up to give a hand and take a look.
LOFAR is looking at the sky, not at any one particular part of it, but the whole sky.
And because it's looking in radio waves, it can do this 24 hours a day.
This site is only one small part of LOFAR, just one in a network of similar sites right across Europe.
10 years ago in the Netherlands, some astronomers built a revolutionary new kind of radio telescope, look at very low radio frequencies which haven't been probed before.
It was realised, in order for the project to be a big success, it would be important to have stations, antenna stations, like this one, in countries beyond the Netherlands, and that gives you the ability to make very high resolution pictures on the sky.
Without the international stations, you re-observe the galaxy, with a black hole at the centre, and this was just a kind of fuzzy blob.
Once you bring in the international stations to get the high resolution, you can see structure in the centre of the galaxy.
Over the past few millions of years, there's been intermittent periods of activity from the central black hole, and that in turn ties in to things we know about - galaxy formation.
So you start to see down into the details of things like activity around black holes.
'LOFAR doesn't look anything like a traditional radio telescope.
' My first contribution to LOFAR.
'For starters, it seems to be made from bits and pieces 'that you'd find in a DIY shop, or in your local garden centre.
' What does this do? This is the heart of the telescopes.
This is a radio mirror.
Doesn't look much like a mirror.
It's for reflecting the radio waves.
In an optical telescope, we spend all the money and energy making the mirror.
In this telescope, it's the cheapest, most rudimentary part of the telescope.
It's the electronics.
And the computers.
'Despite its industrial look, LOFAR's still susceptible to all sorts of interference, 'including from an unexpected source.
' The most important part of the whole antenna are these silver bits This is armour to stop the rabbits nibbling through the wires.
DRILLING 'LOFAR UK is now online, busily scanning the entire sky, whatever the weather.
' Scan the cables.
BUZZING 'In conjunction with the other European sites, it will generate an enormous amount of data 'about a range of astronomical objects from the sun to black holes, and back to the early universe.
' When we look at the history of the universe, we don't really know the teenage years of the universe.
We know the beginning and we know the end, but there's missing photos in the collection of the life of the universe.
And that's what we call the Dark Ages, or the Epoch of Reionization.
So we think at that time was just gas, just neutral hydrogen gas.
Before stars.
Before stars, and so it's just sitting around there, and it emits radio waves.
It emits 21cm The wavelength.
Then that gets heavily stretched .
.
the expansion of the universe Because of the expansion of the universe, and that's why it puts that radiation close to the FM band.
So that's why LOFAR is really looking around this region of the radio spectrum, cos we want to look at the universe about a billion years after it was formed.
CHAMPAGNE CORK POPS THEY CHEER Well, the antennas are all in place, a huge amount of work remains to be done, but it won't be long now before there's real science flowing from this amazing site.
I wonder what that's going to tell us, Chris.
Coming back now to the picture competition, we all have our own favourite choice.
Mine is this lovely green Aurora, but I think you don't agree.
They're from the Astrophotographer of the Year competition, in which both of us were judges.
The Aurora was good.
I liked the winner of the Young Astronomer competition.
Yes.
Which is this beautiful image of an annular eclipse, captured from India.
The eclipse is tiny in the centre, with dramatic clouds around it.
They were lucky to see anything at all.
Taken by a 14-year-old, I believe.
Exactly.
A stunning image.
Well, Chris, we're coming up for a landmark.
Not quite yet, but very soon, it will be our 700th Sky At Night programme.
Patrick, we wanted to make the 700th programme special, so we're putting you, the viewers, in charge.
We'll do a question and answer show, and have a set of eminent astronomers there to answer your questions about the universe.
So if you'd like to submit a question, go to our website If you don't have access to the web, you can send in your question by post AND get your Sky At Night newsletter.
Send a self-addressed enveloped to .
.
and we'll do our best to answer as many as possible on our 700th programme next March.
Well, next month when we're back, we're going to talk about comets.
And so, from my darkened garden, good night.