Good afternoon, everybody. Welcome and thank you for coming to this hidden lecture given by Scott Ransom.
I have just a few introductory remarks to make about the Oxford Centre for Astrophysical Surveys, and so we're kicking this research centre off now.
This is a centre that's funded by the Hennessy Family Charitable Foundation and the aim of the centre of the centre is to
facilitate our involvement in tackling some of their most urgent and important questions of modern physics and astronomy,
the nature of dark matter and dark energy, how galaxies form and evolve,
and the nature and physics of transient objects, which is a new area that's emerging.
The centre that has been set up hosts, research fellows and graduate students.
We have a couple of the research fellows that are joining us in January with us today,
and we have the first of the graduate students with us today as well.
And so we'll be more research fellows coming in the following year and a couple of more graduate students joining us next October.
So if I could, to quickly get the next one. Oh, oh, you did it.
Okay. Oh, back again. Thank you. We should have practised in Denver like Scott.
So in addition to this research team, the gift also funds our involvement in a couple of major international astrophysical surveys.
So they are the Sloan Digital Sky Survey and this is the fourth edition of that survey, but it has completely new equipment.
So this started in July. Its goals are to look for dark matter in galaxies.
Look at the large scale structure of the universe to probe dark energy and map the Milky Way.
An amazing project which we have also been able to join as a result of this gift is the Large Synoptic Survey Telescope.
That's a bit of a mouthful, but it's an enormous telescope, 8.4 metres in diameter in Chile.
It will map 10,000 square degrees of the sky twice a night and cover every three nights.
So this is clearly this is this piece of this telescope won't be available until more or less the end of the decade,
but we can prepare to use it and get involved in the plans.
I'm actually even in the building of this camera, which has 3.2 billion pixels.
So that kind of puts your digital self somewhat in context.
So I think it's true to say that this telescope will revolutionise the study of transient objects,
but it will also produce the deepest image of the sky ever made over the repeated viewing of the southern sky.
So I just wanted to bring this news to you.
Thank the Hennessy Charity Family Charitable Foundation for getting us started on this very exciting route.
So back to the main event. So the since the lecture this year, this term is being delivered by Scott Ransom.
Professor Ransom is an astronomer at the United States National Radio Astronomy Observatory.
This is one of those labs in the world that control some of the key technologies without which nobody can do anything in the field.
It's an incredibly eminent institution, and it's the same town as the University of Virginia, where he's also a professor.
He's an expert in pulsar exploration tests of general relativity in the search for gravitational waves.
And in this quest, he's become an expert in high performance computing and the dealing with big data.
Now, Scott, we're very pleased that Scott is in the department for a few months on sabbatical.
He is a graduate of the United States Military Academy at West Point.
And I think actually there are a few West Point graduates in the audience that are now Rhodes Scholars.
So special. Welcome to you, Scott.
Sandwiched between a master's and a Ph.D. at Harvard, six years in the US, Army is an army as a field artillery officer,
and I think I can say with some confidence that he's certainly the only himself lecturer to be in the US Army,
and I suspect [INAUDIBLE] hold that distinction for quite a long time.
As you can imagine, we've been very nice to him while he's been here.
So he got his Ph.D. in Harvard in 2001, actually in the same class as our previous intellectual, David Charbonneau.
In 2021, there is the American Astronomical Society.
Helen B won a prize, a distinction he shares with another Hennessy lecturer who's in the room today.
That is Chris Reynolds. So all these links are quite fascinating.
Well, having made these introductions, it's my great pleasure to invite Scott Ransome to deliver the hints.
The lecture a millisecond. Pulsars, Magnetars and black holes.
The wickedly cool, stellar undead. Scott Excellent.
Well, thanks a lot. This is a level okay with the microphone.
All right. Great. It's a I like the introduction if for no other reason to hear Rogers say wickedly cool.
That's kind of nice. So you've got the idea of the of the title though, in the talk.
Hopefully by the time the talk is done and you'll see that I really love this subject, pulsars especially,
but the other dead things that come from stars, which is kind of what this lecture is about, really fascinate me.
And I think they're incredible objects that are really interesting.
And so hopefully you'll get some of that out of this lecture. So with it, in order to have the stellar undead, you have to have stars first.
And we live in a big galaxy. That's the Milky Way. I might be to bring the lights down just a touch so you can see some pretty pictures.
Um, just for one or two, and then I'll bring them back up. So this is our Milky Way galaxy.
As we see from Earth, we can only see part of the galaxy. And what we see are stars.
Right. It's the stars that are shining that make up that galaxy.
And this talk is basically about what those stars become after they've exhausted their fuel.
And this in this type of picture, all the stars blend together, basically,
so that if you zoom in on a star field towards the centre of the galaxy, you can see that dozens upon thousands of stars.
And if you use a bigger telescope or binoculars, the numbers just multiply as you go on.
What I want to point out about this is that there's a few things that even without a fancy instrument that your eyes can tell you about the stars.
First off, they have different brightness. That's obvious. Some stars are much fainter than others.
And I'll talk about a few reasons why that is. And the other thing that's not quite so obvious, but this is a nice picture that shows it.
For instance, this star right there, compared to this star right there, they're both about the same brightness.
But this star is much bluer and that one is much redder. The colours of the stars tell us a lot about what's going on with those stars.
And so just these two aspects, the brightness of the star as well as their colour.
Let us infer a whole bunch of information about the stars.
And there's a very famous diagram in astronomy known as the herdsman Russell Diagram, which is up here on the on the on the slide down.
Bring the lights back up a little bit. And this diagram basically tells us how stars live their lives.
And it's a fascinating diagram because, I mean, you can understand it with very little physics understanding.
And it tells you a vast amount about the way our galaxy, how its constituent parts, which are mostly the stars, how they live their lives.
So, for example, this x axis here is temperature.
And astronomers,
we do things a little bit weird here that the highest temperatures are on the left hand side of the the cool temperatures are over here, the y axis.
This is logarithmic. It's a logarithmic plot which makes it a little bit more tricky than usual.
But there are many orders of magnitude, about a billion,
a factor of a billion or maybe even 10 billion between the bottom of the plot and the top of the plot.
So that means the stars up here are the tops of the plot are very bright,
a billion times brighter than the stars that are down here at the bottom of the plot.
And what I want to get out is those two things that we can measure easily.
The colours of the stars and their brightness can be linked to physical properties of the stars.
So the colour of the star is linked to its temperature so that stars that are very hot tend to be blue, stars are very cool, tend to be red.
So that's what the colour tells us. And then their luminosity, which is the intrinsic brightness of the star, how bright they actually are.
Because remember, one of the things that we have to deal with in astronomy is distance.
These are very far objects and stars are spread throughout the galaxy.
So stars that are further away are going to be fainter than stars which are nearby if they're the same intrinsic brightness.
Right. But forgetting about distance here, let's just look at the intrinsic brightness.
So three things determine how intrinsically bright a star is, how massive it is.
And that's a key one, because that also helps determine its temperature and the size of the star.
If it's a really big star, it gives off more light than if it's a really small star.
If everything else is held constant. So those those two things are the two physical properties that we can get from
observations and we can get a lot of information about the stars this way.
And here is, for instance, the main sequence. Most of the stars in our galaxy are along this.
There's lots of little red stars, a whole bunch of stars like our sun and a relatively small number of very, very bright blue stars.
The Giants. All right. So if we take a look at kind of what the relative sizes of those stars are, this is a nice diagram that kind of shows that.
And once again, it lists these weird letters you may have seen on that last diagram that's indicative of the temperatures of those stars.
And there's kind of a famous mnemonic that's kind of not quite politically correct any more to use.
The astronomers know exactly what I'm going to say, because I'm not exactly politically correct usually.
But you can remember this the order from the biggest in the blue with stars down to the littlest and redder stars by Oh, be a fine girl, kiss me.
That's the mnemonic. And there's a whole bunch of variations on that theme.
But the interesting thing is these little puny stars is the vast majority of the stars in our galaxy.
Most of the stars that are near the sun are stars that are very, very small and tiny like that.
The star, like our sun is a g star down here. It's nothing special.
It's kind of run of the mill. Most of the stars I'm going to be talking about in this talk, though, are these guys, the so-called O and D stars.
They're very blue, which means they're hot, they're very big and they're very massive.
And it's the mass that's going to drive. What's what what I'm going to be talking about mostly in this in this talk.
And the important thing is, is that these stars all are glowing because they're burning hydrogen and helium,
and some of them, the more massive ones, even into heavier elements by the process of nuclear fusion.
And that's how stars work. It's fusion. It's very important.
So let's take an example of some of the smaller stars. So how do they live their lives?
So, for instance, here's a life cycle of our sun. Our sun is roughly about four and a half billion years old right now.
It was created out of a gaseous nebula.
And it's been living its life on the main sequence in the little suburbs that it lives in, in the galaxy for about four and a half billion years.
Things are going nicely. If we project forward for a few billion years in the future,
the sun will slowly start expanding and as it starts expanding, the amount of radiation it's given off increases slowly.
This will cause some warming here at the Earth. This is not what's causing global warming now, though, I want to point out.
Okay, so what happens if we push on a little bit further in the future?
Once we get to about 5 billion years in the future,
the star kind of goes a little bit crazy because it's it gets to a special stage at the centre of this
centre of the sun where fusion has some trouble doing its thing and it expands into a red giant.
Okay, this is what our sun will do in about four and a half billion years.
After that time, it gives off the outer part of its envelope and it becomes a white dwarf.
This is how our star, our sun, will die in about four and a half billion years.
And a white dwarf is the special stellar corpse that's left over.
It's what's left over at the centre of the star.
And this is basically the size of what a white dwarf is compared to the earth.
So the sun is dramatically huger, much larger than the earth.
But the white dwarf, the core of the sun is roughly the same size as the earth.
This is basically a giant crystal of helium that's going to glow and cool off over billions and billions and billions of years.
Okay. So this is what our sun is going to do. There is a slight problem is that when this stage happens,
the earth is going to be in the way and this is not going to be a pleasant place to be
when the earth makes this or when the sun makes this transition into a white dwarf.
But most of the stars in our galaxy, the low mass stars, this is how they're going to end their lives as white dwarfs.
And when they do it, they make some spectacular things. Drop the lights.
These are beautiful HST images of what are known as Planetary Nebula.
The outer parts of the stars get ejected. You can see the white dwarfs in the centre glowing.
They're energising all this gas causing the hydrogen and helium and carbon and oxygen
to to radiate that they're causing the atoms to give off these beautiful colours.
And they're incredibly beautiful objects.
You can download for free these beautiful high resolution images from the HST and other telescopes on the web.
So this is what our stars are going to do in a few billion years. It's pretty, but it's not going to be pleasant here.
But so what happens if you have a more massive star? Well, more massive stars.
Fusion can work better. So if you have a much more massive star that hydrogen gets burned into helium, that's what our star is doing right now.
But a more massive star can convert that helium and burn it into carbon and then
into oxygen and then into silicon and eventually down to iron in the core.
And the problem is, once you get to iron, you can't extract any extra energy out of that fusion process.
When you when you create iron and for the most massive stars,
they burn through their fuel furiously so that their lives are very short and they create this iron core.
And gravity keeps pushing down on it. And eventually the pressure of that iron core becomes too much for it to bear and it collapses.
So the most massive stars after the supply of gas.
This runs out. After the fusion pressure gives out, they turn into black holes.
So this is the second part. The most massive stars produce these guys.
Now, one of the problems with black holes, these are very enigmatic objects.
We like thinking about them and talking about them, many of them. Of the astronomers in the audience here, study them.
These things are very special objects.
They have the strongest gravity that we know of in the universe. Almost by definition, they don't have a surface that's called the event horizon.
They're very strange. Physics happens, their light gets bent.
So you can see in this artist's rendition here, all sorts of crazy things happen.
A lot of things don't happen, though. These don't go around eating stars.
They don't go around enveloping galaxies.
They're not out to get you, which is what a lot of science fiction would like to have you believe.
As a matter of fact, a black hole like this would be very difficult to even detect in our galaxy.
So the black holes that we know about are not exactly like this. The black holes we know about look more like this.
There's a massive star or another star near the black hole and the black hole because of the close nature of that other object.
It's steals gas off that black hole.
That gas then glows very brightly and we can see this gas giving off X-rays or gamma rays or optical light or radio waves.
And so by observations of this nature, by those other parts of light that are coming off this disk, we can study black holes.
And there's a whole huge industry of studying black holes of all sizes by looking at this disk of material.
This particular artist's impression is of a very famous object known as Cygnus X-1,
which is kind of the most people would say is the first kind of bona fide black
hole that astronomers really believe that was identified back in the 1970s.
And it caused people to really believe that black holes were real physical objects that were actually out there.
So that takes care of the white dwarfs from the low mass stars. And the black holes are produced by the most massive stars.
What about the stars in the middle? Well, the stars in the middle.
The ones which are more massive than our sun, they burn through their fuel furiously as well.
But they don't make an iron core that's too big. Eventually it's too big so that it will fail.
It causes a collapse, but it's not too big that it collapses all the way to a black hole.
So what happens here when that core collapses, it becomes a neutron star.
And that process of the transition from a star to a neutron star when that core collapses, is known as a supernova.
And this is a picture of a supernova going off in another galaxy.
And what you can see here is this whole galaxy know all of the hundreds of billions of stars in that other galaxy,
their combined light is approximately equal to the light coming out of that one supernova.
These are incredibly energetic, ferociously violent events.
And they mark the very rapid transition of the core of that star from an iron core into a neutron star.
So we obviously see the supernova happen, but do we see what's left over after these supernova?
Do we see the stellar corpses, the neutron star, the neutron stars themselves?
The answer is yes. Do we see objects that will do this in the future?
And the answer is also yes. So here's a very famous constellation.
There's a lot more stars, and you're usually used to seeing if you look with your naked eye, but you should still be able to work out what it is.
Hopefully someone recognise this constellation. It's a riot.
Here's the belt. This is the Orion Nebula. And here is.
What's this guy? The red one. There's the blue one. The red one.
Beetlejuice and then Rigel. This guy right here is a very massive star.
That's Beetlejuice. And in a tiny fraction of the time of time.
Astro, physically speaking. Which means it could be last Thursday.
It could be a thousand years from now. It could be a million years from now.
But all of those are tiny amounts of time, astrophysical speaking, that will go supernova.
And it's relatively nearby in our galaxy. And it will be spectacular when it does.
And that will transition into a neutron star. We see evidence of stars that have already done this nearby in our galaxy.
And this is a very famous object. It's one of the most studied objects in the sky.
This is known as the Crab Nebula. It's kind of hard to tell why it's called the Crab Nebula with modern imaging, because it looks nothing like a crab.
But if you had a really nasty old tiny telescope and you squinted your eyes and the atmosphere was bad, maybe it might have looked like a crab.
But the interesting thing about this object is that historically.
Astrologers and I'm using the term astrologer as opposed to astronomer on purpose.
Astrologer astrologers saw this go off in 1054 A.D. particularly.
There was the the best records are in China, the Chinese astrologers working for the emperor.
They saw the exact date when this went off in the constellation of Taurus.
And it was a guest star that appeared in the constellation, grew very, very bright, was visible during the daytime.
It was also seen by other civilisations. Here's a famous pictogram in the southwest of the United States and New Mexico in Chaco Canyon.
And here is the star itself. This is the supernova.
This is the crescent moon, exactly as it would have been it in the sky at that time.
Excuse me. And this is the artist's signature from the.
From the. The pictogram. Excuse me.
Take a drink. What's going on in the centre of this nebula.
It's being energised and it's being energised because of a neutron star in the centre right there in the centre of this circle.
That neutron star is pumping out an unbelievable amount of energy into this gas around.
And that's that that over 40 years ago, that neutron star was seen to pulse over 30 times per second.
The pulsation is the rotation of that neutron star.
And I can tell you what, 30 times a second I can actually let you hear what 30 times a second sounds like.
If I can get this to work.
That is the from the RC book telescope, the output of the telescope put into a speaker when it's looking at the crab pulsar that that repetition,
every one of those sound blips which sounds like almost like a machine gun fire, is the rotation of a neutron star.
That's pretty, pretty amazing. And this pulsar is putting out an amazing amount of power at all frequencies from the lowest energy radio waves,
through the infrared, through the optical to the ultraviolet, to the X-rays,
to the highest energy gamma rays it's emitting throughout the whole part of the spectrum,
which is one of the reasons why it's one of the most studied objects in the sky,
because every time there's a new telescope that can observe it, a new wavelength.
This object is one of the brightest objects in the sky at that wavelength, and it's always doing something interesting.
And in this case, the red in this image is the radio.
The green is from optical. And the in the the blue is from X-rays.
And if we zoom in on that image, you can see the blue dot in the centre being surrounded by all these wisps.
That's the rotational energetics from that pulsar.
Gas is being blown off that pulsar and is being energised in all that nebula around it to give off all this radiation.
This pulsar is giving off 10,000 times the power that our sun gives off, and it's purely coming from the rotation of that star.
I mean, really amazing objects. And this isn't the only one we know of many other supernovae that have pulsars in the centre.
And here are a few beautiful images from the Chandra X-ray satellite.
These are the supernova remnants themselves on the outside and in the centres.
They're actually there and there are the neutron stars that are giving off pulses of X-rays.
Sometimes these are visible in the radio waves as well.
And they're energising these nebula by the by the gas and the particles that they're giving off.
So amazing objects, these pulsars. So what is the little bit of the story behind the pulsar?
Well, the first one was discovered in a survey that was done by a graduate student and her advisor.
The graduate student is Jocelyn Bell and her adviser, Anthony Hewish at Cambridge University.
They built a radio telescope, which in this case looks a little bit like a fence,
because when you observe at low frequency radio waves, fences actually can detect radio waves quite well.
You don't have to have big, famous, big fancy dishes. And during their their observations, they were looking at how radio sources twinkle.
Basically, they were looking at what's called scintillation and Jocelyn looking through huge amounts of paper printouts.
Basically, the chart recordings saw this regular blip every about every second,
and it appeared at several nights in a row at the correct time each night.
That represented an astronomical, astronomical source.
And that that was very excellent observational work on her part and being able to identify something that otherwise might have been mistaken for,
let's say,
a car driving past, because interference caused by our society can look very much like these pulsar signals and ended up being a huge discovery.
At the very beginning, they thought that it might be due to extra terrestrial intelligence.
But after they found a few more, I should say, after Jocelyn found a few more, they realised that that was not the case.
And this was the the observational discovery that neutron stars, which had been predicted theoretically actually existed, a huge discovery.
And Jocelyn is sitting right here in the second row and is a huge it's been a huge mentor and an inspiration to many astronomers, myself included.
And one of the postscript to the story is that her advisor won the Nobel Prize.
And I'll just leave it at that. We'll talk a little bit of more about that in a little bit.
Okay. So what else? Oh, by the way, I should mention it's a fascinating story that involves in very interesting science,
tells you what it's like to be to be a scientist and how to think about problems and has very interesting sociology in it as well.
And there's many interesting write ups on on the Internet.
If you're if you're interested, I highly recommend that you search for one of those write ups and read about the discovery of pulsars.
So what are these things I've been talking about? Well, they're neutron stars.
So here's the neutron star in the centre.
These things have a mass of about 1 to 2 solar masses, all compressed down into something that's the size of a city.
They have radii that are between ten and 12 kilometres.
So it's about the size of Oxford. But with all the mass of our solar system, including the sun compressed down,
they're basically gigantic nuclei comprised primarily of neutrons, very exotic objects.
Because they're compressed down,
they have incredibly high heat from from when they were in the core of the star and then were compressed about a million Kelvin.
They give off X-rays because of just their high temperatures. There are densities in the centre of those.
Newton neutron stars are much larger than a nuclear density several times.
Matter of fact, we do not understand the nuclear physics in the centres of those neutron stars.
The physics that's going on inside of them is unknown to us.
We have to extrapolate from our best laboratory measurements to try to figure that out.
And that's an area of very active research. What else?
Their surface gravity because they're so dense. Gravity is incredibly strong.
Einstein's theory of relativity. It curved spacetime around these very, very dense objects.
If you were to look at one of those neutron stars, you would be able to see most of the back of the star because light being bent
around it and the surface gravity itself is about 100 billion times the grab,
the gravity of the surface of the earth. So incredibly strong gravity.
The only objects that have stronger gravity are black holes themselves.
They have can spin at prodigious rates. The record holder is 716 times per second.
This is an object the size of a city rotating 716 times per second.
That's faster than a kitchen blender spins. That's faster than a race car engine rotates.
And it's the size of a city. It blows my mind.
And I think about this stuff every day. They have incredibly strong magnetic fields.
So these are a little bit jargony, these units here. But don't worry about trying to understand the units.
The most important thing about these units of the magnetic field strength is that the Earth's magnetic field is about one gauss.
So that means neutron stars in these various flavours can have ten, 100 million up to several, whatever that number is, several trillion quadrant.
I don't even know what that a lot of big number times bigger than than the magnetic field at the surface of the earth.
Extremely large magnetic fields. I think that's a quadrillion isn't it.
12, ten to the 12 is a trillion anyways. I don't know.
Very strong magnetic fields and it's those magnetic fields that lead to the radiation because what happens because we have a rotation and
we have magnetic fields that's represented by these blue lines and they don't have to be lined up just like the earths are not lined up.
The Earth's magnetic pole and rotation axis are not lined up. When you rotate and move a magnetic field,
it creates induces an electric field that accelerates charged particles and those charged particles radiate.
But the details behind that radiation we do not understand very well, even after 40 some years of study.
It's kind of amazing.
And the final thing I've already hinted at, but the most energetic pulsars purely from rotation can give off 10,000 times the total output of the sun.
There's no fusion, there's no chemical energy, there's nothing else.
If this is coming from rotation, these are giant flywheels in space that can pump out incredible amounts of power, fascinating objects.
I mean, hopefully you can see these really are exotic objects and the list can continue, but we don't have time.
So what? How do we know this? Well, we can do something just like people who study stars do.
We can make our own version of the Hertz sprung Russell diagram for pulsars.
And this is a very famous diagram that pulsar astronomers use because just like the Hertz sprung Russell diagram,
it has two things that we can observe for the heard sprung Russell diagram,
we measure the temperature of the star, which is basically its colour and its brightness.
Okay, those are the two things we observe in this diagram, which is also logarithmic.
So things down here, this is the spin period. So this is one millisecond up to 10 seconds.
So this is a factor of 10,000 along this axis. And then how much those pulsar pulsars change their spin rate?
How fast is that rate of their spin change? Well, why does it change?
Well, it changes because all of that energy that's coming off has to come from somewhere and it's coming from rotation.
So the energy that we see gets stolen from rotation and they slow down.
So this is the slow down rate, so to speak, and there's more than ten orders of magnitude on that on that axis right there.
So this diagram, we can do the same type of thing with the third scoring Russell diagram, we can tell how the lives of pulsars, how they go.
So for example, and by the way, here in this diagram, B is the symbol that physicists use to mean magnetic field.
So for those of you who think that's strange, I agree it is strange.
But anyways, beaming magnetic field. So in this diagram, these diagonal lines that you see,
you can use relatively simple physics on kind of undergraduate level electrodynamics to derive physical parameters from these two observables.
And those physical parameters are, for instance, the magnetic field.
So things up here have very strong magnetic field. Things down here have very low magnetic field.
Things up here, we can derive the ages. Our young pulsars things down here are old pulsars.
And this comes from very, very simple physics. And it's interesting.
It works out. If we look at the top, some of the young pulsars that we know are young, they're in supernova remnants.
We see them there. We've even seen when they've been born like the crab, they're up here in the young part of the diagram.
That's nice. You know, it's good that we can verify that our simple theories work.
So what happens? How do they how does this diagram show how pulsars live?
Well, since they slow down because they give off energy, this is spin, period.
And this is slow over here. That means they move to the right along this diagram as they age, as they give off energy.
So pulsar is like the crab. And Vela will become part of this big mass of regular pulsars as they slow down over time.
That's very nice. And this big group of pulsars, they're like the pulsars that Jocelyn and many other people have discovered over the years.
And we know of about 2000 of these so-called normal pulsars.
Now, they spin it about once per second. Well,
they continue to spin and give off radiation and eventually they slow down so much that they move into this part of the diagram where they're rotating
too slowly so that they can't that rotating magnetic field doesn't generate
strong enough electric fields so that the radiation process just shuts down.
And so they're still there. They're still floating and zipping around the galaxy, but we cannot observe them.
There's no pulsations, no lighthouses out there in the sky for us to observe anymore.
They're just all dead and black zipping around the galaxy.
So there's a bunch of empty sources that we do not see here. And we call this the pulsar graveyard.
And it takes between ten and 100 million years for that to happen.
And once again, astrophysical, that's pretty short. Remember the age of the galaxy's about 10 billion years.
So 10 million years is a thousandth of that. So many, many generations of pulsars have been born and died in our galaxy.
And there's huge numbers of missing objects in this part of the diagram.
All right, so that's good. So then what? The problem is we see this happening, but we also see other supernovae that don't seem to produce pulsars.
And here's a great example.
The last supernova that we saw go off very, very nearby to the earth was in our closest little satellite galaxy, and this is called Supernova 1987.
And for various reasons, I don't have time to get into it. Made a neutron star, at least for a little while.
We know that we saw particles that tell us that it made a neutron star.
There should be, we thought, a pulsar there.
But huge numbers of groups with many different telescopes have looked for a pulsar and zero pulsations have been found,
including lots of effort on my part, a bunch of and a bunch of other people as well.
So very interesting what's going on there. We do not know. We do not know.
Hopefully we'll figure it out someday. Here's another great example.
A very famous supernova went off in the galaxy a few hundred years ago.
This is a beautiful composite image of a bunch of different wavelengths, beautiful image of Cassiopeia.
A In the centre of this object is this glowing little dot right here that we're almost positive is a neutron star.
And some of the deepest observations of that neutron star using big telescopes have been made and zero pulsations.
We call that thing this enigmatic object where we know there's a neutron star there, a complex central object.
And many people study these weird objects where we really don't understand what's going on with the neutron stars.
What else? Well, some other supernovae have these things called magnetars.
So here's an X-ray observation of a of a supernova.
And in the centre is this weird object that looks kind of like a pulsar.
The thing is, there's no radio pulsations, and the vast majority of pulsars are only seen in the radio.
This is seen in X-rays, though. So what is a magnetar?
Well, if we go back to our Hertz Brian Russell diagram, basically they live in the very, very top right corner.
They have the most strong magnetic fields of any objects that we know of in the universe.
And they're extraordinarily strong.
They're so strong that the magnetic field, it packs so much energy density that quantum processes end up happening in in the magnetic field.
For example, if you took a cubic centimetre of of the magnetic field of these objects, a cubic centimetre is like a sugar cube.
There's more energy stored in the magnetic field of that cubic centimetre than all of humanity produces in a year.
So if you took this much of the magnetic field, it'd be more power than all of humanity has ever produced.
Ever. And that's the Mac, just the magnetic field, forgetting about all the energy and the gravity and the rotation and everything else.
Very amazing objects. And they're powered by their magnetic field, not their rotation.
And they can do some crazy things. Big magnets, strong magnetic fields like that can twist and do do all sorts of nasty things,
which causes outbursts that are seen in X-rays and gamma rays. Here's the very famous outburst that happened in gamma rays.
Basically, the outer part of the magnetosphere, somehow twisted, released a tremendous amount of energy.
This thing is half way across our galaxy.
And when that wave of of of energy swept over the earth, it caused gamma ray satellites to be completely saturated.
Many satellites turned off because there was so many particles and energy that hit the satellites, the earth itself.
It was night-time on that side of the earth. And when that when the wave of gamma rays hit the ionosphere, this is the ionospheric level.
How much how ionised it was when the gamma rays hit it completely iron, ionised the atmosphere to daytime levels.
This thing is halfway across the galaxy. And then you can see over the next few minutes it the atmosphere recovered again.
But if this thing was next door, this would be bad.
This you could write a science fiction article about about the thing doing bad things to your to your planet.
But luckily, most of these are far away in the galaxy. All right.
So that's basically some of the neutron star objects that are produced in supernova when that when the core collapse happens.
But there's a big part of this diagram that I haven't touched on yet, and these are actually my favourite objects.
These are the so called millisecond pulsars. And you can see the magnet, the young pulsars in the magnetars and the neutron.
The regular pulses are all up here. They evolve down here and they get to this part of the diagram.
These are off on their side.
The other thing you might notice by this diagram is they all have little circles around them, and that's the key to how they get there.
So what are those little circles? The little circles mean they're in a binary.
They have a companion star. So what does that do? Well, the first of these objects was found back in 1982, and it completely surprised everyone.
This is the paper where it was announced. This is one of my mentors, Don Becker, who unfortunately passed away a few years ago.
But he, along with his team, studied an enigmatic object in a huge amount of detail,
looking at using better and better instrumentation to try to find pulsations.
And eventually they did 1.55 millisecond pulsations.
This thing rotates 640 times per second. That was about 20 times faster than the previously fastest known pulsar.
This was a big surprise to everyone. Well, what is 640 times a second sound like?
I told you what the crab sounded like. I'll see if I can make this sound. It's really annoying.
But the interesting thing is it's in the audio band.
It's rotating so rapidly that you don't hear individual pulsations.
They blend together into a tone, and that tone is about a half an octave above concert.
A On The Well-Tempered Scale. This is a rotating neutron star.
That was astonishing. How did it get so fast? The answer is that binary those binaries like I was mentioning.
So if you create a neutron star and a supernova, this becomes a pulsar like the crab.
That crab ages over the next ten or 100 million years and eventually dies.
But if that supernova and the star before it had a companion star, a star may be like our sun and it survives that supernova.
The Pulsar orbits this star, and eventually, after billions of years of the pulsar, is long dead.
After billions of years, because of the binary evolves, that star turns into a red giant just like our sun will.
And when it turns into a red giant, it begins transferring material into a disk around the neutron star.
We see this happening. These are the so-called X-ray binaries.
We see these happen going off around around the sky.
We see their orbits. We see the red giants.
We see the neutron stars spinning faster and faster because as the material gets dumped on the neutron star,
just like a basketball player spinning the basketball faster and faster. That's what happens to the neutron star.
The other thing that happens is that the very strong magnetic fields of the of the pulsar, this material that gets dumped on them,
they get squashed down and pushed into the star so that most of the magnetic field effectively goes away.
When this evolution ends, eventually you're left with a white dwarf,
just like our sun is going to be a white dwarf in a perfectly circular orbit around a millisecond pulsar.
And the reason why the orbit circular is that that process, when you're dumping gas,
is very messy in the minimum energy situation because a tidal forces is a circular orbit.
This process we call recycling. So you take your old dead neutron stars and you recycle them and spin them up to become millisecond pulsars down here.
And this is great because these objects are really useful for us to do basic physics.
And in the last bit of the talk, that's what I'm going to be discussing, the type of basic physics we can do with those objects.
All right. And these because they get recycled, they're rejuvenated and brought back to life.
These really are the stellar undead. So these things, especially the millisecond pulsars, are very precise clocks.
They have much weaker magnetic fields. So they spin very, very stably.
They don't slow down much at all. And that's really, really excellent for us because we can use them as perfect clocks.
Well, just to kind of how perfect of clocks. Well, here's a very famous pulsar.
And at exactly. At 5:30 p.m. on today which was about I'm slightly off I miss estimated.
So 10 minutes ago this pulsar that was its spin period.
And this is kind of like if you do math on a calculator, you know how your kids can like divide like four divided by three.
And the answer is 1.3, three, three, three, three, three. Well, and usually you don't need all those decimal points.
You have to know your errors and stuff. Well, as astronomers and scientists, we have errors on this.
This isn't just a calculator thing. That's our error bar.
Okay. That's the actual spin period of that pulsar.
10 minutes ago, with all those digits of precision.
Now this thing is slowing down with time since it's slowing down with time because it's giving off energy just like the odometer on your car.
This number is changing with time. That means that these digits, especially these last two days, are rotating.
You know, you can think of them ticking off. So how much does this pulsar slow down the time?
Well, that digit changes by about one every half hour.
So even though I was wrong about my timing, I was off by 10 minutes or so.
That number is still the same, at least now. But in a few minutes, it'll click over to become a four.
The amazing thing is, is, well, what happens when you think about what's going on with those other digits?
That means that that digit doesn't change until 500 years from now.
That means that the first six digits are exactly the same for the next millennium.
And this is a natural object that's spinning by itself out there in space.
These are really perfect, amazing celestial clocks.
And by being able to measure them precisely, we can do really neat physics.
And so we use these as tools.
And in my opinion, even though these are great, amazing, exotic objects, this is why I find studying pulsars fascinating.
So how do we do this? Well, the process is known as pulsar timing. I'll quickly go through it.
It's a really neat technique. And if you want to glaze over with this semi technical thing, the important thing is,
is that we unambiguously count every single rotation of the neutron star over timescales of years.
We do not miss a pulse. And that's despite the fact of the Earth's moving around the sun.
We're passing thousands and thousands of pulses as we go around the sun.
The pulsars are moving in their orbits. There's thousands of pulses that are lined up in their orbits, travelling.
As long as you keep track of everything properly, this works and gives you amazing results.
So in this in this demonstration,
this is one observation of the telescope and time is going to be flowing he likely this direction what I mean by he likely.
Well pulses I'm stacking pulses one on top of each other because radio pulsars are very faint objects.
And you have to you have to put lots of pulses on top of each other just to see them.
So. But in order to do that, you have to know the rotation period of the pulsar pretty well.
So if we take our data and we wrap it around on top of itself, so the pulses always line up with each other.
That's this black line.
So we wrap it at the same rotation rate that the pulsars rotating, we can stack all of our data and that's what our measurements actually are.
We rotate our data, basically stack it up. That becomes what we call our times of arrival.
That's when the pulses arrive at our telescope. Once we have those measurements, we have this nice line.
That's actually our model. That's our model for how rapidly the pulsars rotating we can we can mentally.
Matter of fact, you've already done it. You don't even realise that you've made a timing model in your mind.
You can predict when those pulses are going to arrive in the future and it's that line.
So if I go back to the telescope and make an observation several hours later and I can see,
do my pulses arrive right where I expected them to and if I have the correct timing model,
sure enough, this line completely predicts where the next pulsars are.
And I can tell because this line is a diagonal.
I didn't miss any rotations, so I know exactly how many pulses are in that area where I didn't observe.
So you don't have to observe continuously. You just have to observe smartly and you have to space your observations appropriately.
So maybe a week later I go back to the telescope and I make another observation, and this time it's a little bit off.
Not enough. That causes me dramatic worry, but because it's a little bit off, I can make my model a little bit better now.
I can improve my model and then I can come back another week later and to my mind
a little bit more until I've I've got a whole what we call timing solution.
Once I have that, I take my measurements minus my model and this is what we plot in our papers, timing, residuals.
So all this shows right here, this is two years of data along the X axis.
Every day at the telescope is a group of these points. So there's many, many measurements.
It's kind of like all the measurements that I get across here and I subtract off the model from it.
So if my model's good, I should get zero for these residuals.
And you'll notice that the units on the residuals are microseconds here.
Okay. If I were to make a histogram to show how my errors on these measurements are,
what you get is a Gaussian, which shows that I'm just dominated by noise, which is good.
And the the size of that, that histogram would be 200 nanoseconds in width.
That means I can tell you any pulse amongst the billions and billions that occurred over these two years, within 200 nanoseconds.
In a nanoseconds, a billionth of a second.
For those of you who don't know what a nanosecond is, 200 billionths of a second when any individual pulse arrived at Earth.
If you do this right, you get amazing answers, amazingly precise measurements.
Here's an example of what a paper, what a timing model looks like in one of our papers.
And the timing model. These are the parameters such as the position of the pulsar in the sky.
That's what the first to this is. Ten micro arcs.
Second position. Purely from timing when the pulses arrive.
This is not from a high precision image. It's from timing. We get the spin period.
There's all those digits I was mentioning. Here is the orbital parameters we can see.
So how the pulse was moving across the sky. That's called proper motion.
There's some relativistic effects down here.
All these parameters we can measure to exquisite precision by counting the every rotation of the neutron star.
So here's an example of how you can do the basic physics. If you ask the pulsar the right question, you get amazing answers.
So I told you that when you make a millisecond pulsar, you get a circular orbit with with a white dwarf.
Well, here's the orbital parameters, and here is this e centricity that tells how circular the orbit is.
And it's 0.000019.
That sounds pretty circular, but I don't actually think in terms of eccentricity, so I'm not quite sure how circular that actually is.
So how circular is that? Well, we know things orbit in ellipses.
Right? So this is not actually a circle. Circle. And if it's an ellipse, that means it's got a long axis and a short axis.
Well, what's the difference between the long axis and a short axis for this orbit?
Well, this line above it is the size of the orbit in terms of light seconds, and it's 3.3 light seconds.
Well, once again, I don't think in terms of light seconds. So how big is this orbit?
Well, the size of the orbit is about 1.4 times the sun's radius or about ten to the 11 centimetres.
That's pretty big. That's a pretty big orbit. So that's the size of the circle, roughly.
So what's the difference between the semi-major and some minor axis?
The answer is 18.59 plus or -0.01 centimetres.
And this thing is thousands of light years away, and we can measure something with that precision.
Now, yes, this is a really this is kind of I picked the exact right question to ask a pulsar here, but that's the whole key.
If you can figure out the right questions to ask timing, you get amazing answers.
And the first group that really did that was Russell Hulse and Joe Taylor when
they found the first binary pulsar with the Arecibo telescope back in 1974.
And that binary, there was a pulsar and another neutron star in that binary.
And it doesn't exactly follow the full recycling scenario that I told you about.
So it didn't have as perfectly circular orbit. It has an eight hour orbit with high eccentricity.
That's great, because when you have an eccentric orbit, you can watch the ellipse change because of general relativistic effects.
So Einstein's theory of relativity causes changes in the orbit.
For instance, a perception rate due to the relativistic perception of four degrees per year,
which is dramatically larger than the way mercury processes around the sun, which is something that was a test of Einstein's theory of relativity.
But they measured this orbit very precisely using pulsar timing.
They measured using pulsar timing. They were able to measure three relativistic effects.
And you can show these relativistic effects very nicely on this diagram, which is a diagram that shows the pulsar mass versus companion mass.
And according to general relativity, each of these relativistic effects gives you a different line on this diagram.
So when they measured the first two relativistic effects,
which were this one called Gamma and this one called Omega Dot, it gave them the pulsar mass and the companion mass.
That's according to general relativity. Relativity then gave them the masses of the objects,
but then they measured the third effect that's this line and that let them test general relativity.
The fact that that line goes so close to the intersection of the other two was a test of Einstein's theory of relativity in the strong field,
basically. This was a huge thing.
This was this was great. This is a famous plot. This basically shows this curve here,
shows that the orbit was shrinking because that orbit was giving off gravitational
radiation and that gravitational radiation was causing this plot right here.
These are the data points. Their measurements, that line is not a fit to the data.
That's general relativity prediction. It's just beautiful, clean, basic physics.
And for this, they were awarded the Nobel Prize for physics.
And Russell Hulse was a graduate student at the time. Russell Hulse didn't even stay in astronomy.
And Russell got the Nobel Prize. And there were a lot of conversation was started yet again about how Nobel Prizes would be or should be issued.
And there's every year there's there's conversations about this. Anyways, fascinating system.
And it's amazing for us now because it's an indirect detection of gravitational radiation.
And this is a very important part of science right now.
And it's one of the things that myself and many other people doing pulsars are working on right now.
We're trying to directly detect gravitational radiation using millisecond pulsars.
And this project is there's three big groups around the world. One is called Nano Grab in North America, once called the European Pulsar Timing Array,
and the other in Australia is called the Parkes Pulsar Timing Array.
And together we if if we properly joined forces and if we can calm each other's egos down and share our data the way we should,
it's called the International Pulsar Timing Array. So how does this work?
Well, if we have millisecond pulsars spread around the sky,
so say you guys are all millisecond pulsars and there's gravitational waves slowly moving through us,
causing you guys gravitational waves cause space time to jiggle, basically.
So if the gravitational waves are causing you guys to be moving around, they're also causing me to be moving around.
And I have the telescopes. Right. So the telescopes are here.
Did I ever turn the light back up? I just realised, oh, that's okay.
So if I have the telescopes in space, these gravitational waves jiggle me towards you.
You're all pulsars, all of your signals are going to be a little bit early.
All the pulsars behind me are going to be a little bit late. If I get jiggled away, then all your signals are gonna be a little bit late.
These ones are going to be a little bit early. These ones over here don't get affected at all.
We use an array of pulsars around the sky to look for a correlated signal in the timing of the pulsars that indicates gravitational waves.
And we're looking for gravitational waves from the most massive black holes in the
centres of big galaxies that have collided throughout the age of the universe.
And we think that it's likely that we'll have a detection within the next five years.
Things are working really, really well, and so it's a very exciting project.
So the gravitational wave universe, the window on it will be open in the next five years, first probably from advanced Lego in,
second probably from pulsar timing arrays, although there's a chance that we could do it first, as I'd like to keep telling the Lego people.
All right. So this is probably this is what's driving a lot of pulsar research nowadays.
It's really neat, basic physics, and it's being done by these four telescopes as well as several others.
But these are for the of the best known pulsar telescopes in the world.
The 300 metre Arecibo telescope in Puerto Rico. This is huge, is built into movies.
It's so big that they had to use a James Bond movie. It was used in one of those.
This is a 100 metres diameter Greenbank telescope in West Virginia.
That's a big lorry rig or semi-tractor trailer truck right there.
That's really big. This is a 64 metre diameter Parkes telescope and the 70 plus metre level telescope at Jodrell Bank.
Big, big telescopes. We need these because our sources are so faint.
We really need lots of collecting area to do these kind of measurements.
These these telescopes are also searching all of them are searching for new
pulsars because we only know of a small percentage of the pulsars in the galaxy.
And we need more good millisecond pulsars to do the gravitational wave study as well as to do
other neat tests of physics like to figure out what's going on in the centres of neutron stars,
or to test general relativity or to do a whole bunch of other types of astrophysics as well.
So these are all taking huge amounts of data, petabytes and petabytes of data and searching for lots of pulsars.
And we're finding lots of really, really good, interesting objects.
We found objects that are transitioning that are at the end of their recycling scenario.
So they're transitioning from X-ray binaries to millisecond pulsars.
We found objects that are in a centric binaries, but they're millisecond pulsars that totally surprised us.
We found objects that are really interesting, new tests of general relativity, and this is all in the last few years.
Here's the last object I'm going to mention. It's a really neat one that I'm very partial to, and it was a big surprise to all of us.
So this little object here, the white object, is the neutron star, and this is a white dwarf.
And this is the zoom in up here of of that orbit. So the neutron stars orbiting and the the white dwarf, they're both orbiting a centre of mass.
And this happens to be being orbited by another white dwarf out here.
All of these dots are our data points.
What we found in the last about two years ago was a triple system where a neutron star, a millisecond pulsar, has two white dwarf companions.
So this is the first known millisecond pulsar triple system.
And it's incredibly complicated dynamics since there's three bodies that are all interacting due to gravity.
Those three those very interesting three body interactions are being modelled by a post-doc in the Netherlands.
She just came up and visit me during my sabbatical here and we worked together for two weeks.
She's doing fantastic work on the modelling, the timing model of the system to figure out the physics of what's going on with this system.
Here's what our data look like. I got to show you this. It's kind of astonishing.
As the pulsar moves in its orbit, as it moves away from you, its pulses are delayed as it moves towards you in its orbit, they show up early.
That delay is what I'm plotting on this plot right here.
So it's delayed by about 70 seconds, then it's advanced by about 70 seconds over the 327 day orbit of the outer star.
That's the outer star right there. Now, you can see there's a bunch of our data points that all look scraggly right there.
We actually have error bars on all these data points. Our error bars are a million times too small to see.
Okay. That's grogginess. If you zoom in on it is this.
This is the insert. That's the inside orbit, that 1.6 day orbit.
We have a huge amount of data and it completely describes to exquisite detail
using just Newton's gravity and a little bit of special relativity by Einstein.
The behaviour of this system. And this is neat because it's going to provide something.
We didn't expect a test of a really interesting property that general relativity predicts, which is called the strong equivalence principle.
So what the heck is that?
Well, the weak equivalence principle a lot of people have heard of it basically says that all things should fall at the same rate.
That's called the universe fatality of fall. And you can also state that in slightly different ways.
So that basically says that our inertial mass, which is how much mass something has when you push it, is the exact same as the gravitational mass.
The mass it feels in a gravitational field. And this is from just this this just just came out a couple of weeks ago from Brian Cox's new show.
They basically bought out this, the world's biggest vacuum chamber, which is in Cleveland, Ohio.
It's a massive facility, this massive facility. I'm going to turn the lights down.
And in it, they removed all of the air.
And then dropped a bowling ball and a feather at the exact same time.
And you can see that they're falling at exactly the same rate, just like when the astronaut on the moon dropped the hammer in the feather.
Or Galileo did the experiment with the ball in the feather, offered them off the leaning tower.
So that's the weak equivalence principle.
What we're going to test with this system is, is the fact that the neutron star has a huge amount of self gravity.
Strong Cuban principle says that this thing applies.
Also, if you have an object like a neutron star or a black hole where a huge amount of its energy is stored in gravity itself.
So gravity itself gravitates.
And what we're doing then it's like it's like the neutron star is the bowling ball and that inner white dwarf is the feather.
And they're both falling in the gravitational field of that outer white dwarf.
And we're going to have a result out on this within the next, we hope, 4 to 5 months.
And our results should be a factor of between 20 and 100, better than the best results ever published.
And the best results are made by using lasers to bounce off the moon and seeing how the moon and the earth fall in the sun's gravity.
And we can do way better. Even though this pulsar is thousands of light years away because of the beauties of pulsar timing.
So the future. What do I think? I think it's a great time to be a pulsar astronomer.
We only know of a small fraction of the pulsars in the galaxy, and all the major telescopes are going in making surveys.
We're finding lots more now, and we're building new telescopes.
The new telescopes that are being built have a huge amount of sensitivity, and they're going to let us find many more objects.
So here's a big telescope that's being built in South Africa called Meerkat, which will revolutionise Southern Sky surveys.
Here's a 500 metre beast in China called Fast.
Twice the size of Eris, though almost. It's under construction now.
I've seen the most recent construction pictures. It's mindboggling how big this is.
They had to remove a whole village in order to construct this.
I'm not kidding. But we're in these surveys.
We're going to find, we think, some truly exotic, amazing objects of self millisecond pulsar,
one that's rotating faster than 1000 times per second, a pulsar, a black hole system, a millisecond pulsar, a millisecond binary.
These things are probably out there in our galaxy and these new surveys will probably find them and let us do really neat basic physics.
This last thing is one of the reasons why I'm here for my sabbatical, the Square Kilometre Array,
because Oxford is a big part of this big radio telescope that's being built over the next 10 to 20 years.
And a lot of the crucial work dealing with pulsars is being done here by aerospace research,
you and a bunch of other people, many of them in the audience.
And the SKA is going to be phenomenal. It costs a lot of money.
That is a problem and people are still worrying about that.
It's going to have thousands of antennae, thousands of dishes in South Africa and Australia.
These are both you kind of you can kind of switch the locations in these in these pictures.
But basically we know where the dishes are going to be. We know where the the other kinds of antenna are going to be in these countries.
We don't know exactly how many. We don't know exactly what type yet.
But the cool thing is we should find most of the pulsars in the galaxy with the SKA in the next 20 years.
So I think my summary, my summary slide, I think that the Stellar Undead are pretty amazing and I absolutely love what I do.
Thanks.