BRIAN: So Paul, we need something really small
and really hot, and that&#39;s great, because
we have something in our inventory out in space called a neutron
star that just perfectly fits this bill. PAUL: But would a neutron star be this hot?
BRIAN: Well, so a neutron star, if we remember, is 
formed when a massive star, 10- 20 times the mass
of our Sun, runs out of nuclear fuel in its centre and collapses down
and it forms a neutron star, and the centre of that star
is literally billions of degrees when the neutron star is born, so
the neutron star is going to be born really really hot. PAUL: So it sounds like a
really good hypothesis here, that these things, these X-ray sources, are neutron stars
let behind from supernovae. And further evidence for this came from the second one of these
X-ray sources discovered. If you take an optical picture of the part of the sky
where it came form, the X-rays, this is what you see. This is a picture we&#39;ve seen before.
BRIAN: Yes, this is the Crab Nebula. This is where in 1054
the Chinese saw a &quot;Guest Star&quot;
that is, what we would now call a supernova, and we can literally see
the interior of that big star that exploded a thousand years ago
expanding, and so, here we have something hot and small
Seems to me, like, it&#39;s game over, we have our
explanation, it&#39;s a neutron star. PAUL: yes, this would be a very short lesson if that were the case. But
 things get a little bit more complicated, as so often the case in
astronomy. One problem - this was the second X-ray source discovered, and
sure enough, this sits right in the middle of a supernova remnant. But how about
the first one, the one that&#39;s called Scorpius X-1, because it&#39;s in the constellation of Scorpius
an incredibly bright X-ray source. Do you supernovae recently in Scorpius?
BRIAN: Ahh, no, I don&#39;t, and we&#39;ve got a pretty good record of them
as well, we certainly didn&#39;t see anything explode, and it doesn&#39;t sounds like
there&#39;s even any supernova remnant in that part of the sky. PAUL: So that would indicate that if 
it was produced by a supernova, the supernova must have been quite a long time ago - long enough that we have no records
and that the blast wave has faded out of visibility, but that&#39;s - 
how long does it take a blast wave to fade to invisibility? BRIAN: Oh, it&#39;s a couple of hundred thousand
years before a supernova fades, but you&#39;re right, a hundred-thousand
years is a long time, because of course that neutron star is really
hot and really small, it puts out a lot of energy and it&#39;s going to cool
and not be so hot and not so bright. PAUL: Yup, so that&#39;s one
puzzle, that not all these X-ray sources, in fact the majority of 
X-ray sources aren&#39;t associated with nice supernova remnants like this
and there&#39;s also another problem. If it was the neutron star
in the middle that was producing the X-rays, then you&#39;d expect an X-ray image just to show a little dot in the middle
All the radiation would be coming just from that neutron star, smaller than a pixel in the centre. BRIAN: So when they
took their picture, did they see a little dot in the centre? What did they see?
PAUL: Well, to begin with, they couldn&#39;t quite tell more than that it was coming from vaguely from somewhere around here
but they used a very cunning trick. They waited until the Moon came across this part of the sky.
And as the edge of the moon went across, it will block out
X-ryas, and if everything was coming from a little dot in the middle, you&#39;d see
full X-rays, full X-rays, and suddenly, when the limb of the Moon crosses it, it will drop
to nothing right away. But they did their measurement - it was a really hard one to do
because the sounding rockets were very unreliable, it was hard to launch them at exactly the right time
to get precisely to the time when the Moon was crossing exactly the right place, but
they managed it, and what they found was that instead of having an abrupt drop
in X-rays at the moment when the limb of the Moon crossed the middle, they had
a more gentle drop in brightness of X-rays, so it didn&#39;t suddenly 
, it went away more gradually, about the time it takes the Moon to cross
a fair portion of this. BRIAN: So that&#39;s interesting
because, you get a sense of how this would work if you&#39;ve ever seen a total solar 
eclipse. The Sun is really really bright, and then when that final bit of the Sun
gets covered up, and when that final bit of the Sun gets covered up, it just instantly turns black, and what you&#39;re sayings is that as this was
a slow process, that seems to indicate that whatever is glowing
in the X-rays had to be actually, the X-rays
themselves had to be not really really small on the sky but
actually quite broad, so that seems a little different than that idea that
all the X-rays are coming from the neutron star. PAUL: And if you get a modern X-ray image of this
part of the sky, you can indeed see - this is where the X-rays are coming from, and they are not
all coming from a dot in the middle but actually are mostly coming from the sort-of whirlpool-y shape  
around things. BRIAN: OK, so we have something that still makes me happy
there in the centre, but most of the X-rays are coming from outside
it in this sort of windy bit and stuff, so I can imagine
there&#39;s lots of shocks and things, but those only last for
thousands of years, so you get X-rays a little bit, but then there&#39;s got to be
a source of energy that keeps things going, even from those shocks.
PAUL: we need something to get get the nebula and excite it. And curiously enough, about the same
time that these observations were being made, a theorist called Franco
Pacini came up with a possible explanation. The idea was that of course
when a, stars have magnetic fields, and the star that presumably
formed the neutron star had magnetic fields, and it could be that when it
collapsed to form the neutron star, it dragged its magnetic field lines in with it, and so what was
originally a fairly spread-out magnetic field became an incredibly concentrated magnetic field.
BRIAN: a really powerful magnet. PAUL: Yeah, a good analogy of this is rubber bands
Let&#39;s imagine that this whole room was full of rubber bands running from the roof to the floor, and I
get an armful of them, and then pull them tight, and then go around in 
circles like this, I&#39;m going to get this really tangled knot of
rubber bands, that would probably fling me back around again at the end, and the same thing
might happen in this case, you&#39;ve got the star, neutron star, which  is presumably due to conservation
of angular momentum, is going to be spinning really fast by the time it&#39;s formed. BRIAN: So that&#39;s like
the analogy of the person doing the
figure skating and bringing their arms in and speeding up. I always do it with phone
books and a chair - it&#39;s quite fun when you are bored at the office. Take the
phone books in and spin yourself around in your chair. PAUL: So, you have a really strong magnetic field
rotating like crazy. In fact, if you do the calculations, the amount of energy
you get in just one cubic metre of that magnetic field would be enough to power the entire
world. BRIAN: Wow, OK. PAUL: And it&#39;s spinning really fast, that means it&#39;s 
constantly changing, and whenever you get a magnetic field that is changing, it generates an electric
field, which generates a magnetic field, and you&#39;ve got electromagnetic waves, in this case
they would be very low frequency radio waves, with a frequency of maybe only one hertz
which means a wavelength of three hundred thousand kilometres,
so this is far too long wavelength for anything we can pick up, and in fact it wouldn&#39;t get out of
the nebula, but the idea is that these incredibly low radio waves with enormous energy
would come out and get mopped up by the electrons in the nebula, and it
would excite them and give them lots of energy, which would produce shocks and X-rays and everything we see. BRIAN: so you
what you&#39;re really going to do is you&#39;re going to transfer the energy of that one
point four solar mass rotating neutron star
and you&#39;re going to transfer that energy, and there&#39;s a lot of energy, that&#39;s
like the world&#39;s biggest fly-wheel, and through this process you&#39;re going to
transfer that energy out into the nebula, and then you are can get shocks or
something, to give you the X-rays. PAUL: Yesh - it sounds like it almost might work
BRIAN: OK
