Finding Neutron Stars
Can We Detect Them?
As noted earlier, this seems very unlikely:
they are small (asteroid-sized)
 even the nearest is likely to be hundreds
or even thousands of light years away
 even if super-hot, they will be very faint

But: Two Important Factors
Two things come to our aid:
1.
Stars rotate; and
1.
Stars have magnetic fields
Now Suppose a Star Shrinks…
Both the rotation rate and the strength of the
magnetic field are affected!
1. Conservation of Angular Momentum
A Dramatic Example
World Record Figure Skating Spin
Consider the Sun
It rotates every 25 days and is about
1,000,000 km in diameter.
If it shrank to 10 km in diameter (a factor
of 100,000 smaller), it would rotate in a
period of about ~ 20 sec
So: Prediction Number 1
If a star collapses to neutron-star
dimensions, it will probably be rotating
very fast – on a timescale of seconds or
faster.
2. Magnetic Fields
The Sun’s global magnetic field is comparable to
that of the Earth in strength (i.e. quite weak)
If the sun shrank by a factor of 100,000 in radius,
the magnetic field would grow in strength by a
factor of about
100,000 x 100,000 = 1010 (= ten billion).
So: Prediction Number 2
If a star collapses to neutron star
dimensions, it might be expected to have
a fantastically strong magnetic field.
The Wisdom of Hindsight
A really clever astrophysicist might have predicted
how to detect neutron stars, on the basis of
these two considerations:
extremely fast rotation, and
a very strong magnetic field.
Too Bad!
In fact, the discovery came first, completely
by accident.
That was quickly followed by the ‘obvious’
explanation.
Meet Jocelyn Bell
Strip Chart Recording
Jocelyn’s Discovery (1967)
The Discovery
Something is giving rise to ‘blips’ of radio
radiation (not sound!!) about once every
second or so, with metronomic regularity.
What can the source be?
What Do We Learn from
the Rapid Variability?
First, consider city lights! The alternating electric
current comes and goes 60 times a second, too
fast for us to notice. The lights are “turning on
and off” all the time. (Car lights use direct
current from the battery and are steady.)
In slow motion:
Incandescent lights flickering
Now, Suppose You Could
Turn the Sun Completely Off
Assume that the whole surface goes
completely black, all at the same instant!
What would you see? Would it vanish
instantly?
No, For Two Reasons
First, you would not learn about this event
for eight minutes!
Second, the fadeout would take a bit of time
because the sun is big.
What you would see is shown in the next
sequence of panels:
So
The Sun’s Light Dies Away…
…but it does so gradually, taking about 21/2 seconds to do so, with a growing
central ‘blot.’
That’s because the sun is about 2.5 “lightseconds” in size.
Light from the Edge ‘Lags’
Now, What If the Sun
Comes Back On?
Suppose the sun ‘turns back on
(everywhere at once) after about a
second, and this on-off cycle repeats?
What would we see?
And So On: Over and Over
Result: unsteady brightness
….but it never goes out completely!
Summary:
If the sun were able to switch on and off very
rapidly, over and over, we would see:
1) Concentric thin rings of light and dark, moving
outward
2) A fairly steady overall brightness, with only
moderate variability!
Imagine “Turning Off” a Galaxy!
If Every Star in M31 Died at Once
… the galaxy as a whole would take 100,000
years to vanish gradually!
The stars nearest us would disappear first;
the ones farther away would be seen to
vanish much later.
[M31 is about 100,000 light years across]
The Lesson
If the light from an astronomical object
varies dramatically on some timescale, the
“emitting region” can be no bigger than
that (expressed in light-seconds or lightyears, say).
Pulsars Are Fast!
The pulses come once a second, roughly, but the light
turns on and off completely in a fraction of a
second.
(Moreover, much faster pulsars were found later.
Some turn on and off hundreds of times a second!)
Conclusion:
The light-emitting region
must be very small.
These can’t be ordinary
stars!
…and it certainly can’t be
due to eclipses!
So, What Produces the Pulses?
1. Electrical interference? (Car ignitions,
local domestic appliances, …?) Perhaps
something quite mundane.
This was quickly ruled out – the sources
were clearly ‘up in the sky’ and outside
the Solar System.
Another Possibility
2. LGM??
Question: if that’s the case, how do you
handle the news?
This was also ruled out quickly: there are
too many sources in too many different
directions.
Many Thousands of Pulsars
Have Been Found!
Quickly Explained!
The correct astrophysical explanation (next
PowerPoint!) was arrived at within a
matter of weeks.
It has not really changed, except in fine
detail, in almost 50 years.
But Justice Was Not Done!
The Nobel Prize was awarded to Hewish and
Ryle (with no mention of Jocelyn Bell)
Read Jocelyn Bell's gracious remarks