Finding Neutron Stars:

The Discovery of Pulsars

Can We Detect Neutron Stars?

As noted earlier, this seems very unlikely:

u  they are small (asteroid-sized)

u  even the nearest is likely to be hundreds or even thousands of light years away

u  even if formed as super-hot objects, they will be very faint, and fade away quickly as they cool

Surprise! – We Can Detect Them

But we first did so in an unexpected new way, through the chance discovery of pulsars. Remarkably, though, a really clever astrophysicist might have predicted the way in which pulsars would behave and make themselves known to us.

Two Relevant Considerations

Stars rotate, and have magnetic fields Now ask yourself: What would happen if a star were to shrink by some huge factor? (That’s what happens in the formation of a neutron star.)

1. Rotation: The Conservation of Angular Momentum [ASTR 101]

https://www.youtube.com/watch?v=AQLtcEAG9v0

Consider the Sun

It rotates every 25 days and is more than 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, a first prediction:

if a massive star collapses to neutron-star dimensions, it will probably be rotating 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. If the sun shrank by a factor of 100,000 in diameter, the magnetic field would grow in strength by a factor of about

100,000 x 100,000 = 1010 (= ten billion).

So, a second prediction: 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:

n  extremely fast rotation, and n  a fantastically strong magnetic field. But that’s not what happened. The actual detection came

completely by accident, with the physical explanation of their nature to follow almost immediately thereafter.

Jocelyn Bell’s Discovery (Cambridge UK, 1967)

Jocelyn’s 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? Something that

seems to ‘turn on and off’ once a second?

What Do We Learn from Rapid Variability?

First, consider city lights! The alternating electric current comes and goes 60 times a second, too fast for us to notice (thanks to our persistence of vision). The lights are “turning on and off” all the time. (By comparison, car headlights use direct current from the battery and glow steadily.)

Watch this behaviour in slow motion: https://www.youtube.com/watch?v=Fo_1fZfv0P8

Now Think Astronomically

Suppose you could instantly turn the Sun off – that is, assume that the whole surface goes completely black, all at the same instant.

[Of course this is physically impossible. Even if we could somehow shut down the nuclear reactions in the core, the hot material would continue to glow for a long time. This is a purely ‘what-if’ scenario!]

Would you see the Sun vanish instantly?

No, for Two Reasons

First, you would not learn about this event for about eight minutes! Some light is already ‘on the way’ and will continue to arrive for some time: news of the change is delayed. This is just an accident of geometry, because of our location relative to the sun. More importantly, the fadeout would take a bit of time because the sun is big. Here is a crude animation of what you would actually see: https://youtu.be/SWxUqZ-SjgM

So The Sun’s Light Dies Away…

…but it does so gradually, taking about 2-1/2 seconds to do so, with a growing central ‘blot.’

That’s because the sun is about 2.5 “light-seconds” in size, and light from the edges lags behind the light from the [closer] centre of the disk.

Now, What If the Sun Switches On and Off?

Suppose the sun turns off, then back on

(everywhere at once) after about a second -- and then this cycle repeats, over and over?

Here is a brief animation showing what we would

see: https://youtu.be/Ep-Z7W1Thso

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 average brightness, with only

moderate variability! Moreover, if it switched on and off even faster, we would

see thinner rings -- but experience an even steadier and more uniform average brightness!

Now Imagine “Turning Off” an Entire Galaxy!

If Every Star in M31 Died at Once

… as seen by us, that whole galaxy would vanish gradually, taking about 100,000 years to do so! This is because M31 is about 100,000 light years across. The stars nearest us would be seen to disappear first; the ones farther away would vanish much later.

On-and-Off Behaviour

Suppose all the stars in M31 turned on and off, all at the same instant, once every century (say). We would see many thin waves of darkness ‘drifting’ across that galaxy --- but the overall, average brightness would be essentially steady.

The Inescapable Inference If the light from an astronomical object varies dramatically on some

timescale, the principal “emitting region” can be no bigger than that (expressed in light-seconds or light-years, say).

But pulsars vary very quickly. 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!)

So the light-emitting region must be very much smaller than a conventional star.

So, What Can Produce the Pulses?

1. Electrical interference? (Car ignitions, local domestic appliances, …?) Perhaps something quite mundane.

Jocelyn Bell was able to rule this out quickly

– 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? (The Astronomer Royal of Great Britain suggested keeping it secret!)

This was also ruled out quickly: fairly soon, far too

many pulsars were discovered in many different directions. It must be some natural, commonly-occurring physical source.

Many Thousands of Pulsars Have Been Found in our Milky Way!

Quickly Explained!

The correct astrophysical explanation was arrived at within a matter of weeks.

It has not really changed, except in fine

detail, in almost 50 years. We will describe that explanation in a

following presentation.

But Justice Was Not Done! The Nobel Prize was awarded to Hewish and Ryle (with no mention of Jocelyn Bell) You can read Jocelyn’s gracious remarks here:

http://www.bigear.org/vol1no1/burnell.htm