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 FactorsTwo things come to our aid:

1. Stars rotate; and

2. 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 SunIt 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 fromthe 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 ReasonsFirst, 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 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.

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 LessonIf 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 light-years, 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 Possibility2. 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