Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely: they are...

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Finding Neutron Stars

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But: Two Important Factors Two things come to our aid: 1. Stars rotate; and 1. Stars have magnetic fields

Transcript of Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely: they are...

Page 1: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

Finding Neutron Stars

Page 2: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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

Page 3: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

But: Two Important FactorsTwo things come to our aid:

1. Stars rotate; and

2. Stars have magnetic fields

Page 4: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

Now Suppose a Star Shrinks…

Both the rotation rate and the strength of the magnetic field are affected!

Page 5: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

1. Conservation of Angular Momentum

Page 6: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

A Dramatic Example

World Record Figure Skating Spin

Page 7: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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

Page 8: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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.

Page 9: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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).

Page 10: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

So: Prediction Number 2

If a star collapses to neutron star dimensions, it might be expected to have a fantastically strong magnetic field.

Page 11: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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.

Page 12: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

Too Bad!

In fact, the discovery came first, completely by accident.

That was quickly followed by the ‘obvious’ explanation.

Page 13: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

Meet Jocelyn Bell

Page 14: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

Strip Chart Recording

Page 15: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

Jocelyn’s Discovery (1967)

Page 16: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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?

Page 17: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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

Page 18: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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?

Page 19: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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:

Page 20: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 21: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 22: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 23: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 24: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 25: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 26: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 27: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 28: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 29: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 30: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 31: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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.

Page 32: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

Light from the Edge ‘Lags’

Page 33: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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?

Page 34: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 35: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 36: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 37: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 38: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 39: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 40: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 41: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 42: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 43: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 44: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.
Page 45: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

And So On: Over and Over

Result: unsteady brightness

….but it never goes out completely!

Page 46: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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!

Page 47: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

Imagine “Turning Off” a Galaxy!

Page 48: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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]

Page 49: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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).

Page 50: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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!)

Page 51: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

Conclusion:The light-emitting region must be very small.

These can’t be ordinary stars!

…and it certainly can’t be due to eclipses!

Page 52: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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.

Page 53: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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.

Page 54: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

Many Thousands of Pulsars Have Been Found!

Page 55: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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.

Page 56: Finding Neutron Stars. Can We Detect Them? As noted earlier, this seems very unlikely:  they are small (asteroid-sized)  even the nearest is likely.

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