Stars The Discovery of Pulsars Can We Detect Neutron Stars As noted earlier this seems very unlikely they are small asteroidsized even the nearest is likely to be ID: 559669
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Slide1
Finding Neutron Stars: The Discovery of Pulsars
Slide2
Can We Detect Neutron Stars?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
formed as
super
-
hot objects,
they will be very
faint, and fade away quickly as they coolSlide3
Surprise! – We Can Detect ThemBut 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.Slide4
Two Relevant ConsiderationsStars 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.)Slide5
1. Rotation: The Conservation of Angular Momentum [ASTR 101]
https
://www.youtube.com/watch?v=
AQLtcEAG9v0Slide6
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.Slide7
2. Magnetic FieldsThe 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 = 10
10
(=
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
.Slide8
The Wisdom of HindsightA really clever astrophysicist might have predicted how to detect neutron stars, on the basis of these two considerations: extremely fast rotation, and
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.Slide9
Jocelyn Bell’s Discovery (Cambridge UK, 1967)Slide10
Jocelyn’s DiscoverySomething 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?Slide11
What Do We Learn fromRapid 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_1fZfv0P8Slide12
Now Think AstronomicallySuppose 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?Slide13
No, for Two ReasonsFirst, 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
Slide14
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”
i
n size, and light from the
edges lags behind the
l
ight from the [closer]
centre
of the disk. Slide15
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-Z7W1ThsoSlide16
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 outward2) 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!Slide17
Now Imagine “Turning Off” an Entire Galaxy!Slide18
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.Slide19
On-and-Off BehaviourSuppose 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.Slide20
The Inescapable InferenceIf 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.Slide21
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.
Slide22
Another Possibility2. 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: f
airly soon, far too
many
pulsars were discovered in many
different directions
. It must be some natural, commonly-occurring physical source.Slide23
Many Thousands of Pulsars Have Been Found in our Milky Way! Slide24
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.Slide25
But Justice Was Not Done!The Nobel
Prize
was
a
warded 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