1. Dead Star Goes Faster (Up Goer Five)
Lucy Keer
Supervisor: Ian Jones
What is Up Goer Five?
I have only used the ten hundred most used words.
1. A new star?
Ten hundred years ago, people saw a new ‘star’
in the sky. It was very bright at first - so bright
it could be seen in the day. Over the next few
months it became less bright.
We now know that this was a big star at the
end of its life. It ran out of stuff to burn and
could no longer hold together. Some of the
stuff making up the star was thrown out into
space, and you can still see this (right).
That picture shows the kind of light you
see with your eyes. You can also see it with
other kinds of light. The other picture (left)
shows the same thing, but with the kind of
light doctors use to make pictures of broken
arms and legs. With this light, we can see
what is left of the star – it is the bright spot
in the middle.
The star is very small - only as big as a city.
But it is still as heavy as the Sun.
2. Keeping time
There is another kind of light that
is the most important one we use
to see these stars. This is the sort
used to send TV pictures. We see
this light come in bursts - a hun-
dred bursts a second. Always the
same time between each burst -
most of them keep time very well.
2000 50 100 150
Time (days)
Frequencychange
(10-6Hz)
The one in the first picture
is not so good at keeping
time, though. Once a year
or so it gets faster, and we
don’t know why. The pic-
ture (left) shows one time
where it got faster.
3. Why do we care?
Something big must have happened to make the star get faster like this.
We would like to know what happened, and how the star changes.
Most of all, we are interested in whether the
change could make a different kind of wave
from the star. This wave is not light, but it
moves as fast at light. It is made when very,
very heavy things like these stars move about.
These waves are much, much harder to see from here. We look for them
using the thing in the picture (above). But we haven’t seen any yet!
time
+
+
We try to see them by looking
at how the space between things
changes as a wave goes past. This
should happen in the way shown in
the picture (right) – but really it is
only a very tiny change.
The good thing about these waves
is that they would tell us very different things about the star to what we
can find out from light.
4. Cracks in the star
As we don’t know why the star gets faster, we have to make a guess. Our
guess is that it is because the outside of the star cracks. This is how it
would work:
1.
2.
3.
4.
The star has two parts. The inside
of the star can move around, a bit
like water, while the outside part
has a hard layer, a bit more like
rock.
It is not completely round like a
ball. This is because it goes round
and round really fast, which pushes
out the sides. it ends up being a
bit bigger from side to side than it
is from top to bottom.
As the star gets older it goes round
a little more slowly, and so the
star’s sides should be pushed out
a little less.
The hard layer on the outside can’t
change very much. It stays about
the same for a while, and then in
the end when it is pushed too much
it cracks. This suddenly makes it
smaller at the the sides, and it goes
a bit faster. This is a bit like some-
one going round and round on ice,
and then putting their arms in.
5. Making things more simple
After the star cracks the whole thing should ring
like a bell – this should look a bit like the picture
(right).
To understand this better, we want to work things
out in a simple case using numbers. In this simple
case, the star will be the same all the way through.
To do this, we:
Find all the possible ways the star could move about.
Find out how the star looks different after it cracks. We do this by following
each point in the star and finding its new position after the crack.
Use this to pick out which of the possible ways actually happens.
6. What do we find?
-2
-1
0
1
2
3
4
5
6
7
8
0 0.2 0.4 0.6 0.8 1
ξn(x)
x
The picture (right) shows some of
the ways that points in the star
could move after the crack.
The grey lines are ways it can move
because of the hard layer of the
star. The red line would still be
there if the whole star was like wa-
ter, but the hard layer changes it a
bit.
When we pick out which way it actually moves, we find that the red line is
the most important one.
Where pictures are from:
Picture where star gets faster is from Espinoza et al., 2010, with some changes; First two star pictures – NASA; Picture from the air – LIGO Scientific
Collaboration; Dead star – Wikimedia Commons; Other pictures – my own.
Where writing comes from:
I wrote this using splasho.com/upgoer5
The idea is from xkcd.com/1133
lucy.keer@soton.ac.uk
