I'm going to talk about a failed supernova explosion which has resulted
in a weird white dwarf heading out into the
depths of space
okay so a paper came out on the 20th of june
so relatively recently here it is sdss and then the long telephone number that
tells you what the star is the partially burned remnant of a low
mass white dwarf that underwent thermonuclear ignition question mark so
I think we need to back up a little bit and talk about so this is a white dwarf
star and it's associated with a particular
type of supernova uh thing called a type 1a supernova very
famous type of supernova because it's used as a standard candle something
which is the same brightness wherever you see it
so it's used for cosmology where we can actually use it and figure out how far
things are away from how bright it appears
the reason why it's a standard candle is because of the mode
the way in which it explodes and essentially what happens is you have a
white dwarf which is a very compact star it's held
up only by the pressure of quantum mechanics effectively the always
the question with the star is why it doesn't just collapse under its own
gravity when it's burning nuclear processes in its core
that provides the energy source that keeps it up
but the end of its life that goes away and the star will collapse
and so for example for a white dwarf it'll collapse right down to something
about the size of the earth at that point these weird quantum
mechanical effects kick in and stop it collapsing any further but what can then
happen there is a maximum mass for which that
effect will work thing called the chandrasekhar mass and
if you keep adding in some way more and more mass to the
start eventually you're going to approach that mass
and there's a danger that the thing will collapse further
and actually all sorts of turmoil will occur within the white dwarf star it'll
become very convective it'll become very disturbed inside
that's the point at which white dwarf is in danger of actually exploding
by firing off a whole bunch of new nuclear
thermonuclear processes but in a very uncontrolled way
and so it will essentially explode like a bomb
interestingly the the views on this have changed it used to be thought that when
a star tipped over that magical chandrasekhar mass that was
the point at which it blew up as a supernova it's now thought it never
quite gets to that mass but as it approaches that mass as i say it becomes
very disturbed stirs up very energetic processes can
start happening and something then kicks off these uh nuclear
explosions within it so then the question is
what why would a star be putting on weight where's its extra mass coming
from and in order for that to happen there
has to be another star nearby to kind of donate the extra mass and so
it's probably in systems where there's a binary you've got two
two stars orbiting each other and one of them is donating
some of its mass to the white dwarf and so eventually it reaches to this point
where these runaway nuclear processes occur
and it explodes so sort of skimming matter from its
next-door neighbor it's sufficiently close to it it's actually ripping the
mass off it it's you know it's its pull of gravity becomes stronger
than the pull of gravity in the star that was holding onto that matter to
start with so it's literally ripping off the outer
layers of its of its neighboring star and acquiring them for itself usually
that process is thought to be sufficiently disruptive
that there's nothing left at the end that the star just literally blows
itself to bits the interesting case here is that here
there is still a white dwarf left but it has some weird properties and in
particular when they studied the spectra of it
when they split the light up into the colours of the rainbow and looked at
the sort of signature of the different different chemical elements
they found some of those heavier elements that are made in the supernova
explosions so it looks like some sort of
thermonuclear process has occurred with this white dwarf
but it clearly hasn't been quite as destructive as it usually is because
there's still a white dwarf left at the end
and the other interesting thing is that the nuclear processes haven't gone all
the way to the sort of end state that they
usually do usually you end up producing you know so the the carbon in
the white dwarf ends up producing silicon and silicon
turns into nickel and the nickel turns into iron
there's a whole series of nuclear reactions that go on and those produce
all the elements up to iron the interesting thing about
this white dwarf is it doesn't have those heaviest elements in it it doesn't
have iron in it it doesn't have nickel in it
so it looks like the process was sort of incomplete
it didn't destroy the star and it didn't go through all the nuclear reactions
that these things usually go through so it's like a failed supernova it
didn't quite fail clearly something pretty dramatic happened to it
but it didn't quite go all the way so it's sort of yeah it didn't quite
detonate completely it was an incomplete detonation the other interesting thing
about it this particular star is they can
measure its speed you can measure the doppler shift to tell you it's speed
along the line of sight but actually there are also proper motion
measurements for it they're also measurements of its
its changing position on the sky so they know how fast it's going
and it's actually it's going around the wrong way it's not orbiting around the
milky way in the same direction as most stars are
and on top of that it's going at about 200 kilometers per second so it's click
it's going along at a pretty rapid rate which suggests that whatever the
explosion process was gave it a pretty big kick so it's
now traveling at high speed this again is a sign of something to do
with probably the fact that it was in a binary system to start with
it'd be nice to think that you know it was just some failed explosion that
kicked it enormously but it turns out that that probably isn't enough to give
it the speed that we see but what is enough to give it the speed
we see is if it was in a binary system and the explosion was sufficient to
unbind the binary so that the two binary stars were no longer
bound to each other because if for example this was a white dwarf that was
orbiting another white dwarf they can get very close together which means
they're actually traveling around each other
at hundreds of kilometers per second and so if you then suddenly take one of
these white dwarves away the remaining one will then just keep traveling at
that several hundred kilometers per second so
it looks like it was probably in a very tight binary probably with another white
dwarf star this incomplete explosion occurred
because it ripped enough of the material from the
other white dwarf to create a partial explosion and that disrupted
the the second star probably almost
completely and so this first star then shot out uh into the milky way but the
one that's shot out is the one that underwent the supernova of sorts yes
yeah the other one was probably it might have been destroyed completely in the
explosion just because it got caught in the blast or it might
still exist it might itself also be flying off in some other
direction um there's really no way of telling so we haven't we don't know
where the other like the brother is no clue at all so the one that's firing
off that we do know about the one that's
firing off into the depths of space it's not just the recoil from the
explosion it's to do with the kind of uncoupling from the gravitational
interaction exactly it's almost certainly to do with the fact that
what we most of the speed we're seeing is the speed that it originally had in
its binary system but suddenly that binary doesn't exist as a binary anymore
so it's just flying out into space so there's a couple of interesting
implications of this one is that this type of explosion so the reason why we
typically see supernovae isn't because of the original explosion
it's because those explosions produce lots of radioactive material which then
subsequently decays and over the weeks and months that follow
we see the energy that's being released from those
nuclear decay products the main nuclear decay product in type 1a supernovae is
nickel and of course this one never got around
to making any nickel so it wouldn't have had any
material to go through this decay process which means that if we had been
around when the supernova occurred it probably almost certainly worked it
would have been in the very distant past but if such supernovae do go off you
probably wouldn't see them because unless you happen to be looking
at just the right moment when the flash occurred
then you wouldn't see it and then there isn't that then slow subsequent decay
that would allow you to catch the explosion
later on the second sort of slight warning here is this
you know the the simple story we usually tell about type 1a supernovae is they're
all the same and it's very straightforward and that's why we can
use them as standard candles there is at least a little bit of a
warning here that says actually sometimes weird things happen
and you get incomplete explosions or different kinds of explosions
so it doesn't mean we have to be a little careful if we're going to be
using type 1a supernovae as standard candles
mike you refer to this remnant that's barreling through the milky way
as a star is it still acting like a star is it is it undergoing nuclear processes
and emitting light and i mean it's still emitting light because
it's still hot but it's a it is it's still a white dwarf star so it's
still held up by its degeneracy pressure it's not got any new nuclear processes
going on in it so it's a question whether you count
white dwarfs or stars or not if you think a white dwarf is a star then this
thing's still a star if you think it's just the remnant of a star then i guess
at this point it's just the remnant of the star
mike if this is a common process and this happens a lot
would we have seen them before now like should we consider this an oddity or is
it possible these things are everywhere i mean they are relatively faint so
maybe we don't find you know we don't find all of them
necessarily and maybe we haven't studied all stars in sufficient detail but i
think what this is probably telling us is this
this is a kind of a rare process and if the story i've told you is right
that it originated in a pair of white dwarves that was sufficiently close to
each other that one was actually ripping material from the other
and at some point one of them underwent a kind of slightly failed explosion
it's probably a fairly unlikely scenario so i suspect there's probably not a huge
number of them out there but we really don't know at this point it
sounds quite exciting though doesn't it the idea of this
of a white dwarf like you know barreling through the milky way on this sort of
cannonball run and sort of running from the scene of the crime as well actually
right it's clearly you know that it's just
survived this massive explosion and is now heading off as fast as it can
did this finding excite you like do these things still surprise you
absolutely in fact so you know so in fact you told me about this paper i
didn't know about this paper at least you pointed me towards a newspaper
article about it uh which got me to read the paper uh
it's not why you know i usually do galaxies i don't do white dwarfs and
supernovae and things but the more i read about in the paper the
more interesting i got in it because it's clear this is a very intriguing
object
star clusters or if they were galaxies and if there are galaxies then that's
something really really interesting because it's not clear how such a
dense galaxy could form which is really really quite nice it's not a very
impressive image but this is the first image that new
horizons took before it went on to take images of jupiter when it flew past
jupiter and then images of pluto and now even images of an asteroid in
the kuiper belt as well