happy halloween 60 symbols viewers okay
so halloween is upon us and uh so i've
been casting around looking for
something suitably astronomical that
celebrates halloween so our stretch
today is to talk about the witch's broom
nebula because some people think it does
resemble a witch's broom perhaps out of
harry potter
i don't quite see it so much but
i other people do
well in this case we're looking at a
supernova remnant so this is the
expanding shock wave from a star that
probably exploded or 5 000 or 8 000
years ago and when a star explodes it
sort of throws its gas out into space
and that gas then interacts with the
interstellar medium that's already out
there
and heats it up and and makes it glow
and so you have this big almost
spherical ball uh
sphere sort of hollow sphere of material
that's glowing that's how far the
explosions got and one of the sections
of this if you think about a sphere the
bits that you're seeing edge on you'd be
looking right through my hand right the
bit you're seeing face on you'll just be
seeing through my hand whereas here
you'd be seeing the full width of my
hand so actually you see the edges much
more clearly because you're looking
through more stuff and the witch's broom
is one small part of a much larger
nebula called the veil nebula
which is actually quite large it's about
three degrees across
and to put that in perspective
your thumb outstretched
covers an
angle of about two degrees on the sky
so it's slightly larger than your
outstretched thumb and when you think
about it the full moon itself is only
half a degree so this is actually quite
a large object on the sky it's really
like a bubble it's like a big bubble
you're looking at it's not a perfectly
spherical bubble because different bits
have expanded by slightly different
amounts but basically it's a big big
hollow sphere you're looking at you're
talking about anything which is
expanding at sort of a thousand
kilometers per second or they're about
so it's cracking along at a fair old
speed and that's the reason why it glows
at all is because you've got this
material that's expanding outwards and
basically smacking into the material
that's there and shock heating it so
it's only because there are these very
high speeds involved that the thing gets
hot enough to glow in the dark it is
quite bright but that brightness is
spread out over this very large area
which means its surface brightness is
actually quite low so it's very
difficult to see and in fact what you
are seeing if you look closely at the
the pictures you're seeing these sort of
filamentary rope-like structures the
shock wave has hit this diffuse gas
and
although it's happening in a
three-dimensional shell you're really
only seeing it when when you look down
enough of it
that it becomes visible and that only
happens on the edge when you're looking
down a fair amount of material along the
line of sight
it's just chance so if you look at
enough places in the sky you'll start
picking out shapes and this one just
happens to end up looking like a
broomstick if i was in a spacecraft like
the space shuttle and i
flew into the
the shock wave i flew into this thing
head on which looks like this big
beautiful gentle thing
any any ideas what to expect would i go
through it like flying through a cloud
or would it be good night irene it's a
good question it's one of those tricky
ones that you'd have to do a lot of
calculations to figure out in detail my
guess would be you probably wouldn't
notice because the densities are pretty
low i mean it's still pretty much the
vacuum of space and it's only the only
reason you see it is because you know
it's spread out over a huge area and so
over that huge area there's an awful lot
of atoms so there's another one called
the the witch head nebula um which is in
some ways it's the same kind of thing
it's another of these clouds out there
in space
which looks remarkably like a witch's
head i think this is one of those cases
where the human eye is really good at
picking out faces even where there isn't
anything to see like that famous face on
the moon and those kinds of things um
and so this is uh this is that kind of
feature that there's a
face like object uh in in a cloud
and really what you've got there is a
thing called a reflection nebula you've
got a relatively bright star rigel
nearby and then there's just this cloud
of gas next to it and the light from
rigel is lighting up the cloud and then
some of that light is is just being
absorbed by the cloud and then
re-emitted in our direction so it has a
rather bluish sort of tinge to it which
also adds to its veily witchy spooky
nature
but the reason for that that it comes
out the shade of blue is a sort of
interesting piece of physics which is
that the
rigel the star that's lighting it up of
course is emitting light from all across
the spectrum red light green light blue
light but the red light tends to go
straight through it's only the blue
light that gets scattered towards us and
in fact it's the same kind of physics as
why the sky looks blue when you look at
the sky because of course when you're
looking at the sky you're seeing
scattered sunlight and sunlight gets
more scattered by the molecules in the
atmosphere at the blue end of the
spectrum and the red end of the spectrum
and here it's the same kind of physics
but here what's doing the
the scattering is actually dust grains
little dust range but they have the same
property that they tend much more to
scatter blue light than red light red
light goes straight through the blue
light gets scattered towards us
so halloween itself is actually an
astronomical date it's known as a
cross-quarter date in the calendar so
it's halfway between the equinox and the
winter solstice so the equinox being
where there's equal amounts of day and
night the solstice being the longest
night of the year
and so this is halfway in between and
traditionally it kind of marks the
beginning the real beginning of winter