hello my name's Andrew Norton and I'm in
the Astronomy Research Group at the Open
University in this talk I'm going to
tell you about variable stars and a way
in which you can get involved in some
research that I'm doing to identify new
variable stars and search for
particularly rare examples so let me
start with my slides my talks called
data mining the super Wasp variable star
population and it's work that I've done
over the past few years with some
postgraduate students several years ago
Marcus law and then Paul Greer and
currently Heidi teman so Sifl was was
the wide-angle search for planets a
project designed to search for planets
around other stars exoplanets but as
I'll show you one of the spin-offs of
this whole project was the
identification of a huge number of newly
identified variable stars so Sifl wasp
is shown here on the right this is an
image of one of the installations we had
two installations like this one in La
Palma in the Canary Islands observing
the Northern Hemisphere sky and one in
South Africa observing the southern
hemisphere and as you can see it's not
really like a typical telescope in fact
there are eight cameras on a single
robotic mount here each of these cameras
is rather small about 14 centimeters in
diameter
aperture but has a very wide field of
view typically about 8 degrees across
that's 16 times the diameter of the full
moon so with the eight telescopes
observing together we cover an area of
sky about 500 square degrees which means
in a single snapshot we can capture an
image of many hundreds of thousands of
stars and over the course of about
eleven years that super wasp was
operating we observed essentially the
whole sky night after night year after
year and many times per night so the
image on the Left shows a map of the sky
showing the colors there the density of
stars in fact the sort of green u-shaped
region there
is a plot of the Milky Way the band of
our galaxy on the sky where the Scot
where the stars are closer together
and in fact super wasps didn't observe
the very densest parts of the Milky Way
because the stars are simply too close
together to be resolved individually
with these rather small telescopes but
nonetheless over the rest of the sky
over the course of about eleven years we
observed over 30 million stars many
times per night night after night month
after month year after year so the data
that we build up on all these stars is
unique in two ways its first got a long
baseline extending over the 11 years and
the data also have a short cadence so
that's many observations per object Coe
night and this is ideal for studying
variable stars as I'll show you so first
of all what we do is extract light
curves from these super wasps data that
means essentially measuring the
brightness of every star in every image
and joining those measurements together
to make a light curve as shown here so
on the left there's an image of the sky
in false color with an individual star
ringed with a green circle when all we
do is measure the brightness of That
star in every image and then plot all
those brightness measurements as in the
graph at the top and that's a light
curve of this particular star spanning
what 7 years from 2007 to 2014 each dot
is a measurement of the brightness of
That star at a particular time on a
particular night and if we zoom in on
one months of data in the middle panel
you can see each little stripe of dots
is one nights worth of data and then if
we zoom in just on three of those nights
you can now begin to see that this
particular star is varying in brightness
you can see the brightness goes up and
down in a fairly regular manner and
typically for each of these 30 million
or so stars that we've measured light
curves for we would have around 20,000
data points per object so
and got those light curves we need to
then analyze them to look for particular
types of variable stars pulsating stars
and eclipsing binary stars which I'll
explain in a moment now any signal like
a light curve we can analyze it to find
what frequencies what periods are
present in it we call that process
taking a Fourier transform of the data
and it's illustrated schematically here
so on the left is a light curve shown by
that red trace and it goes up and down
in some kind of regular manner but it's
difficult to see what signals are
present in that red red light curve
alone but by this mathematical technique
we can break it down into its component
frequencies its component periods and
that's shown by the three purple traces
stacked behind it as you see each of
those has a different frequency perhaps
a different amplitude and if we add
those three purple traces together we
get back to the red light curve that we
started with and we can display that
information the three periods the three
frequencies that are present as in the
blue graph on the right this is in the
frequency domain where we plot the
amplitude of the signal against its
frequency and there are three spikes in
this particular example showing that
there are three signals three particular
frequencies present in these data and we
can measure what those frequencies are
measure what the amplitudes are and so
understand what variation has gone into
creating that light curve in the first
place so the sorts of signals we're
looking for are shown here these are two
particular types of variable stars at
the top then is an eclipsing binary star
this is where we have essentially two
stars orbiting around their common
center of mass and from our viewpoint
one of the stars passes in front of the
other and then vice versa as they orbit
each other and if the stars have
different brightnesses as in the example
shown here we get a fairly
characteristic light curve that repeats
in a regular manner with first a deep
primary Eclipse as we call it and then a
shallow secondary Eclipse
and that repeats periodically as the two
stars orbit each other with this
regularly repeating motion and that
orbital period of the two stars could be
as short as a couple of hours or even
less in some cases up to many days
months or even years at the bottom is
another type of variable star this is a
so-called pulsating star where we have a
single star which is physically
pulsating in and out getting brighter
and fainter hotter and cooler larger and
smaller as it does so and the light
output therefore varies in the manner
shown typically with a steep rise in
brightness and then a shallow fall in
brightness which again repeats on a
regular cycle might be as short as a few
hours might be as long as many years
different sorts of pulsating star
different sorts of eclipsing binary star
that we can try and find within these 30
million light curves from super wasp now
what super wasp is good for with all
these many many light curves is first of
all finding rare individual objects such
as the one I'm going to tell you about
here and also studying whole classes of
objects where we can find many thousands
of the same type and I'll tell you about
that in a moment but here then is really
my favorite of all the new variable
stars that we found with super wasps but
over the years this particular light
curve of this particular star which goes
by the name one super wasps J zero nine
three zero one two point eight four and
so on and so on as you can see when we
looked at this light curve and made its
power spectrum using the Fourier
transform that I just talked about we
found that it had two distinct signals
in it one signal at a period of five and
a half hours and another signal at a
period of 1.3 days so what we did with
the light curve was fold it at each of
those two periods by folding it we mean
taking say a five and a half hour
segment of the light curve overlaying it
on the next five and a half hours and
the next and the next and so on till
we'd folded the entire light curve to
lay on top of each other in that five
and a half hour period and what we got
when we did that is shown in the top
left here
I showed two cycles of it but you can
see a very clear regular repeating
pattern that recurs every five and a
half hours and what this is is a
characteristic light curve of a very
close eclipsing binary where the two
stars are so close together but as they
orbit around each other we see a
continuous variation in light and the
sort of style that might be like is
shown at the top right of the diagram
where you see the two stars distorted
into these sort of pear shape that
they're physically in contact and as
they orbit around each other so we get
this different projection of their
surface brightness and we get this
continuous variation in light repeating
every five and a half hours five and a
half hours is just the time it takes for
the two stars to go around each other
but then in that same light curve when
we folded it at the longer period of 1.3
days we got the light curve shown in the
bottom left and here you can see there
are very narrow dips a deep narrow dip
and a rather shallow narrow dip but
again they repeat in this case every 1.3
days so what we have here is shown in
the bottom right diagram where we have
two stars that are separated from each
other orbiting around each other every
1.3 days so first one then the other
passes in front of its partner so
whereas we got this light curve of what
was apparently a single star in super
Wasp when we examined its light curve we
found it's at least four stars it's two
binary stars now that wasn't even the
end of the story we then went away and
took spectra of the stars in this system
that means taking the light from the
stars spreading it out into its
constituent wavelengths or colors and
seeing how that varies with time and
what we found there was that the lines
in the spectra the spectral lines due to
the different chemical elements in the
atmospheres of the stars was shifting
backwards and forwards in wavelength
with the five and a half hour period in
one case and with the 1.3
day period in the other case now this is
an example of what's called the Doppler
effect which we'll probably be familiar
with in the context of sound waves you
know that when a vehicle or train or an
ambulance say is coming towards us we
hear a higher pitched sound as the sound
waves get bunched up and as it's going
away from us we hear a lower pitched
sound as the sound waves are spaced
apart the same Doppler effect occurs
with light so here as the stars in these
binaries are orbiting around each other
when one star is coming towards us the
light way to get bunched up and we see
the light coming at a at a higher
frequency more towards the blue part of
the spectrum and when the stars are
moving away from us the light waves get
stretched out to longer wavelengths
towards the red part of the spectrum
here though we know we've got at least
four stars in these two pairs so we can
measure the spectral lines in each pair
moving towards us and away from us and
when we plot those as graphs as shown in
the middle panels here we see the curves
shown these are what we call radial
velocity curves and what they show is
the speed of each star as it moves
towards us and away from there was one
final surprise that this had in store
for us when we looked at the pair of
stars that were orbiting each other
every 1.3 days there were three sets of
spectral lines two of which were
orbiting around each other moving
backwards and forth as the two stars
orbit each other every 1.3 days
but there was a third set of spectral
lines there that didn't move at all at
least on the timescale we were looking
over the course of a few nights so that
eclipsing binary star with the detached
stars was actually a triple star so
there's two stars orbiting around each
other every 1.3 days and a third star
that's presumably orbiting around that
binary with a much longer period maybe
years or more that we couldn't detect on
the time scale we were looking but then
in the same patch of sky there's another
binary the two stars that are in contact
orbiting each other every five and a
half hours
and these are all in the same plane in
the same line of sight as we're looking
that's why we see them eclipsing so what
this system is is not one or two or
three or even four stars it's five stars
it's a doubly eclipsing hierarchical
quintuple star consisting of a binary
and a triple the triple itself contains
another binary and a single and the
whole lot are bound together by gravity
in the manner shown in the right-hand
diagram so at the time we found this and
perhaps even still I think this is the
only EE eclipsing hierarchical quintuple
star known so that was quite something
to find now I mentioned that another use
of the super wasp variable star data is
to look at catalogs of classes of
objects and here's an example of
pulsating stars now there's a class of
pulsating stars called our our library
stars named after that the prototype
star are our Lyra in the constellation
of Lyra
these are pulsating stars that typically
have a period of about half a day so
they get larger and smaller brighter and
fainter on this time scale of about half
a day and there are several thousand of
these known but what we found with Sifl
was was another 5,000 or so around 3000
of which had not been previously
discovered and even the ones that were
previously known we were able to find
out some new information about one thing
we were looking at in particular was a
thing called the Blaschka effect now
this was discovered over a hundred years
ago by an astronomer called sir guy
Glushko and it's still not properly
understood what it is is we see these
pulsating stars pulsating in brightness
with a period of about half a day as I
say but some of them show a modulation
of that period where the period itself
or the amplitude of the period itself
varies periodically with a much longer
period of maybe 20 30 40 days something
like that so these are periodically
varying periodic variable stars
and we found about a thousand of these
stars displaying this so-called blaster
effect and you can see how that
manifests itself in the folded light
curves at the bottom here so what we've
got are three sets of data on one
particular star in the years 2006 2007
and 2008 and if this star was a
constantly pulsating variable all of
these light curves would line up with
each other and we'd see a single
discrete pulse in each set of data but
as you can see the shape of the pulse in
heightened and in duration seems to vary
throughout the year so these light
curves don't quite line up and that is
the Blaschka effect in operation now we
still are not much clearer understanding
what the Blasco effect is but at least
we've now got a thousand new objects
that can try and help us understand it
and in fact what we think may be going
on is perhaps all our library stars may
show the Blaschka effect at some level
if we can only look at them close enough
over enough period of time and with
enough sensitivity so maybe we'll
understand this eventually using the
super watts data now here's where you
can come in and help us with all this as
I said we have these 30 odd million
light curves from super wasp and what I
did a couple of years ago was do a
concerted effort to analyze all these 30
million light curves to look for all the
variable stars that are in there it
actually took me a hundred years of
computing time to do this luckily I was
running it on 50 computers
simultaneously so it only took about two
years of real time but having done that
I found about three quarters of a
million seven hundred and seventy
thousand objects out of those 31 million
that seemed to display period periodic
variations in brightness and between
them there are about 1.6 million
potential periods that I found
some of those probably most of those are
not going to be real they'll be artifact
in the data but many of them will be
real and hidden within all those objects
there will be many unusual objects that
might be just as interesting as the
hierarchical Quinn tuple system I just
told you about so what I've done with
all these light curves is upload them to
a website called Zooniverse this is a
citizen science project that was
originally set up by Chris Linn taught
at the University of Oxford to look at
galaxies he his first project was called
Galaxy Zoo on the Zooniverse now there
are many citizen science projects
including this one of mine called super
wast variable stars and you can see the
address for it there if you just go to
Zooniverse org and search for super wast
variable stars you will find this and
what we ask you to do in this project is
help us classify these variable stars
from their folded light curves so here's
an example shown here this is you
probably guess from what I've told you
previously is by all accounts a
pulsating star you see the steep rise in
brightness and then the shallow fall and
this one has a period of about 24 days
so when you go onto the website and join
in the project you're presented with a
light curve and you have to classify it
as a pulsating star as various types of
eclipsing binary stars as rotating stars
which is something I haven't yet talked
about and unknown types or maybe just
junk and you classify it then you get
another one and another one and another
one and as I say they're up to 1.6
million of these for you to classify now
we don't just rely on one person's
classification of each star we take a
kind of consensus vote if you like once
each object has been voted on classified
by seven people it will be retired from
the system and the most common
classification will take as the the
likely real one and when we download the
results from this we can then
investigate them and see what you found
this is a long project this is going to
go on for a few years I'm sure but over
the next few years we're going to start
analyzing the results for
this and so anything that you find is
really going to help us in understanding
these variable styles as we go forward
now just as an example of one of the
things that we've found so far which is
being investigated at the moment I
mentioned earlier this contact eclipsing
binary style where we had the two stars
orbiting around each other just every
few hours now there are many thousand of
those known and to be orbiting so
quickly in every few hours these must be
rather small stars typically they're red
dwarf stars much smaller than the Sun
but what we found now is about a dozen
or two dozen light curves that look like
contact eclipsing binary stars with this
continuous variation in brightness but
they have periods not of a few hours but
of many tens of days the two examples
here the one on the left has a period of
over 17 days and the one on the right
has a period of over 41 days so if these
are contact eclipsing binary stars with
such long periods they must be very big
stars and what we think these may be is
red giant stars in close contact
binaries and if they are these may be
objects that are the progenitors of
things called red nova these are quite
rare events where we see an explosion on
a star that is very red in color and the
progenitors of these are believed to be
red giant stars that get so close
together that they merge and they
explode in the Nova so what we think we
may have found in some of these systems
is these red giant binary merger
progenitors of red nova now we're still
investigating these so we don't really
know the answer yet but this is just an
example of the type of thing that's
coming out of this super wasp variable
stars project with Zooniverse
so I hope that's inspired you to get
involved in it yourself and I do hope
that you join us in this project and
help us make some new discoveries if I
just stop the slides there thank you for
watching this lecture about variable
stars I hope you've enjoyed it
