- Thank you, welcome to one
of our guest speaker nights at UW Space Place.
Tonight I am pleased to introduce professor Kyle Cudworth
who is the former director of Yerkes Observatory
and was there for many years
and spent a good bit of his career
working on star clusters so he's agreed
to give us a talk tonight
entitled Tales of Star Clusters
and you see some real nice examples of them right here
and he's gonna tell us why they're important
and what sort of research he's done
on star clusters so Kyle.
- Okay, thank you Jim.
Well, as Jim said, I spent many years
at Yerkes Observatory
about 40 years there
and this is not a picture taken at Yerkes Observatory,
but I just threw that in as something
to have on the screen before we got started.
So Tales of Star Clusters
and for one cluster, the tails of that cluster.
We'll get to that eventually.
Just curious, how many of you have been
down to visit Yerkes Observatory at some point?
Well, maybe half roughly.
If you haven't, I encourage you to get down and visit it.
We have at Yerkes, the largest
refracting telescope in the world,
the 40 inch refractor in the big dome here
and I will be showing you some pictures
that were taken with it.
I will also show you
some pictures taken with reflector.
Forty one inch reflector
it was installed in the 1960s.
The refractor goes back to 1897
but the reflectors 41 inch
in the mirror dome there
and the 24 inch in the dome that's almost hidden.
Those were installed in the 1960s.
I won't show anything from the 24 inch.
But 40 inch just for those of you
who have not been down there to see it
this is what it looks
with an astronomer there to show the scale.
Astronomer who might look amazingly like me
and then, since, if you go for a tour
you probably won't get to see the 41 inch reflector
but I should throw in a picture of that as well.
Okay, now to the star clusters.
We usually divide star clusters into a couple of groups.
The open clusters and we got the classic example
up here of the Pleiades
and the globular clusters,
much denser clusters in general
but as we'll see as we go along, not always.
Okay.
Few things about open clusters.
Nearby examples, few hundred light years away.
You can see 'em with the naked eye.
Pleiades, you can see,
well I can see Pleiades and see individual stars
in the the Pleiades from my backyard here in Madison
and of course it was darker down in Williams Bay around Yerkes,
there was easier but Pleiades,
near Peligades in the sky, even closer.
Coma, Praesepe, all of those are ones
that can be seen with the naked eye.
Praesepe, I never convinced myself I saw individual stars
with the naked eye but certainly can see the blur of the cluster.
Typically, open clusters are not
strongly centrally concentrated hence the name "open",
also typically we're talking
anywhere from a few dozen stars
to maybe a few thousand stars
but definitely not millions of stars.
Ages of open clusters range from extremely young,
where stars are still forming,
these would be a few million years,
example the cluster that's associated
with the Orion nebula
and we know from infrared observations
that there are stars still forming
within that nebula.
Going up to extremely old for open clusters
of eight or nine giga years,
a giga year is a million years or
ten to the nine years.
One giga year, ten to the nine years,
a billion years,
the oldest open clusters eight or nine giga years.
And for those of you who don't carry these things
around in your head all the time,
the sun's age is around
four and a half billion years
so oldest open clusters
about twice the suns age
but having said that
most open clusters are much, much younger than the sun.
There are not all that many
that are more than one billion years old.
Typically a hundred million, a few hundred million
is about as old as you find them.
The reason for this is that they're open.
They're not very dense and...
They just don't have enough gravitational pull from
the group of stars together is just
not strong enough gravity to hold the group together
against the pull of everything else in the galaxy
so they slowly migrate outwards
and the cluster just kinda dissolves
into the rest of the galaxy.
Chemical compositions. Well...
Astronomers are not chemist.
Astronomers, well, when I was a undergraduate
my first astronomy professor said
in Astronomy we have hydrogen,
we have helium, and everything else is a heavy metal.
Well I was taking a chemistry course at the same time.
It doesn't quite work that way.
We now as astronomers do distinguish
among the various heavier things
but we do tend to, in many cases,
lump together everything heavier than helium
and rather than discuss in detail
differences among
the different heavier elements
tonight I'm just going to do the typical thing
of lumping together all of the heavies
and for open clusters, we're typically talking
the number of heavies is similar to the number in the sun.
It's only about 1% of all the atoms in the sun
but the numbers in some other stars
can vary considerably from that compared to the sun.
Open clusters might have twice as much of these heavy elements
or might have half as much
but they don't change too much from that
within a single cluster the stars are usually
very nearly all the same
but, cluster to cluster, there can be these variations
and the open clusters are almost all
in the disc of the Milky Way.
We think of our galaxy as a thin disk.
Most of the stars in the galaxy are in this thin disk.
Open clusters are primarily in that disk
and examples here of couple of close ones
in the constellation of Taurus, the Hyades here
and the Pleiades over here
and this was just taken with a very low end
Nikon Digital SLR on a tripod.
Nothing fancy about the equipment to take this.
It goes a little bit fainter
than what you would see with the naked eye but from a dark sight,
a good deal fainter than what you'd see
with the naked eye from here in Madison.
In the city, you don't see quite as faint, of course.
Okay, move on to globulars.
In contrast to the open clusters
where we were talking the nearest
being a few hundred light years,
the nearest globulars
are several thousand light years away
and at best we're talking about things
that are just very faint blurs to the naked eye
from good dark locations
and that's the very brightest of them.
The classic one in the northern hemisphere
is M13, the cluster in Hercules,
and in southern hemisphere
there are couple that are brighter
that actually have names as if they were stars,
47 Tucanae and Omega Centauri.
Classical globulars, the things we usually think of
when we talk globular clusters,
we're talking hundreds of thousands of stars
even millions of stars in some cases.
They also are in general quite centrally concentrated,
much more so than the typical open cluster
but that varies a lot from cluster to cluster.
Even among the very well known globulars
there are variations.
Something I didn't fully realize until
I started taking plates of them
back in the mid 70s and realized,
oh, they really are different, one from another.
Now you pull the plate out of the wash
in the dark room and...
Okay, they don't look alike. That kind of thing.
Ages.
Globluars are all very old.
Greater than 10 billion years.
These are the oldest stars associated
with the Milky Way galaxy.
The age of the universe, but from current data,
is about 13 and a half billion years
so we're talking things that are extremely old,
the oldest globulars, 12 or 13 billion years.
Very close to the age of the universe.
Chemical compositions, here we get to things
that vary greatly relative to the sun.
They are very metal poor,
very lacking in the heavy elements.
Ranging from at the metal rich end of metal poor,
maybe a factor for deficient compared to the sun,
in other words you look at the star in...
a metal rich globular so to speak.
It might have one fourth as much iron as the sun has
but more typical globulars, you'll find they have
maybe about one fortieth as much,
taking iron as an example,
and they get down to things that are
so lacking in heavy elements
about one two hundredth as much as the sun has.
So these things are very different chemical composition
in terms of elements heavier than helium.
And the globulars in many cases
are out of the plane of the Milky Way
in what we often refer to as the halo of the Milky Way.
Yes, some of them are in the disk
but many of them are well out of the disk.
For the ones that are in the disk,
you can raise the question, are they just passing through
or are they ones that stay in the disk the whole time?
We'll come back to that as we go along.
How do we make the distinction?
Well, you've already got kinda of an idea in your head.
Sorry to tell ya but your idea isn't necessarily right.
If you go read the text books
or more less any introductory astronomy book
you find the difference is the structure.
Open clusters are more open,
globulars are more concentrated.
Yes, but in research work
the distinction nowadays, really since the 1960s
has been that it's an age issue.
In the 1950s, when it became possible to actually
put some ages on stars, or on star clusters,
it was realized that all the classical globulars
were extremely old
and so just kinda in the way people worked in astronomy
they started using globular
to refer to any really old cluster
regardless of what it looked like
and open to refer to anything that was not that old
and so we have this bit of a problem
that it's not quite what you think
and just to confuse matters more,
some astronomers who work on clusters in other galaxies
will refer to young clusters that are strongly concentrated
in another galaxy as globulars,
or young globulars.
Well, that doesn't work right
with the working definitions that we use in this galaxy.
Age issues are really the defining characteristic
but now let's go on to some pretty pictures.
This, you look at it, I probably won't have any trouble
convincing you that's an open cluster.
Happens to be M36,
a lot of the clusters that I'm going to show pictures of
have M numbers, that's a catalog put together by
a guy named Messier somewhat over 200 years ago.
His primary interest was said to be looking for comets
and he made a list of things that weren't comets
but looked kinda fuzzy in his little tiny telescope.
So at any rate it's a list of things
that are pretty to look at in many cases.
Now this is a color picture
and they'll be a number of color pictures
here that'll I be showing as we go along.
Most of them will be referenced as
from the Yerkes 41 inch reflector or Skynet.
Skynet is a world wide network of telescopes
and Yerkes 41 inch is on that network.
The color pictures are done by taking images
with CCD camera on the telescope
through blue, yellow, and red filters
and then combining them appropriately
to get the color picture.
I didn't do this. I didn't colorize this.
I didn't make a color picture out of it.
That was done by an eighth grade student
who at the time was the youngest student
in the Yerkes High School program
and that will be true for all of the things
that I reference as Yerkes 41-inch
that are color pictures or Skynet pictures.
She did those as an eight grader.
This is not color.
This is from just a single plate.
This is an open cluster, M11,
taken with Yerkes 40-inch refractor in 1900.
FRy-4 is the plate number up here,
indicates it's the fourth plate recorded
as having been taken with a 40-inch refractor.
As open clusters go, it's fairly dense
but it's still definitely an open cluster.
It's one of my favorites to look at
in the summer sky
with a, well, small telescope but even
with something the size of the Yerkes 40-inch.
It's very much a favorite.
And you want a color picture,
this is one from the CFHT,
Canada, France, Hawaii telescope,
about a three and half meter telescope
on Mauna Kea.
M10, a globular cluster now,
the Skynet picture, the original image
is from the Yerkes 41-inch.
Okay, I promised you that I would
try to convince you that globulars
do not all look alike.
Here are two that are not very far apart in the sky,
not very different in distance.
Distance differences are
between 10 and 20 percent, perhaps.
Basically they're at the same distance from us
that shows the same area of sky,
it happens to be from very old plates
taken with the 40-inch in 1901.
M13, the last dense one,
M92, the more densely concentrated one.
There's no cheating involved in making these look different.
They really are different like that.
It's not a matter of ones a lot further away
so it looks more compact
or that I zoomed in on M13 more.
Nope, they really are different.
In the disk of the Milky Way, in our summer sky,
M71 is...
one of the more metal rich globulars.
It's heavy element chemical composition
is down from solar...
amount of iron in the stars of M71
is one fourth, one fifth of what it is in the sun.
That's toward the rich end of globulars
but obviously the cluster is somewhat sparse
but it is a globular by age
and this is a scan of a single plate
taken with Yerkes 41-inch in blue light.
It's one I took in 1979 I think.
We had just gotten the mirrors
reilluminized and the plate ended up
going a lot fainter than I had expected it to
because the fresh aluminum on the mirrors.
Okay, this is a different picture of it
but it's the same cluster as was up here
on the screen before the talk started.
NGC 2419.
It is an extremely distant globular cluster.
Something like 300 thousand light years away
and this is an image
from the Sloan Digital Sky Survey.
Survey that covers somewhat more
than a quarter of the entire sky at this point.
Everything taken in a very consistent manner
and here we got an image that the Sloan people
put together as one of their
showcase images from the survey.
So, you know, it looks like a globular.
Good healthy globular, just a whole lot further away
than most that we talk about.
At exactly, well, within 10%
at the same distance from us
is this little guy in the center here
we got Pal 3.
Not much of a cluster and...
That was discovered in the 1950s.
NGC 2419 was discovered back in the 1800s.
You can see why it took a little longer
to discover this one but it's about the same distance
as the previous one
but it's also a very old cluster
so it gets referred to as a globular.
Here's another one that's kinda sparse.
It's not as far away as these last two.
Only something like 70,000 light years
but that's still 10 times further away
than things like M4.
Pal 5, again, you can probably convince yourself
that there's a concentration of stars in this area.
This is another one that was nominally
discovered on the Palomar Sky Survey in the 1950s.
In reality, it apparently was discovered on plates
taken before the Palomar Sky Survey because
the Palomar Sky Survey plate of this field
was taken in 1954 but yet observers in Mount Wilson
in Palomar took plates directly on it
not wide field plates starting in 1949.
It was discovered a little bit earlier
than the actual survey plates
but at any rate it's a moderately
distant sparse cluster. We'll return
to talking about that one later.
Again, it's a globular in spite of how it looks.
By age, it's a globular.
Okay.
An earlier version of this talk
was put together for a group of teachers
that I've been working with for several years
and so I had to throw in something
that said quiz time for the teachers
but it be quiz time for you too.
We got two clusters here.
We got...
M35 and we got NGC 2158.
Okay, I think you'll probably all say,
okay, M35...
probably an open cluster.
But what about NGC 2158?
How many want to call it a globular?
Okay, how many want to call it an open cluster?
How many aren't sure? Okay.
Aren't sure you would find lots of professional astronomers
agree with you back in the 1950s.
Once some age work was done on it,
it became clear that actually it's an open cluster by age.
It's only two or three billion years old
so it doesn't qualify as a globular
despite what it looks like.
Now, part of it, looking more concentrated here
is that it is a good deal more distant,
maybe four times further away than M35
but it also really is denser but it's not a globular.
Here's M35.
This was just attaching my
old Nikon visual single lens reflex
on the back end of Meade 8-inch telescope
and maybe a 30 second or one minute exposure,
I don't remember exactly.
2158 couldn't really do it that way.
This is, again, Yerkes 41-inch
colorized by Christi.
So why do we study star clusters?
We looked at a lot of pretty pictures.
We've said a lot of basics about them, but why study them?
One...
is comparison with theoretical calculations.
Star clusters provide us an excellent
test of the calculations of stellar evolution.
As those calculations were done
first in proper detail in the 1950s and 60s,
you got into an era where in the 60s and 70s
there was a lot of star clutter observing done
in trying to test a lot of details
of stellar evolution calculations.
One of the things that comes out
of stellar evolution calculations
is a determination of the age of the cluster
and so clusters also give us things that,
by comparison with theory,
we can get pretty direct determinations of ages
so not only do we start stellar evolution with clusters
but because we can look at clusters
throughout the galaxy that are a variety of ages
we can start looking at things about galactic evolution
because we can look at things of different ages
and we know what those ages are.
It also helps that there tend to be a whole lot brighter
than individual stars because your getting
the joint light of a whole lot of stars,
so you can pick 'em up at great distances.
If you want to look at...
some star in the galactic halo
that is say...
20,000 light years away,
it's really hard to pick out an individual star that is
but you can find a cluster that is
just because you got the conglomeration
of a whole lot of stars.
So a lot more useful for mapping
and again...
you can talk about evolution of the galaxy
coupled with that mapping.
Further more, you get good distances for them
so that helps out in the mapping.
It also means that if you know the distance the thing
you can figure out how luminous
the individual stars in the cluster are
and relatively rare stars like the cepheid variables
or super giants stars that we don't have
other good ways of getting their distances.
Using the clusters, we can calibrate the distances
calibrate the luminosities and thereby
get the distances for these sorts of stars
and then we can go look for these stars in other galaxies.
The clusters thereby form an early run on the ladder
of getting distances throughout the universe.
When we talk about getting distances to other galaxies
the first good work of that sort
was done with cepheid variables
and they still were very important
in the work with the Hubble Space Telescope
that calibrated the hubble constant
on the expansion of the universe.
Cepheid variables critical, good calibrations
of what their luminosities are in order
to get distances to other galaxies.
Lot of those calibrations tie back to star clusters.
And then finally comment on here,
I know at least a few of you are amateur astronomers.
These things are beautiful to look at
whether looking with binoculars at things
like the Pleiades or Praesepe
or small telescope at Pleiades or Praesepe or M35
or 40-inch refractor at M11
or the 200 inch reflector looking at M13 or NGC 2419.
These things are beautiful.
I have been accused of picking
the things I study in my research.
Picking things that are pretty to look at
and that's not why I pick them but it does fit
an awful lot of facts about them.
Okay, some assumptions.
Any one cluster that you pick,
these stars formed
from one dense interstellar cloud originally.
If they did all form from the same cloud,
you'd expect them to have all formed
at pretty much the same time.
So all the stars within a cluster should be the same age
unless your looking at an extremely young cluster.
If the cluster if five million years old
and it took two or three million years
from when the first star formed to when the last star formed,
okay, they're definitely not all the same age.
If the cluster is a hundred thousand years old
and it took two or three million years
from when the first formed till the last formed,
that's two or three percent.
That's lost in uncertanties and everything.
Age differences star to star
should not show up according to this kind of assumption.
Well, you get age differences cluster to cluster
but not within a particular cluster in general.
Further more, we think that all stars
within a particular cluster should at least
start with the same chemical composition.
They're all formed out of the same cloud.
They may have somewhat different chemical composition by now
but they have the elements; shouldn't have change.
Even in very old clusters like globulars,
those stars do not make the heavy elements
and so you'd expect pretty much
the same chemical composition
star to star in any one cluster.
Differences cluster to cluster
but not differences within a cluster.
For some globulars though
it has become clear
that these two statements are not quite true.
In many globulars there are small
star to star variations in chemical composition,
in a few there are very large
star to star differences,
and in some there are maybe some range in ages
among the stars within the cluster.
The most notable example is the one called Omega Centauri
but there's also
quite a few astronomers
who would say, yes Omega Centauri
has been traditionally called a globular cluster
but maybe it shouldn't be.
Maybe it is the remaining
nucleus of a dwarf galaxy
that has mostly merged with the Milky Way
but the nucleus of that galaxy is still there
and you see it as this thing that looks a bit
like a globular cluster named Omega Cen.
Well how do we get distances?
How do we get ages?
We can from very near
by stars get good distances
and we got two different plots here
lets concentrate first on the left hand one.
The vertical axis here is absolute magnitude
or the luminosity of the star.
It's how bright the star would be
if it were 10 parsecs.
32.6 light years if you like from us.
So this left graph shows
a main sequence in red.
Shows were about 90% of stars would fall
in this luminosity, or absolute magnitude,
versus the horizontal axis is color.
It's quantitative way of measuring color.
Don't worry. Details about how it's done, which just...
color of the star, which is related
to temperature of the star.
Over here on the right hand side,
we got actual data from the Praesepe cluster
and we see the main sequence here
but the upper part is missing.
Instead things turn off
and go over and some stars are over here as red giants.
These are actual observed magnitudes here.
If we take something, take the color of around...
B minus B of point 5,
we see that it's about 10th magnitued
in the Praesepe cluster.
We come over here, a 10th magnitude star,
and absolute magnitude is about
should be about 4th magnitude.
Praesepe is far enough away that things appear
about 6 magnitudes fainter
than as they would
at the standard distance as used for plotting this.
The other thing here is at this
the upper end within the cluster,
stars are turning off the main sequence.
Stellar evolution calculations can tell us
what kind of an age that represents.
As stars age, they leave the main sequence
and stars at the upper main sequence
age faster than the stars lower down
and so we can pin ages on the clusters
using the turn off points.
And Globular clusters,
here is the typical color magnitude diagram,
this happens to be one of my favorite clusters, M5.
We got a main sequence down here,
we got a lot of stars
that are off the main sequence here,
up in the red giant branch.
Basically on the main sequence
stars are converting hydrogen
to helium in their cores.
But eventually they run out of helium
and so they got to change.
They've lost...
They don't have enough fuel.
They've lost their main fuel source.
So they shift over to having a different structure.
They have a helium core
and they a shell around that core
that is converting hydrogen into helium.
Those are the red giants.
Eventually, that shell gets far enough out in the star
and the helium in the core gets compressed enough
that things have to change again and it goes
to the core of the star is
doing very different nuclear reactions
that convert helium into carbon.
When that's going on, the stars sit over here
on what we call the horizontal branch.
Now in some clusters, the so called horizontal branch
droops way down like this but still
those are helium burning
if you like in the core.
So we can interpret this in terms of stellar evolution
and I use M5 here
but its a good schematic
of a lot of globular cluster color magnitude diagrams
and I just thrown in a color picture of M5
from the copyrighted by the Anglo Australian Telescope.
Well, what is my research been?
I've been deriving proper motions.
I've been measuring how stars move by comparing
old and new photographs of the clusters
and I've mostly worked on globular clusters.
Partly because when I started on this,
a number of people were working on
most of the well known open clusters.
As a grad student, I did an open cluster project.
Happen to be M35 that I showed you earlier.
But that...
mostly I've worked on a few open clusters,
especially older ones, but mostly globulars
and these are tiny motions
of how the stars have move over even many decades.
The amount of motion over several decades
is much, much smaller then the diameter
of the image on photographs.
So you get only by measuring with high precision
positions on the photographs.
Originally the purpose was cluster membership.
All the members of a cluster should move
pretty much the same
otherwise they wouldn't hold together as a cluster.
Were as the field stars that just happen
to lie in the same direction
that are foreground or background
there's no reason that they're going to move
the same as cluster stars so you can weed out
a lot of the non-members by measuring the proper motions
with sufficient precision.
I got into doing that in the mid 70s on the globulars
because there were friends of mine
that needed that membership information
for other work that they were doing
and it was something I could do, we had old photographs
of some of the bright globulars at Yerkes
and so we were in a good position to try to do this.
It turned out that we had sufficient precision
to look at the internal motions.
Every star within that cluster
feels the gravitational pull of every other star.
So they're moving around inside the cluster.
Very, very small motions but we had enough precision
mostly I would say because of the excellence
of old photographs and the measuring methods
that came along in the 70s and especially the 80s and 90s.
We were able to get these internal motions
and we've also done quite a bit
of looking at how the clusters move
in their orbits around the Milky Way.
And just an example of
comparison of old and new photos.
On the left, these are both M5,
both the same area of sky,
I mean you can probably identify up here
in the upper left corner that same
little parallelogram of stars on both of them.
Well, this is 1900.
Plate FRy-1.
First plate recorded as having been taken
with a 40-inch refractor.
We did use it on our paper on M5.
It was fun to put in that little sentence saying,
we note in passing that plate Fry-1
is the first recorded image photographed
with a 40-inch refractor.
A grad student of mine has improved upon...
round 1990 improved upon what I had done
back in the 70s.
This is the 1988 plate.
The main difference is a sharpness of the image.
That's simply because the atmospheric conditions
were extremely good for months in 1988.
If you were to overlay these, you would probably
not notice a difference in position for anything
but when measured with high precision
we can find the motions.
There's always an issue
of trying to find the appropriate old plates.
Gotta dig around in old plate files
and I sometimes call that astronomical archaeology
although that's...
a term appropriated for other things now.
In 1970s when I started on this,
the lore was the plates
all had to be from refracting telescope.
They'd all better be from the same telescope
or a very similar scope.
No way you were going to be able to use plates taken in the blue
and plates taken in the yellow in the same study.
They were just too many complications
from plates taken in different colors.
Well, in the 1980s,
I realized that we didn't necessarily
have to stick to tradition on some of this.
Yes, there was a reason for these old traditions
but we now had enough computing power
that we could make corrections for the differences
telescope to telescope, color to color, and so forth,
and so since the 1980s, I have been pretty much
freely mixing plates from different telescopes,
plates taken in blue and plates taken in yellow
and sometimes in the red.
Mix them all together in the same measurement of motions.
The wonderful benefit of this is that anything
that has some old plates taken with any telescope
is now...
ripe for a proper motion study.
It doesn't depend on there being old plates
from one of the traditional big ol' refractors
and that meant that...
we were greatly freed up in studying clusters
that had some astrophysical importance,
not just ones that happen to have old plates
from the appropriate telescope that we can still use.
It's not unusual now for me to have 15...
20, 30, even 40 plates in a solution
spanning decades,
typically at least 30 years
but we've gotten up to 100 years
in one case at least.
But for some of the fainter clusters,
we got to go to plates from big telescopes
and I did indeed get to the 200 inch.
Our group were the last users
taking prime focus plates
at the Palomar 200-inch back in the 90s.
So yes, that's me in the prime focus cage
on a cold, as a actually snowy night.
A reason there's opportunity to take a picture
snowing hard out doors.
No way we were going to open the dome that night.
Okay, let's get to some results.
M4.
Those of you who are amateur astronomers
may know M4 as the cluster that is about a degree east
from the bright star Antares in constellation of Scorpio.
If you're an amateur astronomer and you didn't know that,
here's a globular that's easy to find.
It's only about a degree from a first magnitude star.
I have had one observing run
with another professional astronomer that had no idea
where M4 actually was in the sky except by the coordinate numbers
and wondered how I could just step out
and come back in after 10 seconds
and tell her whether there were clouds near our cluster.
I finally let on that, well, it's right next to
something we can see even through broken clouds.
Anyway, it's a messy field. There's a lot of dust here.
M4 is one of the closest clusters here, closes to us.
This is Antares here but all this dust is much closer.
The dust, all this dark stuff in here, is foreground dust.
Much closer to us than the cluster is.
All this diffuse stuff is starlight
reflected off of the dust.
This is a messy neighborhood.
What did we get for our results on membership?
Well, here is a color magnitude diagram,
apparent magnitude
and color for everything we measured
in the area of M4 and here for the stars
that we derived had more than a 90% probability
of being members to the cluster.
It cleaned things up reasonably I would contend.
M71.
This is that one I showed you of 41-inch plate of.
It's very much in the disk of the Milky Way
in a constant, fairly dense area
south of Cygnus if you know the summer Milky Way well.
Clean things up considerably here as well.
This is not final.
Back in the 80s, I did the brighter part
down to about 16th magnitude.
We did that back in the 1980s.
The fainter extension still may
improve matters somewhat on that.
As far as I'm aware the first time
that anybody's been able to do membership,
reaching all the way down
to the main sequence in a globular.
But again I'll raise the question,
what do we do with a disk globular?
Is it something that stays in the disk
or something that just happens to be passing through?
In this case, we were able to get velocity of it
and show that it actually stays in the disk.
It has almost no velocity perpendicular to the disk.
So it is one stays in the disk of the Milky Way.
Bill Morgan, an astronomer at Yerkes
for many, many decades, one of the great
distinguished astronomers of the 20th century
had back in 1957, or there about, suggested
that M71 and a few others might actually be things
that stayed in the disk rather than plunging through.
Very few people believed him at the time.
When I got the result on the velocity of the cluster
it was fun to walk down the hallway
and show him the results
in about 1983 or 84.
It vindicated what he had suggested
that nobody believed back in 1957.
That was one of the fun things
of showing somebody, yeah, you were right.
NGC 6397, this is a southern cluster.
Much too far south to observe
from Wisconsin or, really,
from any northern hemisphere observatory.
This is a color picture
from an amateur astronomer down in Chile.
Here are our measurements. Now what we go here
are just measurements of proper motions
and we got a concentration in the center of this diagram
that a whole lot of stars moving together.
Those are the cluster members
and then a big scattering of stars,
mostly motions quite different from the cluster.
Those are the non-members that just happen
to lie in that direction in the sky.
But the most interesting stuff that came out of 6397,
we went after it because
it was one of the closest globulars.
But most interesting thing that came out of it,
not just the cleaning up the color magnitude diagram
which went pretty well here.
Again, on the left is everything we measured,
on the right of the probable members.
From the proper motions is that this is a cluster
that we found the velocity.
It is plunging through the disk of the Milky Way at the moment.
It is a little bit out of the disk,
it passed through the center plane of the disk
about five million years ago.
This is work that I did with Rick Reese,
who by that time was a former grad student.
He'd been a grad student of mine in the early 90s,
this was late in the 90s or early 2000s that we did this.
This is kind of an oblique view of the Milky Way.
If we do a top down view,
we're then going to blow up the area in that square
to something like this.
The yellow dot is where the sun is.
The red dot is basically
where the cluster NGC 6397, this globular is.
Now the red rectangle
shows the uncertainties of where the cluster
plunged through the disk of the Milky Way
about five million years ago.
But what about the rest of this?
Well, we were thinking,
okay, we got something with a mass of half a million
times the mass of the sun
plunging through the disk.
Are we going to find anything in the disk
that's a remnant of it having passed through?
And we very quickly,
Rick I should give the credit to,
Rick found very quickly,
There's this young open cluster sitting here,
NGC 6231,
and five million years ago it formed.
It is only 4 or 5 million years old
and given the uncertainties of its motion
it was someplace within that green rectangle
at the time it formed.
It formed at the same time as a globular
went plunging through the disk
in very nearly the same place.
After we found this, we discovered
that there was a theoretical prediction done
some years earlier that a globular
plunging through the disk could kick a...
basically kick a dense cloud of interstellar gas
into starting to form stars.
So there was a theoretical prediction
that this kind of thing could happen,
that a globular could cause stars to form
and as it goes through the disk.
So Rick likes to say, well,
we can't prove that 6397 indeed
did cause the formation of 6231.
We can show that they were
in the same place at the same time.
There was opportunity.
There's this theoretical paper that shows
there was a means and will let you speculate on motive.
So, anyway.
It's quite possible that we found a case of a globular
actually kicking off the formation
of a young open cluster.
Okay, go to Pal 5.
I showed you
a Sloan Digital Sky Survey image of Pal 5 before.
This is negative instead of the positives
I've been generally showing you so this is what the plates
actually look like from Kitt Peak 4 meter
and there were plates taken with a variety
of telescopes in the northern hemisphere
in, well, 1949 up into
the 70s on this thing.
The speculation was, okay, it's not much of a cluster.
It's pretty far from us.
Well, in fact, we can...
reading the notes on the plate envelopes,
we can even see some of the speculations.
People wrote it down on the plate envelope
in some cases when you see their speculating on things.
But the thought was, it's really sparse.
It's currently not all that far
from the Milky Way.
From the central mass concentration of the Milky Way.
If it came much closer in,
it would probably get pulled apart by the Milky Way.
So it must be something that's in...
an electrical orbit, and elongated orbit,
that it spends most of it's time much further out in the galaxy.
We're happening to catch it when it's closest in.
Well, okay, first of all
we cleaned up the membership,
same kind of thing as I've shown you several of these
left hand diagram is everything we measured,
right diagram is the probable members
and then we look at the velocity of it.
People thought it was probably as close in as it gets.
What we found from it's orbit though
is that it currently is at the outer end
of it's elongated orbit.
It comes a lot closer in,
it comes in to about half as far from the center
as we are from the center of the galaxy
and that immediately presents a problem
of how does something this sparse survive?
So I talked to one of the...
Solar Dynamics theorists
in my department in Chicago
and he suggested that, well,
it sure would be nice to know what that cluster
looked like before the last time it came around
its orbit through the closest part
but of course we have no way of knowing that
but he could calculate given the orbit
and current star density and so forth
It's not at all clear that it will survive
the next time coming through
close to the center of the galaxy.
It would probably get pulled apart
as it comes through the plane
of the Milky Way next time around
and it probably got a lot of stars
pulled out last time around.
Aha! If it had a lot of stars pulled out last time around
maybe there's a chance at finding those stars
and I was starting to think about how am I going to do that
and grad student at Chicago, Connie Rockosi,
had helped build the camera for the Sloan Digital Sky Survey
and was trying to find a thesis topic,
something to do for a PHD thesis that would use
some of the early data from it.
It so happened that
Pal 5 was within the region,
rather small region that they had scanned
for the test data with the Sloan camera
and I said, okay, you've got by far the best data,
even though it was test data, it's by far the best
for looking for these stars pulled out.
Here's a figure from her PHD thesis.
What we got here are contours on the sky
of stars that fit the color magnitude diagram of Pal 5.
Strong concentration of stars where the cluster is here
and tails ahead of it in its orbit
and behind it in its orbit
exactly where the theorist doing the star dynamics
would have predicted that they should be.
This kind of prediction had been done many times
on other globulars and people
had searched for the tails and not found them.
Here was a case where we knew
it had to be being pulled apart and Sloan data,
Connie having built the camera
and thus having access to the test data found it.
Now, somebody else within the Sloan concertion
actually started putting a paper out on it
and within the consortium, papers had to be agreed to
by everybody in the consortium and...
Connie's thesis advisory said, Whoa!
You tried to do this, even though it had been announced
that it was reserved for Connie's thesis.
At any rate, Connie had a wonderful
piece of work from this.
Now quickly as we're winding up here,
NGC 6791
this is again a Sloan Digital Sky Survey image.
It's one of the oldest open clusters.
Probably around eight giga years
and it actually is metal rich relative to the sun.
It has two or three times as much iron,
for example, as the sun does.
A cluster that old should not be metal rich
it should be metal poor at least a little bit.
This ones metal rich, so that's already a puzzle
about this cluster but we thought, well,
we will at least clean up the color magnitude diagram again,
see the success of the membership determination
that we got a pretty clean color magnitude diagram.
We do have-- If I can get the cursor to work.
We got stars here right above the turn off
are what are called Blue Stragglers
and if you never had Bob Matthew
from your department here at UW
talk about Blue Stragglers you really should
'cause he and his group of grad students have
solved the long time mystery
of why these clusters have Blue Stragglers.
But then here
complicated mass but these are simply
color magnitude diagrams
and just look at the one in the upper left here.
You've got stars plotted in red
and stars plotted in green
and there certainly seems to be an offset
in the sense that the ones plotted in red
would be considered to be probably older.
The ones plotted in red are not plotted in red
because their thought to be older.
Their plotted in red because they were
in the central part of the cluster.
So we may have an age difference within this peculiar
old open cluster.
There are some other possible explanations
for that but we may have
a complication here of something that
isn't suppose to happen in an open cluster
but sometimes they don't do what they're suppose to do.
I guess you could say NGC 2158.
This is that dense cluster near M35.
I did M35 as a first project
as a grad student back in 1970.
I saw this thing down in the corner of my plates,
I've looked up some things about it and thought,
you know, sooner or later I really need to do
a membership study of that cluster.
It needs it, but the plates that showed it
were not old enough yet at that point to think about.
Well, sooner or later came around in the 1980s,
I wasn't very happy with my results
so I got some more plates in the 1990s
and did the analyses in the 2000s
and I still am not very happy with it;
I still don't quite know what's going on.
Yes, we cleaned things up a bit
but it's still a lot more confusing and I don't...
my guess is that some of it is
that there are a whole lot of double stars in there
but I don't know what all is going on in there.
It's still a confusing mess. Okay.
I've been working on these things,
said for, over 40 years now.
What are some conclusions?
Well, from the membership work,
some of the things that got me started
is that there were stars with some peculiar characteristics
that were being identified in some of the globular clusters.
Some of them turned out to be members,
some turned out to be field stars
with nothing peculiar about them,
they only look peculiar if you thought they were cluster stars.
They didn't follow the same chemical composition
as the rest of the cluster but if they weren't members
no problem, it's okay.
Rick Reese's PHD thesis confirmed that
the distant scale that we use for globular clusters
is basically correct but there were a couple,
M4 and M22 that the traditional distances
for them were wrong,
people were making corrections,
you have to make these corrections for the foreground dust.
It dims the stars, it reddens the stars
but the corrections have been done incorrectly.
We got distances independently, we found that.
M71 velocity, I already mentioned that
it's a disk cluster.
It was fun t tell Bill Morgan that,
confirm what he suggested.
NGC 6397 I showed you
as it plunged through the disk of the Milky Way,
it may have triggered the formation
of an open cluster.
Our measurement of the velocity of Pal 5
showed it sparse because its on an orbit
that causes it being pulled apart.
Connie Rockosi followed up on that,
found stars that have actually been pulled out from it.
This has long been predicted show up
but first time it was really found.
And then these last couple.
GC 6791 keeps getting more and more interesting
and we're not sure what's going on
and NGC 2158, and I say here, still a confusing mess.
It's more than 40 years
since I first thought it needed some work
and it still needs work and I'm still scratching my head
at what it needs to be done next on that one.
There are still things even though that
I'm retired, so to speak, at this point.
I'm at Madison more than I am at Yerkes.
Still more things that I want to do.
Some final thoughts.
I didn't think I was gonna work on star clusters
from essentially my whole career.
I thought I would do what I could easily do
with the Yerkes plates in the 1970s,
maybe into the early 80s and move on
to doing some other things
but there kept being more interesting things to work on
so I kept working on them.
I also never expected that doing
really traditional kind of astronomy
of measuring proper motions
would get me on a telescope,
like the Palomar 200-inch that was used
primarily for looking at other galaxies and so forth
but here I was writing
in the prime-focus cage at the 200-inch.
And finally, star cluster research is interesting,
its often fun, and I'm still doing a little bit,
but I'm retired. As my dean told me just before retirement,
it's time to do whatever I want
whether it's astronomy or something else.
So, sometimes its something else.
So with that, we'll leave it.
(audience applauses)