- Thanks for coming to UW Space Place tonight.
We have a guest speaker tonight,
and that speaker is Professor Bob Benjamin
from the UW Whitewater.
Bob and I have a common interest in the history
of astronomy as you can probably tell already.
And so he's linked together an interesting mix here
of the history of our knowledge about the Milky Way galaxy.
So please join me in welcoming Bob Benjamin.
(audience applauding)
- Thank you.
So, yeah, so what I'd like to do is tell you a little bit
of a story about something that's near and dear to my heart,
which is the structure of the Milky Way galaxy.
And I'm going to talk about--
I'm going to try to focus on two years.
I'll have some things to fill in in the middle.
You know, most of the time science is sort of incremental.
We sort of gradually learn things, test things, move on.
But then there are those years where everything breaks all
of the sudden because of some advance in technology
or some insight that allows people
to move forward very rapidly.
And I'm going to describe two years where that happened
in the study of the Milky Way galaxy.
The first year was in 1951,
and the second year is this year, 2018.
So I'm going to try to go through 1951 fairly fast
so that I can get to 2018 because that's all the cool stuff
that's happened in the last few months.
So I'm starting here with a picture
that's actually not from 1951.
It's from 1950.
This was a meeting held at the University of Michigan.
It was a dedication of a telescope in Ann Arbor
where many of the leading lights
of galactic astronomy gathered
to discuss problems of galactic structure.
At the time of this meeting we knew certain things
about the Milky Way.
For one thing, we knew that the Milky Way
had different types of stars.
Basically, Walter Baade, who was a German astronomer
who did his work at Mount Wilson Observatory,
he worked outside Los Angeles during World War II,
and he had sort of the combination of being German,
he wasn't allowed to participate in the war effort,
and being in Los Angeles during World War II
meant that Los Angeles was blacked out.
So he could use his telescope near Los Angeles
to get really great images of the sky, and he discovered,
among other things, that the Milky Way consisted
of a different chemical, stars of different chemistry,
indicating there was a young population of stars
and an old population of stars.
Another thing that we knew is that we did know
that the Milky Way galaxy rotated,
the stars in the galaxy were all rotating
around a common center.
They had measured that.
And another thing we knew, thanks to Harlow Shapley,
was roughly the size of our Milky Way.
By looking at what are called globular clusters
that are spread out spherically around the Milky Way,
we had a sense of how big the Milky Way galaxy was.
Now, lurking in the back row
in this picture is Joel Stebbins.
Joel Stebbins was a professor here
at the University of Wisconsin.
He was actually responsible for correcting
Harlow Shapley's estimate of the size of the galaxy.
So he's most famous for basically shrinking,
well, at the time, the universe.
He shrank the galaxy back down a little bit
by doing precision measurements of stars.
And also from Wisconsin, although working
at the University of Chicago, was Bill Morgan, okay?
So they were all gathered together with many astronomers
to discuss the topics of galactic structure.
What was the Milky Way's structure like?
What would it look like if we were outside?
And the big unsolved, still to be resolved question was,
by looking at other galaxies we knew
that flat galaxies tended to be spiral galaxies,
but in 1950 there was no evidence for spiral structure.
So, in other words, if we looked at the stars
around our sun, we didn't see very clearly
that they outlined any spiral pattern.
And so that was sort of the question of the day.
And so before I get to what happened in 1951, though,
I'm going to just, my overview of the talk here.
First I'm going to give you a little bit of information
about galaxies in general, and the Milky Way in particular.
Then I'll talk about this 20th century effort
to map the galactic structure starting in 1951.
And then I'll describe to you what's been happening
over the past, well, the discoveries that have happened over the last past few months,
although the effort leading up to that started in 2000.
And hopefully, by the end of this talk,
you'll be completely up to date on the structure
of the Milky Way galaxy.
So, what is the Milky Way galaxy?
If you've ever been a dark site, you'll know by looking up
in the sky that you can see this band of diffuse light.
That's actually the light of a bunch
of stars all blurring together.
Galileo saw that when he pointed his telescope
at the Milky Way.
So here's an image of the Milky Way.
It's that band across the sky.
This is a picture taken from Oregon,
so it's fairly high latitude.
One of the particularly interesting things is off
to the right you'll notice that it gets brighter,
but it also seems to get somewhat dustier too.
That dust, by the way, is dust clouds blocking our view
into the center of the galaxy.
And as you go further south on the Earth,
that section of the galaxy, which is the direction
toward the center of the Milky Way,
will get higher and higher above the horizon.
So here's the same sort of picture but taken from Thailand.
And you can see that from the center of the galaxy,
again where that band is the brightest, although,
like I said, there's dust clouds blocking parts of it,
but you can see it's further away from the horizon.
And one of the best places to go is south of the equator.
Here's a view of the Milky Way galaxy from Chile
where the Milky Way arches overhead,
and the center of the Milky Way is pretty close
to directly up. Okay?
So if you really want a beautiful view
of the Milky Way galaxy, and, actually, as it turns out,
some companion galaxies, go south
and you'll really appreciate it.
So, anyway, when you look at this picture of the Milky Way,
it really looks like other galaxies that we see out
in space that are tipped on their edges
because galaxies can be in all different orientations.
They can be face down or they could be oriented
from where we are, edge-on.
And so you look at this picture of the Milky Way galaxy
from inside and you can compare it to this picture
of a galaxy, of the galaxy NGC 891,
and you see a lot of similarities.
Essentially, what we see when we look in the sky,
is we see an edge-on galaxy, our own, right?
We're in it and so we see it from an edge-on perspective.
And so what we would love to have
is the face-on perspective.
By the way, you know, if evolution had proceeded
at a different pace, the sun actually bobs up and down
through the galactic plane.
And we're right almost smack dab in the middle
of the plane right now.
If we had come, if our society had developed, you know,
a few, I guess several million years later,
we would actually be high enough out of the plane
to get a little bit of a top-down view.
It'd still be a long sort of path length.
But we'll never get this kind of view.
These are other galaxies.
And one of the questions is
will the real galaxy that the Milky Way is stand up?
This is the goal to understand,
because once we understand what our Milky Way looks like,
we can make sense of all the details
we see in our own galaxy to understand other galaxies.
That's really the basic idea.
So galaxies by mass, it turns out, are 9% stars,
1% interstellar matter, that's the gas, the nebulae you see
in a lot of the pretty pictures of Milky Way,
and, embarrassingly, 90% dark matter.
So dark matter is simply a placeholder.
We see stars orbiting the galaxy,
we see stars and gas orbiting other galaxies
as if there's more mass than we can detect.
So if you look at the amount of light those galaxies are emitting,
it indicates more mass than we can actually detect.
And that mass has been called dark matter
and people have searched for that dark matter
so far with no success.
But galaxies by function are something just--
It's an island, okay?
Galaxies are an island of stars and gas surrounded
by empty intergalactic space, okay?
So where these little islands of light scatter
throughout the universe.
And understanding this function, how galaxies form
and change and convert their gas into stars,
is one of the big goals of astrophysics now.
So to understand what I'm going to say later,
I should also provide you a little bit of information
about how galaxies form.
This picture shows what's called a space-time diagram.
The time axis is the right.
So as you move from the left to the right,
you're going from earlier times in the history
of the universe to later times.
And then the picture shows what did
the universe looked like at different times.
And so you can see on the left, the left is the Big Bang.
And the Big Bang, about 13.7 billion years ago,
there, all of the matter in the universe
and all of the space in the universe was very small
and compressed to an almost infinitesimal size.
At some point, that started to expand.
Initially it expanded very rapidly,
the period we call of inflation.
And then the gas started to cool, eventually cooling enough
so that the electrons and the protons
which had always been bouncing around without being attached
to each other combined into the first hydrogen atoms.
And the hydrogen atoms and the helium atoms
eventually started to collapse into structures
that formed the first stars
about 400 million years after the Big Bang.
From that point on, as gas started
to fall into these structures, these protogalaxies,
and galaxies started to merge with each other,
the current structure of the universe took shape.
So I'm going to show you a computer simulation.
And the reason I wanted you to see this
is because it's very important to remember
when we study our Milky Way
is that our Milky Way has a history to it.
It's probably eaten other galaxies.
It's probably been interacted with other galaxies as well.
So what you're seeing here is the yellow spots
indicate light and the blue or the red indicates--
Sorry, the blue indicates cold gas.
And so after the Big Bang the universe sort of turned
into this network of gaseous filaments,
and along these filaments you started to condense galaxies.
And when enough gas collected, stars started to form.
And then those collections of stars started to interact
with each other gravitationally and merge.
And out of all that process, somehow
we got our own Milky Way galaxy.
Now this is a picture of a galaxy that we'll never have.
So how did we get it?
I'll tell you a little bit about the story later,
but this is an artist's picture of the Milky Way by Robert Hurt.
I contributed to putting this picture together.
I sort of served as a consultant.
And so it's trying to take data from various sources
and synthesize it into some picture,
some schematic of what the Milky Way might look like.
The order you see here, the beautiful spiral pattern,
is probably a tendency to impart too much order
to the galaxy than it actually has, okay?
So I actually do believe that the galaxy
would look a little messier than this picture.
But many of the things that you see in this picture
I can link back to data.
In other words, I can say, okay, when we look at data
we see some structure that we put into this picture
in a certain way, so in this picture, the sun--
Oh, the first and most important thing you'll notice
about this picture is that there's a bar,
a band in the diagonal band in the center of the galaxy.
That's called a galactic bar, and the reason it happens
is because stars in the very center of a galaxy,
and this is true of many spiral galaxies, have started
to move in elliptical orbits rather than circular orbits.
So the combination of all the stars in the inner parts
of galaxies moving in elliptical orbits means
that you get a band of starlight as opposed
to a sphere of starlight.
But out where we live, which in this picture
would be where this yellow dot is, out where we live,
which is 27,000 light years from the center of the galaxy,
we're going in roughly a circular orbit. Okay?
So somewhere in between where we are
and then the center stars tend to switch
from circular orbits to elliptical ones, okay?
And, again, that's a process we are trying
to understand in astronomy.
Now, something I'd like to do with this picture
I've never done before, some of you may have heard me
show this picture and sort of give the overview
of the Milky Way, but one thing you can think about
in this picture is that the sun is going around the center
of the galaxy roughly once every 250 million years.
That's how long it takes to complete one orbit.
And if you think of that as a galactic year, okay,
since the sun is about 4.5 billion years old,
that means the sun is about 18 galactic years old, right?
So the sun or our solar system is just ready to vote
in galactic elections, okay?
And so, you know, for many of the times around the center
of the galaxy, our Earth was a cooling, you know, hot,
magma-filled, cooling object.
But about the next to last orbit around the sun, okay,
so orbit 17 out of 18, was what
was called the Cambrian explosion.
So at the beginning of our 17th orbit was when a lot
of the life on Earth came into being, okay?
Now, I won't, but I just want to start then
with the last orbit around the sun.
The last orbit around the sun actually started off
with not a good note because this was actually
the largest extinction that we think of
in the history of the Earth, okay?
So the Permian, let's see,
I think it's called the Permian-Triassic extinction event,
happened just as the sun was starting on its last,
its most recent orbit around the center of the galaxy.
So that's the Triassic period.
After the Triassic period, you had another extinction event
where nearly 80% of life on Earth or 80%
of the species were eradicated for some reason.
That left the room clear for the dinosaurs to flourish.
And the dinosaurs then lived during the Jurassic period
and then the Cretaceous period.
And I think most of you are aware that the dinosaurs are not
with us except in bird form, okay?
And so the dinosaurs were wiped out
by another extinction event
that was basically three-quarters
of the way through the most recent galactic year.
And then, after that, we have various epics
where mammals started to take over.
And all of human history is in the last fraction
of a second of that galactic year around this orbit.
Now, as you look at this picture
and think about the sun going around the center
of the galaxy, something that's important
to remember is everything else is going
around the center of the galaxy too.
In fact, the bar that you see here would have rotated.
It would have actually rotated slightly slower than the sun.
So the sun would overtake the bar and come around.
So everything is rotating, I would love to actually take this picture
and put it into motion with our best understanding
of galactic dynamics.
So that takes us to 1951, okay?
Now the dinosaurs are dead. We're here.
We developed civilization. 1951.
We don't know where spiral structures are
but we're about to find out.
540 days was when that meeting in Michigan occurred.
540 days before the announcement of spiral structure in the Milky Way.
At that meeting, Walter Baade, who I pointed out,
described how you might map spiral structure
in the Milky Way, and he was motivated
by a picture similar to the one in the background.
This picture actually shows M31, the Andromeda galaxy.
And if you look at this picture, you'll notice
that there are little red spots along it, okay, that trace out a band.
Those red spots are H2 regions.
And so what Walter Baade suggested to a couple astronomers,
one of whom took him up on it, was why don't we try
to find those H2 regions in the Milky Way,
those ionized nebulae, try to get their distances
by measuring the distance to the stars
that are heating the gas, and then make a map.
And you should see that those H2 regions lie
in some sort of band, which would evidence
for a spiral arm.
And so it turned out that suggestion from Baade--
Oh, I should point out, even at that point
Walter Baade realized that this was going
to be a limited picture of the Milky Way
because of the dust that blocks our view very far
in different directions in the Milky Way.
So at this meeting in 1950, he says of one thing
we can be certain, on account of the heavy obscuration
of the plane of the Milky Way, we will best get only a glimpse
of short pieces of spiral arms.
And one guess that we can probably safely make is
that our sun is located in a spiral arm
because the brightest B stars, our sort of hottest stars,
and dust around our sun in all direction.
I know this argument will not overly impress you
and that you would like to see the arm
or a piece of it demonstrated at oculus. So would I.
At oculus means obvious to the eye.
That's what people wanted.
They wanted a plot or a diagram or some sort
of measurements that made it obvious, oh, yeah,
there's a spiral arm.
And that's what appeared for the first time in 1951.
24 days before spiral structure was announced
one of the most famous astronomers of his time, Jan Oort,
for whom the Oort Cloud is named,
the comets at the edge of our solar system, he, who's shown on the right here,
visited Yerkes Observatory, which is in Williams Bay, Wisconsin.
You can tell it's Williams Bay, Wisconsin,
on December because of the snow, okay?
And so Oort was in the United States to give a lecture
at the American Astronomical Society meeting.
He was receiving an award, and he came
to Yerkes Observatory, which was one
of the premiere observatories in astrophysics
at the time here in Wisconsin.
He talked with several of the people there.
The people, seen from left to right,
is Subrahmanyan Chandrasekhar, who was famed
for figuring out the maximum mass of a star
before it would collapse, the so-called white dwarf,
the Chandrasekhar limit;
Bill Morgan, who I'll say more about in a second, was the person
who found the first evidence for spiral structure;
Gerard Kuiper, who studied the solar system
and other things, the Kuiper belt of our solar system
is named for him; Bengt Stromgren, a professor
at the University of Chicago who understood the theory,
the physics behind these ionized nebulae.
So this is a picture of them sitting on the steps 24 days
before Morgan announced his discovery of spiral structure,
which he shared with Oort.
At this time Oort had a discovery of his own to share,
which is the Dutch had just barely lost to the Americans
but then proceeded to outpace the Americans
in detecting radio waves from hydrogen gas.
And so Oort was primed to start to study the structure
of the Milky Way galaxy using radio waves
from radio telescopes to look at the distribution
of gas in the Milky Way.
Both the first detection of the 21-centimeter line
and the discovery of the spiral arms via these H2 regions
were presented at the same meeting,
but Oort and Morgan were discussing it a month in advance. Okay?
So then, on December 25th, because apparently astronomers
would travel to meetings on Christmas day back then,
on December 25th Morgan arrived in Cleveland
ready to present his discovery of spiral structure,
which I'll show you in a second.
He met with a pair of astronomers who are famous
for their work on the structure of the Milky Way galaxy,
Priscilla and Bart Bok.
This is a picture shown of them later on in 1958.
Actually, Priscilla Bok was a professor of astronomy
at Smith College when she went to Leiden
for an astronomical meeting.
Bart Bok was a graduate student who was assigned
to be her guide to Leiden, to just show her around.
At the end of the one-week long meeting he proposed to her.
The graduate student proposed to the professor.
And she said, "I don't think so."
And so she, but they continued to write and correspond
and eventually they did decide to get married.
And they worked as a team in studying the Milky Way.
And they actually wrote a popular science book starting
in 1941 on the structure of the Milky Way galaxy.
So if you want to learn the history of the structure
of the Milky Way galaxy, reading the successive editions
of this textbook, or not really a textbook,
more of a popular science book from 1941, 47, 57, 74,
and 81, sort of you can see the progression
of the efforts to map out the Milky Way galaxy.
So the night before the meeting Morgan met with them.
They were in a dorm room at Case University.
And Morgan went through his entire presentation
with them while they were sitting on his dorm room bed.
And Bart said, you know, at the time he said,
"I was really grateful that he showed this to me in advance."
He says, "I had been trying to do a similar thing,
"but I was using a different kind of star.
"I was barking up the wrong tree."
So Morgan had beat him to it, but Bart Bok was magnanimous
in this victory of Morgan's.
So at the meeting, on December 26, 1951,
Bill Morgan presented a synthesis
of several different things that all tied together
to show the first map of the Milky Way spiral structure.
And almost all of this data came from Wisconsin.
So the first thing was that they used a camera
to find the H2 regions on the sky.
Like, where were those regions of ionized gas on our sky?
This is a wide field camera
that they had available at Yerkes.
It was eventually, that data was published in 1952,
but it was using a green sign handy camera at Williams Bay.
Then Morgan identified the stars, the hot stars associated
with those H2 regions, those nebulae.
And then, using a spectra, basically taking spectra,
looking at the light from the stars and examining the light
as a function of wavelength to tell you
what kinds of stars you had.
So most of that was done at Yerkes, although some of it was also done
at Warner Swasey Observatory in Ohio.
But then the last thing you had to do to do this process
to get the distance to these stars is you had
to make some estimate of how much dust there was
between us and the star because a star could be dim
because it was far away, or it could be dim
because there was dust, okay?
And you had to know which reason, why was the star dim.
Was it dust or distance?
And so with the dust estimates that came
from Washburn Observatory here in Madison,
he was, Morgan was able to correct for the dust absorption
and estimate the true distance to the stars.
So he presented a synthesis of these results
showing the two bands of H2 regions, the B stars,
the first evidence for spiral structure.
And this is actually the way Morgan remembered it.
He said, "Oort had introduced me, and when he sat down
"to listen, he sat down in my seat.
"It was one of those steeply sloping classrooms
"at Case with all the seats way up high.
"Well, when I got through, the first thing was
"I had no place to sit down.
"And the second thing was that people started to applaud
"by clapping their hands, but then they started stamping their feet.
"It was quite an experience."
Bart Bok proclaimed this a triumph on Morgan's behalf
and astronomers were really excited.
It was the first evidence, the first map
of spiral structure in the Milky Way.
The map that was presented at the meeting
was actually never published in its original form.
The original map that was presented in Cleveland survives
as a board with nails pounded in it, okay?
And then little Styrofoam balls stuck on the nails.
And that board is hanging in the Adler Planetarium.
If you ever go to Chicago, you can see the original map
of spiral arms from Morgan as a board with nails, okay?
A section of that board was photographed
for the April 1952 edition of Sky and Telescope.
So those little cotton balls that you see there
is a band of H2 regions.
That band is one of the spiral arms of the Milky Way, the local arm.
And then the Sky and Telescope magazine put a picture
of Walter Baade's spiral arm section from M31 to say,
"Look, they're just the same."
Now, this is actually a picture of the board I got from Adler.
This is what it looks like now.
And eventually Morgan did publish this in 1953
with two colleagues, Albert Whitford and Art Code,
who were both professors here in UW Madison.
So you'll notice the original publication was actually
a combination of Yerkes and Washburn Observatory.
So that was the first evidence.
And this picture here, if you look, you'll see
that the H2 regions, the nebulae sort
of trace three bands in that picture.
I hope everyone can see that.
So if you look, I didn't add an annotation, but if you see,
there's a lower band, a middle band, and an upper band.
And that was actually the first evidence for spiral structure.
As it turns out, I just wanted to say as a little historical footnote,
this is actually a picture from that camera.
So the picture on the left shows the all-sky view
of our own Milky Way galaxy.
The picture on the right shows a picture
at the time of NGC 891.
But that camera sort of got misplaced or lost.
Yerkes didn't have it.
They didn't seem to be aware they didn't have it anymore.
But it resurfaced here in Madison. (laughing)
And the reason was, the picture here on the left
shows the picture of the camera taken
at Yerkes shortly after its construction.
But a professor here in Madison, Art Code,
borrowed the camera to go down to the southern hemisphere
and take pictures of the sky from the south.
And then after he came back from South Africa, he kept it.
And they used it for lots of different reasons,
including the site Pine Bluff Observatory.
And so it was then put in a closet
in graduate student offices and over time it just sort
of got misplaced and no one knew what it was.
You know, people sort of knew Art Code used that camera,
but it was not realized until Jim Lattis
and the former director of Yerkes and I put it together.
It was like, "Oh, my gosh, we have the camera that made
"the H2 regions for the first map of spiral structure."
So that camera has now been returned to Yerkes Observatory
where it's in the closet again. (laughing)
But hopefully it won't disappear again.
So, you know, Morgan got his discovery in just in time
because he actually was quickly outstripped
in this attempt to look at the structure of the galaxy
by the Dutch effort to measure the radio waves.
So the picture, the little table on the left here,
of this diagram, shows the timeline.
The Americans, Purcell and Ewing
at Harvard College Observatory,
detected a 21-centimeter radio wavelength line
in March 1951.
The Dutch detected theirs in May 1951
after they consulted with the Americans.
The Australians did a crash course
and made their own detection in August 1951,
and all three groups published the first detection
of these radio waves of hydrogen gas in September of '51.
The spiral arms were announced in December 1951.
And then it was off to the races.
Basically a whole frontier of mapping the Milky Way
was opening up before everyone's eyes.
The first effort to map out the hydrogen gas was done
in the Netherlands using not actually
an official radio telescope.
They were using a radar dish leftover
from the German military on the beaches of the Netherlands.
So they retrofitted this German radar to turn it
into a radio telescope
while they were building a proper radio telescope.
And so the original survey of hydrogen gas
in the Milky Way was done
using a leftover German radar dish from World War II.
And so the Morgan, Whitford, and Code paper
that I showed you came out in May 1953.
The first gathering of astronomers to start
to coordinate their efforts on galactic astronomy happened
in June 1953.
That was IAU symposium number one.
And then the famous paper that provided the very first map
of the spiral arms of the gas of the Milky Way
in hydrogen gas, neutral hydrogen gas,
in 21 centimeters was presented in '54.
And that was the paper in which the first spiral arms
that Morgan presented were named.
So Morgan didn't name them in 1951.
But once they thought they were seeing
the corresponding things in the radio waves,
they consulted and so I'll just read from the paper
that Van de Hulst, Muller, and Oort wrote
on their radio surveys.
"After consultation with Morgan,
"the following designations are proposed:
"for the arm passing through the sun, the Orion arm,
"for that passing through H and Psi Persei, the Perseus arm,
"and for the first arm encountered when proceeding
"in the direction of the center, the Sagittarius arm,
"being inside the sun's distance from the center
"of the latter does not appear in our present article."
They stuck in their initial analysis to the gas
outside the sun's orbit because inside the sun's orbit
there is complications.
And so this was their attempt to match their data
with the stars, the dots that Morgan
had measured the distances to.
So I'm going to have to speed up a little bit,
otherwise I'm not going to get through the thing.
So let me just say a couple things
about mapping the Milky Way.
There are several different techniques
to mapping things with distance. Okay?
And the most fundamental and the most reliable
and astronomers won't argue once you have this measurement done right
is what's called parallax. Okay?
Since this being videotaped,
I normally have people demonstrate parallax.
I'll show you a video of this later, how it works.
But, basically, it just uses the idea that
if you have two different lines of sight on the same object,
like if you put your finger in front of your face
and look at, line the finger up with something
in the distance and look at it with one eye
and then switch to the other eye,
you will see the position of your finger shift
against the background wall.
And then if you move the finger closer to your face,
you'll see it shift more.
So, basically, the amount of shift is related
to the distance to your finger, and you can use that
to work out the distance to the source,
finger in this case, using geometry.
Now, it turns out it's very hard to make those measurements,
and the best attempt until just recently
was a European mission called Hipparcos.
But that had very, very limited range.
The white circle indicates roughly how far could you map
with parallaxes before 2018.
Then you could what's called the standard candle,
and that's what Morgan used.
The idea that if you have a source of known luminosity
and it appears to be a certain brightness,
you can relate the known luminosity to the brightness by assuming some distance.
In other words, if you have headlights very close, they'll be very bright.
If you have headlights very far away, they'll be very faint.
And by measuring the brightness or the faintness
of the headlights one can infer the distance of the car
if you know the luminosity of the headlights.
You do have to correct for dust.
And because of the dust, you're limited
to how far out you can see
because the Milky Way does have a lot of dust
in the disc of the galaxy.
And then the last method is using
what's called the kinematic method.
What you're doing is using a Doppler shift of something.
So you see some object moving and if you assume it's moving
in a circular orbit, and you can probably already see
where this is going to be a problem,
if you assume it's moving in a circular orbit,
you can turn the Doppler shift into an estimate of distance.
So I'll just demonstrate this with this little picture.
This shows the artist picture with a contour map
of how you would convert a velocity to a distance.
So, for instance, if you saw a cloud in 30 degrees
to the left of galactic center,
that's along that orange line, and it was observed
to have a velocity of plus 40 kilometers a second,
that would come from measuring the Doppler shift,
it would have to be where I've put it in this diagram
because you can see that's where the 40 line
intersects the orange line.
However, the big problem of this method
is that you can see--
And so the idea here is the cloud is moving
in a certain direction around the center of the galaxy,
the sun is moving in a certain direction around the galaxy,
and that means the sun is moving partially
towards the cloud and partially to the left,
the cloud is moving away from the sun and partially
to the left, but the net effect is that we are not moving
towards the cloud as fast as the cloud
is moving away from us.
So this cloud would appear to be moving away from us
just due to galactic rotation, and it would be red shifted,
it would be Doppler shifted to the red.
And so that's how this method works.
The problem of course is that there's another place
in the galaxy where you get the same Doppler shift.
So basically one measurement of velocity could put the cloud
at two different places.
And that made this method
for mapping the galaxy incredibly complicated.
Not to mention there are certain directions
where it won't work at all.
If you're looking towards the center of the galaxy,
everything is moving parallel to you
because all the orbits are parallel, like in this picture
they'd all be traveling to the left,
and so there's no Doppler shift from anything
towards the center of the galaxy.
And so it really makes it hard
to map the Milky Way reliably.
Nevertheless, astronomers made pictures like this.
This just shows the direction, the galactic longitude,
which is just the direction around the galactic plane,
you're looking, and the Doppler shift velocity
on the Y axis.
The color bands or the color represents the brightness
of the emission.
I'm not going to describe the details here
but if you look at this plot,
you might say that I see patterns.
I see bands. I see hooks.
I see little segments. Okay?
And each of those bands or hooks or little segments
is what astronomers interpreted as sections of spiral arms.
And from the sort of painstaking analysis
of diagrams like this, astronomers came up
with a map of galactic structure,
which I'll share in a second.
But the most important thing to know, I think,
about the Milky Way is that the Milky Way
does have this bar, and that was actually
not really firmly confirmed until the 1990s
when we could use infrared astronomy to see the stars
in the center of the Milky Way for the first time.
So this actually shows a computer simulation
of what that bar would look like viewed from different angles.
You can see as seen on edge-on the bar actually
has a thick part and a thin part.
This is, again, a simulation, not the actual data.
And now the rest of the galaxy has been removed
and just the part, the stars that would be participating
in those long, skinny, elliptical orbits are shown.
So you can see what does the structure of the bar looks like
in a computer simulation of a barred galaxy.
And the Milky Way bears a very strong resemblance
to what computer models predict for this bar
of the Milky Way.
So I'm going to skip over the next section
and jump to 2018, where we are right now.
The effort to map out the Milky Way was something
in 1951 people were incredibly optimistic
and excited about.
By the mid-1980s to late 1980s, people had given up.
The data that we had at the time and the techniques
that we had at the time were really showing
their limitations, and people could start
with the same data and end up with different maps
of the Milky Way, which led to a lot of fights,
a lot of bad blood, and eventually a lot
of people saying I'm going to move on.
In fact, during this time, Priscilla Bok said to her husband Bart,
"Bart, get out of spiral structure. (laughing)
"The future is in star formation."
And Bart followed her advice and moved
from studying the structure of the galaxy
to studying how stars form in these gas clouds
for most of the rest of his career.
Only in the last year before his death did he return
to spiral structure a little bit.
So in 2018, we now have an opportunity to restart some
of what we're doing here using a satellite
that was launched by the European Space Agency,
called Gaia.
Gaia is providing precise measurements of parallaxes
and proper motions, parallaxes I explained to you.
Proper motion just means how do stars move across the sky
due to the relative motion of the sun
and those stars for 1.5 billion stars.
This is not a photo of the satellite,
this is an artist representation of the satellite.
But this satellite was launched in 2013,
and it did something very unusual for astronomical missions,
which is the team that was processing the data
from the satellite was just working
to create a data product.
They wanted to make the data as good as possible.
They didn't skim anything off the top.
They didn't hold onto it until they milked it all dry
of all the cool discoveries.
They just released a data product to all the rest
of us astronomers at one moment on April 25th, 2018.
So they worked for years to get ready.
They worked from 2013 until this last April
to get the data ready for a total
of 22 months of observations.
They were measuring the positions of billions
of stars multiple times to see how they shifted back
and forth due to parallax and how the stars drift
across the sky due to their space motion.
And so, again, the picture here,
this little animation shows how parallax works.
So, in the lower picture, you have something
like the Earth would be the blue dot and the yellow thing
would be the sun, and the red dot
would be the object you're looking at.
And you can see as the Earth goes around the sun,
the red line lines up with different points
along the yellow screen, okay?
And so what it would look like on the sky
is shown on the upper panel.
So you can see when the object is close, it shifts back
and forth a lot; when the object is nearby,
it shifts back and forth just a little.
And so that's how parallax works.
The nearest star has a parallax of less than an arc second.
But Gaia can measure parallaxes as small as five
to 10 micro arc seconds, giving them the ability
to estimate distances, with some bigger error bars
at those things, but out to 60,000 light years, okay,
which is across to the far side of the galaxy.
And the angle they're measuring is roughly the diameter
of a single strand of human hair 600 miles away, okay?
So you can make measurements that precise with this telescope.
And a talk on the technology
of this telescope is fascinating.
They have to do all sorts of amazing--
Every time a dust grain hits the telescope they have
to fire a little bit of fuel to stabilize the telescope
against the momentum from the dust grain.
Every time a piece of dust hits the telescope
they have to correct for it.
The other thing it can measure is space motions.
So this is actually showing the stars of the Big Dipper.
It shows how they, over time, going into the future,
sorry, actually going back into the past
and then going into the future, how the Big Dipper
would warp over time, okay?
Because all of those stars are all moving around the galaxy
just like we are and our vantage points are going to change
on those stars, and so we're going to see them shift
with respect to each other.
And if you know the distance to the stars as well
as their motion across the sky, the motion across the sky
is just an angle change over time,
but if you know the distance to the star,
you can get the true space velocity to the star.
And so that is actually what this satellite was measuring
for 1.5 billion stars, okay,
which is a significant fraction of the total number
of stars in the Milky Way.
So I'm going to show you a Gaia representation
of the same thing.
Although, they'll show you the stellar positions moving
but grossly exaggerated just so you can see what's going on.
So in a second you're going to see all these stars start
to move around in circles.
And you'll notice they're all moving around in circles
in the same way because what you're really seeing
is the Earth's motion around the sun.
And so the Earth's motion around the sun
is causing the nearby stars to move in sort of big circles
and the distant stars to move in small circles.
Now, if you attach the brightest stars to constellation,
you can see how the constellations would wobble
if they were all much, much, much closer
to the sun than they actually are because, again,
this is exaggerated for effect.
By the way, at the very end of the talk I'm going
to leave a link to, you can actually download a simulation
to fly through the galaxy using the most modern data
to see where the stars are and watch them go by
using the modern precise positions.
Now, this thing shows how the constellations
would move over time.
The proper motions of the stars going forward
into the future.
So, of course, this is grossly exaggerated
but you get the idea here, that these motions allow you
to measure the velocities of stars.
So the data was released on April 25th,
and I was forced for the first time in my life
to start following things via Twitter. (laughing)
And so this is a list of all the papers
that have come up using Gaia data since April 25th.
It's a gold rush, okay?
And everyone is rushing to find their things.
I had something I wanted to do with the Gaia data
and I ran into problems and I found out eventually,
just last week, that other people have run into the same problems I did.
They ended up publishing a paper but they ran into the same issues.
So what I'm going to do is describe some of, I think,
the more interesting discoveries
that have been made since April 25th.
I'm going to give a little bit of an overview first
and then talk about some of the more interesting ones.
One thing that we can learn about is the evolution
of stars themselves.
One of the things we get out of Gaia is
with precise positions we can measure precise luminosities
because we know the true distance to the star now,
and so if we know how bright it appears,
we know how luminous it truly is.
And so we can put lots and lots of stars with very,
very well measured luminosities onto a plot
and see what happens.
So one of the things that we found is
we have a complete census of white dwarfs now
for the very first time.
We have evidence that white dwarfs have merged
so that you have two types of white dwarfs.
White dwarf stars are the leftover remnants
of ordinary stars, like the sun.
The sun will be a white dwarf remnant.
But if the sun had a binary companion, you might end up
with two white dwarfs, and you can actually see evidence
that there are white dwarfs that have merged.
So you can have, basically, single white dwarfs
or merged white dwarfs at the end of the star's life.
Okay?
For those of you who have had
or remember introductory astronomy
may remember something called the main sequence.
If you haven't, hold on.
But it turns out there's a gap in the main sequence.
They've detected many, many hyper velocity stars.
These are stars that are moving through the galaxy
at such high speeds that they are on their way out of the Milky Way.
That is, their velocities are higher than the escape velocity
for the gravitational pull for the Milky Way.
So these stars are on their way out.
How did they get going so fast and why?
So we see white dwarfs being ejected
after their companions go to supernovae.
We see regular stars also traveling at hyper velocities.
That's harder to explain.
There's all sorts of stellar clusters that have been identified.
There has also been some clusters that have been de-identified.
There were some clusters of stars that people have been studying that turned out to be
just chance projections of stars on the sky.
So there have been lots--
Luckily there's been more clusters discovered than demoted.
But so we're still going upward on the cluster count
in the Milky Way.
But the study of clusters in the Milky Way
is really undergoing a revolution.
You can look at the stellar kinematics
of the galactic disc.
You can look at how stars orbit around the center
of the galaxy and see, are they moving in circular orbits
or are they not? And the answer is, at least for the sun,
by and large they're moving in circular orbits
but there are groups of stars that are deviating
in systematic ways from those circular orbits.
There are streams, lots and lots of star streams
in the Milky Way, and we don't know why yet.
There's also one big merger, which I'll get to in a second.
We can measure, by measuring the orbits of stars
around the Milky Way galaxy, we can constrain the dark matter
because those stars are basically orbiting
under the influence of gravity, and so we have new constraints
on how the dark matter is distributed around the Milky Way.
Spoiler: so one thing that people have always wondered
about the dark is, is it round around the Milky Way
or is it flattened like the disc of the Milky Way?
And it seems to be, as best you can tell, pretty round.
So the dark matter is distributed
in a round sphere around the galaxy.
There's been lots of satellites
and streams found around the galaxy.
My favorite one is called Phlegethon.
It's a stream of stars 60 degrees long across the sky.
That would be about this long, if you just hold your arms out.
And it's probably a disrupting globular cluster.
If you don't know what Phlegethon is,
it's actually the river in the underworld
in Greek mythology.
And the kinematics of stars in the local galaxies,
you can actually see evidence for the stars
in other galaxies rotating for the first time.
Well, not for the first time.
Actually, there was some evidence from Hubble before that.
And then I'll talk about spiral structure in a second.
So we're going to rush through a few of these.
So for those of you who know introductory astronomy know
that stars are distributed across a diagram
if you plot brightness on the vertical axis or temperature
or spectral class on the horizontal axis.
This is called the--
And the band of stars that goes through the middle
is called the main sequence.
That's where stars sit when they're burning hydrogen
and helium during their normal main sequence lifetime.
And so that sequence is now understood
to be a sequence of mass.
In other words, on that diagonal band,
stars that are on the upper left part of the band are stars
that are very massive, very luminous, very hot.
They burn through their material very quickly
so they're very short-lived.
Then if you go down to the bottom right along that band,
you have stars that are faint, low mass, low temperature,
and a star that was formed even at the beginning
of the universe wouldn't have finished burning up all its fuel.
In fact, it would have billions of years to go.
So one of the surprises is we thought that was a continuous sequence
but if you actually use the precise positions
and you zoom in on the section of this diagram shown
with the arrows, the data that was used from Gaia,
does everyone see the picture on the right?
Do you see a little bit of a band cutting through that?
There's a gap in the main sequence.
This was completely unexpected.
Although some people said, oh, well maybe I thought of it.
It seems to indicate there's a temperature
in luminosity that stars just avoid.
And it's thought to be telling you something
about the structure of these low mass stars
and how they go from, how they carry their energy out to the surface.
Basically, stars at that mass range should carry most
of their energy from the center of the surface
using convection, just like boiling and water.
So it turns out there's a moment where you go
from partially convected to fully convected
where the stars just might never be able
to be at a combination of temperature.
But in that band there's a 24% deficit of stars,
or that gap, and that's probably an underestimate
because every effect that you could think of
that would affect this would tend
to fill the band in, not empty it out. Okay?
So that was a surprise on stellar evolution.
Hyper velocity stars, I'm not going to tell you
about all the discoveries of hyper velocity stars,
but this one I found particularly interesting.
There were three white dwarfs that were found
to be traveling at somewhere between 1,000
and maybe 3,000 kilometers a second.
For reference, it takes about 500 kilometers per second
to escape the gravitational pull of the Milky Way.
And so the curves here sort of just show the probability
of a star having, of these three stars having a given speed
because you can't measure it exactly.
You have to make some assumptions.
But one of the stars, shown in the picture on the right,
appears to have come from a supernova
or from the vicinity of a supernova remnant.
By the way, I didn't read this until today, and I was like,
oh, my gosh, we know a lot about that supernova remnant,
I should jump on that.
So there's a supernova remnant at a galactic longitude
of 70 degrees and then below the plane of the galaxy.
That's shown with that green circle.
And it looks like the star that they're seeing
that's a hyper velocity may have been ejected
in that supernova explosion.
So that's sort of an interesting find.
Globular clusters, some of you may know in astronomy
we have something known as globular clusters.
These are dense balls of stars, super dense,
and thought to have formed early on
in the history of the Milky Way galaxy.
Now you can measure the motions of all those stars
across the sky and you can see evidence that they rotate.
Okay, so you can actually see that ball of stars
isn't just sort of randomly,
the stars aren't randomly swinging around.
There actually is a net sense of rotation for the stars
in the globular cluster which gives you some sense
of how that cluster might have formed in the first place.
The kinematics of stars in the galaxy, this is an attempt
to look at just what are the stars doing
in terms of their vertical motion.
And this is, actually, this was done over a weekend
by a graduate student in Italy.
She just took the data and said, I wonder what I can do
with this, and out popped the paper the next week.
And so this is an attempt to basically look
at the distribution of young stars in the Milky Way.
The sun is in the center of the picture,
and the galactic center is at zero-zero.
But it shows, the top two panels show the density
of young stars, like the O and B stars,
and then the density of the old stars,
these are red giant stars.
And then the bottom plot shows
the vertical motions of those stars.
And what you can see is that, as you go away
from the center of the galaxy, which is to the right
on these plots, that the color becomes increasingly red,
and that means the stars are getting a net vertical velocity,
meaning stars are not just orbiting in these nice, flat orbits.
There seems to be this vertical motion,
and that is actually correlated to the fact
that we know the Milky Way galaxy is warped.
In this direction that they're seeing
that the stars are moving upward, we actually know
that the mid-plane of the galaxy warps up.
So we're going to get a sense of why does the galaxy warp
and how does the galaxy warp by studying this data.
And this is probably to me the most interesting
and surprising result.
And this has been found by two groups.
They found evidence that there are stars spread out
through the inner part of the Milky Way
whose orbits are all off.
That is, they actually are not rotating
in the same sense as all the other stars in the Milky Way.
They're actually going slightly the opposite way.
Slightly what's called retrograde.
And so the plot here, the color, the orangish color
shows the density of those stars plotted on the sky.
And then the blue stars show particularly useful stars,
what are called (mumbling) stars,
that are part of this weird kinematic motion.
But what they think is happening here
is they're seeing a collection of stars by looking
at their difference in these stars orbits compared
to all the other ones, that we're seeing the evidence
of a galaxy that was swallowed by the Milky Way
some billions of years ago.
A galaxy roughly the mass of the small Magellanic cloud
rotating a slightly retrograde to the Milky Way's motion,
and some of the globular clusters, which are indicated
with numbers, may actually be part of this swallowed galaxy.
When this object was first found it was called The Blob.
Then another group found it and called The Sausage.
And so now the first group has decided they wanted
to call this Gaia-Enceladus, that this was the name
for the galaxy that the Milky Way swallowed.
And in case you're wondering where that comes from,
Enceladus was one of the offsprings in Greek mythology
of Gaia, the Earth, and Uranus, the sky, okay?
So Gaia-Enceladus.
But that seems to be the biggest, in my opinion,
the biggest discovery so far.
But another neat one shows the motions of,
the actual motions of stars in the Andromeda galaxy,
M31, and in M33.
So you can actually measure the motions
of stars across the sky.
The blue lines here are the model predictions,
and the black lines are what they're measuring with the data.
So you can see the agreement is not perfect
and there's some areas where you think, well, I'm not sure,
but the data will only get better with time,
and we'll have more and more reliable maps
of how rotation is actually occurring
in other distant galaxies.
And then, finally, the spiral structure of the Milky Way.
One of the biggest things I wanted to do with this data
I knew is I wanted test Morgan's maps of the spiral arms.
I was ready to jump on the O and B stars
and measure their distances precisely
and make sure Morgan was right back in 1951.
As it turns out, Gaia has a problem in this area.
The very brightest stars have some problems
in the measurements of the parallax.
They're hoping to correct it for the next data release.
But it means that the bright OB stars are not reliable,
do not have reliable distances right now.
And so there was a group here that started off
with about 5,772 stars.
Those are the red dots.
And they were comparing to the models of spiral arms,
the Sagittarius, the local arm, and the Perseus arm
that I showed you in the beginning.
Although the data points shown here
are now maser parallaxes testing the distances of these arms.
But they wanted to do it with those stars,
but they found that if they only used the O stars
with 10% distance uncertainties, that left this--
Sorry, this plot is after they do the first cut
where they remove the most uncertain stars.
So they cut their sample down from 5700 to 2800.
But then if they only use stars with distance uncertainties
that are, I think, and they remove all the stars
that are moving fast, anomalously fast.
By the way, I mentioned the hyper velocity stars,
there are other stars that are basically called runaway stars.
They're not going to escape the galaxy,
but they're anomalously fast
and they seem to have been kicked out
of star formation regions.
So you remove those and they are left with 241 stars.
Here's their map with 241 stars.
And they say, well, you know, even those have some problems
so let's go down to another cut, and that's all we have left.
So I'm not sure we can test Morgan's picture
of spiral arms quite yet.
But hopefully by data release three we will.
So that's it. Things to remember from this talk:
the Milky Way is a strongly barred, partially mapped spiral.
It's about 10 billion years old.
I didn't mention that but it's true, roughly.
The sun formed approximately 4.5 billion years ago
and it's made 18 galactic orbits.
You know, one thing I do hope you remember
is the first evidence for spiral structure was here
from Wisconsin and using distances
to local star forming regions.
Much of the mapping of gas was done, the hydrogen
and molecular gas, but the Gaia measurements are going
to open up a new frontier to give us new tests,
new challenges, new problems, and hopefully new maps.
And then, if you are interested in downloading
that Gaia sky thing where you can fly through the Gaia data,
the modern data released in April,
I've included the link down here at the bottom
so you can download it and try to set it up on your computer
and tour the galaxy yourself.
Thank you.
(audience applauding)