Kay Kriewald: This evening's
speaker is Bob Benjamin. Bob has been with the Physics Department here at the
UW-Madison and is currently with the Physics Department at
a reintroduction UW-Whitewater, and he's going to talk to us about
to the Milky Way Galaxy. [APPLAUSE]
know if my title >> Robert Benjamin: So, I don't
is appropriate for everyone. Maybe some of you have never met
[LAUGHTER] the Milky Way Galaxy before.
In which case this will be your first introduction to the Milky Way.
But in case you've learned
something about the Milky Way in the past, or you've heard me
wanted to reintroduce you to the speak about it in the past, I
Milky Way. What we know about our own
changing, and the parts, the galaxy, what's changed, what's
by a Belgian painter, history oRene Magritte.uture of it. So I've started with a picture
were actually written on the Rene Magritte was a surrealist, and the title, the words that
painting above this apple, it says "Ceci n'est pas une pomme," which means
"This is not an apple." Which seems like a strange thing
to put right over a picture of an apple.
And of course, the thinking that
Magritte was trying to emphasize is this is, in fact, not an apple, it's a representation
of an apple.
And what he wanted to draw your attention to is not oh, what a pretty apple, but look at the
lighting, look at the shading,
look at the way the composition of this representation of an apple.
So another image that he showed was this. It says ceci n'est pas une pipe.
And he very famously said about this image, "Of course, it's not a pipe, This is not a pipe.
I love the title for this painting, the title for this you can't put tobacco in it."
painting is called "the Treachery of Images."
Images can be misleading. And that's something to bear in mind as we get to the next image
which is this. that I wanted to show you,
And actually, this is not the Milky Way. And one way you know this is not
the Milky Way is because you're not inside this picture.
If you were inside this picture, you would be located about right
here, and this is, of course, not the Milky Way.
was involved with in 2008 trying This is our best representation of the Milky Way or one that I
to synthesize the work of decades of astronomers with some new results, some very old
results, but just an attempt to put it together as one sort of
encapsulating image. Some people look at this and
say, oh, well that's what the Milky Way looks like. What I want to tell you during
definitely you can expect changes, some in the short-term future and even in the the course of this lecture is
I would hope that this is not the final picture of the Milky long-term.
Way. So before I talk about the Milky
Way itself, I'm just going to remind you what a galaxy is.
of the basic facts about that. Then I'll do a little brief overview of the Milky Way, some
And then I'll talk about the history of mapping the Milky
It's a long and tortuous tale. Way.
do. AnSo here's a set of images, ad future discoveries. And so that's what I'm going to
rogues gallery of spiral galaxies.
If you were trying to identify
which galaxy was yours and you had looked, you can tell these
These galaxies all have different structures. Although, one thing that unites them all is they are, in fact,
spiral galaxies.
Most people when they think Gabout galaxies they just think of their mass is in the form of stars.
it's a bunch of stars. But actually 9% of the mass of a
galaxy is stars.
About 1% is in the form of gas and dust in between the stars,
is this form that we call dark and the remaining 90%, roughly,
that 90% of the galaxy is in a matter, which physicists have yet to figure out what that is, and I'll explain why we think
form yet unseen shortly. But by function, the way you can
stars grouped together in large collections called galaxies with think of these is basically
compared to the vastness of intergalactic space. huge gaps between them. So the size of a galaxy is tiny
And for some reason nature has seen fit to organize our
universe in such a way that we
have these islands that we call galaxies in a sea of intergalactic space.
Understanding how these galaxies
gradually convert their gas into stars over time is really one of
this century of astrophysics. People are trying to get a sense the frontiers of this decade,
of what determines what a galaxy will look like, what determines
how it evolves. So the sun is just one of 200 to
400 billion stars in the Milky Way Galaxy.
Sorry I can't be more exact than
that. But most of them turn circular orbits around the center of the
galaxy. So the total mass in the form of
So if you added up the total stars is about 45 billion times the mass of the sun.
number of stars, we have 45 billion times more than just our single sun.
Interstellar gas, which is only 1% of the total, is still five
throughout the galaxy. billion times the mass of the sun when you add up all the gas
And actually, this gas is the source of all the new stars that not be as much as 45 but it's a lot of stuff.
also, and this is another area form in our galaxy. It's actually thought, though
So even though some of the gas of my research, that some gas continues to rain in from intergalactic space.
gets used up, there's a steady supply of new stuff.
As its name indicates, dark matter has yet to be detected, And then finally dark matter.
stars and gas around the center of galaxies. but its presence is inferred by looking at the rotation of the
Actually, this is better done in other galaxies but you can see
evidence for it in the Milky Way
too. If you look at the rotation, the
going around the center of the velocity at which things are
galaxy as a function of the distance from the center, if you tried to look at all the stars
and add up their gravitational
effects, you would get a curve that goes down like this. But, in fact, in most galaxies
you have a curve that flattens
out. Indicating if we tried to add up
the stars and all the gas, it's not enough to explain how things the gravitational effect of all
deficit, they gave it the name orbit around. And so astronomers gave this
dark matter. We assume that there's matter there but it's not emitting
light of any type that can be seen by astronomers.
modern astrophysics. So what is this dark matter is, again, one of the challenges of
It's still not solved. So another thing to know about
galaxies in particular, is galaxies in general, and spiral
they're very thin. You look at those beautiful spiral patterns but if you tip
them on their edge, they would
nearly disappear. They have an aspect ratio of a DVD.
So if you look at the thickness of a galaxy compared so its radius, it's about the same dimensions in terms of the
ratios as a DVD.
So here's an edge-on galaxy. This is NGC 4565 and you can see
It's not quite tipped exactly edge on. it's fairly thin.
galaxy pokes above and below, but most of the rest of the Yogalaxy is actually veryer part, thincredibly flat.of this
So that's really important
because when we live in the Milky Way, some of you have been at a dark site and seen the
Milky Way crossing across the sky.
Going out in July or August in a very dark site, just look to the
south and you'll see a band arcing from the south above you.
Around midnight in July is about the best time to look.
And you'll see this band of light sort of diffused light, in
addition to all the stars in the night sky, and that band is the
Milky Way.
And what you're seeing there is the fact that we are in a galaxy but we're right in the middle of
it, and so when we look along the plane of the galaxy we see
the Milky Way edge on. So essentially the Milky Way is
an edge-on galaxy because we're tipped on the side. So this is a picture, an optical
minus 30 degrees, 60 degrees, of picture of the inner, plus or
sky. So if you were to just take your
this, that would be about 60 degrees from this direction hands and hold them out like
to this direction. So if you were to just look at
the sky and hold your hands and look at a strip, this is what you would see if you had really good eyes.
This is a photographic image in different colors, so it's better
than what you could do with your eyes.
But you'll notice there's all sorts of things.
If you're an amateur astronomer, you'll actually recognize things
in this.
Telescope picture. This over here is the Eagle Nebula which actually has a very, very famous Hubble Space
This thing right above the center is actually called the Pipe Nebula, and it's a dark
cloud.
It's actually a very dense cloud of gas nearby.
galaxy from edge-on perspective. And so we view the Milky Way
We're in the middle of it. We're looking along the galaxy
And this means we can't see reliably across the galaxy using rather than from face down.
optical light. This cloud, for instance, is
only about 300 light years away so it blocks out everything beyond that distance. And 300 light years, as you'll
find out, is nothing. So, of course, what you do to get your bearings in the picture
draw in constellation boundaries. So I've drawn the of the constellations.ple will
Here's the constellation Scorpio
and Ophiuchus and Scutum and Sagittarius.
more regular grid, so we use a And an astronomer would prefer a
the galactic center. rectangular grid. So this is the galactic plane and these are angles away from
would be great. So the image I showed you, as I said, comes from the inner 30 Ifdegrees of the Milky Way.at
And so the inner 30 degrees on
this artist's picture would be this section of the galaxy here.
shown you the night sky, if we So the galaxy, now that I've
distance of about 28,000 light try to go to this face-on view, which no one could ever truly have, the sun is located here, a
years from the center of the galaxy. The sun is one of
200 to 400 billion stars. It goes around the center of the
And since the sun is four billion years old, galaxy every 250 million years, approximately.
the galaxy since it was first formed. So we've had 16 galactic years, if you want to think of it that
way.
And so this is what one would see in this section of the sky right along the plane of the
galaxy. This is our view in the optical.
If you want to blow this picture up, and here I'm just giving you
a few basic parameters about our
galaxy, our Milky Way is what we call a barred spiral galaxy.
within 14,000 light years of the And so as it turns out the stars
They actually orbit in long skinny ellipses for reasons that here, they don't orbit in circles.
How do you form, how to you get a whole bunch of stars to all simultaneously start to change astronomers.
long skinny ellipses? But when you have many, many their orbits to produce these
of stars that produces a stars doing that, what you have is you produce an over-density
diagonal along the ellipse and so you get a barred galaxy. So the Milky Way, it turns out,
depending on what wavelength of light you use, somewhere between
30% and 60% of galaxies have bars, these long sort of groups
of stars crossing through the center of the galaxy. So there was actually indirect
1950s, but it wasn't until evidence for this back in the
people could actually see evidence for the stars using infrared observations in 1991
that people said, okay, yeah, the Milky Way is definitely a barred galaxy.
evidence. Here's the night sky again as seen in the optical in the inner And here's actually some of that
And I can actually over-plot. These color are basically lines of constant infrared brightness. part of our Milky Way galaxy.
there's lots of dust. It's all sort of patchy. So you can see if I go back,
also a bright region here, so it's hard to really tell where It does seem like there's a bright region here, but there's
there really are the stars because of all that dust in the
way.
But in the infrared, each one of these contours is a factor of two fainter than the next one.
So basically from here to here
is a factor of two, then a factor of two and so on.
most of the stars in the center of the galaxy are, in fact, here. That there's this concentration
of stars in this area, and that is what is referred to as the galactic bar. Any questions at this point?
I'm happy to entertain questions
if people want to jump in. And so one of the things we've
Telescope is we've surveyed this, again in the infrared, but done with the Spitzer Space
So this red boundary indicates even higher resolution, meaning crisper, sharper images.
the section of the galaxy that we've re-observed with the
And I'll talk about that later. So the galaxy is barred. Spitzer Space Telescope.
I said it's like a CD. As it turns out the galaxy is also warped.
For those of you who remember vinyl records, since they warp a lot easier, leave a record out
in the car in the sun and it can get warped.
It turns out the Milky Way does, in fact, have a warp.
If you get far enough out from
the center, if you get out to a distance of about 42,000 light years, which puts you about here
from the center of the galaxy,
defined by the center of the you can actually see that the gas starts to warp up or warp down below sort of the midplane
Milky Way Galaxy. So this is actually going to show, I'm going to show a little
movie here. You're going to see a little
image of the galaxy so you get a region here of the sky and it's going to rotate around this
sense of where you're looking, and you're going to see a layer,
gas, the thickness and the the estimates of the layer of
version and a red version based on different estimates, but So you're going to see if the's two different versions, a white
this line would just lie right galaxy were completely flat,
here in the midplane. And then as we go around the Milky Way, you'll see the lines
fluctuate.
So now we're scanning across the galaxy. And you'll see that the layer of
gas is actually going up above
And then as we move towards looking at exactly the opposite Sdirection of the galaxy, we'll of the picture in this section.
technique, I'll explain why shortly. have problems using this
other side, you'll see that this So we lose it for a little bit, but then as we come out the
layer starts to warp down a little bit, especially in this region.
And it doesn't warp down on this
side as much as it warps up on the other side. So the warp is very asymmetric.
And again, I'm sorry to say,
just like the bar, the origin of the warp is still debated.
other galaxies to have warp is What causes the Milky Way and
still not clear. But the best guess is that it has something to do with the
fact that the Milky Way suffers bombardment.
Other small galaxies come near the Milky Way, and that tends to
sort of unbalance the outer parts of the galaxy and they
sort of flop up and down sort of like a sail.
And I have seen some really nifty simulations of this process.
of the Milky Way. If you remember nothing else, I hope you'll remember the Milky So, that's sort of the overview
galaxy. Okay? And so let's talk now about some Way is a barred, warped, spiral
to map it because as it turns out that spiral thing, that's sort of uncertain. of the history of the attempts
it's a spiral galaxy because all Not that it's uncertain that
flat galaxies are spirals, we observed that in other galaxies, but to map out the spiral
structure has been a challenge. And it's sort of bedeviled
astronomers for decades. So the early attempts to map out
the galaxy, before we had the opportunity of seeing the gas,
a technique that I'm using now. But it works a lot better when was done with counting stars, which actually, oddly enough, is
list at the end of this talk if anyone is sort of interested. But you can read about the story Actually, I'll have a reading
Herschel. William Herschel was better known for discovering Uranus, the planet Uranus, which made
him the preeminent astronomer of
all times. He was the first sort of person to discover a planet in our
solar system since the ancients.
And so what he did is he would climb up a ladder, he developed a telescope, he'd look through
the eye piece and he'd count of number of stars in a certain
direction. He'd call out to his sister
Caroline. She would mark it down carefully
in the notebook. Then William would reset the
telescope, point it another direction, count the stars, Caroline would record it, and then the two of them together
tried to create a picture of the
numbers of stars as a function of direction to get a sense of
the universe was because at that what, at the time, they thought
point they didn't know about the existence of other galaxies.
stars. All you could see were the
So this was the picture that actually in their 1765 paper
The galaxy was a big splotch they came out with.
where the sun was located here. So there were certain directions where you saw a lot more stars
and other directions where you
didn't see so many stars. And as it turns out, we now know that was entirely due to the
dust. William and Caroline Herschel did not know that there was dust
Now Jacobus Kapteyn, who was a in space blocking our view.
Dutch astronomer, did this in the early 20th century, it's sort of a sadder story because
he, in fact, new about dust, but
when he did a very similar project as William and Caroline Herschel except rather than
counting stars by eye he got
photographic plates, the new technology of the day.
place to observe. So he would have his By the way, Leiden is a terrible
photographic plates shipped from nicer places with better
telescopes. And then he would just sit there
And he tried to work out the and count the stars, he and his assistants.
shape of the galaxy. And, again, he got sort of an oval shape in 1922, but it turns
out it was again the wrong
there was dust out there, he picture because he didn't correct for the dust. Even though he was aware that
didn't try to correct his data for dust distinction, the fact
Although, as it turns out, doing that dust obscures your view.
So the first discovery of spiral that correction would have been nearly impossible with the data at the time.
structure, or the claims of spiral structures in the Milky Way, was actually a Wisconsin
discovery.
I just want to point that out. Of course, it was made by a professor from the University of
Chicago, but he was using Yerkes
Telescope here in Wisconsin in Williams Bay. And what he did is he measured
the distance, he didn't count all stars, he just focused on these very bright, hot stars, in
he estimated the distance of astronomy we call O stars, and
So here's a picture of Yerkes thObservatory in the winter.n map and he found that his O stars lay in three bands.
That's fun science in Wisconsin. And when Morgan, basically what
Morgan did is he used this
telescope in Yerkes, the refractor to estimate the distances so these hot stars.
Now, I don't actually have the
original data, though I'm looking to get that, but this is a blow up of the small section
of the galaxy where the sun is, what he found is there's three
groups, the stars lay in three bands which he named the
Perseus, Orion and Sagittarius
arms. The moment he discovered this, and he presented it in the 1951
meeting of the America
Astronomical Society, he was a hero. It was the very first evidence
that anyone had ever had for a
spiral structure in the Milky Way. We knew the Milky Way was a
spiral galaxy, but he had found the very first evidence for it. So that's why he got a standing ovation.
He actually shortly thereafter
had a nervous break down, and it took him years to actually
presented in Cleveland. But any rate, that was a start. And it turns out he scooped a group that was about to map the
Milky Way using a completely
different technique. And that technique and that was group was based in the
Netherlands.
During World War II, Jan Oort, a very famous Dutch astronomer, he's responsible for many
important discoveries in
astronomy, but one of the things he's known for in particular is the pioneer of what we call
21-centimeter radio astronomy.
A student of Oort's realized that, I said that there's all this gas between the stars and
mostly it's hydrogen gas. And hydrogen being the most common element in the universe
atoms. by 90% of the atoms are hydrogen
And it turns out that hydrogen atoms emit radio waves. There's a particular, what
happens is a hydrogen atom
consists of a proton and an electron, and the electron actually can go from one state
to a different state, we call it
the spin states of hydrogen, so it can go from spinning this way to flipping over and spinning
this way, and when it does that it loses a little bit of energy and emits it as light, and that light has a particular
wavelength, a particular
frequency, and the wavelength is 21 centimeters which put it into
So it's a radio emission. And because it's a particular line, you can actually measure radio waves.
Many of you know that if the Doppler shift.
something comes towards you, like an ambulance, the pitch is shifted to a higher pitch, and
if it goes away, it drops to a
lower pitch. That's because the wavelength of
the source comes towards you or goes away from you. The same is true with the astronomical sources.g as
We have red shifts where the
And we have blue shifts when an object is coming towards you and and the wavelengths are shifted to longer wavelengths.
the wavelength is shifted to a shorter wavelength.
And by looking at the wavelength
that you actually observe in space, if it's not exactly
coming towards you or gas coming 21 centimeters, it's either gas
you could look at the hydrogen away. And so Oort had the idea that since you could have this line,
emissions throughout the galaxy, measure the Doppler shifts and
velocity to estimate the use the Doppler shift, the
distance to the gas. I'll explain how that works in a
So they originally, to do this second.
project, they actually recommissioned a German radar dish that had been left in the
Netherlands.
So eventually Oort immediately started work on building his own
out they thought, well, maybe we radio telescope, but it turned
can use the Wurzburg dish. So they repurposed this radar dish and started to use it to
search for 21-centimeter radio
emission from space. And, in fact, it's one of the things I've learned over the
past year because I've been trying to educate myself more about the history of this field,
galactic structure were all in most of the major discoveries in
place by 1958 with this technique. There was still a lot of
fighting to come, but what we learned about mapping, all of the hints of galactic structure that were later found out were
in place even before 1960.
So, as I said, basically these works traced the interstellar gas, not the stars, but they
didn't suffer from dust distinction.
And the flaw is related to the They did, however, have, as I mentioned, one fatal flaw.
following. I'm not going to go through the vector diagram, but basically
here's the artist picture of the
Doppler shift corresponding to Milky Way, and what these lines show is at a given place in the galaxy we would observe a
the numbers, these are contours. So as an example, actually, so
rotation of the sun combined basically at any given point the
with the rotation of the gas means that the gas is either
or away from you with some tends to be moving towards you
speed. And by assuming that the gas
everything is assuming rotates in a circular motion, so
everything rotates in a circle, you can make an estimate you can convert velocity to distance
with one major problem. And that major problem is if you
look in this picture and imagine you look in a certain direction.
So let's say that we look in
this direction. 30 degrees to the right of galactic center.
So again, if you're starting in the center of the galaxy, let's look 30 degrees.
would be at minus 20, gas here And we will see that gas here
This is all do to the relative would be at minus 40 kilometers a second, minus 60 kilometers a second.
motion of the gas compared to the sun. But if we look in a given
direction, we see a problem.
And the problem is the following.
appear to be rotating at minus Here, this point, gas would
40 kilometers a second. That is, it would be coming at you at 40 kilometers a second.
But, along the same direction,
at this point gas here also would appear to be coming at you at 40 kilometers a second.
That is, blue shifted at 40 kilometers a second.
technique. So we had a fundamental unresolvable problem using this
And the problem was is that the only thing we can measure is the Doppler shift, the velocity.
But if you want to convert the
velocity into distance, you have a problem here. And the problem is that one
velocity could be at two
different places. And without any additional information, you don't know when
you see an object or some gas moving at one velocity, you don't know where to put it. It sounds like a detail but it
was absolutely, it was a show stopper.
convert your velocity to And the reason is because you had to make assumptions to
I'm sure some of you have already picked up on it. Now actually there's some other problems with this too.
into nice circles. The assumption that went into this was that everything rotates
If that's not the case, you also have problems.
directions you may notice where if you look you can't use this And of course, there's certain
method at all. Like if you look at directly
towards galactic center, the sun
is moving this way, everything else is moving this way, so you are neither moving towards the
material or away from it. You're basically moving in
parallel tracks. So everything along the line
connecting us to the center of
material. theSo you can start to get aler picture of where things might be So there's no way to place that
galaxy but in other sections of the galaxy you are out of luck. Which didn't mean that people didn't try for the next 30
years.
So center part of the Milky Way, basically notice that this only affects in this circle where you
have this double value problem, and that circle is defined by the location of the sun.
orbit inward becomes hard to So everything from the sun's
map. Bummer. On the other hand, anything
outside the sun's orbit, you can
look in a certain direction and you can say, okay, say here or here or here there's a nice
mapping, a one to one mapping of
the velocity, the Doppler shift, to the distance.
the Milky Way with some confidence without having to So you can map the outer part of
make lot of assumptions. So that's what the regular
astronomers did.
They were happy. They mapped the outer part of the galaxy first. They got some results.
Decades later people had forgotten some of their results and were still rediscovering
them. But in the inner part it became a slug fest because astronomers had to debate each other on the
quality of deciding between near
and far among other things. And it all basically boiled over in 1985.
In 1985, this is actually, I
Harvey Liszt showed this figure din his talk, everyone threw up paper really was at the moment. I don't know that the moment
going to work on this problem anymore. their hands, stormed out of the meeting and said no, we're not
But in retrospect, it was sort of the moment where people said,
you know what, we're just not getting anywhere here.
a spiral structure of the Milky Way. This actually shows several different astronomers' models of
And the amazing thing about this picture, so Harvey basically
collected all of these pictures from people's different papers, tried to put them on the same scale so that you could get a
sense of how easy it was to compare them, and all the models
In other words, all of this group, except this one are based on the same data, okay?
and this group, all were and this group, and this group, and this group, and this group,
looking at the same data and trying to convert it into a map of the Milky Way.
And so you can see. That's a really frustrating moment. And in astronomy when you read
the textbooks you always read about the victories, you very rarely read about the failures
and we just moved on. [LAUGHTER] like this really sucked,
And what made it worse was the But this is one of the failures of galactic structure.
personalities. This was the frontier and there were some bitter, angry
arguments over this.
I'm told people would yell at each other and call each other names at the meetings.
And so eventually people just gave up.
me and called me names, especially when it would be hard I would never want to go to a meeting where people yelled at
to actually uniquely convert the data.
So we sort of limped along from 1985 to 2003. People collected more information.
They would try to ask that we find more and more regions where
stars are forming. We get their velocities, we try
to plot them on to a map using
this kinematic distance technique, but if I were to take away these lines, I think you
would have a hard time placing
lines on to this picture. The lines are really guiding your eye.
So take away this line and would
you really put an arm here? So this is sort of an attempt to compare the distribution of
star-forming regions throughout the Milky Way with this picture.
By the way, this is an interesting sociological
problem.
Is a bad map worse than no map at all? [LAUGHTER]
Which would you rather be,
trapped in a forest with no map or trapped in the forest with a really cruddy map?
Well, I guess with a really cruddy map because what happened
is over time for some reason people gravitated to this one
picture because it was the one
estimate the distance to picture that was not based on the neutral hydrogen gas. It was based on trying to
star-forming regions. So people thought, well, maybe that's a better one to work with
although there was no real good justification for that, but it's
what we did. And you'll notice in the
Artist's Conception that those
arms are still here. So this should not give you a warm feeling about this picture.
[LAUGHTER]
Essentially, this is the best attempt. And this is the artist's
conception again, done by Robert Hurt.
And Tom Dame, who I will mention in a second, said, okay, in this
picture I have shaded in color
things where I think I can point at data and arguably make the case that this data should be
identified as that arm. Now, the conversion of the arm
to an actual position on the map, that is very problematic.
But there are patterns in the data. It's that last step of
into a face-on the map that has stopped astronomers. converting the Doppler shifts
So Tom basically did this, and if you divide this arm and this
image, you can actually divide
it into two zones. Everything sort of in here was
Ask actually, really, I think, discovered before 1970.
before 1966. This last one might have been a late comer.
So that is not to say that, so
there are patterns in data that you can make an argument that we saw in the data then, we see in
the data now, we think they're
spiral arms, we're not quite sure how to map them, but if we were to map them, this is
approximately where they would
First of all, there should be something fishy about this. Now that I'm sort of pulling. hFirst of all, notice anything.
that doesn't have color is
imaginary. It was filled in by the artist. Except, as it turns out, this
image was made in 2008. In 2004, this segment of arm was
found by Naomi McClure-Griffiths in Australia.
She found it in H1. We have yet to see it in other
things that tend to trace spiral arms like molecular gas,
molecules in the bright star-forming regions. But I look at the data and
in the data. I say, okay, I see something
So it's probably a spiral arm, we think. So let's go ahead and try to put
it on to an image.
And then you'll notice at this point that there's something
have been discovered. Here's a line through the center interesting about things that
of the galaxy. Everything that we found is on
the near side.
actually mapped, or at least Essentially, there's this daunting realization on astronomers that we've only
attempted to map, half the galaxy. So there's the other half, the
half that's beyond galactic center. Now this thing is shaded but it was actually discovered in 2008
just in time to be put on the artist's conception by about two and a half weeks.
discovered this, Tom Dame and Pat Thaddeus, they contacted us because they knew we were putting together the artist's
conception and all the speakers at this meeting had been asked
image coming along? And he says, well, because I to contribute, and Tom Dame said, by the way, how is that
have something new to add to it. The interesting thing about this
discovery is it's exactly symmetric to this thing.
This was discovered in 1957. This was discovered in 2008.
In terms of the properties of the spiral arms, how much gas do
they contain, they're almost identical.
And the reason this was not discovered before is because
And it fell below the sensitivity of many surveys of it's further away, it's smaller and it's fainter.
here was discovered in 2011. the Milky Way Galaxy. And then finally, this arm right
And this is sort of an interesting case because you'll notice that the artist's picture
was made in 2008 and this was discovered in 2011. And it turns out if you actually
on to this galaxy, this is try to take the data and map it
approximately where it goes. So this is a case where the art seemed to precede the science.
And it may indicate something profound about the galaxy.
down and think, okay, this There's always some fighting to do, but if astronomers settle
really is a spiral arm, Tom Dame and Pat Thaddeus, once again
hydrogen gas, they actually found some molecular clouds along it as well, which is they found it in neutral
arm in the Milky Way, I think. almost a lock for being a spiral
But the interesting thing about it is it turns out that if you say why did the artist do that?
Why did the artist put something
in there? All they did is they took this side of the galaxy and flipped
it over.
So what it seems to indicate is there's some hope, and, by the way, spiral structure is a field
of broken dreams. So maybe 5 to 10 years from now I give this talk and I say, well, here's where we were
wrong.
things we have on the near side, But there is some hope now that the Milky Way has, basically, some underlying symmetry that
real segment, this is still wimaginary, but there are three And there's more to test. So for instance, if this is a
groups now searching to see if they can see evidence.
It would be hard because of the Doppler shifts that you would expect, but they're looking for evidence that maybe we can fill
this arm in and connect it up with the arm that we believe is on the near side of the galaxy. Now, I know that this is a
figure that would challenge many
astronomers, but what this shows, this is two visions of that arm in terms of data.
One thing it shows is the arm as
a function of the longitude. So from 0 degrees to 70 degrees,
And one of the things you notice so this section.
notice that this arm actually iarcs upward as a function of you say where is it above and below the midplane, you'll
longitude. What that means in terms of this
picture is that this arm is actually coming out of the
picture. It's actually coming towards you
a little bit which is associated
This arm is actually in the warped part of the Milky Way Remember I said the galaxy is warped?
was missed in the passed is it galaxy. So that's part of the reason it
warps above the plane in the galaxy. And then this shows the
direction you're looking
pattern here. And the yellow dots are where Dame and Thaddeus have actually Anddetected molecular clouds.ice
Each molecular cloud, by
the galaxy there has to be a definition, is what astronomers call a giant molecular cloud. To detect it on the far side of
large concentration of molecules.
The Milky Way has new arms. I saw one of the headlines said Milky Way grows a new arm, So, big surprise.
So this arm may have grown all or grows an arm.
the way out to here if we can start to fill in this center part.
And so that's the hypothesis and
we'll see where it goes. Another thing we're finding, and
GLIMPSE data, is that this is data from the Spitzer Space Telescope, and we have in our Spitzer Space Telescope, and you
can go look on the wall on the
thing, you're seeing a way out there and see some of our data. If you find a tiny little fuzzy
star-forming region. And it turns out all of the tiny
uncatalogued. So Tom Bania, of Boston University, and Loren Anderson, his graduate student who's now
at Marseille, they went to these
objects, they used radio telescopes to try to measure
they put their, they used the Doppler shift once again as best their Doppler shift, and then
circle, you have the problem of not knowing whether to put it they couldnear or far.distance. Interior to circle, this solar
try to estimate whether an Although, they found a way to
object is near to the sun, I apologize by the way, the sun is
So I flipped the galaxy over. There's a debate among us astronomers as to whether the nsun is properly put at the top
of the picture or at the bottom of the picture. Maybe after the end of this talk
But these H2 regions, these objects are actually way far we can all vote and see.
away from the sun, and, in fact, if you look at their positions,
they actually are all above the plane.
same section where Tom Dame and Pat Thaddeus discovered their new segment of spiral arm, the this section of the galaxy, the
star formation regions are galaxy is warping up and the
warping above the plane to the galaxy. So we're following up on this
discovery with the Spitzer Space
Telescope. The red shows, this is one of those "doh" moments because we
actually, with the Spitzer Space
So this is actually showing 360 degrees of sky and the red band shows the section that we observed.
But we've neglected to take into
account that when you get to-- we thought we were seeing,
galaxy is further away and will appear to be smaller an angle, because the distant part of the
we thought we were basically getting the whole galaxy in one
shot. We neglected to take in account the fact that the galaxy curves up.
And so these green areas are
areas that we've been approved to now resurvey to try to find
far side of the Milky Way. the star formation on the
little bit of this, but I do So this is an ongoing project that we're now doing. So I'm actually going to skip a
is that you can see through the want to say tdust.nefits of the So the point is that radio waves lately an infrared astronomer,
On a foggy day your radio reception doesn't get worse. pass through dust.
And on a foggy day you can still see the sun, although the sun
tends to look a lot redder. And the reason sunsets, the sky is blue and sunsets are red is
effect of dust and also molecules at some level scattering light. aSo when the sun is on thee
horizon or gets close to the
scattered away but the red light horizon, you're looking through a lot more air. The blue light tends to get
actually penetrates through. And so red light is absorbed
blue light. Infrared is absorbed even less less and scattered less than
is also sometimes called heat radiation, but that radiation basically just goes right So it tuthrough dust.red light
Firefighters actually use infrared goggles or infrared
vision to see through smoke. And so that infrared light
penetrates through. And infrared light also tracks
were to shut off all the lights here, your body would glow at thabout 10 microns,m. So10 micrometers, which is notwe
visible unless you have thermal goggles.
So if you can detect the But you're glowing as bright as about a 100-watt light bulb.
infrared, you can see it. So the Spitzer Space Telescope is an infrared telescope, and
this is just a fun picture, a fun NASA movie. It actually was put in orbit
So it was kicked out of Earth's orbit and in 2003, launched in around the sun.
August 2003, and so it's actually trailing behind the
Earth, getting further and
detectors, but other detectors, including the one that we've In 2009, I guess, it ran out of coolants to operate some of the
Ultimately, it will get far enough away from the Earth that bit's too difficult to keep upe radio contact to transmit the
data, and at that point our mission is over.
But then what it does is it will observe a section of the sky.
This is supposed to be light
coming in, not light going out. And then it aims its transmitter back to the Earth, transmits the
data back to Earth, to NASA's Deep Space Network, gets a few
We've been keeping this telescope very busy because we've been taking snapshot after observing again.
sky. snapshot after snapshot of the
Very fast cycling. And one of the things we've been
Just like William and Caroline able to do is count stars.
galactic plane in the mid-infrared in the optical. We actually, this is a picture, a section of the sky in the
I never get tired of showing this image.
I know some of you have seen
or so of observing with an this before but to me it's just so wonderful. This is actually about an hour
optical telescope down at -- done with some students that I had.
And so now let me show the exact
same section of the sky in longer wavelengths of light.
So it's just beyond the range of What's called the near infrared.
human vision. So now let me run this back. Pick one of the bright stars
here that you see, particularly some of these redder bright stars, and let's run it backwards.
And it should have gone away. So stars that were invisible in the optical start to become visible in the near infrared.
And the wavelengths we're using are even further into the infrared. So now go ahead and pick some
faint red star, and my favorite one, let's see, it's a little hard to see.
let's see what the sky looks Just pick a star, any star, and
like in the mid-infrared wavelength. So about the same wavelengths
where your bodies are glowing the brightest.
Pick one of the bright stars Now, I'd like to run it backwards.
that you see here. These are among the more
Galaxy. And now let's see what your star luminous stars in the Milky Way
Okay? So what the mid-infrared has does if we go back to the optical view of it.
allowed us to see is most of the stars in the galaxy.
And what I am now doing and continue to do and have been
the stars as a function of doing for years now is dividing the sky up into bins, counting
plot in detail. A friend of mine actually says And I don't expect you to understand or to explain this
[LAUGHTER] it reminds him of a scan of his thyroid at the doctor.
But what this shows is, this the axis that tells you the direction you're looking,
and this is the axis that tells you the brightness of stars that
you're seeing. And the color basically tells
you whether or not you're getting an excess bump of stars.
And in particular, you'll notice there's a big blue band going
through this image and this is
sort of our newest discovery with this technique.
the Milky Way, the disk of This is actually evidence that
stars, you go out to a certain distance away from the galaxy,
comes to a fairly abrupt end. Effectively, an edge to the galaxy. tAlthough the analysis of thisy
was complicated by the fact that
the galaxy is also warped. So sometimes you don't whether
or that your line of sight just left a curved galaxy. But at any rate, this is you're seeing the end of stars
simulation, a model I made. This is what that diagram would look like if there were no actually not data, this is a
sort of gradually died off as a function of distance from the center of the galaxy. And then this plot is what it
would look like if you had some place at a certain distance out in the galaxy, in this case about 40,000 light years, where
you just ran out of stars.
And, in fact, this doesn't look, this looks fairly similar to the
So the galaxy is filled with data.
surprises. It's the only spiral galaxy where we can make out a wide
variety of details. We can see the individual stars.
We can look at how stars are forming.
We can look at the vertical thickness of the bar and the
spiral arms. We can't do that in any other
galaxy. The Milky Way is unique in that regard. And for many years the attempts
to work out this big picture
have been stymied. First by dust and then by the Doppler shift problem for the
gas.
But we're starting to make progress again thanks to
through dust and for people basically looking at the older infrared telescopes that can see
Thaddeus results of the spiral radio data with new understanding like the Dame and
arm on the far side of the galaxy. So this is the picture in 2008.
And this is the picture in 2015.
You might wonder why there's a Tyrannosaurus Rex here, and I have to thank Tom Dame for
making this analogy.
There's a question really how to think about this artist picture of the Milky Way.
It has had an influence among astronomers.
But on the other hand, we know it's partially made up and in
many ways suspect.
So Tom Dame said the way you should think about this is
in the same way that this is it's a model of the Milky Way
a model of the Tyrannosaurus Rex And you look at a model of the Milky Way
and you think, wow, isn't it
amazing that astronomers can figure that out.
that. The same way, you look at the Tyrannosaurus Rex and you think It'sisn't it amazing thatn do
paleontologists can look at the bones and they can figure this
out. And you'll say is that what
you found, when you dug it up,
did it look like that? And they'll say, well, no.
What did you find? And you'll say, really?
And they'll say, well, I found an arm. And they'll say, well, I assumed
the dinosaur was symmetric, so I added the other arm. There's a lot of guess work and reconstruction.
Just like you can only
understand a Tyrannosaurus Rex in pieces and put it together,
galaxy in pieces and put it together. And the data can change, the we're trying to understand the
could be changes ahead. I expect and I hope there will synthesis can change, so there
be. [APPLAUSE]