It's really a pleasure for me to be participating in Cal Bay.
For many years I've come to Cal Bay and heard a great diversity of talks, ranging from,
I remember how insects fly, a wonderful talk by Professor Dickinson here some years ago
who did very high speed movies, motion of insect wings, I heard a wonderful talk on
how children's literature is illustrated by a real illustrator, a real artist, and just
a huge variety of things here.
I think it's really a wonderful event.
So thank you all for turning out today.
I have devoted the last 25 years of my life to answering a single question.
Talk about OCD, something like that.
And that is, why are there stars?
How do stars form?
It's one of the deepest mysteries we have in science.
We see stars all around us.
It turns out, remarkably enough, that stars did not form just close to the Big Bang billions
of years ago.
Stars are forming all the time, all around us.
And that's how the subject holds such fascination for me, because we're not just picking through
the archeological debris of events that happened eons ago.
That's very difficult to do.
We see this happening, yes, stars did form eons ago, but they're still forming now,
which makes it a really extraordinarily fascinating topic, I think.
Star formation is one of the three big origins questions that is exciting astronomers today.
And I've listed them here from top to bottom in sort of size scale.
The largest origins question of all concerns the entire universe.
How did the universe come to be?
That's the whole question of the Big Bang and the aftermath of the Big Bang 14 billion
years ago.
And that's the branch of astronomy called cosmology.
In the middle and sort of size scale is my topic, the formation of stars.
And then on the smallest scale, the third origins question is the origin of planets.
Not only the planets in our own solar system, people have pondered that for many years,
but that question has now received new urgency and much more information, of course, because
of the hundreds of planets that have been discovered just within the last decade, circling
other stars.
So star formation, the question of how stars form, doesn't just occupy the middle in this
list in terms of size scale, it also occupies a central position intellectually.
It informs these other two origins issues, and let me illustrate that for you.
The people who are concerned with the Big Bang, with the origin of the universe and its aftermath,
the holy grail for these cosmologists is to create galaxies.
How do galaxies form?
Galaxies are enormous systems of stars scattered throughout the universe.
We live in a galaxy, of course, our own Milky Way that contains something like a hundred
billion stars in it.
Those are typical numbers for all galaxies, and they want to find, they want to tell us
ultimately how galaxies are formed.
But galaxies are composed, at least the visible parts of galaxies, are composed entirely of
stars.
So there's no chance of answering that question of how galaxies form, at least in any detail,
until we can learn much more about how individual stars form.
And then on the small end of things, there is the question of how planets form.
But the formation of a planet is intimately related to the formation of stars, because
planets form out of discs, rotating discs like CDs or records that rotate around a star.
And when a star forms, that record, that disc of gas and interstellar dust particles, also
forms around it at exactly the same time.
And it is from those discs, those rotating discs, that planets congeal.
So again, there is no chance that we will know in detail how planets form, including
the planets of our own system, until we understand much better how stars form, because it's part
and parcel of the same process, the same process within physics and throughout the universe.
If you were to get into your comfortable spaceship and travel out, what would you see?
Let's suppose you have plenty of food to last you for this voyage, very comfortable and
a nice temperature inside your spaceship.
You can live an indefinite period of time, that's very important, you can travel these
great distances, you would see nothing for most of the time.
But then often the distance, you'd see some fuzzy speck of light, and as you accelerated
your ship close to that speck of light, it would break up into countless numbers of stars.
And what you're seeing are star formation factories known as galaxies.
Our universe contains millions and millions of galaxies, it probably contains, according
to current theories, an infinite number of galaxies.
We've photographed literally millions of them, and there are plenty more out there that
we haven't yet imaged.
Our own galaxy, the Milky Way, is something like this.
It's called a spiral galaxy, you can see the beautiful spiral pattern.
What you're seeing here is a galaxy face on.
This is a galaxy that we're looking at directly, but galaxies are actually paper thin objects.
Most of them we see more nearly edge on.
We're lucky in this case we happen to see it close to face on.
This is the nucleus of the galaxy, that's where most of the stars are concentrated,
it has the highest density of stars, but these hundred billion stars are spread out throughout
this galaxy.
And it's rotating in a counter-clockwise direction.
And as this giant pinwheel rotates, and it takes about a hundred million years to rotate
around once, these spiral arms rotate it along with it.
What are the spiral arms?
Well, the thing that lights up the spiral arms, the reason that we can see them so clearly
is that they contain O-stars.
O-stars are the brightest, the most brilliant, and the most massive stars that are created.
A typical O-star has a mass, amount of matter, somewhere between 30 and 100 times the mass
of our own sun.
And they have luminosities tens to hundreds of thousands of times the luminosity from
the sun.
And we can see O-stars clear across our own galaxy and well into other galaxies, and it
is the O-stars that light up the spiral arms of the galaxy.
Now an O-star lives for a relatively short time.
An O-star lives for just a few million years, a few million years sounds like a very long
time in human terms, but a few million years is the blink of an eye.
Remember that a galaxy takes a hundred million years just to turn around once.
So if we're seeing O-stars in galaxies that live for just a blink of an eye, those events,
the formation of O-stars must be happening all the time.
We can't be so lucky that we're seeing just this lift of an event just now.
There's nothing special about us, about our time in cosmological history.
So that's the basis of my statement that star formation is happening all the time all around
us.
And it's not only O-stars that are being formed, only a very small fraction of the stars that
are being formed are this spectacularly massive and this tremendously bright.
Most stars that are formed are more like our sun, dimmer objects of lower mass that are
pretty hard to see.
We can only see these lower mass stars in our own galaxy and in relatively nearby galaxies.
We can't see them in far away galaxies like this one, but we know that they must be there.
Because every time we see a group of stars, a young group of stars that contains O-stars,
if it's relatively nearby, we see scads of other stars of other mass.
But these bright stars that are the beacons of star formation far across the universe
are just the tip of the star formation iceberg.
There are many, many other stars of lower mass forming all along with them at the same
time.
The other thing that is helping to delineate these spiral arms are these dark curves here
that are sort of the inside track of the spiral arms.
And they're called dust lanes.
And they're called that because what they are are clumps of these interstellar dust
particles that I've mentioned to you before.
These are really tiny microscopic flecks of solid material that partially block the star
light behind them.
And that's why they show up these spiral arms so nicely.
Well is there anything interesting about dust grains?
Well there is something rather interesting about them.
We're made out of them.
Our Earth was made out of them.
All planets are made out of dust grains.
They're extremely interesting.
Not only do they block external star light, but they are what form planets.
I mentioned to you that planets can heal out of the disks that orbit around stars to be
more precise, not the gas in the disks that congeals to form planets, but the dust in
the disks that congeals to form planets.
Earth, Mars, Venus, Mercury, they're all just conglomerated dust grains.
Jupiter, Saturn, Uranus, and Neptune, they're also conglomerated dust grains, but only in
the center of those giant gas planets and then extra gas is piled on to form planets,
the full giant planets.
So in a sense, since we are formed from the Earth ultimately, we are dust grains.
So they're extremely interesting objects.
Now not all galaxies are forming stars, and not all galaxies are forming stars in the same
way that our own Milky Way and other spiral galaxies are forming stars.
About a third of galaxies are spiral galaxies, and the other two thirds fall into two categories.
And here on the left, this sort of football shaped object is called an elliptical galaxy.
Elliptical galaxies are not paper thin and wheel spirals, they're footballs.
They're full three dimensional objects made of stars held into gravitational orbit around
each other.
Elliptical galaxies are not forming stars at all.
And we can tell that because of the red color of the elliptical galaxies.
Well what does that mean?
What does the color of the galaxy have to do with whether or not it's forming stars?
Red stars are low mass stars that are dying out.
They live a very long time and they're slowly fading away.
Hot stars, massive stars that only live a short time, these O stars are very blue.
So when you see a very red galaxy, star formation is over.
Elliptical galaxies formed stars billions of years ago and are no longer forming them.
Why is it that some galaxies are these hot spiral galaxies that are forming stars today
like our own Milky Way?
And why is it that many other galaxies, at least as many, are elliptical galaxies that
are dead as far as star formation goes?
We don't know.
That's one of the big questions that cosmologists are trying to answer.
An equal number of stars are even more amazing in their star formation activity and they're
called irregulars.
And from their name, they have no particular shape.
They're just sort of blobs with tentacles sticking out at odd angles.
And they tend to be pretty small galaxies, not very luminous.
But their bright blue color tells you they have a lot of hot, young stars in them.
But they can't be forming stars at the same rate that our Milky Way is forming stars or
else they would have used up their star fuel a long time ago.
They could no longer be forming stars today, and yet they are.
So astronomers think that irregular galaxies form stars in kind of bursts with resting
periods in between.
How that comes about, why some galaxies are irregulars, others are ellipticals, and still
others are spirals that form stars at a steady rate, is not really well understood.
So let's go into our own galaxy and look at the birthplaces of star formation right here
at home within the Milky Way.
Well, first of all, where is the Milky Way?
We're inside it.
So how do we see it?
Well, the name Milky Way comes from earlier observations of the sky.
If you look out, as you all know, you see a band of thick band of stars across the sky.
And here's the northern sky during the winter.
And that thick band of stars is the Milky Way galaxy.
We don't have an external view, a view from the outside of the Milky Way.
We're inside the flat record, so we see the edge of the record looking out.
That edge is the band of stars that we call the Milky Way across the sky.
And here are some well-known constellations, Orion, Monoceros, Taurus, Perseus, so on,
Gemini, the twins, some of the brightest stars have been marked there.
But you don't see in an ordinary photograph like this, and what's difficult to discern
just with optical light are the actual birthplaces of stars, but they're right in our cosmic
backyard.
And let me show you what they look like.
Here is one in Taurus, and this lighter blue sort of tarantula-looking object there, that's
called a molecular cloud.
It's a cloud of gas, and that cloud of gas is churning out new stars like the sun all
the time.
Taurus is one of the nearest by star formation factories, and it's turning out stars that
are a lot like our own sun.
Another hot star formation region is in the constellation Orion, and if you look, there's
the belt of Orion there, there are his shoulders and so forth.
Hanging from his belt is Orion's sword, and near the tip of Orion's sword is a fuzzy patch
called the Orion Nebula.
The Orion Nebula is the most well-studied region of star formation in the solar neighborhood.
It's churning out not only stars like the sun, but also very massive O-stars, like the
ones I showed you that light up the spiral arms of other galaxies.
And also the Orion Nebula is the densest region of star formation.
It's a real hotbed of star formation activity, and I'll talk about it a bit more.
Let's look at one star formation region that's making solar-type stars, sun-like stars, and
this is the star formation region in the constellation Taurus the Bull.
So if you were to take an optical photograph of the Taurus region of the sky, what hints,
if any, would you have that it's forming stars?
Very little.
What you would see is this.
You see the background stars here, a sort of uniform gray of background stars, countless
numbers of them really, but then on top of the background stars we see these dark splotches
here.
So for a long time people didn't understand what those dark splotches represented.
Could they be holes in the background array of stars?
Well, that's not very likely, because if they were holes or tunnels, those tunnels would
have to be directed exactly to us.
That really doesn't make any sense, but people suspected for a long time, and then this was
confirmed in the 1930s, that these are not really tunnels or holes in the collection
of background stars, really they're objects that are blocking the light from the background
stars.
And by now that's familiar to you.
What's blocking the light is dust grains, and those dust grains are accompanied by gas
clouds, and those gas clouds called molecular clouds are what are actually forming stars.
So the biggest hint, and the first hint that we have, that regions like Taurus are actively
forming stars today, is this splotchy silhouetted cloud formation that we see superposed on
the background field of stars.
This optical means of looking at molecular clouds just by their silhouette, you don't
get much information that way, of course.
You have to turn to other means, other eyes, so to speak, to look at these clouds.
These clouds, although they don't emit light in the optical regime, they emit light at
radio wavelengths, molecules within the cloud are rotating around and emit radio radiation
at much longer wavelengths than we can see with our human eyes.
But with radio telescopes we can pick up that emission, and then we can study the structure
of these clouds in considerable detail.
So these contours here, this is the same region of the sky, the Taurus region, and these contours
are contours of radio intensity, showing the shape and size of these molecular clouds that
are forming stars.
We can also use regular telescopes, optical telescopes, to image these same regions, and
we see here clumps of stars.
These are young stars that are being formed even now in that region.
Some of them were formed up to a million or so years ago, and some of them are continuing
to be formed now.
And you can see they're not spread uniformly in this region, they're clumped together.
And where they're clumped together, the gas is clumped together, indicating again very
strongly that that is the gas that is the fuel.
That's the material out of which young stars are being made.
There is the cloud gas, there are young stars.
Some of the young stars are so embedded within this interstellar dust that we can't see them
with optical telescopes, and we need infrared telescopes to see them.
Because what happens is that the light from the stars gets absorbed by the dust, just
like light gets absorbed by dust here on Earth, and then the dust re-radiates that
light at longer infrared wavelengths.
So with radio telescopes and infrared telescopes, we can probe inside these clouds and detect
not only the gas that forms stars, but the most embedded stars themselves.
Star formation is part of a grand evolutionary cycle that's happening all the time in our
own galaxies and many other galaxies throughout the universe.
And this is a little sketch to show you how the cycle operates.
First we have these interstellar clouds that I've called molecular clouds.
And by the way, the reason for the name molecular clouds is that they're mostly made out of
hydrogen molecule.
Hydrogen, the first element in the periodic table, one proton and one electron, is by far
the most common element in the universe.
It was made in the Big Bang 14 billion years ago, and it's also the most common element
naturally in these clouds.
But it's not atomic hydrogen in these clouds, it's hydrogen molecule.
Two hydrogen atoms bound together, orbiting each other.
These clouds of molecular hydrogen congeal in a process that I'll talk about a little
bit more later on in the talk, and they form stars.
Stars have a finite lifetime.
They're born, the subject of my talk.
They go through middle age.
Our sun is a middle age star, it's about halfway through its life.
Our sun is about four billion years old, four and a half billion years old.
It will probably live for another four or five billion years old.
So there's no reason to worry now about the death of the sun.
When the sun dies, it will do a number, when stars die, they do a number of things.
When our own sun dies, it will balloon into something called a red giant, where its radius
is so large that it will swallow up the inner planets of the solar system.
But interestingly enough, it will also do two things.
It will slough off a wind, a gentle wind from that star will be ejected into space, and
the inner part of our sun, during these death throes, four or five billion years from now,
will shrink into a very hot object called a white dwarf.
A white dwarf is a dying star, fading out from view.
In some millions of years, it will fade away completely, and then just be a dead piece
of matter floating into space.
So it's a sink of matter.
If a star is more massive than our sun, say about five or six times the mass of the sun,
it will not be able to form a white dwarf.
When it, after it blows off its outer envelope in a much more spectacular event called a supernova,
it leaves behind a much more dense object.
A white dwarf is something that has roughly the mass of the sun, but it has the size of
the earth.
Now the sun has a million times the mass of the earth, so if you compress that into the
size of the earth, you get an object of enormous density.
But a neutron star is even more spectacular still.
It has about the mass of the sun again, but the diameter of a neutron star is roughly
the distance from here to San Francisco, just 10 miles or so.
So that's an object of such high density that the density is comparable to the density inside
the nuclei of atoms.
It's held together by the pressure of neutrons inside that star, a very spectacular way for
a star to end its life.
But from my point of view, just another dead object.
White dwarfs and neutron stars are called compact objects.
They're reservoirs of matter, they're sinks of matter, they're just pieces of matter floating
in space, nothing else much interesting happens to them.
And yet, and again, if a star is even more massive, say eight or ten times the mass of
the sun at its birth, when it dies, it cannot form a white dwarf, it cannot form a neutron
star, its gravitational pull is so great of that remnant object that it collapses forever
and forms the exotic object called a black hole.
Black holes so called because no light can escape from them, the crushing force of gravity
is irresistible.
From my point of view, from a star-former's point of view, yet another compact dead object.
So part of the cycle is that interstellar clouds form regular stars that have a finite
lifetime, a birth, a middle age, and a death.
Some of those regular stars form these sinks of matter, these compact dead objects, but
along the way, as I've indicated, other things happen.
The stars can blow winds off into space, like these red giant winds I described.
You can have nova events where matter from one star is spilled over onto another, then
explodes in a thermonuclear flash, on the surface of that star, that spreads matter
out into space, and then the most spectacular way of dispersing matter into space is a supernova
explosion.
During the supernova explosion, which forms a neutron star or a black hole, the light
from that supernova explosion is so intense that it out shines all the other stars in
the galaxy.
And remember, there are a hundred billion stars in a typical galaxy, so that's a lot
of light.
It's a very tremendous explosion, but that is a way of also returning matter into space.
So from that debris, from that matter, spewed out into space from supernovae, novae, and
stellar winds, guess what?
New interstellar clouds are formed.
Those new clouds then form new stars, which then start the cycle all over again.
Our sun has been around the block many times.
How do we know that?
How do we know that we are not new, that our sun not formed from primordial matter?
Primordial matter is just hydrogen and helium.
That was formed in the Big Bang.
That was not formed by stars.
But as a star ages and goes through its life, it produces all the other elements in the
periodic table, oxygen that's so important to us, carbon that's also so important to
us, silicon, neon, nitrogen, you name it, they're all formed within stars, and then
eventually spewed out into these interstellar clouds, which form more stars, which create
more of these so-called heavy elements.
Everything beyond hydrogen and helium, we astronomers just lump together as a heavy
element or a metal.
So these metals are an indication that stars have been there before.
Our sun has metals in it.
About 2% of the mass of the sun is in the form of these heavier elements.
So we are not primordial.
We have gone through this cycle several times before.
And our neighboring stars, the same thing.
They have also been around the block many times before.
Let's zoom even further into a star formation region and look at what's going on.
So let's go to the tip of the sword in the Orion constellation, the sword hanging from
Orion's belt, and zoom in on that fuzzy patch, the Orion Nebula that I mentioned to you earlier.
In the Orion Nebula, with even a modest telescope, it breaks up into a beautiful pattern as shown
here.
You see a very bright region in the middle here, and that bright region is a group of
four O-stars called the trapezium.
So these, again, are very bright, very massive stars that are very short-lived, but we're
catching them right after they were formed, so we're lucky enough to see them being very
active.
The reason that you see all these colors here surrounding the O-stars is that they give off
intense ultraviolet radiation, and that ultraviolet radiation is ripping apart atoms of gas around
the stars.
It tears the electrons off the nuclei of these atoms, turns that gas into a plasma, a floating
sea of electrons and nuclei, and then those electrons jump back down to rejoin those nuclei,
and when they do that, they emit radiation at different wavelengths or colors.
That's the multicolored image that you're seeing there.
It's from the glow of an ionized plasma recombining, electrons rejoining their parent nuclei.
The O-star is tearing apart its environment.
In another million years or so, this region will be gone.
We won't see any of the O-stars will be gone, the glow will be gone, because we're just
seeing the last wisps of gas that are being driven away by those very destructive O-stars.
That's a very important lesson that I want to convey to you, that O-stars, although they're
so spectacular, and tell us their giveaways for star formation, they destroy the environments
for further star formation.
They destroy the molecular clouds from which they were born.
They're the most ungrateful of children.
Once the clouds spawn them, their children destroy them.
What's not obvious again in a photograph like this is that the O-stars, again, are the tip
of the star formation, iceberg.
There are many, many other stars formed in the Orion Nebula cluster, as it's called.
There are about 2,000 other stars here, in addition to the four O-stars, and with strong
enough telescopes, you can pretty easily find them.
A graduate student at Berkeley, Eric Huffani, did the following map of the remaining stars,
the full population, in the Orion Nebula cluster.
This shows the density as a function of position of these stars.
As I said, it's a very crowded star formation region, so here in this graph, you can see
the density piling up toward the middle, where the O-stars are found, where the trapezium
stars are located.
And again, this density, this matter, consists of sun-like stars, much less spectacular stars
that are formed all along with the O-stars, but they just don't show up as readily.
You need to appear a little bit more carefully with your telescope to pick them out.
You'll see there's a little cavity here in the center.
It looks like there's a dip in the density of stars.
That's really not true.
There's an apparent dip there, because we can't see the stars that are very, very close to
the trapezium, they're lost in the glare of the trapezium stars themselves.
As Dan mentioned in his talk, it's hard to pick out planets next to stars because these
stars outshine the planets.
Similarly, very hard to pick out ordinary stars next to extra-bright stars because of
the extreme contrast in brightness.
So stars really are piling up right up to the trapezium stars that are formed out of
the densest region of stars.
So now I'm going to show you a movie that Eric and I made of the process of star formation
within the Orion Nebula itself.
And let me say a word before I run the movie about how we did it.
If you look at a star, one of these low mass stars that I'm talking about, like the sun,
from its brightness and from its temperature, in other words how bright it is and how hot
it is, you can discern its age.
And we did that for about 300 of these stars.
And if you know the age of a star, you can make a movie like we just did of the process
of birth in that region.
We place stars into, we plop stars down in this diagram according to their ages.
So if we see a star that's 5 million years old, as judged from its luminosity and temperature,
we put it in 5 million years ago.
We see a star that was 2 million years old.
We put it in 2 million years ago and so on.
So now when I start the movie, you'll see how star formation proceeded within this cluster.
How did the stars that we see today get to be there?
And I think you'll see the answer is quite interesting.
Here are the stars slowly coming into view.
The trapezium is at the center, by the way.
I'm going to play that for you one more time.
The thing I want you to notice is that there's kind of a spattering of stars in the beginning.
There's no real clear pattern to it.
But then notice two things.
The rate of star formation is picking up with time.
We're getting more, I hope you notice that.
More and more stars are forming per time, accelerating, and also it's concentrating more and more
toward the center where the O stars are forming.
Here you see it leisurely building up stars and more and more, and especially in the middle.
So lesson, the O stars are the last gasp in a star formation region.
Yes, they're the most massive and most spectacular.
They form at the end and they form in the most crowded region after a lot of other stars
have formed around them.
And I think that's a clue to how O stars form.
Maybe they form by coalescence, other stars that have already formed around them.
That's my idea at least.
Here's another OB association.
The name OB association Orion is a, we call it an OB association because it consists of
O stars and B stars.
B stars are the next level down in terms of brightness.
Also spectacularly bright stars, just not quite so much.
Most OB associations, unlike Orion, don't have any gas in them anymore.
It's been blown away.
And here's one relatively nearby, much closer than Orion, called the Scorpius OB association.
And again, the brightest lights that we see here are the O stars and a few B stars, but
they again are just the tip of the iceberg.
Not so evident in this photograph are the thousands of other solar type stars that are
being born along with the O stars.
And the reason for me showing this, I want to show you something interesting that's happening
in this and in other OB associations.
Here is again as a map of the brightest stars in the OB association.
If you wait a decade or so, you don't have to wait very long and take a region of that
same piece of the sky, excuse me, a photograph of that same region in the sky, the stars
move very slightly.
And these arrows show the motion of the stars.
So it's obvious that this thing is expanding into space.
Now you can play the following game, which as was first done about 40 years ago by the
Dutch astronomer Adrian Blau, you can run the movie backward.
This is their velocities today.
Run the movie backward in time.
The whole association, the whole group of stars contracts as you go backward in time.
If you keep going back, keep running the movie backward, it will pass through itself and come
out the other side.
But if you go back to the smallest position where the group occupies the smallest volume
possible, that probably was the initial state of the group.
And using that reasoning, you can tell how old the group is.
How many years did it take to go back to that initial state, running the movie backwards
given its current rate of expansion?
And Blau played that game and found the age of this OB association.
It's about four million years old.
Again a blip of an eye in terms of galactic ages, four million years, not very old.
So the lesson here is that O-stars destroy their parent clouds.
And once they do that, the whole group expands into space.
It disperses.
And after about 10 million years, we don't see any OB associations.
Everyone that we see, they're all very young groups that are dispersing, actively dispersing
into space now.
Here's a beautiful picture that made the headlines some years ago, a Hubble Space Telescope picture
of something called the Eagle Nebula, where you really see a cloud in the act of being
destroyed by an O-star.
You don't actually see the O-star here.
It's up to the upper right in this photograph.
But what it's done is it's eroded away the cloud through this harsh ultraviolet radiation
leaving behind these pillars, these pillars that all face toward the O-star.
And they're just regions of gas that have been shadowed by other dense clumps within
that clouds and just are the last to go.
And at the tips of these pillars, you can see new stars that are being formed.
Those are the glows at the tips of the pillars, the tops of the columns.
And those new stars are solar-type stars that are forming all along with the O-stars.
But the O-stars wipe out the whole background.
They're taking away that parent cloud gas and they're ending the star formation process.
Well, if what I said is true, that raises a puzzle right away.
If we look into the outskirts of our own galaxy, we see spectacular clusters of stars that
are far more populous than anything I've talked about so far.
A typical OB association contains a few thousand stars.
We see something called globular clusters in the outskirts of our galaxy that can contain
up to a million stars.
So, from what I said, globular clusters must have contained many, many O-stars, perhaps
hundreds of O-stars when they were very young.
But globular clusters are not very young.
In fact, they are the oldest stars we know about.
A typical globular cluster like this one can be five or ten billion years old.
They're the oldest stars in the galaxy.
What are they doing there?
Why didn't they disperse?
They created hundreds of O-stars.
All those O-stars were gone billions of years ago, but why didn't those O-stars blow apart
the parent cloud and cause the whole group to disperse into space?
They're not dispersing.
They're happily circling each other around even today, billions of years later.
We think that the answer is that these globular clusters were formed from clouds, molecular
clouds that were so tremendously massive that their gravity was strong enough to hold them
together despite this hot activity of O-stars underneath them.
It's like boiling gas inside this enormous oven, but the oven is so strong that it can
withstand this explosive boiling within it.
On the other hand, what can drive these things apart eventually are supernovae.
The stars really blow themselves apart.
That can disperse one of these groups, eventually.
This globular cluster and others like it contains only low-mass stars because they're the only
ones that can live long enough so that we can see them billions of years later.
How do globular clusters form?
Well, I've sketched to you a little bit of the theory.
There's more than theory now.
We can actually see so-called super star clusters being formed in other galaxies.
It's very interesting.
What we see is this kind of thing.
There are two galaxies in the act of colliding.
Galaxies can actually hit each other.
Now, galaxies, as I said, are mostly composed of stars.
Stars are so far apart that when galaxies collide, for the most part, just pass through
each other.
They don't just pass through each other.
They create tails like this.
They pull the stars out and create these tails.
This is called the antenna galaxy because the tails look like wispy antennas.
These two blobs in the middle are the nuclei.
Perhaps these were spiral galaxies that have been so distorted from their spiral shape
by this passage, by passing through each other.
In addition to passing through each other, however, the gas within the galaxy, this fuel
for creating new stars, does not pass through itself.
It slams together and creates, guess what, new stars.
So the most spectacular activity of star formation that we see, so-called starburst galaxies,
are all galaxies that are actually ramming into each other.
They're very far away, but with the aid of very powerful telescopes, again, like the
Hubble Space Telescope, we can actually image clusters being formed.
All of these white splotches here are new superstar clusters being formed, and we think,
it's not certain, but we think these could be the progenitors, the massive things like
globular clusters that we see in the far outskirts of our own Milky Way.
And finally, we have bound clusters within our own galaxy, not so spectacular, that have
also been held together for a long time, and here's one example, Pleiades cluster.
A lot of you are familiar with the Pleiades, again, it's kind of a fuzzy patch in the Taurus
region, it turns out, again, and if you look at it, even with under modest magnification,
you can see that it's composed of at least these seven bright stars here, called the
Seven Sisters, and about a thousand other stars that are less spectacular and less bright.
And it's been held together for about a hundred million years, that's the age of the Pleiades.
It's not like a globular cluster that has far less stars, it may not have ever had any
O stars in it.
Here's a movie of the history of the Pleiades that I did with another graduate student at
Berkeley, Joe Converse, and I just want to show you how these clusters are held together,
what keeps them intact.
They're all orbiting each other, every star in there is held in orbit by the gravity of
every other star, so the orbits are not nice elliptical or circular orbits like the orbits
of the planets in our solar system, they're complicated orbits, because the gravity is
kind of lumpy, that gravitational force from all those other stars, and they're all moving
around just like the star that you're interested in.
So in this movie, you can actually see this simulation on the computer, you can actually
see the stars moving around in space, like bees, right, exactly, and they're all attracted
to the honey, and the honey is the gravitational force of all the other bees.
So that movie lasted, of course, a few minutes on computer time, but 100 million years supposedly
in real time.
What's happening to the cluster during that time, it's a very stable object, here's
a graph of the radius of that cluster as a function of time over that 100 million years,
it's not changing very much, it wiggles a little bit up and down, and it's probably
expanding slightly, but it's quite stable.
So these clusters of stars can last a very long time.
So that's enough about groups of stars, let's go to individual stars and talk about how
any given star forms, especially a star like our sun, which we understand the best.
So let me start with the most basic question of all, what is a star?
A star to a astrophysicist is a ball of gas held together by its own gravity, they're
all nearly perfectly spherical, and the center of that ball of gas is a nuclear furnace,
where hydrogen is being fused into helium, just like in a hydrogen bomb, it's a slow
process, the hydrogen is slowly turning into helium, and that's why these stars live for
such a long time.
So it's composed of this ionized gas or this plasma, how big is a star?
Well in light units, a star is about one light second across, if diameter of the star is
such that it takes light one second to go from one side to the other, the temperature
in the middle of a star is enormous, about 10 million degrees in the Kelvin scale, which
in this case is close to 10 million degrees centigrade, and it's in hydrostatic balance,
which is a fancy term to mean it's in equilibrium between the crushing force of gravity and the
hot pressure created by the nuclear energy being released from inside that object.
So that's what a star is to someone like me, and to investigate how a star, an individual
star forms, we have to look to its parent.
I've said that stars form out of molecular clouds, but it's, we can be much more specific
than that.
With that tarantula blob that I showed you in Taurus for example, remember it's not
forming stars all over the place, it's forming stars in very specific locations, and those
locations are called dense cores, they're little nuggets, cloud nuggets that are denser
than their surroundings, and they are the things that form individual stars.
The typical diameter of a dense core is about one light month.
A month is a lot longer than a second, it's about a million seconds, so the physics question
that I and other people set out to answer some years ago now, what started me off studying
star formation, is how does something one light month across collapse on itself to produce
something one light second across?
How do you get something to collapse to a millionth of its size without breaking into
many, many pieces along the way, because they don't break into many pieces, they only formed
one star or maybe two stars, each of these dense cores.
I will address that question in a moment.
To summarize the stages of early evolution of stars, there are three stages that we recognize.
In the first stage, the dense core, the kind of thing that I showed you, this sort of lumpy
potato about a light month across, collapses on itself due to its own gravity, and it forms
a very dense object in the middle, and that object is the first phase in the life of a
star, and it's called a protostar.
This protostar lasts as long as the collapse lasts, and that's something like 100,000 years.
So protostar doesn't live for very long, consequently it's very hard to find them, it's also very
hard to see them, because the light from that protostar gets eaten up by the dust grains
in the gas that's collapsing down on it, in the dense core, gets re-radiated in the infrared,
so we have to look with infrared telescopes for protostars.
We see many infrared objects today that look like young stars, we're still not sure, even
today in 2009, we're still not sure which embedded infrared objects are protostars,
which are more mature stars.
The next phase in early stellar evolution is after that collapses over, and you have
that central protostar in the middle, with no collapsing gas around it, it evolves much
more slowly, and it slowly sinks together because of its own gravity, and that's called
a premain sequence star.
Our sun, when it went through the premain sequence phase, four and a half billion years
ago at the time of its birth, took about 10 million years in this phase.
This is kind of the troubled teen years of a star.
Protostar is its infancy, lasts a very brief time, during the premain sequence star it's
troubled, it's giving off all kinds of blips and burps, it's giving off winds that I'll
show you in a moment, it has quite erratic behavior, and it's visible optically.
So we've known about premain sequence stars for 50 years, there are thousands that have
been catalogued, they're not hard to find.
And finally, as that object contracts, eventually the center of that object becomes so hot that
the nuclear fire starts, and when the nuclear fire starts, that's a very stable phase in
the life of a star.
A star is then called a main sequence star, our sun is a main sequence star.
99% of the stars that you see in the sky at night are main sequence stars.
They are burning hydrogen, fusing it into helium, that's a very stable, long live phase
in the life of a star, typically lasting 10 billion years.
That's where the star formation story ends, really.
So one question I asked was, how does this dense core not break up into many pieces?
How does it go into one piece in the middle?
And the theorists discovered that about 30 years ago, they figured out that the collapse
doesn't happen all at once, it happens in an inside out fashion.
First the center collapses to form a little baby protostar, and then the gravity from
that protostar pulls in the matter in the dense core around it, and then that falling
region spreads out throughout the dense core.
And as it spreads out, it's releasing pressure.
The pressure is dropping inside that region as stuff comes cascading down, and that causes
matter outside it to know, oops, my turn to fall, I don't have any pressure supporting
me anymore, so it comes down, and then it caves in bit by bit, and the in falling region
then spreads out, and that spreading time is the 100,000 years that I mentioned to you.
That's the collapse of a cloud to a protostar, and that explains, again, how you get an object
so tiny out of something so big, one light month, one light second.
When a young star forms, as I said, they're very erratic objects at first, and one of
the most amazing things they do is they emit winds, and they emit winds not spherically
symmetrically in all directions, but in two nozzles called bipolar flows, or jets.
And here is a wonderful example of a jet.
This is the dense core.
The jet has gone far out beyond the dense core.
Star, we can't see.
It's too buried inside there to photograph, even in the infrared.
And here are the jets viewing out on either side, and these green regions here are regions
where the jet is slamming into material around it and creating shock waves that illuminate
in these bright patches.
We don't really understand what causes these jets, but we think that the role of the jet
in the whole story is to clear away the cloud material around the star, thus rendering it
optically visible.
That's why we don't see clouds around stars more than a million years old or so.
And a few words about this troubled teen stage of stars.
Remember I said that the star is slowly contracting during that phase.
As it contracts, the interior of the star gets hotter and hotter until the nuclear fire
lights up.
But let's just pause and think about that process for a second.
What is causing the star to contract is that it's emitting energy into space.
It's not burning any nuclear energy yet.
Emitting energy into space because of gravity.
Gravity squeezing it together and causing it to heat up on the inside, which causes
it to emit energy into space.
And once it emits energy into space, it can contract a little bit further, emit more energy,
contract further, and so on.
That's the way the contraction proceeds.
To summarize this process, the star is emitting energy into space.
As a result of emitting energy into space, it's getting hotter and hotter.
Temperature is climbing to higher and higher values.
How many objects to know about on the Earth, which as a result of losing energy into space,
radiating into space, get hotter?
None.
All objects that we know about on Earth as a result of radiating into space get colder.
Imagine a coal in a fire, a hot coal.
It's glowing red because it's emitting that radiation, visible radiation, in the red region
of the spectrum.
And as a result of that emission, it gets colder until it's no longer emitting.
And so all objects act on Earth, stars do not act like that.
As stars radiate into space, they get hotter.
But as they get hotter, they tend to emit more.
And so you can see that there's no good end to that story.
That's why the endpoint of stars is an spectacular explosion or something else.
Because this is gravity that's doing it.
Gravity causes a star to get hotter as it emits energy into space.
That is ultimately a runaway process.
And finally, the story would not be complete unless I mentioned the smaller end of the
scale, the formation of planets.
So where do the planets in our own solar system come from?
And where do the planets come from in all the other planetary systems that we're now
seeing in abundance?
The big clue comes from the fact that in our own solar system, all the planets number one
orbit in the same direction around the sun.
Number two, they all lie in the same plane or very close to the same plane.
We've now seen dozens of multiple planetary systems around other stars, not our own sun,
but around other nearby stars.
And in every case, the planets orbit in the same plane.
That's the big clue.
We are being tapped on the shoulder and told what is going on and how those systems arise.
They must have originated from a disk of dust and gas that's circulated around the mother
star, parent star.
That disk then coagulated and congealed into these orbiting planets.
No question that must have been the story.
Details, we are just eking out now.
That's the basic story of how it happens.
To corroborate that, it would be very, very nice if we could see one of these primitive
disks around young stars.
Well, guess what?
We've seen many of them.
We see lots of disks around young stars.
Let me show you one example.
Here is one in the Orion Nebula, that region that I talked about before.
Here's a star just poking out over its disk, which we're seeing in silhouette here and
seeing it almost edge on that disk.
Ironically, the reason that we can see disks like this is because they're being destroyed.
They're being destroyed by the ultraviolet radiation from those O-stars that I talked
about, the trapezium stars.
These disks won't last very long, but we're lucky enough to see them glowing as they're
being destroyed, and that's how we can pick them out.
There's other more indirect evidence for disks around scads of other young stars, so we're
absolutely sure that that general story is the way that planets form around stars.
Why do you get disks around stars?
The original dense core was not only collapsing, it was rotating as it collapsed.
It did this and then spun up and formed a central star, a disk around that star.
Let me end with some of the outstanding questions, the field of star formation, and I'll give
you some of my hopefully educated guesses as to what the answers might be to some of
these mysteries that still remain.
One is, how does a dense core get into a state of collapse?
Most dense cores that we observe have no stars in them.
Some have young stars, most don't.
How do you get from one to the other?
The ones that have stars in them are collapsing now in that inside-out way.
How do you get from pre-collapse state to collapse state?
No one knows.
My guess is that these dense cores are slowly adding in matter around them.
Matter is drifting onto the dense core until gravity wins and they go into a state of collapse,
and I'm working on it to try to flesh out that picture.
Second question, I haven't talked about this, but most stars are not like the sun.
Most stars are not single stars but are binary stars, two stars orbiting each other because
of their mutual gravity.
We think roughly two-thirds of stars are binary stars.
What creates binary stars?
Why does nature prefer to form stars in doublets rather than singlets?
I think the answer here is that dense cores are elongated.
They're more cigar-shaped than round, and when they condense and form stars, they tend
to do it in pairs.
Again, we don't really know the answer, but we're working on it.
We're looking, in fact, for very young binaries inside of dense cores.
And finally, I've mentioned clusters of stars ranging from things like the Pleiades, very
massive popular clusters, like the globular clusters containing a million members.
What creates clusters of stars?
We know how individual stars form, but why do stars always form in groups?
We think that every star forms in a group.
Most of those groups disperse into space, like the Obi Association, some stick around,
but the fact remains, stars are not born alone.
They're born in groups.
How does that happen?
If we don't know, I think the answer is that they form from bigger clouds.
The bigger clouds form these groups of stars, not just individually but in their densest
regions.
So I'll end there and be happy to take your questions.
I've given you just an overview of this very exciting frontier field of astronomy research.
Thank you.
Thank you.
