Over the past few weeks I’ve talked about
our Sun, the planets, and other objects in our solar system, stars (both big and small), brown dwarfs,
exoplanets, supernovae, and clouds of gas and dust. What do all these things have in common? Well, they — WE — all live in the same
town. It’s a big town, and it’s called the Milky Way galaxy. Let’s cut to the chase: The Milky Way galaxy
is a vast, flat, disk-shaped collection of gas, dust, and stars; a sprawling megalopolis
that’s a mind-crushing 100,000 light years across and several thousand light years thick.
The Sun is but one of HUNDREDS of BILLIONS of stars in it, and we’re located halfway
out from the center, in the suburbs. The disk has two major spiral arms in it,
and two minor ones. These come together more or less in the middle of the galaxy. Located
in the center of the disk is a bar; an elongated roughly cylindrical collection of old red
stars. I know, this is a lot to take in. I’ll go
over it piece by piece. And how we know all this, like with so many other such deep
astronomical discoveries, begins simply enough: going outside and looking up. If you happen to be in a dark spot on a moonless
night, you’ll see a faint fuzzy glow stretching across the sky. It’s brighter in some spots
than others, and if the constellations of Sagittarius and Scorpius are visible, you’ll
notice the stream broadens there, forming a bulge apparently bigger than your outstretched
hand. This glow’s resemblance to a pathway or
river is strong, and many ancient cultures saw it that way as well. Because it glows
a dull white, the ancient Greeks called it Galaxius, or “milky”. Over the millennia
that name stuck, and we now call it the Milky Way, or (redundantly) the Milky Way galaxy. But what is that glow? It was thought to be
nebulous, cloudy, until one day a guy by the name of Galileo pointed his newly crafted
telescope at it. He was astonished to see that it was actually made up of stars, thousands
of them. They were so close together and so faint that our naked eyes couldn’t see them
individually; instead they blended together to create that glow. Clearly, there were far more stars in the
sky than the 6000 or so visible to our unaided eyes. But how many more? And what overall
shape did they form? Astronomers reasoned that counting stars in
different directions might reveal the shape; if the galaxy is longer in one direction than
others, you should see more stars in that direction. Over the centuries maps were made
this way, and they generally showed the galaxy to be oval shaped, or a flattened disk, with
the Sun near the center. This star counting method is a good idea,
but it’s fatally flawed: It doesn’t account for interstellar dust! As we saw in our episode
about nebulae, dust absorbs visible light from stars, hiding them from our view. It’s
like being in a smoky room where you can’t see the walls. It looks like the room is smaller
than it really is, and you look like you’re in the center, even if you’re near one of
the walls. Globular clusters were a good clue that there
was more going on. These spherical balls of stars orbit the galaxy well outside the main
body of the galaxy itself. There are more of them on one side of the sky than the other,
which you wouldn’t expect if they were evenly distributed in space. The simplest explanation
was that they actually WERE evenly distributed in space, but it was WE who were off-center.
Measurements put the Sun about 25,000 light years from the galactic center, which was
in the direction of Sagittarius. Hey, that’s where the glow of the Milky
Way in the sky gets brighter and broadens out, too! Coincidence? So at this point things were coming together.
The Milky Way must be a flattened disk, with a bulge in the middle. The Sun is in that
disk, far from the center. From inside it, we see it as a broad line in the sky, and
when we look toward the center, toward Sagittarius in the sky, we see that bulge as a big blob.
The disk is thinner than it is broad, so we see more stars looking INTO the disk then
we do looking up or down OUT of it. But what about the disk itself? Does it have
structure in it, or is it more like a smooth flat, plate? That’s hard to determine using visible light.
Dust severely restricts our view; looking into the disk we can only see about a thousand
light years before dust blocks everything behind it. But with the advent of radio astronomy we
figured that out too. Star forming gas clouds emit a very narrow wavelength range of radio
waves, and this could be used to measure their Doppler shift, how quickly they were moving
toward or away from us. Making a few other basic assumptions, their rough distance could
be found as well. An amazing result was found: The locations of these nebulae indicated that
the disk of the Milky Way was divided into SPIRAL ARMS; huge, sweeping, curving arms
that were tens of thousands of light years along their lengths. Interestingly, all the young, massive, bright
stars appeared to be in these spiral arms too, with none in the apparent gaps between
the arms. But that makes sense: Like all stars, these massive ones form in giant nebulae,
but unlike most stars they don’t live very long before exploding as supernovae. If most
nebulae are in the arms, then these massive stars will be too; they don’t live long
enough to get very far from their nurseries before they explode. Here’s where things get a bit weird. You
might want to think of the arms as structures; huge collections of stars and nebulae moving
together around the galaxy’s center. But that can’t be right: If that were the case,
they’d wind up like string on a spool, since the stars and nebulae closer to the galactic
center orbit faster. But we know the spiral arms have been around a while, and AREN’T
wound up. That’s because they aren’t actual structures.
They’re traffic jams. Think about a highway with heavy traffic on
it. For some reason one car slows down a little. The car behind it slows, and the car behind
that one hits the brakes, too. It’s like a wave that moves backwards through traffic,
slowing everyone down. Where they slow down they tend to clump up, too, so you see more
cars and trucks in the jam. The funny thing about traffic jams is, they
tend to persist for a long time even as cars move out of them. The car at the front might
slow for a moment, then accelerate away. But the cars behind it are still moving slowly,
and more cars are piling up in the rear. For every car that leaves the jam at the front,
another enters from the back. Even though the actual INDIVIDUAL cars come and go, the
jam persists! Spiral arms are the same sort of phenomenon.
The initial wave, the jam, may be started by some sort of disturbance in the disk, perhaps
an overdense region created when a couple of giant molecular clouds smash together.
Once created, this effect propagates outward like a ripple in a pond, and gets sheared
into a spiral wave pattern by the rotation of objects in the disk. Stars and nebulae
enter the wave, pile up a bit, move through it, and then resume their merry way once they
leave it. Our Sun takes about 250 million years to orbit the galaxy, which means it’s
made about 20 orbits since it was born; it’s moved in and out of spiral arms many times
over its long life. That’s why spiral arms don’t wind up:
The arms are waves made up of objects in them, but the objects themselves are transient. However, there’s more to it. A nebula entering
a spiral arm might hit another one already in there — a cosmic fender bender. But when
that happens the nebulae can collapse, forming stars. Some of them are massive: luminous,
blue, and hot. These stars are very easy to spot at great distances, making the galaxy’s
spiral arms look blue. They also light up the gas around them, accentuating the spiral
arms. Spiral arms are where the majority of star
birth occurs in the galaxy, which helps maintain the spiral pattern. They’re the ultimate
crowd-sourced projects. After all that, it’s actually hard to say
how many spiral arms the Milky Way has! It’s been thought to have two for a long time,
but a recent study of the locations of star clusters seems to indicate there are actually
four! Milky Wayologists are still arguing it, but
either way, there are also short spurs that stream off of the major arms as well. Our Sun
is located just outside one of those smaller arms, called the Orion Arm. Mapping the galaxy’s spiral arms is ongoing
work and it’s kind of a mess. It’s like being inside a hurricane and trying to figure out
its structure. But we’re doing pretty well so far. In the center of the galaxy is the bulge.
Recent observations indicate this is ACTUALLY a cylindrical bar of redder stars that’s
pointed nearly directly at us, so it looks more circular to us. This region is very old;
all the star birth in there happened long ago, and ceased when the gas ran out. Unlike
the spiral arms, where stars are still actively being born, all the blue stars in the bar
are long dead, exploded, leaving behind billions of lower mass, redder stars. The bar is probably about 20,000 light years
long. This odd structure is due to the weird gravity of the galactic disk. Unlike our solar
system where the central Sun dominates the gravity, the gravity of the galaxy is spread
out over a huge area. This distribution of stars and gravity means weird structures can
pop up and self-maintain, like spiral arms, and central bars. Unlike the arms, though,
the bar rotates as a single unit; stars near the edge make one orbit in the same time as
the stars closer in. In the very center of the galaxy sits a VERY
massive black hole, one with four million times the mass of the Sun. This might seem
huge, but remember the galaxy has hundreds of BILLIONS of stars in it. The central black
hole is actually just a teeny tiny part of the galaxy by mass, but we’ll learn more
about it in a future episode, and see that it may have played a very big role in our
galaxy’s formation and evolution. The last component of the Milky Way is its
halo, a vast, spherical cloud of stars surrounding it out to great distance, more than 100,000
light years. We don’t know much about the halo, but we know the stars in it are old.
No star formation occurs there, so any stars we do see WOULD be old. It’s possible some
stars in the halo formed there, while others were flung out into the halo after collisions
with other stars in clusters. Speaking of which, most of the globular clusters orbiting
the Milky Way are in the halo. There are more than 150 in total. So, there you have it, our local galactic
town. But like any denizen of a small town, it’s natural to wonder if this is all there
is. What else is out there? And at the beginning of the 20th century, astronomers started to
ask the same thing. Is the Milky Way THE galaxy, or A galaxy? Oooh, we’re about to take a big step here with
Crash Course Astronomy. A REALLY big step. Better strap in. Starting with the next episode,
the Universe is about to get a LOT bigger. Today you learned the Milky Way is a disk
galaxy, a collection of dust, gas, and hundreds of billions of stars, with the Sun located
about halfway out from the center. The disk has grand spiral patterns in it, formed by
the traffic jams of stars and nebulae, where stars are born. The central region is shaped
like a bar, and is mostly old, red stars. There’s also a halo surrounding us of old
stars. Crash Course Astronomy is produced in association
with PBS Digital Studios. Head over to their YouTube channel to catch even more galactically
cool videos. This episode was written by me, Phil Plait. The script was edited by Blake
de Pastino, and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins,
edited by Nicole Sweeney, the sound designer is Michael Aranda, and the graphics team is
Thought Café.
I love that series! Phil [the speaker] makes the subject extremely accessible even to casual people who even remotely have interest in space. Even as someone who's been in love with space from childhood, I'm still learning new things from watching these relatively short videos. I recommend it to anyone who wants a good introduction to astronomy.