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.