The Milky Way Demystified

Alright, people…time to finish off our exploration of the Milky Way Galaxy, our home in the cosmos!

For the past nine weeks, we’ve covered everything from how our galaxy was “discovered” to how it may have formed. But there’s so much more to explore–and, starting next week, we’ll begin covering the vast universe of galaxies beyond our own!

But before we do that…I want to wrap up our discussion of our own galaxy with an overview to tie the last nine posts together.

(By the way, has anyone noticed I actually managed to chug out a post a week for the entire Milky Way “module”? I’m a bit impressed with myself for that!)

Anyway…on to the Milky Way!

Discovery of the Milky Way

Technically, humanity has known about the Milky Way since the dawn of human existence–but we didn’t always know that it was a galaxy. All we knew was that mysterious milky-looking spread that traverses the night sky. The ancient Romans dubbed it the milky way.

In 1610, Galileo Galilei observed the Milky Way through his telescope. He was the first to realize that the Milky Way was actually made up of a crap ton of stars.

Later astronomers realized what the shape of the Milky Way meant. It wrapped around the entire sky, so it was easy to imagine that humanity–that is, Earth–was sitting in the middle of a disk of stars. Thomas Wright described it as a “grindstone universe,” comparing it to the technology of the time.

But we still didn’t understand much about the Milky Way itself. Astronomers began to call it the great star system. It was a major expansion of humanity’s view of the universe. Previously, all we had known was the sun, planets, and “fixed stars.” Now, the sun and planets were sitting amid a broader “star system”–which, for all we knew, was the whole universe.

Enter Sir William Herschel and his sister Caroline, who mapped the star system to try to figure out what it looked like.

The Herschels called their method “star gauging.” They counted the number of stars visible in different directions, and hypothesized that the number of stars visible could determine the distance to the edge of the star system.

But just as the classical astronomers were shackled by the idea of uniform circular motion, the Herschels were shackled by an incorrect assumption of their own: that they could see the entire star system from Earth.

The Herschels’ model was inaccurate…by a long shot. But soon enough, modern astronomers would begin to figure out better ways to measure the size of the galaxy…

How Big Is It?

The key to finding the true size of our galaxy came in the form of variable stars.

These are stars that have burned through all the hydrogen fuel in their cores and lost stability. (For a review of star life cycles, check out my overview of stellar evolution. I didn’t include variable stars, but they fit into the “expansion into a giant” section!)

For this post, what’s important to know about variable stars is that they regularly fluctuate in brightness. There is a clear relationship between their average luminosity and the period of their fluctuations. (A star’s average brightness–that is, luminosity–is a result of its size, so brighter stars are represented here as larger spheres.)

What does that mean?

Astronomers can measure a variable star’s period to find its luminosity. Once we know how bright a star actually is (absolute visual magnitude), we can use how bright it appears from Earth (apparent visual magnitude) to measure its distance from Earth.

And if we know the distance to a star in our galaxy…well, we can figure out how big the galaxy actually is!

Around 1920, astronomer Harlow Shapley used variable stars within globular star clusters to determine the size of the Milky Way.

Shapley’s work had a critical weakness: his measurements of the variable stars’ distances were inaccurate.

This is because not all variable stars were the same. Shapley used calculations for the period-luminosity relation of a specific type of variable stars, Cepheids, but he didn’t realize that not all the variable stars he used were Cepheids.

Also, some of the variable stars he used were partially obscured behind interstellar dust clouds, and others weren’t. Shapley didn’t realize this, either–so he inaccurately recorded some of the stars’ luminosities.

Still, Shapley’s work accurately demonstrated that the Milky Way Galaxy was quite a bit bigger than the Herschels had thought.

Here, we see Shapley’s globular cluster distribution on top of the Herschels’ model. Quite a difference, huh?

Shapley estimated the distance to the center of the galaxy as up to 30 kiloparsecs (98,000 light-years). The modern estimate is only 8.5 kiloparsecs (28,000 light-years). But still, his work played an important role in humanity’s growing understanding of our home in the universe.

Structure of Our Galaxy

The Milky Way Galaxy is a spiral galaxy. Two main spiral arms are anchored to the central nucleus and swirl outward to the galaxy’s outer edge. Smaller arms and “spurs” of stars branch off from the main arms.

Viewed from the side, the Milky Way is a disk–it looks a whole lot like a frisbee.

The spiral arms are contained within the disk. The disk itself is ridiculously thin. In proportion to the full diameter of the galaxy, it’s thinner than a pizza crust.

At the center is the central bulge, which is home to the nucleus, the gravitational center of the galaxy.

If you’ve looked up images of our galaxy before, the disk (and spiral arms) and bulge are probably the parts you’re most familiar with. They’re certainly the most photogenic!

But that’s not all there is to the galaxy. Surrounding the disk and bulge is a halo of very diffuse material. Unlike the disk, there’s not a lot of stars, gas, or dust in the halo. The halo is where we find most globular clusters–the star clusters Shapley used to measure the size of the galaxy.

The disk of the galaxy is often called the disk component, and the bulge and halo are together called the spherical component.

How Massive Is It?

First things first: mass is not the same as weight.

Mass is the amount of physical material an object is made of. For any one object, the mass never changes. Weight, on the other hand, is a force. Weight only exists in the presence of gravity. And gravity, by the way, is a result of mass.

The mass of the Milky Way is the combined mass of all the stars, dust, and gas within it. And in order to find that mass, we need to model the orbits of objects–like stars–around the center of the galaxy.

So how does that work?

To find the mass of our own sun, we only need to examine the orbits of the planets of our solar system. Each planet orbits the sun, so their orbits will tell us our sun’s mass.

Technically, we’re finding the entire mass that is enclosed by the planet’s orbit–but more than 99% of that mass happens to be concentrated in the sun.

In a galaxy, the mass is not concentrated in the central bulge. It’s spread out across the disk. So, if we examine the orbits of stars at the edge of the galaxy, we can find how much mass is in the whole galaxy.

That’s where things start getting…weird.

Using both Kepler’s laws of motion and the observed luminosity of stars in our galaxy, we can predict how much mass should be there. And the actual amount is much more.

Yes, you read that right.

We’ve measured our galaxy’s mass using the orbits of its outermost stars. We’ve checked that number against predictions. We’ve repeated the measurement time and time again–but the result is always the same. All the stars in our galaxy can’t account for its total mass.

*not the Milky Way–but the same weird effect is happening, so it works for my point*

There’s mass in the outer reaches of our galaxy that we can’t see.

It’s not faint stars or cool, dark interstellar dust–neither are massive enough to explain this.

That is where the idea of dark matter comes from. It’s matter we can’t see at all, but we can detect its effects–the effects of its gravity–on visible mass.

We’ll come back to dark matter in future posts. But for now, let’s move on to explore our galaxy’s spiral arms.

The Spiral Arms

The spiral arms are the single most defining feature of a spiral galaxy like the Milky Way. But what are they?

One thing’s for sure: they are not bound by gravity. Stars, dust, and gas travel around the galaxy in independent orbits, passing through the arms on their way. That is to say, the actual stars and interstellar clouds that make up the arms are constantly changing…but the shape of the arms themselves stays the same.

The explanation? The spiral arms are like traffic jams.

Imagine a slow-moving truck cruising along the highway. Passenger cars slow as they approach, work their way out from behind the truck, and then speed up as they clear the knot of traffic.

The spiral arms are just the same: great streamers of slow-moving interstellar clouds, often giant molecular clouds, are gravitationally dragged by the rotation of the central bulge.

Faster-moving stars, gas, and dust overtake the spiral arms from behind. They get stuck within the “traffic” of the spiral arm and slow down. Then, millions of years later, they emerge on the other side and continue their higher-velocity orbits about the galaxy.

The spiral arms are considered dynamically stable–a structure whose parts change, but whose broader pattern remains the same. This theory of how the spiral arms work is called the spiral density wave theory.

But something much cooler happens when the faster-moving stars and gas get caught in the spiral arm’s traffic…

Passing through the denser gas and dust of the spiral arm causes the interstellar clouds to lose stability.

And when a giant molecular cloud loses stability…stars are born.

Essentially, the cloud collapses and fragments into smaller dense “cores.” Each of these cores continues to collapse under its own gravity, and a protostar is born. (You can explore a star’s full life “cycle,” from dust cloud to stellar remnant, here.)

The star-forming regions within the spiral arms will look something like this…

By the way, this image isn’t visual–protostars are not detectible at visible-light wavelengths.

Anyway…how do we know? What evidence is there that spiral arms are actually places of star birth, triggered by the collapse of giant molecular clouds?

Because molecular clouds, bright young stars, and other young objects are what we call spiral tracers: objects that help us map the shapes of spiral arms.

Stars of the spectral classes O and B only live for a few million years, which isn’t long enough for their orbits to carry them clear of the spiral arms. They remain in the star-forming regions of their birth.

That means that, theoretically, we should be able to use such young stars to trace out the shapes of spiral arms.

Sure enough, when we observe other spiral galaxies, O/B associations–groups of sibling O/B stars–are primarily (and almost exclusively) found within spiral arms.

Many other young objects are also found almost exclusively within spiral arms. Look at M51 above. See how the dark brown dust clouds pretty much define the shapes of the arms?

There’s your giant molecular clouds, right there.

Another defining feature of M51’s spiral arms are the hazy pink regions–emission nebulae. These are interstellar clouds that have been heated to produce their own light…by nearby O/B stars. They can’t exist without being close to young stars.

Other young objects found in the spiral arms are variable stars–yup, the very same variable stars that helped us measure the size of the Milky Way!

Despite having nearly reached the end of their lifespan, variable stars have only lived for a few million years–and thus are very young objects, just like O/B associations, giant molecular clouds, and emission nebulae.

Spiral arms are home to the galaxy’s star formation…but what’s going on in the galaxy’s central bulge?

The Nucleus

In a word: chaos.

The central bulge is home to some star forming regions and supernova remnants–evidence of some super intense gravitational activity.

After all, we don’t have spiral arm traffic jams here, and it’s surprisingly hard to get giant molecular clouds to collapse. Also, only the most massive stars produce supernova remnants, and massive stars are quite rare.

Besides which, there isn’t even a whole lot of interstellar dust in the nucleus in the first place. The nucleus doesn’t have much star-forming material to work with. So the fact that there’s ongoing star formation–and the nucleus is producing massive stars–is a bit weird.

There must be some serious gravitational perturbations going on here, wreaking havoc on the stability of the clouds of gas and dust that are there.

Which brings me to Sagittarius A, or Sgr A–the intense radio source at the center of the nucleus.

Sgr A controls all gravitational activity in the Milky Way. It holds more than 100 billion stars in orbit, along with countless interstellar clouds. It has a reach of at least 300,000 light years, necessary to hold the galaxy’s halo.

With that kind of powerful gravity, it can explain the star-forming activity in its immediate orbit within the nucleus.

But…what the heck can explain that kind of powerful, far-reaching gravitational pull?

The answer lies deep within Sgr A, in a region 3 parsecs across.

Meet Sgr A*: the supermassive black hole at the heart of our galaxy.

Sgr A*, distinguished from its surrounding region by the asterisk (*), is a black hole of at least 4 million solar masses. It’s 4 million times the mass of our sun.

Astronomers honestly don’t have a clear idea how such a black hole even forms. It’s way too massive to be the end state of a massive star.

But one thing is increasingly clear: most galaxies have central, supermassive black holes, which must all have formed nearly 14 billion years ago, when the universe was still very young.

Which brings me to the story of the Milky Way’s formation, beginning with stellar generations…

What’s the Galaxy Made Of?

Mostly hydrogen.

The linear scale on the right gives you a good visual impression of the abundance of elements in the universe. And there really is a crap ton of hydrogen.

As we’ll explore shortly (after galaxies, when we cover cosmology), the universe started out with almost only hydrogen, a small amount of helium, and almost negligible trace amounts of some other heavy elements. Most of the “building blocks” of the universe didn’t exist yet at all.

The first generation of stars, then, were made up almost entirely of hydrogen. But it turns out that stars–particularly massive stars–have a very important job…

Massive stars are engines of nucleosynthesis, which is how heavier elements are created. It’s thanks to massive stars and their death throes that pretty much all the stuff in the universe exists.

When massive stars die, they spread newly-produced heavy elements out into interstellar space. Subsequent generations of stars, then, form from interstellar clouds that contain more heavy elements. These stars begin with slightly higher concentrations of heavy elements–and through nucleosynthesis, they produce even more.

When those stars die, they even further enrich interstellar space. And the cycle continues.

As a result, there are two main populations of stars within our galaxy: Population I stars, which are from younger generations and are richer in heavy elements, and Population II stars, which are from older generations and are poorer in heavy elements.

(By the way, this graphic refers to heavy elements as “metals.” That’s astronomy jargon. In astronomy, “metals” means any elements heavier than helium.)

We already know that the youngest stars are found in the Milky Way’s disk component, particularly the spiral arms–because that’s where star formation takes place.

But we can also tell how old the different components of the galaxy are, as a whole, using stellar populations.

Population II stars are metal-poor, from the earliest generations of stars. Components of the galaxy where these stars are found must have formed quite some time ago, when the galaxy was very young.

Population I stars, on the other hand, are too young to have moved far from their birthplaces and would be found in only the youngest parts of the galaxy–formed “recently,” on an astronomical timescale.

Based on stellar populations, the galactic disk is quite young, and the spiral arms are even younger–which fits with what we know about star formation!

The central bulge, on the other hand, is quite a bit older than the disk component, and the halo is practically ancient.

This evidence of our galaxy’s history provides clues to its formation…

How Did It Form?

Astronomers first began to answer this question in the 1950s, with the monolithic collapse hypothesis, also known as the top-down hypothesis.

This was an early hypothesis–it doesn’t explain more modern evidence. But we’ll get to how astronomers modified their hypothesis in just a second.

According to the monolithic collapse hypothesis, the galaxy formed from a single large protogalaxy cloud, made up of turbulent, metal-poor gas.

Over the course of billions of years, the cloud would have collapsed and flattened under its own gravity, just like you see above.

This hypothesis attempts to explain the stellar populations: the first, metal-poor generations of stars would have formed in the outer reaches of the cloud. The deaths of massive stars in this generation would have enriched the gas of the cloud.

The gas would continue contracting to form the present-day galaxy. Subsequent generations of stars would form closer and closer to the plane of the disk, from metal-enriched gas. Less massive, longer-living stars from the first generation would be “left behind” in the halo surrounding the new galaxy.

The end product: a flat, disk-shaped spiral.

There’s just one problem…

The monolithic collapse hypothesis predicts that all objects in the halo should be roughly the same age. After all, they formed at the same time, from the same cloud.

But they’re not.

The halo is populated with globular clusters–yes, the star clusters Shapley used for his measurements of the galaxy’s size. Globular clusters are unique objects with mysterious histories, but one thing is certain: they are very old, close to the age of the universe.

Still, globular clusters have quite a wide range of ages, too wide a range for monolithic collapse to make sense. These clusters couldn’t have formed at the same time.

So, astronomers proposed the bottom-up hypothesis.

According to the bottom-up hypothesis, there was indeed an original “monolithic collapse.” This collapsing gas cloud would have formed the galaxy’s spherical component (the central bulge and halo). That would explain why the spherical component is, as a whole, quite a bit older than the disk component.

But, after the monolithic collapse, new material enriched the galaxy.

Clouds and star clusters–such as globular clusters–would have formed outside the galaxy, at different times, and drifted into the Milky Way’s gravitational influence. They would have been absorbed into the galaxy.

Smaller galaxies, especially dwarf galaxies, would also have been absorbed–and even today, the Milky Way has quite a menu of dwarf galaxy morsels at its disposal.

In fact, the galaxy isn’t done building itself–we can still see this happening, almost in real time.

I’ll leave you with the image that began this post, to bring it all full-circle: the “big picture” of the Milky Way.

Several dwarf galaxies are visible here, such as the Large and Small Magellanic Clouds, Segue 1, and Ursa Major II. We can even see a galaxy that is currently in the process of being absorbed–the Sagittarius Star Stream, a stream of stars that was once the Sagittarius dwarf galaxy.

This galaxy must have swung just a bit too close to the Milky Way, sometime in our galaxy’s distant past. And any one of the intact dwarf galaxies you see here could go the way of the Sagittarius dwarf, too.

There’s definitely plenty more of our own home galaxy to explore…but now, we’re ready to move on to explore the vast universe of galaxies beyond our own. But we’ll be coming back to the Milky Way–next time, in the context of the greater picture!