The Composition of the Milky Way

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What is our home galaxy made up of?

In the broadest sense, it’s made up of stars, clouds of dust and gas, and the mysterious dark matter.

We could also get a little more detailed. We could say that it is a great wheel of stars, made up of a thin disk component, a central bulge, and a broader spherical halo that surrounds the disk.

We could even build on that, and say that the thin disk is where all the youngest stars are found. We could say that within the thin disk are spiral arms, where the star formation actually happens. We could say that the oldest stars are found in the central bulge and the halo, where there is very little dust and gas to make new stars.

But…what about its chemical composition? If we could explore our galaxy and bring home test tubes of “star stuff,” what would we find? And what can that tell us about our galaxy’s history?

It’s certainly not what we humans would expect, having spent our lifetimes bound to our own Earth.

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Earth is what we call a terrestrial planet. It’s made up of rocks and metals. Earth’s surface is home to a crap ton of carbon and biological material, but below the paper-thin crust is a massive ball of molten rock.

When humans mine resources for their infrastructure, they barely scrape the surface of the sheer volume of rocks and metals our planet is made up of.

But, compared to the universe, Earth is tiny.

Carl Sagan once described our planet as “a mote of dust suspended within a sunbeam.” Even on the scale of our solar system, Earth is just a “pale blue dot,” barely visible to a spacecraft’s camera within a beam of sunlight.

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There’s a sizable fraction of the rocks and metals in our solar system, right there in that pale blue dot. The other terrestrial planets are no bigger than Earth, and there are only four of them (including Earth). The scattered moons and asteroids are even smaller.

In fact, most of the material in our solar system is hydrogen, and 99% of it is in the sun.

That’s what the rest of the galaxy looks like, too–and, indeed, likely the rest of the universe. The primary element is hydrogen, found in stars and the gases used to make them. Helium is the next runner-up–but there’s not nearly as much of it.

You’d be surprised just how little there is of non-hydrogen elements in the universe…

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Here, we see the abundance of elements in the universe plotted twice: first on a logarithmic scale, then on a linear scale.

Astronomers usually use logarithmic scales. They’re easier to work with, for research purposes. But the linear scale gives you a much clearer impression of just how rare elements heavier than helium are.

So why the heck is there so much hydrogen, and so little of everything else?

The answer is found within the stars themselves.

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Stars start out made of mostly hydrogen and some helium. But as they burn through their nuclear fuel, “ashes” from those nuclear reactions are deposited in their cores–in the form of increasingly heavier atomic nuclei.

This is called nucleosynthesis, and it’s the primary way that elements heavier than helium are created.

When massive stars lose stability and go supernova, that explosion creates even more heavy elements–and scatters all the newly created elements back into space.

The material that supernovae return to the interstellar medium–the clouds of gas and dust between the stars–can then be used to create more newborn stars.

The first “generation” of stars in our galaxy wouldn’t have had much of the heavier elements to work with. They would have been primarily hydrogen with some helium, and extremely poor in heavier elements.

But can we see any evidence of this?

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Above is a stellar spectrum of HD 122563, a star that is extremely poor in “metals”–astronomy jargon for any element heavier than helium.

This graph plots wavelength (of the electromagnetic spectrum, the spectrum of all light) according to intensity. What we are seeing here is the intensity–the brightness–of each wavelength of light the star emits.

A perfect stellar spectrum–known as a continuous spectrum–would have no dips in intensity. The dips show where certain elements in the star’s atmosphere have blocked certain wavelengths of light. Thanks to those elements, those wavelengths never reached astronomers’ instruments on Earth.

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The core of this star has no doubt undergone nucleosynthesis. But the atmospheres of stars do not, and that is exactly what a stellar spectrum looks at. With a spectrum like the one above, we can see what the star was made of when it first formed.

As you can see, HD 122563 has very few dips in its spectrum. That tells us that it’s very poor in “metals.”

It also indicates that this star is from a very early generation, before nucleosynthesis had spread heavy elements into space. But is this really an old star?

This star is, in fact, a red giant. It is 12.6 billion years old.

But…what if all stars look like this, not just the old ones?

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Well…here’s a spectrum taken from a star much richer in heavy elements.

If these graphs are confusing to interpret, I’ll point out the important difference: HD 122563 up above had a very flat spectrum, not a whole lot of bumps and dips. If you scroll back up, you’ll notice that save for those few dips, the line is practically as flat as a tabletop.

This star, however, has a very wiggly line. That’s because more of its light is getting blocked from reaching astronomers’ instruments–and that’s because there are far more heavy elements in its atmosphere.

Clearly, this star formed from interstellar clouds much richer in heavy elements than HD 122563. But is it younger?

This star happens to be our own sun. It formed around the same time as the Earth, roughly 4.6 billion years ago. It’s definitely much younger than the 12.6-billion-year-old HD 122563.

So, back to nucleosynthesis…

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All stars produce new elements through nucleosynthesis. But as the graphic above puts it, massive stars in particular are “engines of creation.”

As I’ve explained before, massive stars lead very short, explosive lives. Not only are they engines of nucleosynthesis, they only live for a few million years–so many generations of newborn massive stars can spread new material throughout the galaxy in an astronomically short time.

In the beginning of our galaxy’s history, its interstellar clouds would have been poor in heavy elements. But when the first massive stars went supernova, those explosions would have enriched the surrounding clouds with more heavy elements.

Each subsequent generation of stars would have formed from interstellar clouds that were just a bit richer in heavy elements than before. And each of those generations would have had more and more heavy elements to offer the interstellar medium.

Generally, we can divide stars into two different categories, by age: the creatively named Population I and Population II stars.

Population I stars are young and rich in metals (elements heavier than helium). Population II stars, on the other hand, are old and poor in heavy elements.

Of course, like most concepts in science, these are descriptors that humans use to make things easier to describe and study. Stars exist on a continuum from metal-rich to metal-poor. So we also have “intermediate” and “extreme” categories for both populations of stars.

Now for the real test: do these populations line up with where we already know young and old stars are found?

Our galaxy’s spiral arms are home to its youngest stars, since that’s where star formation takes place. And, indeed, these youngest stars are rich in metals, clearly from the galaxy’s youngest generation. These are the extreme Population I stars.

Intermediate Population I stars can be found in the disk between the spiral arms. These stars, slightly less rich in metals than extreme Population I stars, are old enough that they’ve had time to travel away from the star-forming regions of their birth–but they have not yet had time to leave the galactic disk.

How about the central bulge, then?

The central bulge is home to the nucleus of our galaxy, a chaotic region of very little gas and dust and mostly old stars.

Sure enough, this is where intermediate Population II stars are found. These stars are significantly poorer in metals and from older generations of stars.

Last but not least, we have the extreme Population II stars. These stars are from our galaxy’s first generations and would be extremely poor in metals. The most massive of them would have gone supernova billions of years ago, enriching the interstellar medium for future generations. We would expect most of them to be the longest-lived stars.

And sure enough…the galactic halo, an expansive region far from any star-forming interstellar clouds, is full of small, faint, and extremely metal-poor red dwarfs. Red dwarfs are so low-mass that they can live longer than the current age of the universe.

Our galaxy’s history is written in its stars. All the evidence matches up: the spiral arms are young, the disk is a bit older, the central bulge is quite old, and the halo is practically ancient.

But here’s the million-dollar question: why?

Why would the different parts of our galaxy be different ages? What does that tell us about how the Milky Way formed?

We’ll explore that question in my next post.

Questions? Or just want to talk?

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