Category Archives: chemistry

astronomy, chemistry, starbirth, vocabulary

Stars and Heavy Metal, Dude

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Metallic-*

Astronomers, like people in any other special discipline, have their own language. You’ve probably heard some of it, or read about it from time to time. Terms like “supernova” and “quasar” and “black hole” are all part of our everyday language, but they originated in astronomy. But, ever hear an astronomer talk about metallicity in a star? The word makes it sound like there are stars out there with full metal jackets or heavy metal interiors or something like that.

In truth, “metallicity” is a sort of shorthand way for astronomers to tell you how much of a star’s elemental inventory is made up of things other than hydrogen and helium. If you dig a little deeper, however, you find out that metallicity is also another way of indicating the age of star and what sort of birth cloud it formed in. For that reason, metallicity comes into play in a lot of discussion about the evolution of stars and galaxies and–of particular interest to those of us who live on planets–solar system formation.

Globular cluster M80, as seen by Hubble Space TelescopeHow does this metallicity thing work? First, think about star birth. Stars form from whatever materials are in their birth clouds. If those clouds are purely hydrogen, then that’s what the stars will be mostly made of. If their birth clouds have other elements like calcium, carbon, lithium, and sodium–in essence, any elements heavier than hydrogen and helium, then the stars will have those elements, too. Normally we wouldn’t think of sodium and carbon and calcium as metals, but in this context, that’s what they’re called.

Some older stars in the universe, notably the ones that formed not long after the Big Bang (some 13.7 billion years ago) are metal-poor. That is, they are mostly (or all) hydrogen and helium, with a few traces of other elements. We find those stars in globular clusters like M80 (left), or in some of the earliest galaxies. They’re also often referred to as “Population II” stars.The Sun, as seen by SOHO

Now, as older stars die (really, as any star dies), they enrich the interstellar medium with heavier elements. Where do those come from? I won’t go into the details of star death here, but it’s enough to know for now that as stars evolve, they create heavier elements in their cores. When a star dies, all that material eventually finds its way to interstellar space, either through mass loss (stellar winds) or through supernova explosions.

Those elements get mixed into clouds of gas and dust (called nebulae), and from those clouds come the next generations of stars; these all are born with heavier elements in them and are called “metal rich” and “Population 1” stars. The Sun (seen here in a SOHO satellite image) is a Population 1 star. If you look at its light through a spectrograph, you’ll see that the Sun has hydrogen, but it also has calcium, sodium, and traces of neutral iron and other “heavy” elements. These all reflect the conditions of the Sun’s birth cloud and place it firmly in the ranks of Population 1 stars.

If our star hadn’t formed in a metal-rich cloud, that is, if it had formed from a cloud of nearly pure hydrogen, no planets would have formed. You need those heavy elements to make planets. And those heavy elements were born inside other stars that lived and died billions of years before the Sun formed. Which makes us, as Carl Sagan once said, “Star stuff.” So, we (and the planet we live on) are also part of the “metal rich” universe, a pure byproduct of metallicity.

astrophysics, chemistry

Cosmic Mother’s Milk

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The Big Bang and Hydrogen

 

The progress of evolution from the Big Bang to the present…

 

I’ve got a project coming up in a few weeks that involves learning more about the conditions under which life might form throughout the universe. So, I’m busily studying various papers and articles so that when I get to one of the meetings involved with the project, I’ll be able to ask some intelligent questions.

There are so many factors that can play a role in the formation of life that I could spend dozens of blog entries talking about them. One of the most important sciences we can study to suss out the role elements play in the cycle of life is chemistry. Which is, of course, the study of the elements that make everything the universe, and how they work together to do so.

The typical study of chemistry starts with a student learning the chemical elements, starting with hydrogen, shown here as an atom with probable locations of its single electron. Why hydrogen? Because it was the first element created in the Big Bang, the creation event thatstarted the universe on its evolutionary journey some 13.7 billion years ago. The second element was helium, followed by lithium. All the rest of the elements depend on some sort of action that takes place inside stars, or at the end of a star’s life. And, those elements, along with hydrogen— a star’s first “food”— play a huge role in shaping how new generations of stars—and planets (and us) — are formed.

Hydrogen, however, is ubiquitous, which is a fancy word meaning that it’s everywhere. And, it was all there was in the early universe to feed the first rounds of starbirth, and thereafter nourish the formation of more stars. Those stars consumed hydrogen in their nuclear furnaces for much of their lives, but also created new elements as they went along, using their fuel to do so. When they died, they spit back into space all the elements they’ve made, along whatever hydrogen they have left. All that stuff becomes the seed material for MORE stars, and whatever planets those stars form. It’s going on today, this chain of starbirth and stardeath, using hydrogen as the formative “star food” and then churning out heavier elements at the other end of the stellar life cycle. So, like babies that survive on mother’s milk early in life, but turn to other foods later to grow and thrive, the universe continues its reliance on hydrogen for the early phases of star life, creating the heavy elements needed for later phases where more metal-enriched stars, planets… and life… are formed.

So, hydrogen is everywhere, even in you! How so? To paraphrase a life form that once spoke in an episode of Star Trek: The Next Generation, we are “bags of mostly water.” Water molecules are made of two atoms of hydrogen and one atom of oxygen.

So, think about THAT the next time you’re out under the stars. You have hydrogen to thank for your existence… the mother’s milk of the cosmos.