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.

How 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.

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.