Thursday, June 16, 2011

Dribs and drabs


Here is a detail, one little corner, of the much larger Carina Nebula in the southern sky. This dazzling mountain of gas and dust is about 4 light-years tall, about the distance between the Sun and the nearest other star. It is about 8000 light-years away, part of our own Milky Way Galaxy. (Click to enlarge.)

This is the sort of place where stars are born, as gravity pulls knots of gas into ever denser globs.

Like most of the universe, the nebula is about 99 percent hydrogen and helium, the two lightest elements, and the only elements (with a little lithium) created in the Big Bang. But there is a percent or so of other stuff present, atoms of oxygen, nitrogen, sulfur, iron and so on, and even some molecules. These heavier elements were forged by stars that lived and died in earlier stages of the galaxy's history.

It was in a nebula like this that our Earth was born 4.5 billion years ago.

But the Earth has a very different composition than the nebula out of which it was born. The Earth is about 30 percent iron (mostly at the core), 30 percent oxygen (most common element in the crust), 15 percent silicon, 15 percent magnesium, and a percent or so of sulfur, nickel, calcium, aluminum, and so on.

Why the difference?

OK, start with a cloud of hydrogen, helium, and smidgens of everything else. Squeeze it down it with gravity. Whatever rotational motion the cloud had will speed up, for the same reason an ice skater spins faster when he draws in his arms. The whirling glob spins out a disk -- like the dough in the hand of a pizza maker. Most of the gas and dust is pulled to the center to form a star. The whirling disk will become the planets.

When the temperature at the squeezed-down core of the proto-star reaches 10 million degrees, fusion begins, producing energy and halting the collapse. The star turns on, emitting a wind of energy that blows out through the surrounding disk -- the disk that, like the star, is mostly hydrogen and helium and dribs and drabs of everything else.

In close to the new star -- the Sun, say -- the light hydrogen and helium in the protoplanetary disk is mostly blown away when the star turns on, into the outer solar system where it became part of giant outer gassy planets, leaving behind the one percent or so of iron, oxygen, and so on. Which gravity eventually collects into small rocky and metallic inner planets.

Our Earth, our bodies -- we are the smidgens of heavy elements that billions of years ago were scattered in a vast, beautiful, glowing nebula like the one above.

In transit to Ireland tomorrow. As usual, I won't know what I have for internet until I get there. Hope to be back by Monday at the latest.