The Life of Super-Earths (23 page)

Angular momentum is the product of mass, velocity, and size of a rotating body; left on its own, the body will conserve its angular momentum. If its size is shrinking, the body has to rotate faster to compensate for the decrease in size. The mass of gas and dust that surrounds a young star shrinks toward it while orbiting around it and attains a flat disk shape.

Here I use the word “mass.” The mass of an object measures the amount of stuff (atoms or matter) in that object. Mass and gravity are interrelated: the strength of the force of gravity depends on the mass—a more massive body exerts a stronger force (or pull). Colloquially, we often say weight or heavy, instead of mass or massive. This is imprecise and leads to confusing statements, so I'll insist on saying mass and massive.

Walt Whitman's poem “When I Heard the Learn'd Astronomer” has upset generations of scientists, as they will tell you that math does not keep them from looking up in awe at the stars.

Note the new term I have introduced: “scale.” In astrophysics this term means a characteristic range—of size, or of time, energy, and so on. For example, we speak of a galactic scale (sizes of 10
²⁰
to 10
²⁵
m) versus a quantum scale (sizes of 10
^—10
to 10
^—15
m); and we use shorthand notation—powers of ten—to write those very large (or very small) numbers.

The definition of system as a combination of related elements organized into a complex whole is what I use here to refer to life for a lack of a better word.

A network is a pattern of branching and interconnected lines, circuits, things, and so on; the pattern does not have to be ordered.

An isotope occurs when the atomic nucleus of the same element (same number of protons) has a different number of neutrons.