Bad Astronomy: Misconceptions and Misuses Revealed, from ...

THE GRAVITY OF THE SITUATION 75
moons all affect each other tidally, too. When one moon passes
another, the differential gravity squeezes and stretches the moons,
flexing them.
Have you ever taken a metal coat hanger and bent it back and
forth really quickly? The metal heats up, possibly enough to burn
you. The same thing happens when these moons flex. The change
in pressure heats their interiors. It heats Io enough to actually melt
its interior. Like the Earth, Io’s molten insides break out of the surface
in huge volcanoes. The first was discovered when the Voyager I
probe cruised past the blighted moon in 1979. Many more have
been found since then, and it looks like there are always volcanoes
erupting on the poor moon.
The tidal friction also warms the other moons. Europa shows
evidence of a liquid-water ocean buried under its frozen surface.
That water may be heated by tides from passing moons.
If we look even farther out, we see more tides. Sometimes stars
orbit each other in binary pairs. If the stars are very close together,
tides can stretch them into egg shapes. If they are even closer, the
stars can exchange material, passing streams of gas from one to
the other. This changes the stars’ evolution, affecting the way they
age. Sometimes, if one of the stars is a dense, compact star called
a white dwarf, the gas from the more normal star can pile up on
the surface of the dwarf. When enough gas accumulates, it can
suddenly explode in a cosmic version of a nuclear bomb. The
explosion can rip the star to pieces, creating a titanic supernova,
which can release as much energy in one second as will the Sun in
its entire lifetime.
And we can take one more step out, to a truly grand scale.
Whole galaxies are affected by tides, too. Galaxies, collections of
billions of stars held together by their own gravity, sometimes pass
close to each other. The differential gravity of one passing galaxy
can not only stretch and distort but actually tear apart another
galaxy. Sometimes, as with the binary stars, the more massive galaxy
actually takes material—stars, gas, and dust—from the less
massive one in an event called galactic cannibalism. This is hardly
a rare event. There’s evidence our own Galaxy has done this before,
and as a matter of fact, we are currently colliding with a tiny