Water in the Cosmos - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 12

seeded by random fluctuations in the drifting clouds. Or, as seems to
have happened in the case of our own solar system, the pressures gen-
erated by a nearby supernova can trigger gravitational collapse. The
clues to this trigger event lie within ancient meteorites that have fossil
chemical traces of short-lived, highly radioactive isotopes that could
only have been generated in a supernova that erupted just prior to our
own solar system forming.
As the portion of cloud begins to collapse, any initial movement is
converted into a slow rotation of the cloud. As the cloud continues to
shrink, it begins to rotate more rapidly. The mass of material now
occupies a smaller space, and therefore any original movement is
speeded up, much as a skater spins more quickly when they pull their
arms in tightly to their body. The compression of the gas in the core
of the collapsing cloud begins to release heat, from the release of
gravitational energy, and this heats up the interior part of the spin-
ning cloud that is now flattening into a thin disc. The cloud becomes
a luminous proto-sun, with a fitful output of heat and light reflecting
the growing pains of a star in the making.
The densely packed, colliding atoms at the core of the young star
generate temperatures—simply through the immense compression—
of millions of degrees Celsius. This compressional heating in itself can
'burn' lithium, atomically transmuting it into beryllium. Eventually—
usually after tens of millions of years—the inexorable rise in pressure
and temperature makes hydrogen atoms begin to fuse into helium,
releasing a truly gargantuan source of energy that can last for billions
of years. A true star (or more prosaically, a 'main sequence star') is
then born.
Starbirth is a thing of beauty and violence, with intense X-ray emis-
sions and powerful stellar winds of outflowing atoms and ions. The
outburst of energy powers the outflow of radiation and gas from the
inner parts of the star system, driving them to its outer regions. Shock