Our Milky Way is an elliptical galaxy, with five spiral arms peeling off
a central bulge. New stars originate in dense regions of interstellar gas and
dust called giant molecular clouds. Several times a century, a giant star more
than 100 times larger than the Sun explodes, producing a supernova a bil-
lion times brighter than an ordinary star.When a star reaches the supernova
stage, after a very hot existence spanning several hundred million years, the
nuclear reactions in its core become highly explosive. While the star sheds
its outer covering, the core compresses to an extremely dense, hot body
called a neutron star, similar to condensing Earth down to about the size of
a golf ball.
The expanding stellar matter from the supernova forms a nebula com-
posed mostly of hydrogen and helium along with particulate matter that com-
prises all the other known elements. About a million years later, the solar
nebula collapses into a star.This process begins when shock waves from nearby
supernovas compress portions of the nebula, causing the nebular matter to
collapse upon itself by gravitational forces into a protostar. As the solar nebula
collapses,it rotates faster and faster, and spiral arms peel off the rapidly spin-
ning nebula to form a protoplanetary disk. Meanwhile, the compressional heat
initiates a thermonuclear reaction in the core, and a star is born.
A new star forms in the Milky Way galaxy every few years or so. About
4.6 billion years ago, the Sun, an ordinary main-sequence star, ignited in one
of the dusty spiral arms of the galaxy about 30,000 light-years from the cen-
ter. Single, medium-size stars such as the Sun are a rarity. Due to their unique
evolution, these stars appear to be the only ones with planets. Thus, of the
myriad of stars overhead, only a handful might possess a system of orbiting
planets, and fewer yet might contain life.
When the Sun first ignited, the strong solar wind blew away the lighter
components of the solar nebula and deposited them into the outer regions of
the solar system. The remaining matter in the inner solar system comprised
mostly stony and metallic fragments,ranging in size from fine sand grains to
huge boulders. In the outer solar system, where temperatures were much
colder, rocky material along with solid chunks of water ice, frozen carbon
dioxide, and crystalline methane and ammonia condensed. The outer planets
are believed to possess rocky cores about the size of Earth, surrounded by a
mantle composed of water ice and frozen methane, and a thick atmosphere,
mostly composed of hydrogen and helium.
The Sun was extremely unstable during its first billion years of existence.
The solar output was only about 70 percent of its present intensity. The Sun
provided only as much warmth on Earth as it presently does on Mars. The
early Sun rapidly spun on its axis, completing a single rotation in just a few
days as compared with 27 days today. Periodically, the Sun expanded up to a
third larger than its current size, and enormous solar flares leaped millions of
PLANET EARTH