We live in an enormous galaxy containing 100
billion stars. This photograph of the nearby galaxy M31, is what we
believe our Milky Way galaxy looks like.
The Big Bang
It is generally believed by most scientists that about 15 billion years ago,
all matter and energy in the universe was confined to an extremely
small area, which suddenly exploded. The universe has been
expanding in size ever since.
Creation of Matter
In the first few seconds of the universe, energy in the form of
photons, condensed out to form the matter of the universe. After a
few minutes, the density and temperature of the universe was such
that it fused some of the Hydrogen into Helium (about 25% by mass).
Creation of Stars and Galaxies
The universe continues to expand, cooling as it does so. Regions of
higher density began forming first stars, when the universe was
about a million years old, then galaxies when it was almost a billion
years old.
Creation of Elements Necessary for Life
Only a few million years after forming the most massive
stars, their central cores will have fused elements such as
C, Si, and finally Fe. Upon going Supernova, these elements are thrown
back into interstellar space, from which the next star will form.