Chapter 1 – Our Place in the Universe
1.1Our Modern View of the Universe
The Sun, Moon, planets and stars appear to circle around our sky each day,
and we cannot feel the constant motion of Earth as it rotates on its axis and
orbits the sun. It therefore seems quite natural to assume that we live in an
Earth-centered or geocentric universe.
What is our place in the universe?
Earth is a planet in our solar system, which consists of the sun and all the
objects that orbit it: the planets and their moons, and countless smaller
objects including rocky asteroids and icy comets.
The sun and all the stars we can see with the naked eye make up only a small
part of a huge, disk-shaped collection of stars called the Milky Way galaxy.
A galaxy is a great island of stars in space, containing from a few hundred
million to a trillion or more stars.
The milky way galaxy contains over 100 billion stars
Our solar system is located a little more than halfway from the galactic centre
to the edge of the galactic disk
Our milky way is one of the two largest among the 40 galaxies in the Local
Groups of galaxies with more than a few dozen members are often called
The regions in which galaxies and galaxy clusters are most tightly packed are
Together, all these structures make up our universe
How did we come to be?
The Big Bang and the Expanding Universe
Telescopic observations of distant galaxies show that the entire universe is
expanding, meaning that average distances between galaxies are increasing
with time. this fact implies that galaxies must have been closer together in
the past, and if we go back far enough, we much reach the point at which the
We call this beginning the Big Bang, and from the observed rate of expansion
we estimate that it occurred about 14 billion years ago
Structures such as galaxies and galaxy clusters occupy regions where gravity
has won out against the overall expansion That is, while the universe as a
whole continues to expand, individual galaxies and galaxy clusters (and
objects within them such as planets and starts) do not expand. Stellar Lives and Galactic Recycling
Within galaxies and like the Milky Way, gravity drives the collapse of clouds
of gas and dust to form stars and planets.
Stars are not living organisms, but they nonetheless go through “life cycles.”
A star is born when gravity compresses the material in a could to the point
where the centre becomes dense enough and hot enough to generate energy
by nuclear fusion, the process in which lightweight atomic nuclei smash
together and stick (or fuse) to make heavier nuclei.
The star “lives” as long as it can shine with energy from fusion, and “dies”
when it exhausts its usable fuel.
Massive stars die in titanic explosions called supernovae. The returned
matter mixes with other matter floating between the stars in the galaxy,
eventually becoming part of new clouds of gas and dust from which new
generations of stars can be born.
Galaxies therefore function as cosmic recycling plants, recycling material
expelled from dying stars into new generations of stars and planets
Stars Manufacture the Elements of Earth and Life
By studying stars of different ages, we have learned that the early universe
contained only the simplest chemical elements: hydrogen and helium (and a
trace of lithium)
Earth is primarily made of other elements, such as carbon, nitrogen, oxygen,
and iron. Where did these other elements come from? Evidence shows that
they were manufactured by stars, some through the nuclear fusion that
makes stars shine and others through nuclear reactions accompanying the
explosions that end stellar lives, manufactured them.
Most of the material from which we and our planet are made was created
inside stars that lives and died before the birth of our Sun.
How can we know what the universe was like in the past?
We can actually see into the past by studying light form distant stars and
The speed of light is 300,000 km/second
Light from the stars takes many years to reach us, so we measure distances
to the stars in units called light-years. One light-year is the distance that
light can travel in 1 year – about 10 trillion km.
The father away we look in distance, the further back we look in time.
Can we see the entire universe?
The distance of 14 billion light-year therefore marks the boundary of our
observable universe – the portion of the entire universe that we can
Note that this fact does no