COSMIC STRUCTURES
ISLANDS OF STARS
A galaxy can contain millions, billions,
or even trillions of stars. Galaxies are
among the largest gravitationally bound
structures in the Universe.
WHAT IS A GALAXY?
A galaxy is a huge gravitationally bound
system containing stars, stellar remnants,
gas, dust and dark matter.
Galaxies range from relatively small
dwarf galaxies to enormous systems
containing hundreds of billions of stars.
They can also contain supermassive black
holes at their centers.
HOW DO GALAXIES FORM?
Galaxies began forming in the early
Universe as matter gradually collected
under gravity.
Small structures merged together over
billions of years, growing into larger
galaxies.
Dark matter played a major role by
providing gravitational structures that
helped ordinary matter gather and form
stars and galaxies.
THE MILKY WAY
The Milky Way is the galaxy that contains
our Solar System.
It is a barred spiral galaxy with a
central region surrounded by a large
disk of stars, gas and dust.
The Solar System lies in one of its
spiral-arm regions, roughly 26,000
light-years from the galactic center.
GALAXY COLLISIONS
Galaxies can interact and collide because
of their mutual gravity.
Despite the enormous number of stars
inside galaxies, direct star-to-star
collisions are extremely rare because
the distances between stars are so large.
The Milky Way and Andromeda are expected
to interact in the distant future.
MAIN GALAXY TYPES
SPIRAL
Spiral galaxies have a rotating disk
with spiral arms extending from a
central bulge. The Milky Way and
Andromeda are examples.
ELLIPTICAL
Elliptical galaxies have smooth,
rounded shapes and generally contain
older stars with relatively little
cold gas and dust.
IRREGULAR
Irregular galaxies do not have a
simple organized shape. Gravitational
interactions can strongly influence
their appearance.
LENTICULAR
Lenticular galaxies have a disk and
central bulge like spiral galaxies,
but they generally lack prominent
spiral arms.
FAMOUS GALAXIES
MILKY WAY
Our home galaxy and a barred spiral
galaxy containing the Solar System.
ANDROMEDA
A large spiral galaxy approximately
2.5 million light-years from Earth.
TRIANGULUM
A spiral galaxy in the Local Group
and one of the nearest large galaxies
to the Milky Way.
DID YOU KNOW?
When you look at a distant galaxy,
you are looking into the past because
its light can take millions or
billions of years to reach Earth.