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Organizing the cosmos
& our
There is still uncertainty on exactly how the
universe and our solar system formed. There
are many theories but no one knows for sure.
By studying distant galaxies astronomers may
one day determine how the universe was
What is a Galaxy?
A large group of stars,
dust, and gases held
together by gravity
Interstellar Matterdust and gases found
between stars
Our Milky Way
Size: large spiral disk 100,000 light
year wide & 10,000 thick
Structure: Spiral disk-like shape like
our solar system
Galaxy is Greek for milk
**Our Sun is 1 of 100 Billion stars
in the Milky Way Galaxy
The sun rotates around the galaxy
once about every 240 million
Pinwheel Galaxy
Largest galaxy known to date
The giant spiral disk of stars,
dust and gas is 170,000 lightyears across or nearly twice the
diameter of our Milky Way.
The galaxy is estimated to
contain at least one trillion
stars. Approximately 100
billion of these stars alone
might be like our Sun in terms
of temperature and lifetime.
Andromeda Galaxy
Our closest neighbor
It is visible as a faint
smudge on a moonless night,
it is one of the farthest
objects visible to the naked
eye, and can be seen even
from urban areas with
2.3 million ly away
Did You Know ?
The Milky Way galaxy, and its closest neighbor, the
Andromeda galaxy, are on a collision course with
each other! This collision between these two spiral
galaxies will likely occur in about 3 billion years, and
the resulting galaxy is expected to be an elliptical.
Galaxy Classification
In 1924, Edwin Hubble
divided galaxies into
different “classes” based
on their appearance.
Galaxies differ in size,
shape, and ages of their
•Hubble classification
serves as the basic language
of the field.
Types of Galaxies
Spirals & Ellipticals & Irregulars
(oh my!)
1. Spiral Galaxy
Disk shaped (looks like a pinwheel)
Contain mixture of young/old stars
Greater concentration of stars near the middle
(older stars)
 Arms (younger stars) are seen extending from
central nucleus
Orderly rotational motion
Our own Galaxy is a spiral.
Types of Spiral Galaxies
Barred Spiral Galaxies- a
galaxy having straight arms
extending from its center
Normal Spiral Galaxies- a
flattened, rotating galaxy
with pinwheel-like arms of
interstellar material and young
stars winding out from its
center or nucleus
Spiral arms attach to the bar
instead of the center disk
2. Elliptical Galaxies
(not a disk) and often
football shaped
Smooth round shape
No arms
Very little interstellar
matter leads to minimal
star formations
Composed of old stars
Random star motion
60% of all galaxies
“Space footballs”
3. Irregular Galaxies
No exact size or shape
Composed of mostly young stars
Lacks symmetry
Small and faint
Final Galaxy vocabulary….
1. Most Galaxies are
located in groups
called a cluster.
2. Local Group – the name
for the cluster of
galaxies the Milky Way
is in
3. Super cluster –
Gigantic Cluster of
Skip Quasars
Hubble’s Tuning Fork
diagram classifying different galaxies
by shape
Use your Hubble’s Tuning Fork Diagram
to help you identify these Galaxies: