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Galaxy Classification
 Edwin
Hubble was the first person
to establish the distances to other
galaxies.
 Because he studied so many galaxies,
he began to classify them into
different types.
 The shape and color of a galaxy tell
us a lot about the events that have
happened within that galaxy.
4 General Types of Galaxies
Elliptical – spherical to footballshaped
 Spiral – like a propeller
 Barred Spiral – similar to a spiral, but
with a straight bar through the
nucleus
 Irregular – no well-defined shape

Elliptical Galaxies
Look like enormous globular clusters.
 Elliptical galaxies are often bigger
than spiral galaxies.
 Stars are yellow & red, with very little
free dust & gas.
 These galaxies are old & no longer
forming new stars.

Elliptical Galaxies
Elliptical galaxies may form from the
collision of 2 or more spiral galaxies.
 Spherical elliptical galaxies are E0’s.
Very elongated ellipticals (football
shaped) are E7’s.
 All ellipticals fall in the E0 to E7
range.

M87 galaxy in Virgo – an E0 type
M32 in Andromeda, an E2 type
M110 in Andromeda,
a dwarf elliptical
galaxy of E6 type.
Spiral Galaxies
Like our own Milky Way, all spiral
galaxies have a nuclear bulge, and at
least 2 spiral or propeller-shaped
arms, with a halo of gas, dust, stars,
and globular clusters.
 Spiral galaxies are classified as Sa,
Sb, or Sc depending on the size of
the nucleus & how tightly the arms
are wrapped.

M104 – the Sombrero Galaxy, an Sa type,
large nucleus & very tight spiral arms.
The Andromeda Galaxy, M81, is a type Sb.
It has a medium nucleus & less tight arms.
This type Sc has a small nucleus and very
open arms.
Barred Spiral Galaxies


Barred Spiral galaxies (SB) are
classified just like ordinary spiral
galaxies: SBa, SBb, SBc.
Our own Milky Way is probably an
SBb type galaxy, similar to
Andromeda, but with a central bar.
The bar is very noticeable in this SBc type.
In a barred
spiral, the
arms don’t
“grow”
directly
from the
nucleus.
This is NGC
1365.
Irregular Galaxies
Several famous galaxies are irregular
and have no definite shape.
 The Large & Small Magellanic Clouds,
and the Cartwheel Galaxy are
irregular.
 These galaxies are full of regions of
new star formation!

Irregular Galaxies 2
Irregular galaxies are often the result
of collisions between galaxies.
 Our own Milky Way cut a single
smaller galaxy like a buzz saw blade.
 The Large & Small Magellanic Clouds
are the “chips” left over from that
round of cannibalism.

The Large Magellanic Cloud.
The Cartwheel
Galaxy.
Now you try Hubble’s scheme.

Homework: Classify the 15 galaxies
on the handout.