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 football-shaped

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 veryopen 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 barredspiral, thearms don’t“grow”directlyfrom thenucleus.

This is NGC1365.

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 CartwheelGalaxy.

Now you try Hubble’s scheme.

Homework: Classify the 15 galaxies on the handout.