Galaxy Classification

Edwin Hubble was the first person to establish the distances to other galaxies. 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. 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. 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 Elliptical – spherical to football- shaped Spiral – like a propeller Spiral – like a propeller Barred Spiral – similar to a spiral, but with a straight bar through the nucleus Barred Spiral – similar to a spiral, but with a straight bar through the nucleus Irregular – no well-defined shape Irregular – no well-defined shape

Elliptical Galaxies Look like enormous globular clusters. Look like enormous globular clusters. Elliptical galaxies are often bigger than spiral galaxies. Elliptical galaxies are often bigger than spiral galaxies. Stars are yellow & red, with very little free dust & gas. Stars are yellow & red, with very little free dust & gas. These galaxies are old & no longer forming new stars. These galaxies are old & no longer forming new stars.

Elliptical Galaxies Elliptical galaxies may form from the collision of 2 or more spiral 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. Spherical elliptical galaxies are E0’s. Very elongated ellipticals (football shaped) are E7’s. All ellipticals fall in the E0 to E7 range. 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. 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. 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. 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. 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. Several famous galaxies are irregular and have no definite shape. The Large & Small Magellanic Clouds, and the Cartwheel Galaxy are irregular. The Large & Small Magellanic Clouds, and the Cartwheel Galaxy are irregular. These galaxies are full of regions of new star formation! These galaxies are full of regions of new star formation!

Irregular Galaxies 2 Irregular galaxies are often the result of collisions between galaxies. Irregular galaxies are often the result of collisions between galaxies. Our own Milky Way cut a single smaller galaxy like a buzz saw blade. 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 & 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. Homework: Classify the 15 galaxies on the handout.