Galaxy Classification n Edwin Hubble was the first

Galaxy Classification

n Edwin Hubble was the first person to establish the distances to other galaxies. n Because he studied so many galaxies, he began to classify them into different types. n 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 n Spiral – like a propeller n Barred Spiral – similar to a spiral, but with a straight bar through the nucleus n Irregular – no well-defined shape n

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

Elliptical Galaxies Elliptical galaxies may form from the collision of 2 or more spiral galaxies. n Spherical elliptical galaxies are E 0’s. Very elongated ellipticals (football shaped) are E 7’s. n All ellipticals fall in the E 0 to E 7 range. n

M 87 galaxy in Virgo – an E 0 type

M 32 in Andromeda, an E 2 type

M 110 in Andromeda, a dwarf elliptical galaxy of E 6 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. n Spiral galaxies are classified as Sa, Sb, or Sc depending on the size of the nucleus & how tightly the arms are wrapped. n

M 104 – the Sombrero Galaxy, an Sa type, large nucleus & very tight spiral arms.

The Andromeda Galaxy, M 81, 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 n n 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. n The Large & Small Magellanic Clouds, and the Cartwheel Galaxy are irregular. n These galaxies are full of regions of new star formation! n

Irregular Galaxies 2 Irregular galaxies are often the result of collisions between galaxies. n Our own Milky Way cut a single smaller galaxy like a buzz saw blade. n The Large & Small Magellanic Clouds are the “chips” left over from that round of cannibalism. n

The Large Magellanic Cloud.

The Cartwheel Galaxy.

Now you try Hubble’s scheme. n Homework: Classify the 15 galaxies on the handout.