3.02 Galaxy Classification Lab

Galaxies can also be classified by their shape.

Elliptical galaxies appear smooth and featureless, with their shapes ranging to circular to very elongated.

Spiral galaxies have distinctive arms that curve outwards from a central bulge. Spirals are defined by the tightness of their arms and whether a "central bar" is present in the center.

Lenticular galaxies feature a central bulge and flattened disk, but lack spiral arms.

Irregular galaxies lack a defined shape and may have once been another type of galaxy distorted by the gravitational pull of other galaxies.

In 1927, Edwin Hubble created a classification scheme for galaxies based primarily on their shape that is still used today. It is sometimes called Hubble's "tuning fork" because its shape resembles a sideways tuning fork.

• Elliptical galaxies are classified as E0 (perfectly round) to E7 (extremely oval shaped).

• Lenticular galaxies are classified as S0, or galaxies in between the elliptical or spiral classiciation.

• Spiral galaxies are divided into 2 categories: unbarred and barred

  • Unbarred spirals are classified as Sa (tightly wound arms with bright central bulge) to Sc (loosely wound arms with fainter central bulge)

  • Barred spirals are classified as SBa (tightly wound arms with a distinct central bar) to SBc (loosely wound arms with a fainter central bar)

• Irregular galaxies are not part of the Hubble sequence.

Galaxies can be classified by the color of their stars.

Think about what you know about star lifecycles.

Remember that most stars can be found in the Main Sequence for the majority of their lives.

Young blue and white stars are the hottest and brightest. There are young yellow, orange, and red stars, but they aren't as hot or bright.

Once stars are older and run out of hydrogen to fuse to helium, they become red giants or supergiants. These stars are cooler, but still very bright.

White dwarfs are also older stars that are very hot, but they are very dim and hard to see.