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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 27: Spirals & Ellipticals & Irregulars
(oh my!)

Readings: Ch 26, section 26-3

Key Ideas

Key Ideas:

Hubble Classification of Galaxies
Spirals (Ordinary & Barred)

Differ in terms of
Relative Stellar & Gas content
Star Formation Histories
Internal Motions

Dwarf Galaxies

Hubble Classification of Galaxies

All bright galaxies fall into one of three broad classes according to their shape:

Basic classification system developed by Hubble (1926 & 1936).

Hubble Classifications (Tuning Fork Diagram)

Type E: Ellipticals

Show little internal structure:

Classified by the degree of apparent flatness:

Some images of Ellipticals

Type S: Ordinary Spirals

Classified by relative strength of the central bulge & tightness of the spiral arms

Types: Sa, Sb, and Sc

Some images of Ordinary Spirals

Type SB: Barred Spirals

Parallel group to the ordinary spirals:

Feature a strong central stellar bar:

Same subclasses:

The Milky Way is thought to have a weak bar, and is classified by some as SBbc (part way between SBb and SBc), though this is still a matter of some controversy.

Some images of Barred Spirals

Type I: Irregulars

Show an irregular, often chaotic structure.

Little evidence of systematic rotation.

Catch-all class:

Significant dwarf irregular population, classified as "dI"

Some images of Irregular Galaxies

Spiral Galaxies


Structure & Dynamics:

Elliptical Galaxies


Structure & Dynamics:

Irregular Galaxies


Structure & Dynamics:

Relative Stellar & Gas Content




Dwarf Irregulars:

Dwarf Galaxies

Low-luminosity Ellipticals & Irregulars.

There are no (convincing) Dwarf Spirals.


Cosmic Building Blocks

Galaxies of all types are the basic "units" of luminous matter in the Universe. Differences in the types of galaxies reflect the difference in their formation histories and environments.

Return to [ Unit 4 Index | Astronomy 162 Main Page ]
Updated: 2006 February 12
Copyright Richard W. Pogge, All Rights Reserved.