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

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

Key Ideas

Key Ideas:

Hubble Classification of Galaxies

Spirals (Ordinary & Barred)

Ellipticals

Irregulars

Differ in terms of

Relative Stellar & Gas content

Star Formation Histories

Internal Motions

Dwarf Galaxies

Hubble Classification of Galaxies

• Spiral Galaxies (~75%)
• Elliptical Galaxies (20%)
• Irregular Galaxies (5%)

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

Type E: Ellipticals

• Elliptical in shape
• No disks, spiral arms, or dust lanes
• Brightest stars are red

Classified by the degree of apparent flatness:

• E0 is circular
• E7 is flattest (~3:1 aspect ratio)

Type S: Ordinary Spirals

Types: Sa, Sb, and Sc

• Sa: strong bulge & tight, indistinct arms
• Sb: intermediate type
• Sc: small bulge & loose, well-defined arms

Some images of Ordinary Spirals

Type SB: Barred Spirals

• About as many barred as ordinary spirals.

Feature a strong central stellar bar:

• Bar rotates as a unit (solid body rotation)
• Spiral arms emerge from the ends of the bar

Same subclasses:

• SBa, SBb, and SBc

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

Little evidence of systematic rotation.

Catch-all class:

• Proposed systematic subclasses, but many irregulars defy classification.

Significant dwarf irregular population, classified as "dI"

Some images of Irregular Galaxies

Spiral Galaxies

• Mass: 10⁹ - 10¹² M_sun
• Diameter: 5 - 50 kpc
• Luminosity: 10⁸ - 10¹¹ L_sun

Structure & Dynamics:

• Disk + Spheroid
• Supported by relatively rapid rotation, but spheroid is puffed up by random motions.

Elliptical Galaxies

• Mass: 10⁵ - 10¹³ M_sun
• Diameter: 1 - 200 kpc
• Luminosity: 10⁶ - 10¹² L_sun

Structure & Dynamics:

• Spheroid of old stars with little gas or dust
• Supported by random motions of stars with some very slow rotation

Irregular Galaxies

• Mass: 10⁶ - 10¹¹ M_sun
• Diameter: 1 - 10 kpc
• Luminosity: 10⁶ - few x 10⁹ L_sun

Structure & Dynamics:

• Chaotic structure, lots of young blue stars
• Moderate rotation in Irregulars, but very chaotic motions as well.

Relative Stellar & Gas Content

• Range is ~10-20% gas
• On-going star formation in the disks
• Mix of Pop I and Pop II stars

Ellipticals:

• Very little or no gas or dust
• Star formation ended billions of years ago
• See only old Pop II stars

Irregulars:

• Can range up to 90% gas
• Often a great deal of on-going star formation
• Dominated by young Pop I stars

Dwarf Irregulars:

• Very metal poor (<1% solar)
• Forming stars for the first time only now.

Dwarf Galaxies

• Significant number of dwarfs
• Most common type of galaxy by number

There are no (convincing) Dwarf Spirals.

Possibilities:

• Small versions of their larger cousins
• Different population of objects despite their superficial similarities to larger E's and Irr's

Cosmic Building Blocks

• Basic units of larger, organized structures
• Sites of star formation from raw gas
• Factories that synthesize heavy elements from Hydrogen & Helium