How Stars are grouped together
1. Open Clusters- loose groupings of stars; few stars;
constellations.
2. Globular Clusters- close groupings of many stars.
3. Nebulae- thin, wispy- colored glowing gas clouds.
4. Galaxies- three main types.
Types of Galaxies
Spirals & Ellipticals & Irregulars
Key Ideas:
Three basic types of Galaxies:
• Spirals
• Ellipticals
• Irregulars
http://hubblesite.org/discoveries/hubble_deep_field/
Hubble Classification of Galaxies
All bright galaxies fall into one of three
broad classes according to their shape:
• Spiral Galaxies (~75%)
• Elliptical Galaxies (20%)
• Irregular Galaxies (5%)
Basic classification system developed by
Hubble (1926 & 1936).
Elliptical_3 Elliptical_8
Type E: Ellipticals
Show little internal structure:
• 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)
Elliptical Galaxies
M87: Giant Elliptical
Leo I: Dwarf Elliptical (Spheroidal) Galaxy
Sprial_Galaxies_4
Sprial_Galaxies_7
Type S: Ordinary Spirals
Classified by relative strength of the central
bulge & tightness of the spiral arms
Types: Sa, Sb, and Sc
• Sa: strong bulge & tight, indistinct arms
• Sb: intermediate type
• Sc: small bulge & loose, well-defined
arms
The Milky Way is probably type Sb.
Ordinary Spiral Galaxies
NGC 2997: Sc Galaxy
NGC 4321 (M100): Sc Galaxy
NGC 891: Edge-on Sb
Galaxy
Type SB: Barred Spirals
Parallel group to the ordinary 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
Barred Spiral Galaxies
NGC 3351: SBb Galaxy
NGC 1365: SBc Galaxy
M83: SBc Galaxy
Irregular_9 Irregular_6
Type I: Irregulars
Show an irregular, often chaotic structure.
Little evidence of systematic rotation.
Catch-all class:
• Proposed systematic subclasses, but
many irregulars defy classification.
Significant dwarf irregular population,
classified as "dI"
Irregular Galaxies
Large Magellanic Cloud (LMC)
Small Magellanic Cloud (SMC)
Sprial_Galaxies_7
Sprial_Galaxies_4
Spiral Galaxies
Properties:
9
12
• Mass: 10 - 10 Msun
• Diameter: 5 - 50 kpc
8
11
• Luminosity: 10 - 10 Lsun
Structure & Dynamics:
• Disk + Spheroid
• Supported by relatively rapid rotation,
but spheroid is puffed up by random
motions.
Elliptical_8 Elliptical_3
Elliptical Galaxies
Properties:
5
13
• Mass: 10 - 10 Msun
• Diameter: 1 - 200 kpc
6
12
• Luminosity: 10 - 10 Lsun
Structure & Dynamics:
• Spheroid of old stars with little gas or
dust
• Supported by random motions of stars
with some very slow rotation
Irregular_9 Irregular_6
Irregular Galaxies
Properties:
6
11
• Mass: 10 - 10 Msun
• Diameter: 1 - 10 kpc
6
9
• Luminosity: 10 - few x 10 Lsun
Structure & Dynamics:
• Chaotic structure, lots of young blue
stars
• Moderate rotation in Irregulars, but very
chaotic motions as well.