http://seds.lpl.arizona.edu/messier/

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 GalaxiesAll bright galaxies fall into one of threebroad classes according to their shape:• Spiral Galaxies (~75%)• Elliptical Galaxies (20%)• Irregular Galaxies (5%)

Basic classification system developed byHubble (1926 & 1936).

Elliptical_3 Elliptical_8

Type E: EllipticalsShow 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 GalaxiesM87: Giant Elliptical Leo I: Dwarf Elliptical (Spheroidal) Galaxy

Sprial_Galaxies_4 Sprial_Galaxies_7

Type S: Ordinary SpiralsClassified by relative strength of the centralbulge & tightness of the spiral armsTypes: Sa, Sb, and Sc• Sa: strong bulge & tight, indistinct arms• Sb: intermediate type• Sc: small bulge & loose, well-defined

armsThe Milky Way is probably type Sb.

Ordinary Spiral GalaxiesNGC 2997: Sc Galaxy NGC 4321 (M100): Sc Galaxy NGC 891: Edge-on Sb

Galaxy

Type SB: Barred SpiralsParallel 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

barSame subclasses:SBa, SBb, and SBc

Barred Spiral GalaxiesNGC 3351: SBb Galaxy NGC 1365: SBc Galaxy M83: SBc Galaxy

Irregular_9 Irregular_6

Type I: IrregularsShow 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 GalaxiesLarge Magellanic Cloud (LMC) Small Magellanic Cloud (SMC)

Sprial_Galaxies_7 Sprial_Galaxies_4

Spiral GalaxiesProperties:• Mass: 109 - 1012 Msun

• Diameter: 5 - 50 kpc• Luminosity: 108 - 1011 Lsun

Structure & Dynamics:• Disk + Spheroid• Supported by relatively rapid rotation,

but spheroid is puffed up by randommotions.

Elliptical_8 Elliptical_3

Elliptical GalaxiesProperties:• Mass: 105 - 1013 Msun

• Diameter: 1 - 200 kpc• Luminosity: 106 - 1012 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 GalaxiesProperties:• Mass: 106 - 1011 Msun

• Diameter: 1 - 10 kpc• Luminosity: 106 - few x 109 Lsun

Structure & Dynamics:• Chaotic structure, lots of young blue

stars• Moderate rotation in Irregulars, but very

chaotic motions as well.