Hubble Classification of Galaxies (3)
All bright galaxies fall into one of 3 broad classes
according to their shape.
75% - Spiral
20 % - Elliptical
5 % - Irregular
Basic classification system developed by Edwin
Hubble “Tuning Fork” Diagram
I. Spirals (2)
a) Normal Spirals
Classified by its relative strength of the central
bulge & tightness of the spiral arms.
Types: Sa, Sb, and Sc
Strong bulge and tight, indistinct arms.
Small central bulge and loose, well defined arms.
The Milky Way (130,000 light
Andromeda (our nearest galaxy
is 2-3 million light years away).
b. Type SB Barred Spirals
Features a strong central stellar bar.
Bar rotates as a unit (solid body
Spiral arms emerge from the ends
of the bar.
Structure and Dynamics:
Spheroid of old stars with little
gas or dust.
Supported by random motions
of stars with a very slow
Lots of young blue
in Irregulars, but
motions as well.
Our Milky Way
Our sun is just one of the perhaps 100 billion
stars that make up the Milky Way.
The sun is located about 28,000 light years from
the center of the Milky Way.
The Milky Way galaxy is ~100,000 – 130,000
light years in diameter.
3 parts of the Milky Way
1. Disk – flat rotating frisbee shaped part of the
galaxy. 100,000 – 130,000 light years in diameter.
2. Bulge – the dense roughly spherical
concentration of stars at the center of disk.
Roughly 24,000 light years from the sun.
3. Halo – is sparsely populated extended region
that encompasses the disk and bulge of the
galaxy. The brightest members of the halo are
large star clusters, known as globular clusters.