There are three main classifications of galaxies: spiral, elliptical, and irregular. Spiral galaxies make up 75% of bright galaxies and are further divided into normal spirals and barred spirals, classified by the strength of their central bulge and tightness of spiral arms. Elliptical galaxies comprise 20% of bright galaxies and have a spherical structure without clear spiral arms. Irregular galaxies make up the remaining 5% and have a chaotic, non-uniform structure. The Milky Way is a barred spiral galaxy over 100,000 light years wide that contains over 100 billion stars, including our sun which is located 28,000 light years from the galaxy's center.

1. Galaxies
Galaxy Field – Hubble Telescope

3. Hubble Classification of Galaxies (3)

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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 (1936).

4. Hubble “Tuning Fork” Diagram

5. 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

6. 
Sa:
Strong bulge and tight, indistinct arms.

7. Sb

Intermediate type

8. 
Sc
Small central bulge and loose, well defined arms.

9. Examples
 The Milky Way (130,000 light
years wide)
 Andromeda (our nearest galaxy
is 2-3 million light years away).

10. b. Type SB Barred Spirals
Features a strong central stellar bar.
 Bar rotates as a unit (solid body
rotation).
 Spiral arms emerge from the ends
of the bar.

11. Same subclasses
a.
b.
c.
SBa
SBb
SBc

12. II. Ellipticals
Structure and Dynamics:
 Spheroid of old stars with little
gas or dust.
 Supported by random motions
of stars with a very slow
rotation.

13. E0

14. E1

15. E3

16. III. Irregulars
Structure and
Dynamics:



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Chaotic structure
and movement.
No definite
pattern.
Lots of young blue
stars.
Moderate rotation
in Irregulars, but
very chaotic
motions as well.

17. Our Milky Way
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Spiral Galaxy.
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.

18. You
are
here.

19. 3 parts of the Milky Way
1. Disk – flat rotating frisbee shaped part of the
galaxy. 100,000 – 130,000 light years in diameter.

20. 2. Bulge – the dense roughly spherical
concentration of stars at the center of disk.
 Roughly 24,000 light years from the sun.

21. 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.