Western Washington University   College of Sciences and Technology   Physics/Astronomy Dept.   Bellingham, WA USA
 
ASTRO 101

Analemma
Asteroids
Aurora
Big Bang
Black Holes
Bode Titius
Brightest Stars
Comets
Constellations
Coordinates
Cosmology
Cruithne
Dark Matter
Eclipses
Galaxies
Historical
HR Diagram
Hubble's Law
Intelligent Life
Kepler's Laws
Leap Year
Light Waves
Lunar Libration
Messier Objects
Meteors
Milky Way
Moon
Moon Phases
Planets
Precession
Rainbows
Redshift
Seasons
Stellar Evolution
Stardust
Sun & Fusion
Telescopes
Tides
Time of Day
Twilight
Zodiac
 
Galaxies
"The infinitude of creation is great enough to make a world, or a Milky Way of worlds, look in comparison with it what a flower or an insect does in comparison with the Earth."   Immanuel Kant

M31, Andromeda Galaxies are collections of stars, interstellar dust and gas, and other matter. They contain anywhere from millions to trillions of stars. They are extremely large compared to our solar system. If our solar system could fit into a coffee cup, then our galaxy (the Milky Way) would be the size of the North American Continent!

During the 1920's when most astronomers thought the Milky Way was the whole of the Universe, Edwin Hubble realized that the Milky Way was only one of a Universe full of galaxies. He classified them into a system that is known as the "tuning fork."

Edwin Hubble 1889-1953
Edwin Hubble 1889-1953
Elliptical galaxies are sorted by how round or or stretched out they appear. An E0 galaxy is very round and an E7 galaxy is very elliptical. The higher the number, the more eccentric the ellipse.

Some galaxies have a spiral shape, and a bright bar running across the middle. Hubble named these galaxies "barred spiral galaxies."

Galaxies that have the spiral shape without the central bar are classified simply as "spiral galaxies."

Both barred and non-barred spiral galaxies are more finely sorted by how tightly their arms are wound to the core. Type "a" have arms wound very tightly to the core and have large central bulges. Type "c" have arms wound loosely and have small central bulges.

Some galaxies appear to be between the elliptical and spiral galaxies, they are labeled S0 on the tuning fork. These are called "lenticular galaxies" which means "lens-like." Lenticular galaxies have a central bulge and a disk like the spiral galaxies but no spiral arms.

A third class is called "irregular." Irregular galaxies are difficult to classify as spiral or elliptical, and can have almost any shape. Many of these probably result from two galaxies colliding, or by being reorganized by the gravity from a near miss. Galaxies that are extremely odd are sometimes classified as "peculiar."