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|Hubble Tuning Fork|
The Hubble Tuning Fork
After he discovered what galaxies really were, Edwin Hubble became the first person to classify galaxies. Astronomers use his system, called the "Hubble Tuning Fork," even today. First, Hubble divided the galaxies into two general categories: elliptical and spiral galaxies. Elliptical galaxies are shaped like ellipses, and spiral galaxies are shaped like spirals, with arms winding in to a bright center.
Elliptical galaxies are classified by how round or flat they look. An E0 galaxy is very round and an E7 galaxy is very flat. In detail, the number after the "E" is determined by the galaxy's ellipticity - the ratio of the ellipse's major axis to minor axis. Galaxies with higher ellipticities have higher numbers.
Hubble noticed that some spiral galaxies have a bright line, or bar, running through them. He called these galaxies "barred spiral galaxies." Galaxies with spiral arms, but without the bar, are just called "spiral galaxies."
Spiral galaxies are further classified by how tightly their arms are wound. Type a galaxies have their arms wound very tightly and have large central bulges. Type c galaxies have very their arms would loosely and have small central bulges.
Some galaxies are a transition type between the elliptical and spiral galaxies, labeled S0 on the tuning fork. These are called "lenticular galaxies." Lenticular galaxies have a central bulge and a disk but no spiral arms.
The third class of galaxies is irregular galaxies. Irregular galaxies are neither spiral nor elliptical, and can have any number of shapes. They are frequently the product of two galaxies colliding with each other, or at least affecting each other through the force of gravity.
You can see why this diagram is called the Hubble tuning fork. Hubble believed that galaxies started at the left end of the diagram and evolved to the right. He called the elliptical galaxies "early galaxies" and the spirals "late galaxies."
We now know that he was wrong: galaxies do not move down the forks of the diagram as they evolve. We know this because spiral galaxies rotate quickly (on an astronomical scale), while elliptical galaxies do not. There is no way that an elliptical galaxy could spontaneously begin rotating, so there is no way an elliptical galaxy could turn into a spiral galaxy.
Although Hubble was wrong about his theory of galaxy evolution, his diagram provides a useful way to classify galaxies. In fact, astronomers today still use his terminology: elliptical galaxies are still referred to as "early galaxies" and spirals as "late galaxies."
Hubble tuning fork diagram courtesy of the Space Telescope Science Institute.