1. Hubble's Tuning Fork
By: Tali Yahalom
March 28, 2006

2. How It All Began… Charles Messier 1730 - 1817
Scientists today know about the existence of galaxies, but a couple of hundred years ago, the only known celestial objects were point-like sources, including planets, comets, and relatively steady, distant stars.
But curiosity spread and knowing about point-like sources was not enough.
While working in an observatory in Paris in 1758, Charles Messier was looking for Halley’s comet when he discovered faint-looking objects in the sky.
Charles Messier

3. Messier called these objects “NEBULAE,” and started to catalog them in order to aid his observations of comets.
The first entry in his catalog—which he noted as “M1”—is what we today call the CRAB NEBULA.
When Messier published
his catalog in 1781,
he listed 103 nebulae.
7 more objects were
added later on.
4 Different Views of the Crab Nebula

4. BUT…
Harlow Shapley
  
Heber Curtis
  
The nature of the nebulae was constantly questioned because the telescope resolution was not good enough to see the objects’ details clearly.
So, in 1920, the National Academy of Sciences in Washington organized a debate for astronomers Harlow Shapley and Heber Curtis concerning the nature of the nebulae.
The 2 opposing theories were that:
1) The Universe is rather small, containing nebulae that are nearby gas-clouds (Shapley)
2) The nebulae are very distant “island universes” (Curtis)
This “Great Debate” incorporated important issues, like how to measure distance to nebulae, how to determine the material composition (gas or many stars), and why there is a “zone of avoidance,” i.e., a region of the sky where nebulae are not found.

5. Uh Oh!!! A few years later, Edwin Hubble settled the issues… How can I
classify the
Universe?

6. Spiral Galaxies…
Hubble isolated Cepheid variable stars* in the Andromeda galaxy (M31) and measured the distance to these stars.
*Remember: A Cepheid variable star is a member of a particular class of variable stars (varying luminosity), notable for a fairly tight correlation between their period of variability and absolute luminosity. The greater the period, the greater the luminosity.)
The distance turned out to be much bigger than the possible size of the Milky Way!
Hubble, therefore, concluded that M31 must be a separate and distant “island Universe,” i.e., a spiral galaxy similar to our own.

7. Hubble’s Pictures Hubble started to take pictures of many galaxies.
He soon realized that they come in many shapes and sizes.
In order to make sense of his findings, he implemented a system of classification based on the appearance of these systems…

8. Discovery  Classification
The two types of galaxies he discovered were Elliptical and Spiral.
Hubble divided spiral galaxies into two groups: Normal and Barred.
Normal Spiral = bulge in the center, spiral arms extending out from the central regions.
Barred Spiral = spiral arms originate from the tips of a straight bar and go through the center of the galaxy.

9. Normal Spiral vs. Barred Spiral

10. There’s More…
Hubble also introduced the classes of lenticular and irregular galaxies.
Lenticular = between ellipticals and spirals, consist of a large bulge with a small flattened disk around them without visible spiral structure.
Irregular = account for 5% of the galaxies we observe nearby, are forming, colliding, and do not fit in with any of the other 3 categories.

11. Lenticular and Irregular Galaxies

12. ORGANIZATION?!?!
Hubble constructed a “tuning-fork” diagram to illustrate his classification system.
Elliptical galaxies (E) are on the stem and are subdivided into classes 0 – 7. The higher the number, the more elongated the galaxy.
Lenticulars are denoted S0 (or SB0, with a bar) and are subdivided into classes 1, 2, and 3 depending on dust absorption within the disk.
Spirals are denoted S (normal) and SB (barred) and are subdivided into classes a, b, and c. “a” signifies a large bulge and relatively weak, tightly wound spiral arms. As the letters continue, these traits change to their opposite.

13. Visualization:
Normal Spiral galaxies that begin with large bulges and small arms that change
Elliptical galaxy with a round looking elliptical
Elliptical galaxy with an elongated elliptical
Barred Spiral galaxies that begin with large bulges and small arms that change

14. How Does Hubble Apply Today?
Since Hubble’s time, tens of thousands of galaxies have been imagined using different kinds of telescopes and instruments.
Many of these galaxies do not fit into Hubble’s relatively simple classification system.
Many more extensions to the tuning fork diagram came about after Hubble’s discovery.
De Vaucouleurs, for example, distinguished between s-shapes and rings, thus making the tuning fork 3-D.
While Hubble’s diagram is neither finite nor as simple as it may seem, it is nevertheless the backbone for all observations and classifications that still go on today.

15. A Diagram that Includes Lenticular and Irregular Galaxies:
A CLOSER LOOK:

16. Bibliography
Foundations of Modern Cosmology, John Hawley and Katherine Holcomb
(“Cepheid Variable Stars”)