1. Hubble’s Classification of Galaxies

2. The Accumulation of Information Hubble set out to derive distances to as many of the nebulae (now called ‘galaxies’) as he could. But he also noted that they do not all look the same. What sorts of galaxies exist? Can they be classified in some useful way? What can we learn from this?

3. Three Main Types of Galaxies 1. SPIRAL of two kinds ‘ Regular’ ‘ Regular’ the spiral arms come out right from the centre

4. and ‘ Barred’ the spiral arms start at opposite ends of a central ‘bar’ feature

5. 2. Elliptical they can appear round or elongated, with very smooth brightness (no spiral arms or structure)

6. …and 3. Irregular

7. What is a Galaxy’s True Shape? Both of these (‘thick cigar’ Both of these (‘thick cigar’ or ‘fat pancake’) will look or ‘fat pancake’) will look round, if seen pole-on! round, if seen pole-on! It’s hard to determine! It’s hard to determine!

8. Remember! In observing any given galaxy, we have only one point of view – we can’t ‘move around’ and look at it from a different perspective! Nor can we look for long-term changes (except in very limited ways, like noting the explosion of a single star as a supernova, or the periodic variability of Cepheids within a galaxy).

9. Hubble’s ‘Tuning Fork’ Diagram

10. In Cartoon Form (easier to read):

11. …and Even with Some Numbers

12. How Should We Interpret Hubble’s Diagram? Hubble was attempting to show that there is a continuum of properties -- at least in appearance, but possibly nothing more. (He knew that the detailed understanding of stellar contents, masses, motions, and so on could only be determined over the course of time.)

13. The Obvious Question Is the ‘tuning fork’ indicative of an evolution of galaxies from one form to another? To answer this, we need to know much more about the properties of galaxies. To answer this, we need to know much more about the properties of galaxies.