Galaxy Types

1. Galaxy Types

2. Hubble Tuning Fork

3. Elliptical

4. Elliptical • On the left of the diagram are the Elliptical galaxies • Elliptical galaxies contain very little gas and dust, and the gas that is present is very hot and diffuse. • Consequently, there is no current star formation in elliptical galaxies. • The stars in elliptical galaxies are old Population II stars. • Unlike the disks of spiral and lenticular galaxies, elliptical galaxies are NOT rotating rapidly.

5. Elliptical galaxies are subclassified according to how flattened they appear. • Let ``a'' be the diameter of an elliptical galaxy along its longest dimension (its major axis, in the language of mathematicians). • Let ``b'' be the diameter along the shortest dimension. The elliptical galaxy is then given the label ``En'', where ``n'' is a number given by the formula:n = 10(a-b)/a

6. For instance, a galaxy which appears circular has a=b, and hence n=0. Galaxies which appear circular are thus given the label ``E0''. Slightly more flattened galaxies are labeled ``E1'', and so forth, up to the most flattened elliptical galaxies, which are called ``E7''.

7. Since elliptical galaxies appear elliptical in the sky, they must be ellipsoidal in three dimensions. Just as an ellipse is a distorted circle, an ellipsoid is a distorted sphere. There are three types of ellipsoid: • 1 - a stretched sphere (like a hot dog) • 2 - a squashed sphere (like a hamburger) • 3 - a sphere squashed in one direction, stretched in another (like a baking potato)

8. Hubble Tuning Fork

9. Lenticular

10. Lenticular • After these simple types of galaxies the diagram splits into two. On the upper branch are the S0 galaxies first - called lenticular galaxies because they are shaped like a lens in a magnfiying glass. • The description is made up of the "S", meaning lenticular, the "0", meaning no arms, and the subscript number indicates how heavily a stripe is absorbed out of the image of the galaxy by dust in the galactic disc.

11. Lenticular galaxies have • flat, rotating disks • central bulges • VERY LITTLE gas and dust in the disk • NO spiral arms

12. The fact that lenticular galaxies lack both gas and spiral arms has led to the hypothesis that interstellar gas is necessary for spiral arms to form.

13. Hubble Tuning Fork

14. Continuing along that branch the next 3 types are all have spiral arms, and they are grouped by how tightly those arms are wound and how large the central bulge is - the two happen to be closely related. The name is defined by the "S" and the lower case letter after which indicates how wound up the arms are: from "a" to "d". • Sa galaxies have big central bulges, tightly wound spiral arms, and a relatively small amount of interstellar gas. • Sc galaxies have small bulges, loosely wound spiral arms, and a relatively large amount of gas. • Sb galaxies are intermediate between Sa and Sc. Our own galaxy might be an Sb galaxy

15. All spiral galaxies have • flat, rotating disks • central bulges • gas and dust in the disk • star formation in spiral arms

16. Hubble Tuning Fork

17. Barred

18. Barred • The lower branch of the tuning fork diagram is largely a copy of the upper branch, but its occupants all have a line of stars through the center - a bar. The first two pictures shown on the diagram are the SB0 galaxies. The B stands for barred, and a subscript number indicates how heavily defined the bar is. • In a barred spiral galaxy, the spiral arms wind away from an elongated central bar rather than from a spherical central bulge. • subspecies'' of spiral galaxy

19. Summation • Spiral galaxies have rotationally flattened disks, and contain moderate amounts of gas and dust. • Elliptical galaxies are slowly rotating ellipsoids, and contain little gas and dust.

20. Irregular

21. Irregular • Irregular galaxies tend to contain lots of gas and dust. As a consequence, irregular galaxies contain copious star formation. • The star formation is patchy (tending to occur in clusters), as is the distribution of dust. • Therefore, irregular galaxies are given their characteristic irregular, patchy, raggedy appearance. • The Large and Small Magellanic Clouds, about 50,000 parsecs away from our own galaxy, are examples of irregular galaxies

22. The Hubble tuning fork does not show an evolution of Galaxy formation. • By this I mean that it doesn’t start out with one type and evolve into another type.

23. On a piece of paper. Label the following ten galaxies by type using the Hubble tuning fork

24. #1

25. #2

26. #3

27. #4

28. #5

29. #6

30. #7

31. #8

32. #9

33. #10