Galaxies

1. Galaxies Astronomy 115

2. First, which of the following is a galaxy? • Open cluster • Globular cluster • Nebula • Interstellar medium (gas and dust) • Supernova remnant • None of these

3. First, which of the following is a galaxy? • Open cluster • Globular cluster • Nebula • Interstellar medium (gas and dust) • Supernova remnant • None of these

4. What is a “star cluster”? • stars formed together at same time • stars are at least weakly gravitationally bound together • two types: open (galactic) and globular (shown to right) Image at http://hubblesite.org/newscenter/archive/releases/star%20cluster/globular/2007/18/image/a/format/web/results/50/

5. Open Clusters • dozens to thousands of stars • young stars! only a few million years old • may still be surrounded by nebula from which they formed • located in the spiral arms of a galaxy • example: Pleiades • Fate: generally, the stars drift apart (not enough gravity) Image at http://hubblesite.org/newscenter/archive/releases/star%20cluster/open/2004/20/image/a/results/50/

6. More open star clusters

7. Globular Clusters • millions to hundreds of millions of stars • old! 6 to 13 billion years • mostly red giants and dwarfs • stars are clumped closely together, especially near the center of the cluster (densely); stars don’t drift apart • surround our disk as a halo Image at http://hubblesite.org/newscenter/archive/releases/star%20cluster/globular/1999/26/image/a/results/50/

8. What is a “nebula”? • A cloud in space • Made of gas and dust • Can have stars inside • Most of the ones we see are inside our Milky Way Galaxy • Different types

9. Large, massive, bright nebulae • Emission Nebula • The hot gas is emitting light

10. Colder, darker nebulae Dark dust blocking the hot gas behind it

11. Leftovers from an Explosion Supernova remnant (smaller, less gas)

12. So, what is a “galaxy”? • A large group of stars outside of our own Milky Way • Made of billions to trillions of stars, held together by its own gravity, with all different ages of stars • Also may have gas and dust • Spiral, or elliptical, or irregular shaped

13. Galaxy Classification E0, …, E7 Sa Large nucleus; tightly wound arms E0 = Spherical E1 Sb Sc Small nucleus; loosely wound arms E7 = Highly elliptical E6

14. Spiral galaxy--Andromeda NOAO/AURA/NSF Images at http://www.noao.edu/image_gallery/html/im0606.html and http://www.noao.edu/image_gallery/html/im0685.html

15. Elliptical Galaxies

16. Irregular Galaxies NASA and NOAO/AURA/NSF Images at http://hubblesite.org/newscenter/archive/releases/galaxy/irregular/2005/09/results/50/ , http://www.noao.edu/image_gallery/html/im0560.html , and http://www.noao.edu/image_gallery/html/im0993.html

17. Irregular Galaxies Often: result of galaxy collisions / mergers Often: Very active star formation (“Starburst galaxies”) The Cocoon Galaxy NGC 4038/4039 Some: Small (“dwarf galaxies”) satellites of larger galaxies (e.g., Magellanic Clouds) Large Magellanic Cloud

18. Galaxy Diversity Even seemingly empty regions of the sky contain thousands of very faint, very distant galaxies Large variety of galaxy morphologies: Spirals Ellipticals Irregular (some interacting) The Hubble Deep Field: 10-day exposure on an apparently empty field in the sky

19. Gas and Dust in Galaxies Spirals are rich in gas and dust Ellipticals are almost devoid of gas and dust Galaxies with disk and bulge, but no dust are termed S0

20. Barred Spirals • Some spirals show a pronounced bar structure in the center • They are termed barred spiral galaxies • Sequence: • SBa, …, SBc, • analogous to regular spirals

21. Our Galaxy: the Milky Way • has about 200 billion stars, and lots of gas and dust • is a barred-spiral (we think) • about 100,000 light-years wide • our Sun is halfway to the edge, revolving at half a million miles per hour around the center of the Galaxy • takes our Solar System about 200 million years to revolve once around our galaxy

22. The Milky Way

23. Mapping the Milky Way How do we know what our Galaxy looks like? We can see: • Stars and star clusters – microwaves generated by water from H II regions (called the MASER technique) traces the Milky Way’s spiral arms • Nebulae – infrared light (detected by the Spitzer Space Telescope) shows the outline of the heat generated by the bar • Other galaxies (analogous structure as our galaxy)

24. Do galaxies evolve over time? Edwin Hubble (whom we’ll hear more about next lecture) in 1926 classified known galaxies according to shape, and suggested an “evolution” of galaxies from elliptical to spiral as they aged. The diagram was called a “tuning fork” due to its shape

25. No one evolutionary path for galaxies As detection methods grew more sophisticated, using the infrared (Spitzer telescope), radio (Very Long Baseline Array) and gamma ray (Compton telescope) portions of the EM spectrum, the tuning fork is no longer regarded as containing an evolutionary sequence – it’s simply a way of classifying galaxies. It is true that irregular galaxies seem to form from galactic collisions, and that some spiral galaxies lose their arms to become elliptical (Milky Way + Andromeda fate), there is no good model to describe galactic evolution.