Galaxies and The Milky Way

Transcription

1 Galaxies and The Milky Way

2 Attendance Quiz Are you here today? Here! (a) yes (b) no (c) To infinity and beyond!

3 Next Tuesday, 5/30, I will be away at a meeting. There will be a guest lecture by Dr. Jorge Moreno

4 Galaxy types Spiral Elliptical Irregular Today s Topics Hubble s classification scheme Structure of the Milky Way

5 Hubble Ultra Deep Field (11.3 day exposure) Between September 3, 2003 and January 16, 2004, the Hubble Space Telescope stared at a single, seemingly empty patch of sky in the constellation Fornax This observation lasted for 1,000,000 seconds, or about 11.3 days The resulting image was the deepest ever taken by a telescope, and covered a patch of sky only 1/100th the size of a full moon It revealed over 10,000 objects, most of which are galaxies

6 Hubble Ultra Deep Field (10,000 galaxies)

7 The Beginning of Time As we look at distant galaxies, we are looking back in time, due to the finite speed of light Thus, when we see a galaxy 13 billion light years away, we are seeing it as it was 13 billion years ago Since the expansion rate of the universe tells us the universe is about 14 billion years old, we are seeing these galaxies very early in the history of the universe

8 Types of Galaxies Galaxies fall into 3 main categories Spiral Galaxies - flat white disks with yellowish bulges at their center. These disks contain gas and dust, and show spiral arms, where SF is occuring Elliptical Galaxies - redder, ellipsoidal collections of stars with varying degrees of flatness (some spherical, others football shaped) Irregular Galaxies - no coherent structure; may be the result of a galaxy collision or other gravitational interaction with a nearby galaxy

9 Hubble s Classification Scheme Each galaxy is assigned a letter - E=elliptical, S=spiral, Irr=irregular Ellipticals are given a number to describe their roundness (0=spherical) Spirals are given a capital B if they are barred, and a letter a-c a = tight spiral arms, large bulge, little dust in the disk c = loose spiral arms, small bulge, lots of dust in the disk

10 Hubble s Classification Scheme Hubble (and others) thought this classification scheme might be an evolutionary sequence However, the existence of all types of galaxies together at all times, as far back in time as we look (by looking further away), makes this unlikely (click for image)

11 Lecture Tutorial: Galaxy Classification, pp Work with one or more partners - not alone! Get right to work - you have 15 minutes Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you all agree on. Write clear explanations for your answers. If you get stuck or are not sure of your answer, ask another group. If you get really stuck or don t understand what the Lecture Tutorial is asking, ask me for help.

12 A cool website Go the website and learn how to classify galaxies Then create an account and try your hand (actually eye & brain) at classifying real galaxies (try to do at least ; you may find you can t stop!)

13 Galaxy Collisions Galaxies form by collisions and mergers Irregular galaxies are thought to be galaxies undergoing such mergers Shown here are a pair of galaxies merging called the Antennae Galaxies Giant Elliptical galaxies ( times the size of the Milky Way) form by cannibalizing their neighbors over billions of years This movie shows that the fate of the Milky Way is to collide and merge with the Andromeda Galaxy MW-Andromeda collision movie

14 Many views of a galaxy collision

15 Structure of the Milky Way We now have a pretty clear idea of the structure of our home galaxy We live about halfway to the edge of a highly flattened spiral galaxy Not an actual photograph! Our galaxy is a typical spiral galaxy Highly flattened disk (like a CD) Stars in this disk are in circular orbits There are both old and young stars These stars contain ~2% heavy elements (heavier than He), like the Sun Roughly spherical bulge in the center Halo of globular clusters and other stars Stars in the bulge/halo move in 3-D elliptical orbits All old stars with little or no heavy elements

16 Difficulties of our position in the Milky Way Because we live in the plane of the galaxy, we have a very obscured picture of things in the Milky Way In particular, there is interstellar dust obscuring our view, particularly towards the Galactic Center Nonetheless, we can learn quite a lot, if we remain aware of our surroundings

17 Our Position in the Milky Way In the late 1700s, William and Caroline Herschel (brother and sister) mapped nearby stars and concluded that the Sun was near the center of a slightly flattened distribution of stars Later, more quantitative studies, in the early 1900s, showed that this distribution is about ~500 x 2500 light years in size Some astronomers were suspicious of this (remember Copernicus) Q: What could make us think were were near the center of the Milky Way when we aren t? A: Interstellar medium!

18 Our Position in the Milky Way This modern image of the Milky Way clearly shows the dark patches caused by interstellar In the early dust, 1900s, which Harlow is responsible Shapley for the pervasive dimming and obscuring of observed the distribution of globular objects far from the Solar System clusters, and found that they were centered on a point 10s of thousands of light years from the Sun He also found that these clusters avoided the plane of the Milky Way From these facts, he concluded 1. The Sun is not at the center of the Milky Way 2. The Milky Way is much bigger than had been previously thought In the 1920s, Robert Trumpler, observing open clusters, demonstrated that there is an obscuring fog which makes distant objects seem dimmer than they should - like car headlights in a fog

19 Milky Way Quiz I Why is it difficult to get a good picture of what our Milky Way galaxy looks like? a) We re in the middle of one of its arms no perspective b) It is very large and the edges are far away and faint c) Dust blocks our view when we look toward the center or in the plane d) No one has ever taken a photo from both the top and side of it e) All of the above

20 Milky Way Quiz II You observe two identical stars at the same distance. One is in the disk of the Milky Way, the other in a direction perpendicularly out of the disk. Chances are the disk star will be a) brighter and be reddened by dust b) less bright and be reddened by dust c) brighter and be less reddened by dust d) less bright and be less reddened by dust

21 Lecture Tutorial: Milky Way Scales, pp Work with one or more partners - not alone! Get right to work - you have 15 minutes Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you all agree on. Write clear explanations for your answers. If you get stuck or are not sure of your answer, ask another group. If you get really stuck or don t understand what the Lecture Tutorial is asking, ask me for help.

22 Milky Way Quiz III Where is our Sun located in the Milky Way Galaxy? a) at the exact center b) near the center c) about half way between the center and the outer edge d) near the outer edge

23 Milky Way Quiz IV Which letter represents M45 on the MW diagram (Q5)? a) A b) B c) C d) D e) E

24 Milky Way Quiz V Which letter represents M71 on the MW diagram (Q5)? a) A b) B c) C d) D e) E

25 Milky Way Quiz V Does the Andromeda Galaxy fit on the diagram? a) yes b) no