1. Units to cover: 72,73,74, 75, 82, 84

2. Model of AGN Depends on our observational geometry. Superluminal motions are possible.

3. The Origin of Helium Immediately after the Big Bang, only protons and electrons existed Shortly after the BB, temperature and density was high enough for deuterium to form by fusion After 100 seconds or so, temperature cooled enough so that deuterium could fuse into helium nuclei The temperature continued to cool, and fusion stopped after a few minutes. Big Bang theory predicts that around 24% of the matter in the early universe was helium, which matches what we see.

4. The Epoch of Inflation Modern technology allows us to test theories back to a time 10 -33 seconds after the Universe Birth (UB). Physics as we know it ceases to function at 10 -43 seconds after the UB, called the Plank Time Using particle colliders, scientists have uncovered a number of clues about what happened in the early universe, after the Plank time The early universe underwent a period of very rapid expansion By 10 -33 seconds, the universe expanded from the size of a proton to the size of a basketball This expansion is called inflation

5. Expansion Forever? Or Collapse? The fate of the universe is ultimately controlled by its total amount of energy –Energy of expansion (positive) –Gravitational energy that can slow the expansion (negative) –Binding energy If the total energy is positive or zero, the expansion continues forever If the total energy is negative, the expansion will halt, and the universe will contract and eventually collapse.

6. Dark Energy Dark energy may provide the solution to the mystery Dark energy remains constant everywhere, regardless of the universe’s expansion Provides an outward push to accelerate expansion Dark energy must make up around 70% of all of the energy in the universe Much work remains to be done on this frontier…

7. The Formation of the Milky Way Our galaxy likely began 13 billion years ago as a huge cloud of pure hydrogen and helium, slowly rotating and collapsing The first stars formed within this cloud, burning out quickly and violently. This added heavy elements to the cloud Population II stars formed next, capturing some of the heavy elements and settling into elliptical orbits around the center of the cloud As the collapse continued, a disk formed, and Population I stars formed from the ashes of dying Pop I stars

8. Galactic Cannibalism There are a few observations that are not explained by this model –Some stars follow unusual orbits in the galaxy –Not all Pop II stars are the same age –Model predicts that the first stars might not have been very massive, and should still be around! Galactic cannibalism provides some answers –The Milky Way may be absorbing another galaxy! –Observations show streams of stars coming from our galaxy’s “victim”.

9. Composition of Interstellar Clouds Light passing through an interstellar cloud can hold clues as to the cloud’s composition Atoms in the cloud absorb specific frequencies of starlight passing through, creating absorption lines Astronomers can analyze these spectra to determine what the clouds are made of. Spectra show that interstellar gas clouds are made of mostly hydrogen and helium, just like the Sun Dust particles do not absorb light the same way that gas atoms do, but using similar methods tells us that the dust is made of silicates

10. Interstellar Reddening As starlight passes through a dust cloud, the dust particles scatter blue photons, allowing red photons to pass through easily The star appears red (reddening) – it looks older and dimmer (extinction) than it really is.

11. The Galactic Center and Edge Despite the appearance of being closely spaced, stars in the Milky Way are very far apart –At the Sun’s distance from the center, stellar density is around 1 star per 10 cubic parsecs Density is much higher at the core –Exceeds 100,000 stars per cubic parsec! X-ray and gamma ray telescopes reveal a supermassive black hole at the Milky Way’s core –Called Sag A* –5 million solar masses!

12. Sag A*

13. Star Formation in Spiral Arms

14. A History of Galactic Discovery In the early 20 th century, the existence of other galaxies was unknown –The Milky way was the Universe! –Other galaxies were called nebulae Light from galaxies always appears fuzzy and diffuse, due to the vast separation between the Sun and the observed galaxy, as well as the separation between the stars of that galaxy! –The paleness of visible light from distant galaxies is called the surface brightness. Galaxies are therefore difficult to observe, even with good telescopes.

15. More History… In the 1700’s, Charles Messier was observing comets, and kept finding objects that while fuzzy, were not comets –He made a list (or catalog) of these undesired objects, so he could avoid seeing them –They became known as Messier Objects, a number preceded by an M. –M31 (the Andromeda galaxy) is one such object William and Caroline Herschel (1800’s) developed a catalog of faint objects in the heavens –Now known as the New General Catalog –Objects are known by a number preceded by the letters NGC –Objects can appear in both the Messier and NGC catalogs! M31

16. A Sky Full of Galaxies Technology has advanced to the point where we have found as many galaxies as there are stars in the Milky Way! Note the gap running along the zero latitude line –Called the zone of avoidance –Puzzled astronomers!

17. The Zone of Avoidance Dust and the center of our own galaxy merely blocks our view – there is no zone of avoidance!

18. Distances to other galaxies We can use Cepheid variable stars to measure the distance to other galaxies A Cepheid’s luminosity is proportional to its period, so if we know how rapidly it brightens and dims, we know much energy it emits If we see a Cepheid in another galaxy, we measure its period, determine its luminosity, and calculate its distance! Distance between galaxies is huge! –M100 is 17 million parsecs away.

19. The Sun’s position in the galaxy is A. unknown B. in the disk of the galaxy C. in the spherical halo of the galaxy D. in the galactic nucleus

20. Spiral Galaxies Spiral arms and a central bulge Type S

21. Elliptical Galaxies No spiral arms Ellipsoidal shape Smooth, featureless appearance Type E

22. Irregular Galaxies Stars and gas clouds scattered in random patches No particular shape Type Irr

23. Galaxy collision and merger

24. The Mice These two interacting galaxies are tidally distorting each other.

25. Which two quantities are shown to be related to one another in Hubble Law? A. distance and brightness B. distance and recession velocity C. brightness and recession velocity D. brightness and dust content

26. How are galaxies spread through the Universe? A. They are grouped into clusters that in turn are grouped into clusters of clusters (superclusters) B. Galaxies are spread more or less evenly throughout the Universe C. They are grouped around our galaxy D. none of the above