2. The Milky Way Appears as a band of light stretching across the sky There are dark regions along the band, giving the appearance of a lack of stars This is caused by dust and gas obscuring the light of stars behind

3. The Milky Way The gas and dust limits our view to about 6000 light years Much of this gas and dust can be observed in the form of nebulae A nebula is a cloud of gas seen in visible light How the gas is seen depends on the stars around it

4. The Milky Way Types of Nebulae 1. Reflection Nebula: –this type of nebula reflects the light of the stars in and around it –it appears blue because the nearby stars are luminous, young main-sequence stars –Ex: the Pleides

5. The Milky Way Types of Nebulae 2. Emission Nebula –This type of nebula is caused by high energy radiation (UV) from nearby stars exciting the gas into emitting its own light –These regions are known as HII regions (hydrogen has been ionized

6. The Milky Way Types of Nebulae 3. Absorption Nebula –this type of nebula is made of cooler gas and dust –it doesn’t glow; it obscures our view of things behind –it appears as a dark silhouette

10. The Nuclear Bulge Radius: 16,000 light years Contains Population II stars –Older stars, K type stars Generally free of gas and dust Nucleus: 10 light years across –Studies indicate the presence of a massive non- stellar black hole

11. The Disk Extends 50,000 light years beyond the central bulge Forms spiral arms that contain a lot of gas and dust Population I stars are found in the spiral arms –these are young O and B main-sequence stars –they are often found in open clusters The Sun is about 2/3 of the way out from the center

12. The Disk The disk is very thin Its “thickness” is only about 2% of its width Spiral structure has been determined through radio observations –Radio observations have shown how the spiral arms move around the center of the galaxy It takes 250 million years for the Sun to make one orbit around the galactic center

15. The Halo The halo completely surrounds the disk of the galaxy It contains Population II stars Most of the stars are found in globular clusters

16. The Galactic Corona 200,000 - 300,000 ly beyond Contains 5 - 10 times as much mass as the rest of the galaxy Can only detect it through gravitational effects

20. Elliptical Galaxies Most common type of galaxy, trillion solar masses; Largest: 10 - 100 kpc across; these are very rare Dwarf ellipticals are the most common –2 - 3 million solar masses; 2000 pc across Shape is based on optical appearance: –circular: E0 –most elongated: E7

21. Spiral Galaxies Brighter & larger than ellipticals 25,000 - 800,000 pc across 10 9 - 10 12 solar masses 1/3 of all spirals are barred Classified based on the size of nucleus & how tightly arms are wound

22. Spiral Galaxies

24. Irregular and Peculiar Galaxies Irregular galaxies show no regular shape Could be remnants of a collision Ex: Large magellanic clouds Peculiar galaxies are very rare may look like a regular galaxy exploded May be a result of a collision

25. Irregular and Peculiar Galaxies

26. Hubble Classification Amount of interstellar gas and dust differs in each type of galaxy Ellipticals have very little gas or dust; spirals have a lot In spirals, amount of gas increases from Sa to Sc In spirals, amount of star formation increases from Sa to Sc O and B stars formed mostly in Sc, SBc, and Irr galaxies

27. Clusters of Galaxies The Local Group: –Contains about 2 dozen of the nearest galaxies –1 megaparsec in diameter Distant Clusters –Contain 100’s to 1000’ of galaxies Virgo Cluster: –Closest cluster to us; 20 Mpc distance, 2 Mpc across Superclusters –Many clusters form a supercluster –There are 16 superclusters within 2 billion ly

28. Clusters of Galaxies