The universe contains ~ 100 billions galaxies including our Milky Way Galaxy. Along the plane of Milky Way, dust clouds block our view of distant galaxies. Elliptical galaxies: Spherical or elliptical in shape, lacking in gas and dust, they contain  relatively old, low-mass stars (Fig. 15-5) Disk component is not obvious or missing Dwarf elliptical galaxies contain only ~ 10 million stars; they are similar to large globular clusters and are very common in the universe. However, they are dim and not easy to find. Fig. 15-5 The giant elliptical galaxy M87 is surrounded by a swarm of globular clusters. Objects to the right are smaller galaxies. Spiral galaxies contain gas, dust, and hot bright stars outlining spiral arms, having a mixture of star types (Fig. 15-6). e.g., Andromeda Galaxy M31 is the nearest (2.2 million ly from Earth) giant spiral galaxy. Obvious disk component They are very luminous and therefore easy to find ~ 2/3 of all known galaxies are spiral, but they may make up only a small fraction of all galaxies Fig. 15-6 The Andromeda Galaxy M31 is about 2.2 million light years from Earth. It is one of our closest neighbors in the galactic scale. Barred spiral galaxies: About 20% of spirals have an elongated nuclear bulge with spiral arms springing from the ends of bars. Irregular galaxies: Irregular in shape, clouds of gas and dust mixed with both young and old stars. Fig. 15-7 Modern astronomers use the tuning-fork diagram as an organizing framework for galaxy classification. Elliptical (E), spiral (S), barred spiral (SB) and irregular (Irr) are placed form left to right. e.g., the Large Magellanic Cloud and Small Magellanic Cloud are neighbors of the Milky Way Galaxy. Hubble classification of galaxies (Fig. 15-7) Distance indicators Periods of Cepheid variables (Hubble Space telescope can resolve Cepheids in distant galaxies out to ~ 120 Mly) Largest of H II regions have predictable diameters. Supernovae reach about the same maximum brightness - a clue of estimating the distances of very remote galaxies. Collisions of galaxies Sizes of galaxies not much smaller than the distances between galaxies e.g., the Milky Way Galaxy is ~0.1 million ly in diameter, but distance to the Andromeda Galaxy M31 is only ~2.2 million ly. the universe is crowded on galactic scales galactic collisions are common! Fig. 15-8 (a) A pair of interacting galaxies with peculiar tails (false color image). (b) A computer simulation of a close encounter between two normal galaxies produces similar tails. Individual stars do not collide, but strong gravitational fields twist and deform galactic shapes (Fig. 15-8) Compression generates shock waves that trigger star formation consume much of its gas and dust in making new stars Evidence of galactic collisions Dense clusters have more elliptical galaxies. some elliptical galaxies might be formed from the merging of spiral ones Some giant elliptical galaxies have multiple nuclei cores of several smaller galaxies were absorbed by a large one Spiral galaxies have not encountered major collisions they should have been more common in the past (verified by observing very distant galaxies) New and better observational data, more sophisticated computer simulations lead to better understanding of the collisions.