ASTR 1514 1st Edition Lecture 32Galaxy Classification- Classified according to their shape- Empirical classification scheme- Three classes: Elliptical, Spiral, Irregular 1.) Elliptical Galaxies- Have centrally-peaked stellar distribution - Classified by shape, E0 are circular, E7 are cigar shaped. - Are Redder (Old stars)- Have little gas and dust- Range in a lot of different sizes, for example the smallest elliptical galaxy is 1% the size of our galaxy and the largest one is 5X the size of ours. 2.) Spiral Galaxies- Contain a disk component - Can also have a bulge- Contains gas, dust, hot bright stars and areas of star formation – therefore they appearblue- Three different types of Spiral GalaxiesThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.1. Normal Spiral – has a round or elliptical bulge in the center and has spiral arms2. Barred Spiral – Similar to normal spiral but there is a bar in the center and spiral arms emerge from the bar3. S0 spiral – Round or elliptical bulge in center. Has spiral arms. - Three subtypes of arms –Classified by how large/comparatively bright the center is and how tightly wound the arms are1. Sa/SBa – Large bright center, arms are tightly wound2. Sb/SBb – Intermediate3. Sc/SBc – small dim center, loosely wound arms3.) Irregular Galaxies- Have an irregular appearance - Contain large clouds of gas and dust mixed with old young stars - No obvious nuclear bulge or spiral armsMore about galaxies- Elliptical galaxies are more common that spiral galaxies- Irregular galaxies make up only 25% of the galaxiesProperties of Galaxies- The shapes of galaxies are influenced by the motion of stars- Stars in elliptical galaxies orbit in many different directions maintaining the elliptical shape, with random directions- Spiral galaxies have 2 different star motions:1. Thin Disk – orbits are circular and all nearly in the same directions2. Central bulge – orbits in many directions (the bulge itself is similar to an elliptical galaxy)- In spiral galaxies there are “dust lanes” (dark) indicating cold gas- Cold gas is necessary for star formation – hot gas clouds don’t collapse- Spiral galaxies have active star formation- There is little new star formation in elliptical galaxies because the temperature of gas in ellipticals is hot, to form stars gas has to be cold to collapse. - Stars form along spiral arms- Clouds are compressed in the arms- Hot yound type O and type B stars produce bright blue light. - Mass doesn’t matter much when classifying galaxies. Spiral Arms- Stars exist outside of spiral arms- Arms are the site of star formation – they trace young stars which emit in the UV- Differential Rotation – Stars near the center take less time to orbit the center than those farther from the center- Differential rotation cannot explain spiral arms, if it did then we would expect the spiral arms to be so wound up that the spiral structure disappears. Spiral arms are a Density Wave- Density wave – a fluctuation in a density that travels through the galaxy, somewhat like a sound wave.- Compression in the density wave causes molecular cloud collapse and star formation. - You see young stars along the portions that were recently compressed. - Old stars drift away. - Spiral arms are density waves: waves of compression moving through the disk- Waves trigger star formation- Existing stars are not as affected as much as the gas- Spiral arms are not a galaxy winding itself up , it is a