AST 115 1st EditionFinal Study Guide Lectures: 28 - 38Lecture 28 (April 10)1. Describe the structure of the Milky Way Galaxy.  The Milky Way is the irregular band of faint white light that encircles the entire inside of the celestial sphere. We are in a flat, disk-shaped Galaxy, and we are positioned betweenthe middle and the edge (the Sun lies about 26,000 LY from the galactic nucleus/center). The structure of our Galaxy is similar to a flat, wheel-shaped disk that is about 100,000 LY in diameter and 2,000 LY thick. Inside the disk is a high concentration of interstellar dust and gas and about 200 billion stars. The center, or galactic nucleus, of our Galaxy is still somewhat unexplained, but we believe that a supermassive black hole exists here. The galactic nucleus is surrounded by a central bulge, which is about 20,000 LY in diameter. The entire Galaxy is surrounded by two halos of matter. These spherical, concentric halos are a few hundred thousand LY in diameter. The inner halo includes a spherical distribution of globular clusters and field stars. It also contains dark matter and orbits in the same direction as the disk. The outer halo contains very old stars that have retrograde orbits. 2. Describe the populations in the Galaxy. What Population is the Sun a member of? There are two general categories, or populations, of objects in the Galaxy. They are categorized based on their chemical makeup and location. Population I is called disk components. These include: Helium, Hydrogen, and a small amount of heavier elements.Population II is called old halo components. These include: only Hydrogen and Helium. The Sun is a member of Population I.3. Where is the Sun’s location in the Galaxy? Who discovered the location? How was this determined?  The Sun is located in the disk of the Milky Way Galaxy at about 26,000 LY from the center. This was determined by Harlow Shapley using the RR Lyrae Variable stars in theglobular star clusters. He discovered that the center of the Galaxy is associated with the concentration of globular star clusters. The concentration of these star clusters is displaced from the location of the Sun; therefore, the Sun is not at the center of the Galaxy.4. Describe the Great Spiral Nebula in Andromeda. Explain how Edwin Hubble figured the distance to M31. What did Henrietta Leavitt discover in the 20th century? What is the distance to the Great Spiral Galaxy in Andromeda? The Great Spiral Nebula is referred to as M31 in the Messier Catalog and NGC 224 in the New General Catalog. M31 looks like a cotton ball in small telescopes. Hubble discoveredthat Cepheid variable stars can be recognized by spectra and a regular cycle of brightness variation. Some are bright enough to see at distances of 10s of millions of LY. Henrietta Leavitt discovered the Period-Luminosity Relation. This can be described as “the longer the period of brightness variation that a Cepheid has, the more luminous it is.” The Great Spiral Galaxy in Andromeda is about 2,500,000 LY.Lecture 29 (April 13)5. Describe the Spiral Structure of the Galaxy’s disk. At optical wavelengths, dust prevents a complete view of the disk. We can only see about 1 to 3 thousand LY into the disk. Optically, we can see bright blue O and B spectral class stars along the spiral arms near to us (just as they are seen along spiral arms of other galaxies). In order to map out the spiral structure of our Galaxy, astronomers must use radio observations of cold HI clouds in the disk. These clouds emit “21 – cm radio waves.”6. Describe the nucleus of the Galaxy. What is the strong radio source in Sagittarius called? Where is it located? What lies at the center of the Galaxy? There is a strong radio source in Sagittarius called “Sagittarius A”. It lies about 25,000 LY beyond the stars that make the pattern of the constellation Sagittarius. The Very Large Array (VLA) radio telescope system in New Mexico has resolved this strong radio sources into components as small as 3 AU across. The part known as “Sagittarius A*” (pronounced A-star) lies at the center of the Galaxy and is believed to contain a supermassive black hole. The diameter of the event horizon of a black hole with 2 million solar masses would be about 12 million km (7.2 million miles). As matter gradually falls towards this large black hole, an accretion disk has formed and is responsible for strong radio signals.7. Describe the rotation of the Galaxy. The motion of the Sun with respect to distant globular star clusters using the Doppler Effect in their spectra gives a roughly circular orbit with the velocity of about 230 km/sec(toward Cygnus). The Sun’s galactic orbit has a circumference of2 π r=2 π(26,000 LY). To travel this distance at 230 km/sec requires about 230 million years. 8. What is dark matter? How is it distributed throughout our Galaxy? The disk of the Galaxy contains a lot of interstellar dust and gas. However, this dust is notthe dark matter. Dark matter can be described as material that is not emitting any kind of radiation, but has gravity. It is also visible at all wavelengths. The presence of this darkmatter has been deduced by studying the orbital motion of various objects in Galaxies. A“galactic rotation curve” shows that the velocity of objects does not fan off, or disperse more, with increasing distance from the center. It stays flat. It has been concluded that about 90% of the mass of the Galaxy may be this “dark matter”. No one has been able tocome up with a solid explanation of what dark matter is, but we do know for a fact that it exists in space.Lecture 30 (April 15)9. What are the different types of galaxies that have been discovered? Elliptical (E)  Normal Spirals (NS) Barred Spiral (SB) Irregular (Irr) Flocculent Spiral Galaxies  Grand-Design Spiral Galaxies10. Describe Elliptical Galaxies (E). Elliptical galaxies range from circular (E0) to elongated (E7). They contain little to no interstellar gas or dust and show no evidence of current star formation. However, elliptical galaxies contain primarily Population II, intermediate-mass, long-lived stars. They tend to be the largest of the galaxies and have the greatest range in size, brightness, and number of stars. 11. Describe Normal Spiral Galaxies (NS). Normal Spiral Galaxies range from a large nucleus with tightly-wound arms (Sa) to a small nucleus with loosely-wound arms (Sc). Moderately wound