Review 4 Chapters 22: The Milky wayHow do we know that there is gas and dust in interstellar space?Just as dust in the disk of our own galaxy can be seen on a clear summer night as a dark band slicing the Milky Way in two, the dust in an edge-on spiral galaxy appears as a dark, obscuring band running down the midplane of the disk. The cold gas that accompanies the dustcan also be seen in radio observations of spiral galaxies. In contrast, elliptical galaxies contain large amounts of very hot gas that we see primarily by observing the X-rays it emits. How were the size and shape of the Milky Way discovered? It was discovered ultimately through trial and error when astronomers tried time and time again to understand the galaxy. Early Astronomer William Herschel’s computations were flawed due to the underestimated deceiving power of interstellar dust, which led them tobelieve we were in the middle of the galaxy. A better attempt to determine the shape and size of our galaxy was made in the early 20thcentury, by an astronomer called Harlow Shapley. He was studying globular clusters (dense clusters containing hundreds of thousands of stars). There are a couple of hundred globular clusters in a spherical swarm surrounding our galaxy. If the Sun were at the center of our galaxy, we would see equal numbers of globular clusters in all directions. However, Shapley found that globular clusters all tend to lieon one side of the sky. Therefore, Shapley concluded, the center of ourgalaxy lies in the direction of Sagittarius. Shapley was, in short, able to measure the distances to globular clusters. What is the shape, form, structure, and various parts of the Milky Way? Recent studies show the galaxy is a barred spiral galaxy. The structure of the milky way is as follows: 1. Galactic disk, 2. global clusters, 3. Halo. 1. This is where most of the Milky Way’s stars are located as well interstellar gas and dust. Stars within the disk orbit the galactic center in roughly circular orbits. The parts of the disk are broken up into the Nucleus (center of disk), the Bulge (area around nucleus), and the spiral arms (one of these spiral arms is where our solar system is located).2. A few hundred of these are scattered above and below the plane of the disk. Globular clusters orbit the galactic center in elliptical orbits in which the directions are randomly scattered. The stars in the globular clusters are much older stars than those in the galactic disk, and there's little or no gas and dust.3. This is a large, dim, region that surrounds the entire galaxy. The halo is made of hot gas and possibly dark matter.Where is our sun located in the Milky Way? The Milky Way Galaxy is approximately 100,000 light years in diameter, and the sun is located about 28,000 light years from the center.Why do astronomers think that much of the Milky Way consistsof “dark matter”? Based on our current understanding of gravity, the amount of visible matter only accounts for a small amount of the mass present in the galaxy (Keplers laws are used for these calculations, the rotation of themilky way actually stays relatively constant unlike that of solar system bodies). As a result, scientists assume that, since there is little evidence that the theory of gravity is wrong, the galaxy must consist ofmatter that does not produce light, hence the term "dark matter."Why does the galaxy have spiral arms? The rotation in a disk galaxy naturally produces spiral structure. Any disturbance in the disk of a spiral galaxy will cause a spiral pattern because the disk rotates. Disks do not rotate like a solid body, instead material close to the center takes less time to travel around the galaxy.Is there a massive black hole at the center of the galaxy? Very accurate stellar positions can be estimated in order to keep track of the motions of the stars in the compact central cluster, which are zipping around Sgr A* at speeds up to 3 million miles per hour! Using Kepler's laws of motion, the orbital velocities and the positions of the bright stars an be used to estimate the mass that must be contained within their orbits. The resulting enclosed mass is 4.6 ± 0.7 X 10^6 solar masses--4.6 million times the mass of our Sun! This large mass combined with the minute size of Sgr A* in radio emission suggests taht the stars must be swiftly circling around a supermassive black hole…. Basically, through Sagittarius A, astronomers found that theirobservations must lead to a black hole but the formation of such a large black hole and how it affects the evolution of its host galaxy are not well understood.What is an HII region and how does it evolve? HII regions are the most luminous objects in the Milky Way at mid-infrared to radio wavelengths and can be seen across the entire Galactic disk. Regions of hot, thin hydrogen emission nebulae that glow from the fluorescence of hydrogen atoms. What is the origin of the 21 cm line and how do we utilize it in astronomy? The hydrogen in our galaxy has been mapped by the observation of the 21-cm wavelength line of hydrogen gas. The observation of the 21cm line of hydrogen marked the birth of spectral-line radio astronomy. It was first observed in 1951 by Harold Ewen and Edward M. Purcell at Harvard, followed soon afterward by observers in Holland and Australia. Dutch astronomer H. C. van de Hulst made the prediction that the 21 cm line should be observable in emission in 1944.What are the various types of nebulae in the Milky Way? The remnants of exploded stars, vast clouds of gas and dust, and star nurseries where new stars are forming out of clouds of gas and dust. Chapter 23:What are the different kinds of galaxies? There are many types of galaxies: elliptical, spiral, bar spiral, irregular, and quasars. Irregular galaxies have no set shape, while quasars are galaxies that produce about 100 times as much energy as our own. Regular galaxies:SpiralBarred spiralElliptical Irregular galaxies: Large Magellanic cloudHow have astronomers determined the distances to galaxies? To determine the distance a galaxy is from Earth, astronomers look for celestial objects within that galaxy they can determine the luminosity (or absolute magnitude) of. These objects are known as standard candles. Once the luminosity is known from observation they can makea measurement of how bright the object appears here on earth and determine the distance. If one has two objects of the same luminosity, the one further