CHAPTER 26: (Galaxies)1. Describe the main distinguishing features of spiral, elliptical, and irregular galaxies.Spiral galaxies, also called disk galaxies are distinguished by their spiral arms but they also consist of a central bulge, a halo, and a disk. Interstellar material is usually spread throughout the disks of spiral galaxies. The disks are often dusty, which is especially noticeable in those systems that we view almost edge on (933). Elliptical galaxies have shapes that are spheres or ellipsoids. They have no arms and are generally smooth and featureless. Lastly, irregular galaxies have neither arms or uniform appearance, they generally have stars and gas clouds scattered in random patches5. What is one of the best ways to measure the distance to a nearby spiral galaxy, and how would it be measured? The rotation rate of the spiral galaxy can be used to determine the distance using the Tully-Fisher relation. Take a spectrum of the galaxy. The line widths of the 21-cm line can then be used to determine the rotation rate of the galaxy.10. Does an elliptical galaxy rotate like a spiral galaxy? Explain. No, Spiral galaxies are composed of collapsing cloud of gas while elliptical galaxies are formed by the merging of other galaxies. Thus spiral galaxy possesses angular momentum because of gas from its collapsing into disk, while stars in the elliptical galaxy have random orbits for stars.11. Why does the disk of a spiral galaxy appear dark when viewed edge on?It appears bright because of the presence of bright emission nebulae and hot young stars in the arms or disk which means that new star formation is still occurring. Disks are also very dusty.19. Which is redder—a spiral galaxy or an elliptical galaxy?Elliptical galaxyCHAPTER 26:7. Why is Hubble’s law considered one of the most important discoveries in the history of astronomy?The first law to put forth the idea of an expanding universe in astronomy Led to the determination of many properties of galaxies explained that nearest galaxies appear to be in motion away from our galaxy demonstrates relativity distance measured by velocity.8. What does it mean to say that the universe is expanding? What is expanding? For example, is your astronomy classroom expanding? Is the solar system? Why or why not?The objects aren't expanding, it's the space around them. Our solar system is gravitationally bound and so the planets aren't moving away from the sun.15. If all distant galaxies are expanding away from us, does this mean we’re at the center of theuniverse? No, the expansion follows a simple proportional relationship (Hubble's law), then all points in space within the expanding universe could make the same observation and claim to be the center.16. Is the Hubble constant actually constant? The Universe expands at the same rate at every location in the Universe: the Hubble constant isconstant throughout space, however, the expansion rate, and therefore the value of the Hubble constant, changes with time.CHAPTER 284. Explain what we mean when we call the universe homogeneous and isotropic. Would you saythat the distribution of elephants on Earth is homogeneous and isotropic? Why? Homogeneous means the same material in all directions and isotropic means it looks the same in all directions. 7. When astronomers make maps of the structure of the universe on the largest scales, how do they find the superclusters of galaxies to be arranged? They're found in filamentary structures that are huge but occupy a very small portion of space.CHAPTER 291. What are the basic observations about the universe that any theory of cosmology must explain?The basic operations are universe expansion, cosmic microwave backgrounds, an abundance of cosmic elements, and primeval explosion.2. Describe some possible futures for the universe that scientists have come up with. What property of the universe determines which of these possibilities is the correct one?The universe might continue to expand into the indefinite future and grow thinner, colder, and darker based on the information that came from the big bang.5. Describe at least two characteristics of the universe that are explained by the standard Big Bang model.8. What is dark energy and what evidence do astronomers have that it is an important component of the universe? Dark energy is an unknown form of energy that makes up large portions of the universe. The evidence there by the pull its gravity exerts on the stars and raw material we can observe, but what this dark matter is made of and how much of it exists remain a mystery.9. Thinking about the ideas of space and time in Einstein’s general theory of relativity, how do we explain the fact that all galaxies outside our Local Group show a redshift?That there is some random motion of galaxies within the local group. Since the Hubble flow for the local group is zero, you some random motion around zero which results in redshifts less than