The Universe Essay Research Paper Student — страница 2

  • Просмотров 286
  • Скачиваний 5
  • Размер файла 18

absolute magnitudes of stars. A Hertzsprung-Russell, or H-R diagram is a plot of the absolute magnitudes of stars as a function of their surface temperatures. On an H-R diagram, different types of stars, such as main sequence stars, red giants, and white dwarfs, are located in well defined areas, which makes the diagram useful in assigning characteristics to stars for which we know only the absolute magnitude and/or surface temperature. 9. What are Cepheid variables? Your Answer: Stars in which their absolute magnitude varies. Cepheid variables are stars that exhibit a periodic variation in their absolute magnitude or brightnesses. A linear relationship has been discovered between the length of these periods and the absolute magnitudes of the stars, so we need only measure the

time between brightness maxima to estimate the absolute magnitude of a given star. Then using the inverse square law of intensity we can calculate the distance to this star. This is one of the best methods astronomers have for finding the distance to faraway stars and the galaxies that they inhabit. 10. How are galaxies classified? Your Answer: By their appearance to an observer on Earth. Galaxies are classified by their appearance to an observer on Earth. This determination is usually made after studying photographs of the galaxies taken by our largest telescopes. 11. What are the three main classifications of galaxies? Your Answer: Elliptical, spiral, and irregular. Galaxies are classified as either elliptical or spiral, depending on their appearance as seen from Earth. Any

galaxies that do not fit into these two main categories are simply called irregular galaxies. 12. Which type of galaxy is most abundant? Your Answer: Most are spiral galaxies Of the brighter galaxies observed from Earth, over 75% are of the spiral type, but in any given volume of space there are more elliptical galaxies than spirals. This is the case because elliptical galaxies are made up of older stars that are generally dimmer than the young stars that predominate in spiral galaxies. This makes the spiral galaxies easier to see at large distances. The third type, irregular galaxies, appear to make up only about 3% of all known galaxies. 13. What are the principal phases in the life cycle of a star? Your Answer: Begin as a cloud of gas called main sequence stars, then depending

on their size, they enter white dwarf, neutron, or black hole stage. Stars begin their lives as large clouds of gas and dust that collect under the pull of gravity into main sequence stars, during which phase they produce energy from the fusion of hydrogen in their cores. When the hydrogen becomes depleted, they evolve into their red giant phase where helium is the main core fuel. When the helium in the core also gives out, stars end their lives in one of three final states, depending on their masses at this point in their lives. These final states are white dwarf stars, neutron stars, and black holes. 14. What are the physical dimensions of the Milky Way Galaxy? Your Answer: 100,000 light-years in diameter and 2,000 light-years thick. The Milky Way is 100,000 light-years in

diameter and 2,000 light-years in thickness in the region where our solar system is found. There is also a much thicker central bulge (10,000 ly) surrounding its center of rotation. The Milky Way contains about 100 billion individual stars. 15. What is a black hole? Your Answer: a collapsed star with strong gravitational pull. A black hole is believed to be a collapsed star with a gravitational field so strong that nothing, not even light, can escape from it. Black holes are the final stage in the life cycles of the most massive stars. 16. What are Seyfert galaxies? Your Answer: They have bright centers and tend to give off high levels of radio emissions. Seyfert galaxies are galaxies that exhibit very bright centers and whose spectra show broad emission lines. They tend to give

off large amounts of radio emissions from their cores, but they look like normal spiral galaxies through an optical telescope. 17. What are superclusters? Your Answer: Large clusters made of individual clusters of galaxies. Superclusters are huge clusters made up of many individual clusters of galaxies lumped together. They vary from 50 to 300 million ly in diameter and contain masses that are estimated to be greater than 1015 solar masses. 18. What is the Hubble constant? Your Answer: Constant of proportionality in Hubble s law which relates to galaxies distance from Earth. The Hubble constant is the constant of proportionality (H) in Hubble’s law (v = H d), which relates the speed of recession (v) for remote galaxies to their distances from Earth (d). The present value for H