6: Galaxies and the Universe
- Page ID
- 55006
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- 6.1: The Architecture of the Galaxy
- The Milky Way Galaxy consists of a thin disk containing dust, gas, and young and old stars; a spherical halo containing populations of very old stars, and globular star clusters; a thick, more diffuse disk with stars that have properties intermediate between those in the thin disk and the halo; a peanut-shaped nuclear bulge of mostly old stars around the center; and a supermassive black hole at the very center. The Sun is located roughly halfway out of the Milky Way.
- 6.2: Types of Galaxies
- Most bright galaxies are either spirals or ellipticals. Spiral galaxies contain both old and young stars, as well as interstellar matter, and have typical masses in the range of 109 to 1012 MSun. Our own Galaxy is a large spiral. Ellipticals are spheroidal or slightly elongated systems that consist almost entirely of old stars, with very little interstellar matter. Elliptical galaxies range in size from giants, more massive than any spiral, down to dwarfs, with masses of only about 106 MSun.
- 6.3: The Expanding Universe
- The universe is expanding. Observations show that the spectral lines of distant galaxies are redshifted, and that their recession velocities are proportional to their distances from us, a relationship known as Hubble’s law. The rate of recession, called the Hubble constant, is approximately 22 kilometers per second per million light-years. We are not at the center of this expansion: an observer in any other galaxy would see the same pattern of expansion that we do. The expansion described by Hub
- 6.4: The Beginning of the Universe
- The universe cools as it expands. The energy of photons is determined by their temperature, and calculations show that in the hot, early universe, photons had so much energy that when they collided with one another, they could produce material particles. As the universe expanded and cooled, protons and neutrons formed first, then came electrons and positrons. Next, fusion reactions produced deuterium, helium, and lithium nuclei.
- 6.5: The Cosmic Microwave Background
- When the universe became cool enough to form neutral hydrogen atoms, the universe became transparent to radiation. Scientists have detected the cosmic microwave background (CMB) radiation from this time during the hot, early universe. Measurements with the COBE satellite show that the CMB acts like a blackbody with a temperature of 2.73 K. Tiny fluctuations in the CMB show us the seeds of large-scale structures in the universe.
Chapter 6: Galaxies and the Universe — Introduction
Stars do not exist in isolation. They are members of enormous systems called galaxies, and galaxies themselves are part of the larger structure of the universe. Our Sun is just one of the many stars in the Milky Way Galaxy, so understanding our own galaxy provides a starting point for exploring the universe on its largest scales.
In this chapter, we will examine the structure of the Milky Way and compare it with other types of galaxies. We will then look beyond individual galaxies to one of the most important discoveries of modern astronomy: the universe is expanding. By observing the motions of distant galaxies, astronomers can investigate how the universe has changed over time and trace its history back toward a much hotter and denser beginning.
Finally, we will examine the cosmic microwave background, radiation left over from the early universe. Together, the expansion of the universe and the cosmic microwave background provide important observational evidence that allows scientists to reconstruct the history of the cosmos.