Astronomy chapter summaries for ASTR Chapter 19 PDF

Preview

Summary

Astronomy chapter summaries for ASTR Chapter 19...

Description

Chapter 19 1. The Milky Way Revealed What does our galaxy look like? The Milky Way Galaxy is a spiral galaxy consisting of thin disk about 100,000 light years in diameter with a central bulge and a spherical region called the halo that surrounds the entire disk. The disk contains most of gas and dust of the interstellar medium, while the halo contains only a small amount of hot gas and virtually no cold gas. How do stars orbit in or galaxy? A spiral galaxy like ours may look as if it should rotate like a giant pinwheel, but the galaxy is not a solid structure. Each individual star follows its own orbital path around the center of the galaxy, with nearly all stars following one of two basic orbital patterns. Orbit of Disk Stars The general orbit of a star around the galaxy arises from its gravitational attraction toward the galactic center, while the bobbling arises from the localized pull of gravity within the disk itself. A star that is “too far” above the disk is pulled back into the disk by gravity. Because the density of interstellar gas is too low to slow the star, it flies through the disk until it is “too far” below the disk on the other side. Gravity then pulls it back in the other direction. This ongoing process produces the bobbing of the stars. The up-and-down motions of the disk stars give the disk its thickness of about 1000 lightyear. In the vicinity of our Sun, each star’s orbit takes more than 200 million years, and each up-and-down bob takes a few tens of millions of years. In our galaxy’s rotation, the orbital velocities of stars near the edge and those near the center are about the same. Stars closer to the center therefore complete each orbit in less time than stars further out. Orbits of Halo and Bulge Stars The orbits of stars in the halo and the bulge are much less organized. Individual bulge and halo stars travel around the galactic center on more or less elliptical paths, but the orientations of these paths are relatively random. Neighboring halo stars can circle the galactic center in opposite directions. They swoop from high above the disk to far below it and back again, plunging through the disk at velocities so great that the disk’s gravity hardly alters their trajectories. These swooping orbits explain why the bulge and the halo are much puffier than the disk. Halo and bulge stars soar to heights above the disk far greater than the heights achieved by disk stars as they bob up and down. Stellar Orbits and the Mass of the Galaxy By measuring the speeds of globular clusters relative to the Sun, astronomers have determined that the Sun and its neighbors orbit the center of the Milky Way at a speed of about 220 kilometers per second. It takes the Sun about 230 million years to complete one orbit around the galactic center. The orbital motion of the Sun and other stars gives us a way to determine the mass of the galaxy. Newton’s version of Kepler’s third law allows us to determine the mass of a relatively

large object when we know the period and average distance of a much smaller object in orbit around it. 2. Galactic Recycling How iss gas recycles in our galaxy? The galactic recycling process proceeds in several stages (star-gas-star cycle). Stars are born when gravity causes the collapse of molecular clouds, they shine for millions or billions of years with energy produced by nuclear fusion, and in their deaths they ultimately return much of their material back to the interstellar medium. Gas from Dying Stars All stars return much of their original mass to interstellar space in two basic ways: through stellar winds that blow throughout their lives ,and through “death events” of planetary nebulae (for low-mass stars) or supernovae (for high-mass tars). Low-mass stars generally have weak stellar winds while they are on the main sequence. Their winds grow stronger and carry more material into space when they become red giants. By the time a low-mass star ends its life with the production of a planetary nebula, it has returned almost half its original mass to the interstellar medium. High-mass stars: the powerful winds from supergiants and massive O and B stars recycle large amounts of matter into the galaxy. At the ends of their lives, theses stars explode as supernovae. The high-speed gas ejected into space by supernovae or powerful stellar winds sweeps up surrounding interstellar material, excavating a bubble of hot, ionized gas. These bubbles are not easy to detect. While some emit strongly in visible light and others are hot enough emit large amounts of X rays, many bubbles are evident only through radio emission from the shells of gas that surround them. Supernovae generate shock fronts-abrupt, high-ga-pressure “walls” that move faster than the speed at which sound waves can travel through interstellar space. When we observe a supernova remnant, we are seeing the aftermath of its shock front, which compresses, heats, and ionizes all the interstellar gas it encounters. The distinctive shapes of hot bubbles make them easy to spot with X-ray telescopes when they lie at some distance from us. In addition to their role as the movers and shakers of the interstellar medium, shock fronts from supernovae can act as subatomic particle accelerators. Some of the electrons in supernova remnants accelerate to nearly the speed of light as they interact with the shock front. These fast electrons emit radio waves as they spiral around magnetic field lines threading supernova remnant. Supernovae can affect life by generating cosmic rays that can cause genetic mutations in living organisms. Cosmic rays are made of electrons, protons and atomic nuclei that zip through interstellar space at close to the speed of light. Superbubbles and Fountains The bubble created by a single supernova can grow to a diameter of about a hundred lightyears before it slows and merges with surrounding interstellar gas. But in some regions of the Milky Way, we see cavities of hot gas that are more than a thousand light-years across. These huge cavities presumably arise when many individual bubbles combine to form a much larger superbubble. The hottest, most massive stars in a cluster can end their lives and explode within a few hundred thousand years of one another. The shock fronts from the individual

supernova soon overlap, combining their energy into one very powerful shock front that we see as the superbubble. When the superbubble escapes the Milky Way, it explodes into the form of galactic fountain. Cooling and Cloud Formation The gas eventually cools and mixes into the surrounding interstellar medium, turning into atomic hydrogen gas and then cooling further, producing molecular clouds. Tese molecular clouds then form stars, completing the star-gas-star cycle. Where do stars tend to form in our galaxy? Active star-forming regions, marked by the presence of hot massive stars and ionization nebulae, are found preferentially in spiral arms. The spiral arms represent regions where a spiral density wave has caused gas clouds to crash into each other, thereby compressing them and making star formation more likely. 3. The History of The Milky Way What clues to our galaxy’s history do halo stars hold? The stars of the bulge and the halo, together known as the spheroidal population of stars, are old low-mass stars with a much smaller proportion of heavy elements than stars in the disk population. Halo stars therefore must have formed early in the galaxy’s history, before the gas settled into a disk. How did our galaxy form? Halo stars probably formed in several different protogalactic clouds of hydrogen and helium gas. Gravity pulled those clouds and stars together to form a single larger cloud of stars and gas. The collapse of this cloud continued until it formed a spinning disk around the galactic center. Stars have formed continuously in the disk since that time, but stars no longer form in the halo. 4. The Mysterious Galactic Center What lies in the center of our galaxy? Orbits of stars near the center of our galaxy suggest that it contains a black hole about 4 million times as massive as the Sun. The black hole appears to be powering a bright source of radio emission known as Sgr A*....