Chapter 1 Here and Now 1-1 Where are you? • To find your place among the stars, we can take a cosmic zoom — a ride out through the Universe — to preview the kinds of objects you are about to study • From each frame to the next, we will zoom out by about a factor of 100 A Campus Scene (16 m) • An area about 50 feet (16 m) across on a college campus. Includes small objects (trees, people, sidewalks) with sizes we can understand. Note: Each successive picture in this “zoom” will show you a region of the Universe that is 100 times wider than the preceding picture. That is, each step will widen your field of view, which is the region you can see in the image, by a factor of 100 . A City View (1.6 km) • An area 1 mile (1.6 km) across in the next image. Small objects (people, trees, sidewalks) are too small to discern, but now you can view an entire college campus plus surrounding streets and houses. The dimensions of houses and streets are familiar to us. The Landscape of Pennsylvania (160 km) • Infrared image (field of view is 160 km) in which healthy green leaves and crops are shown as red. Cities and towns appear as patches of gray. Wilmington, Delaware is visible at the lower right. At this scale, you can see some of the natural features of Earth’s surface. The Allegheny Mountains of southern Pennsylvania cross the image in the upper left, and the Susquehanna River flows southeast into Chesapeake Bay. What look like white bumps are a few puffs of cloud. The Earth (Diameter: 12,756 km) • In this photograph, we can see our entire planet, which is nearly 13,000 km in diameter. Earth → Moon (3.8 × 105 km) Earth-Moon system, view is 1,600 ,000 km wide. Earth is the small blue dot in the center, and the Moon, the diameter of which is only one-fourth of Earth’s, is an even smaller dot along its orbit 380,000 km away. (The relative sizes of Earth and Moon are shown in the inset at the bottom right.) Earth → Sun (1.5 × 108 km or one AU) • This image (1.6 x 10⁸ km) shows the Sun and two other planets that are part of our Solar System. • An Astronomical Unit is the average distance from the Earth to the Sun • A Planet is a spherical, nonluminous body that orbits a star and shines by the reflecting light. • A Dwarf Planet is small object that orbits the Sun and has pulled itself into a spherical shape but has not cleared its orbital lane of other objects. The Solar System (Mars → Sun: 1.5 AU) • This image shows the entire planetary region of our Solar System. The Sun, Mercury, Venus, and Earth lie so closely together that you cannot see them separately at this scale, and they are lost in the red square at the center of this diagram that shows the size of the previous figure. You can see only the brighter, more widely separated objects (Mars, Jupiter, Saturn, Uranus, and Neptune.) • Our Solar System consists of the Sun, its family of planets , and some smaller bodies such as moons, asteroids, and comets. The Solar Neighborhood (~11,000 AU) • In this image, the Sun is only a point of light, and all the planets and their orbits are now crowded into the small red square at the center. The planets are too small and too faint to be visible so near the brilliance of the Sun. The Solar Neighborhood (17 Light Years) • This image (FOV 1.1 million AU) shows our Sun at the center, and at this scale you can see a few of the nearest stars. Common Misconceptions • Misconception: A light-year is a unit of time • Truth: A light-year is a distance, not a time • 1 light year (ly) = distance traveled by light in 1 year = 1.1 million AU • Misconception: Stars look like disks when seen through a telescope • Truth: Stars are so far away that astronomers only see them as points of light The Solar Neighborhood (1,700 ly) • Expanding your field of view by another factor of 100, and the Sun and its neighboring stars vanish into the background of thousands of other stars. The field of view is now 1700 ly in diameter. The Sun is a relatively faint star that would not be easily located in a photo at this scale. • An Extrasolar Planet is a planet orbiting a star other than our Sun. The Milky Way Galaxy (~80,000 ly D) Expanding your field of view by a factor of 100 , you see artist’s conception of our galaxy, with a visible disk of stars about 80,000 ly in diameter. • A galaxy is a great cloud of stars, gas, and dust held together by the combined gravity of all its matter. • Galaxies range from 1000 ly to more than 300,000 ly in diameter, and the biggest ones contain more than a trillion (10¹²) stars. The Milky Way Galaxy • How can we visualize the Milky Way? • In the night sky, our galaxy looks like • Artists use descriptions based on scientific a band of stars known as the Milky evidence to create images Way. These images are much better • Better than guesses than guesses; they are scientifically • Show events that our senses cannot detect based illustrations guided by the • Search the Internet for images of other best information astronomers can galaxies gather. • A Spiral Galaxy is a galaxy with an obvious • How else do we know what our disk component containing gas; dust; hot, galaxy looks like? Indeed, galaxy bright stars; and spiral arms. NGC 1073 is known as the Milky Way • Spiral Arms are long, spiral pattern of twin (a barred spiral galaxy located 55 bright stars, star clusters, gas and dust that million light-years away in the extends from the center to the edge of the disk of spiral galaxies. constellation of Cetus). The Local Group: Our Cluster of Galaxies • Distance to the nearest large galaxies: • Expanding the field of view by several million light years another factor of 100, we can see a few of these other galaxies. Our galaxy appears as a tiny luminous speck surrounded by other specks in a region 17 million light-years in diameter, with each speck representing a galaxy • Notice that our galaxy is part of a cluster of a few dozen galaxies. Galaxies are commonly grouped together in such clusters. Common Misconceptions • Misconception: People often say Galaxy when they mean Solar System, and they sometimes confuse both terms with Universe • Truth: The Milky Way Galaxy contains our Solar System plus billions of other stars and whatever planets orbit around them, or billions of planetary systems The Universe On Very Large Scales • Clusters → superclusters → filaments/walls • If you expand the field of view one more time, we see that clusters of galaxies are connected in a vast network. • Clusters are grouped into superclusters — clusters of clusters — and the superclusters are linked to form long filaments and walls outlining • nearly empty voids. These filaments and walls appear to be the largest structures in the Universe. Scientific Notation • Sometimes scientists work with very large numbers and they have to simplify the numbers. Rather than writing such astronomical numbers they use scientific notation. • Scientific Notation is the system of recording very large or very small numbers by using the power of 10. • Example: 580,000 AU • 5.8x10⁵ 1-2 When Is Now? • The Universe is ~14 billion years old • Began with the Big Bang • Our solar system formed 9 billion years later (4.6 billion years ago) • ~3.4 billion years ago: Earliest forms of microscopic life • ~0.4 billion years ago: Emergence of life on land • ~65 million years ago: Dinosaurs go extinct • ~10,000 years ago: Civilization began • Humanity is very new to the Universe 1-3 Why Study Astronomy? • Two fundamental questions are • To understand the world around addressed throughout this text you and to make wise decisions • What are we? for yourself, for your family, and • How do we know? for your nation, you need to • In what ways will this knowledge understand how science works. helps you enjoy a richer life and be a better citizen? How Do We Know? • The scientific method • Form hypotheses that explain how nature works and test with experiments/observations: • If contradicted, the hypothesis is revised or discarded • If confirmed, the hypothesis is tested further • How is a statement in a political campaign speech different from a statement in a scientific discussion?.