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What Is Astronomy? Ryan Hickox Astronomy,at its heart,is exploration on the grandest scale.Humans are natural explorers, and intrepid adventurers have now journeyed to the deepest ocean trenches and the highest and most remote mountains.We have even sent a handful of people to the moon,and our robotic surrogate explorers have now reached the edge of the solar system.These achievements are fascinating to people of all ages—in elementary school I learned by heart every book in the library about the Apollo missions to the moon and the Voyager probes to the outer planets.As I got older and studied math and physics I began to focus my exploration inward, and my college senior thesis used data from a particle collider to study the world on the smallest subatomic scales. However, in my heart I always remembered that our home planet comprises only a tiny portion of the much, much larger universe. In my final semester of college I was fortunate to take a wonderful course on astronomy, which set me on a path to exploring beyond our backyard into the vast expanses of reality. By carefully observing the sky, astronomers discover new stars, planets, galaxies , and gas clouds. I’m particularly interested in huge black holes that reside at the centers of galaxies, and much of my work involves searching for the signatures of these massive and exotic objects. However, we astronomers don’t simply look to see what’s out there—we also seek to understand what we find, using the knowledge of physics we have determined here on Earth. In this way astronomy is broadly synonymous with astrophysics, and indeed astronomers have a boundless curiosity about the greater universe around us as well as deep appreciation for the principles of physics that explain the fundamentals of the natural world.We have little hope of actually traveling to faraway stars and performing experiments on them,so how do we explore the universe while staying What Is Astronomy? 47 at home? The answer is simply by looking, using telescopes as enormous eyes to detect and study the light from faint, distant objects. What are the objects we see in the cosmos? An image showing several interesting objects is shown in figure 1. The most familiar objects are stars, huge balls of gas like our sun powered by nuclear fusion in their cores. Orbiting the stars are planets, rocky bodies like the Earth or gaseous giants like Jupiter. We Figure 1 The universe is teeming with a wide variety of wonderful objects. This is a Hubble Space Telescope image of Stephan’s Quintet, a group of five galaxies in the direction of the constellation Virgo.The galaxies contain huge clouds of gas known as nebulae (shown as blobs, particularly visible in the galaxy to the upper left).The small, bright objects (such as the one halfway up the image on the left edge) are stars in our own Milky Way galaxy that we see in the foreground. Courtesy of NASA 48 Ryan Hickox can now identify and study billions of stars and many thousands of planets, but these represent just the tip of the iceberg of what we can observe with telescopes. We see the huge nurseries in which stars and planets form, diffuse clouds of gas known as nebulae, and we see the remnants of dying stars as they puff off their outer layers forming beautiful patterns of gas, or explode and drive shock waves into the surrounding space.These stellar explosions produce exotic objects known as neutron stars and black holes, in which material the mass of the sun is compressed into a size of less than ten kilometers. Rotating neutron stars can create radio-wave “lighthouses” known as pulsars, while black holes have such strong gravity that they dramatically warp the space around them and even light cannot escape. When we look deeper out into space we see galaxies, enormous “cities” of billions of stars like the Milky Way galaxy in which we reside. Some galaxies are beautiful pinwheel spirals like the Milky Way, while others (the elliptical galaxies) are smooth and round like rugby balls. Galaxies are collected together into a vast cosmic web of which the densest regions are massive clusters of thousands of galaxies.Inside almost every galaxy resides a huge black hole,weighing from a million to a billion (or more) times the mass of the sun.Sometimes these black holes “eat”surrounding gas, which is heated to hundreds...

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