In this chapter, you will compare objects of different sizes to grasp the scale of the universe.

1. In this chapter, you will compare objects of different sizes to grasp the scale of the universe.

2. You can begin with something familiar. • The figure shows a region about 52 feet across occupied by a human being, a sidewalk, and a few trees—all objects whose size you can understand.

3. Each successive picture in the chapter 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—the region you can see in the image—by a factor of 100.

4. In this figure, your field of view widens by a factor of 100, and you can see an area 1 mile in diameter. • The arrow points to the scene shown in the preceding photo. • People, trees, and sidewalks have vanished, but now you can see a college campus and the surrounding streets and houses. • The photo in the figure is 1 mile in diameter. • A mile equals 1.609 kilometers.

5. The view in this figure spans 160 kilometers. • In this infrared photo, the green foliage shows up as various shades of red. • The patches of gray are small cities, with Wilmington, Delaware, visible at the lower right. • The Susquehanna River flows into Chesapeake Bay. • What look like white bumps are a few puffs of clouds.

6. At the next step in your journey, you will see your entire planet—which is 12,756 km in diameter. • The photo shows most of the daylight side of the planet. • The blurriness at the extreme right is the sunset line. • The rotation of Earth carries you eastward. • As you cross the sunset line into darkness, you say the sun has set.

7. Enlarge your field of view by a factor of 100, and you will see a region 1,600,000 km wide. • Earth is the small blue dot in the center. • The moon—whose diameter is only one-fourth that of Earth—is an even smaller dot along its orbit 380,000 km from Earth. • These numbers are so large that it is inconvenient to write them out.

8. Astronomy is the science of big numbers, and you will use numbers much larger than these to discuss the universe. Rather than writing out these numbers as earlier, it is convenient to write them in scientific notation. This is nothing more than a simple way to write numbers without writing lots of zeros. In scientific notation, you would write 380,000 as 3.8 x 105. The universe is too big to discuss without using scientific notation.

9. When you once again enlarge your field of view by a factor of 100, this diagram has a diameter of 1.6 x 108 km. • Another way astronomers deal with large numbers is to define new units. • The average distance from Earth to the sun is a unit of distance called the astronomical unit (AU), a distance of 1.5 x 1011 m.

10. Using this unit, you can say that the average distance from Venus to the sun is about 0.7 AU. • The average distance from Mercury to the sun is about 0.39 AU.

11. The orbits of the planets are not perfect circles, and this is particularly apparent for Mercury. • Its orbit carries it as close to the sun as 0.307 AU and as far away as 0.467 AU. • You can see this variation in the distance from Mercury to the sun in the figure. • Earth’s orbit is more circular, and its distance from the sun varies by only a few percent.

12. Enlarge your field of view again, and you can see the entire solar system. • The details of the preceding figure are now lost in the red square at the center of the diagram.

13. The sun, Mercury, Venus, and Earth lie so close together that you cannot separate them at this scale. • Mars, the next outward planet, lies only 1.5 AU from the sun.

14. When you again enlarge your field of view by a factor of 100, the solar system vanishes. • 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 reflect too little lightto be visible so near thebrilliance of the sun.

15. Notice no other stars are visible except for the sun. • The sun is a fairly typical star, and it seems to be located in a fairly average neighborhood in the universe. • Although there are many billions of stars like the sun, none is close enough to be visible in the diagram—which shows an area only 11,000 AU in diameter. • Stars are typically separated by distances ~10 times larger than the diagram diameter.

16. Now, your field of view has expanded to a diameter a bit over 1 million AU. • The sun is at the center, and you can see a few of the nearest stars. • These stars are so distant that it is not reasonable to give their distances in astronomical units.

17. To express distances so large, astronomers define a new unit of distance—the light-year. One light-year (ly) is the distance that light travels in one year—roughly 1013 km or 63,000 AU. It is a common misconception that a light-year is a time. A light-year is a distance, not a time.

18. The diameter of your field of view in the figure is 17 ly. • The nearest star to the sun, Alpha Centauri, is 4.2 ly from Earth. • In other words, light from Alpha Centauri takes 4.2 years to reach Earth.

19. Another common misconception is that astronomical telescopes reveal the stars as disks. • Although stars are roughly the same size as the sun, they are so far away that astronomers cannot see them as anything but points of light—even if they looked through the largest telescope on Earth. • Any planets that might circle those stars are much too small and too faint to be visible directly. • Using indirect methods, astronomers have found nearly 200 planets orbiting other stars.

20. In the figure, the sizes of the dots represent not the sizes of the stars but their brightness. • This is the custom in astronomical diagrams, and it is also how star images are recorded on photos. • Bright stars make larger spots on a photo than faint stars. • The size of a star image in a photo informs you not how big the star is but only how bright it looks.

21. Now, you expand 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 1,700 ly in diameter.

22. Of course, no one has ever journeyed thousands of light-years to photograph the solar neighborhood. • So, this is a representative photo of the sky. • The sun is a relatively faint star that would not be easily located in a photo at this scale.

23. If you expand your field of view by a factor of 100, you see our galaxy—a disk of stars about 75,000 ly in diameter. • A galaxy is a great cloud of stars, gas, and dust bound together by the combined gravity of all the matter. • Galaxies range from 1,500 to over 300,000 ly in diameter and can contain over 100 billion stars.

24. Of course, no one can journey far enough into space to look back and photograph our home galaxy, the Milky Way. • So, the photo shows a galaxy similar to our own. • Our sun would be invisible in such a photo. • However, if you could see it, you would find it in the disk of the galaxy about two-thirds of the way out from the center.

25. As you expand your field of view by another factor of 100, our galaxy appears as a tiny luminous speck surrounded by other specks. • The diagram includes a region 17 million ly in diameter, and each of the dots represents a galaxy. • Notice that our galaxy is part of a cluster of a few dozen galaxies.

26. If you again expand your field of view, you see that the clusters of galaxies are connected in a vast network. • Clusters are grouped into superclusters—clusters of clusters. • The superclusters are linked to form long filaments and walls outlining voids that seem nearly empty of galaxies. • These appear to be the largest structures in the universe.