# Ch01 - Chapter 1 The universe at different scales Poets say...

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Chapter 1 The universe at different scales Poets say science takes away from the beauty of the stars - mere globs of gas atoms. Nothing is mere . I too see the stars on a desert night, and feel them. But do I see less or more? The vastness of the heavens stretches my imagination - stuck on this carousel, my little eye can catch one-million-year-old light … Or see them [the stars] with the greater eye of Palomar, rushing all apart from some common starting point when they were perhaps all together. What is the pattern, or the meaning, or the why? It does not do harm to the mystery to know a little about it. For far more marvellous is the truth than any artists of the past imagined! Why do the poets of the present not speak of it? Richard Feynman 1.1 Units of astronomical distances Scientific notations: Because of the large range of scales we will encounter, we will use powers to express very large or small numbers. e.g., 10 5 = 100,000, 10 3 = 1,000, 10 -3 = 0.001, 10 -5 = 0.00001. e.g., 300,000 = 3 × 10 5 , 4,500 = 4.5 × 10 3 , 0.000062 = 6.2 × 10 -5 . Metric Prefixes: These prefixes specify powers in a convenient way. e.g., kilo (k) = 10 3 = 1,000, mega (M) = 10 6 = 1,000,000. e.g., 1 kilometre = 1 km = 1,000 metres, 1 megaton = 1 Mton = 1,000,000 tons. Astronomical unit (AU): The average distance between the earth and the sun. 1 AU 1.5 × 10 11 m Light year (ly; 光年 ): One light year is equal to the distance travelled by light in vacuum in one year. In vacuum, light travels at a speed of 3 × 10 8 m/s. 1 ly 6.3 × 10 4 AU 9.5 × 10 15 m Parsec (pc; 秒差距 ): Another common astronomical unit for distance. 1 pc 3.3 ly 1.2 A quick tour of the cosmos To understand the universe, we must understand the relative scales of planets, stars, galaxies and the universe as a whole. We shall journey from a campus scene to the limits of the cosmos in 13 steps. In each step we shall widen our view by a factor of 100. That is, each successive picture will show a region of the universe that is 100 times wider than the preceding picture.

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UGB 240M Astronomy The universe at different scales 1-2 Fig. 1-1 This scene shows a region about 16 m across. It is occupied by a human being, a sidewalk and a few trees. Only 13 steps separate this scene from the universe as a whole. Step 1 We start with objects whose sizes we can understand. This helps us to comprehend astronomical distances relative to ourselves.
The universe at different scales 1-3 Fig. 1-2 We now increase our field of view by a factor of 100. Individual people, trees, and sidewalks vanish, but now we can see a college campus and the surrounding streets and houses. Step 2

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Ch01 - Chapter 1 The universe at different scales Poets say...

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