Size and Scale Attendance Quiz II
Are you here today? Here! (a) yes (b) no (c) are we still here? Today’s Topics
• “How do we know?” exercise • Size and Scale • What is the Universe made of? • How big are these things? • How do they compare to each other? • How can we organize objects to make sense of them? What is the Universe made of? Stars
• Stars make up the vast majority of the visible mass of the Universe • A star is a large, glowing ball of gas that generates heat and light through nuclear fusion • Our Sun is a star Planets
• According to the IAU, a planet is an object that 1. orbits a star 2. has sufficient self-gravity to make it round 3. has a mass below the minimum mass to trigger nuclear fusion 4. has cleared the neighborhood around its orbit • A dwarf planet (such as Pluto) fulfills all these definitions except 4 • Planets shine by reflected light • Planets may be rocky, icy, or gaseous in composition. Moons, Asteroids, and Comets
• Moons (or satellites) are objects that orbit a planet • An asteroid is a relatively small and rocky object that orbits a star • A comet is a relatively small and icy object that orbits a star Solar (Star) System
• A solar (star) system consists of a star and all the material that orbits it, including its planets and their moons Star Clusters
• Most stars are found in clusters; there are two main types • Open clusters consist of a few thousand stars and are young (1-10 million years old) • Globular clusters are denser collections of 10s-100s of thousand stars, and are older (10-14 billion years old) Nebula • A nebula is an interstellar cloud of gas and/or dust • Nebulae are where stars are born and are also created when they die Galaxy • A galaxy is a great island of stars in space, all held together by gravity and orbiting a common center • In addition, there is gas and dust between the stars, some in the form of visible nebulae Cluster of Galaxies
• A collection of galaxies all held together by gravity Universe
• The Universe is the name for the sum total of all matter and energy; that is, everything within and between all galaxies How big are these things? Object Size (km) Moons, asteroids, comets 1-1,000 (100-103) Planets 1,000-100,000 (103-105); Earth ≈ 104 Stars (middle-aged) 100,000-10,000,000 (105-107); Sun ≈ 106 Stars (extreme) 10 (101) - neutron stars 1,000,000,000 (109) - red giants Solar (Star) System 10,000,000,000 (1010) Stellar Clusters 100,000,000,000,000 (1014) - open clusters 1,000,000,000,000,000 (1015) - globular clusters Nebulae 1,000,000,000,000-1,000,000,000,000,000 (1012-1015) Galaxies 1,000,000,000,000,000,000 (1018)
Clusters of Galaxies 100,000,000,000,000,000,000 (1020)
Universe ? Scientific Notation • In order to write these very big numbers, it is convenient to use a shorthand known as Scientific Notation • A number such as 100 can be written as 102, where the 2 in the exponent means multiply 10 two times, i.e, 100=10x10 • Likewise, 10,000 = 104 and 100,000,000,000 = 1011 and so on • To multiply two such numbers, we simply add the exponents; to divide we subract the exponent in the denominator from the exponent in the numerator 103 x 107 = 10(3+7) = 1010 109 / 104 = 10(9-4) = 105 • Thus, to figure out how many times larger than the Solar System is the Milky Way: 1018 km / 1010 km = 108; thus, the Milky Way is 100 million times larger than the Solar System Astronomical measures of distance • Since astronomical distances are so large, it is convenient to define some other measures of size and distance
1 astronomical unit (AU) = the average distance from the Earth to the Sun = 150,000,000 km = 1.5 × 108 km 1light - year = (speed of light) × (1 year) ⎛ km⎞ ⎛ 365 days 24 hr 60 min 60 s ⎞ = ⎜ 300,000 ⎟ ×⎜ × × × ⎟ ⎝ s ⎠ ⎝ 1 yr 1 day 1 hr 1 min⎠