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Chapter 1 Our Place in the Universe

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Chapter 1 Our Place in the Universe 1.1 Our Modern View of the Universe Topics we will explore: • What is our place in the universe? • How did we come to be? • How can we know what the universe was like in the past? • Can we see the entire universe? • First, let’s take a look to some basic astronomical objects Solar System (in general planetary system) A star, in our case the Sun and all the material that orbits it, including its planets, moons, asteroids, comets, small bodies and gas and dust. Star (Sun) A large, plasma sphere that generates heat and light through nuclear fusion in its core. In the case of the sun, it fuses hydrogen into helium Plasma: Is a state of matter in which the matter is in ionized form. Composed of particles electrically charged. Normally composed of electrons (negative) and protons and ions (positive) Planet Mars Neptune A moderately large object that orbits a star; it shines by reflecting light from the star. The constitution of planets may be rocky, icy, or gaseous in composition. We will discuss the formal definition given by the IAU (International Astronomical Union) later in the semester Moon (or Satellite) An object that orbits a planet Ganymede (orbits Jupiter) Asteroid A relatively small and rocky object that orbits a star Comet A relatively small and icy object that orbits a star Nebula What is outside the solar system? An interstellar cloud of gas and/or dust. Parts of a nebula may reflect light from a star (s), some part may emit light from exited gas atom by UV radiation from a star (s). Part may absorb light . (In the picture, the Orion nebula) The Orion nebula belongs to the Milky way (our Galaxy) Galaxy What is outside the Milky Way? A huge collection of stars in space, all held together by gravity and orbiting a common center. The Milky Way and the Andromeda galaxy are examples of galaxies M31, the great galaxy in Andromeda Clusters of galaxies and superclusters • Groups of galaxies with more than a few dozen members are called galaxy clusters • Regions where galaxies and galaxy clusters are most tightly packed are called superclusters • Galaxy clusters and superclusters form giant chains and sheets with large voids between them Universe The sum total of all matter and energy; that is, superclusters of galaxies, voids and everything within and between all galaxies The observable Universe • The portion of the universe than can be seen from Earth. The observable Universe is probably only a portion of the entire Universe What is our place in the universe? The Big Bang and the expanding of the Universe • Observations of galaxies show that the entire universe is expanding, the average distance between galaxies is increasing with time. • This means that galaxies ( or at least matter) must have been close together in the past. • If we go back far enough, all the matter was concentrated in a small radius from which the expansion began. • That is called the Big Bang. From the rate of expansion it is estimated that it occurred about 14 billions year ago. • The universe has continued to expand and evolve since then. • On a small scale, the force of gravity has drawn matter together forming galaxies, stars formed inside the galaxies and planets formed around some of those stars. • The only two chemical elements created during the Big Bang were hydrogen and helium • The rest of the elements heavier than H and He are generated inside the stars • As Carl Sagan said: We are “star stuff”, all the chemical elements on our body came from stars How did we come to be? Let’s address the question: How can we know what the universe was like in the past? • Light travels at a finite speed (300,000 km/s). Destination Light travel time Moon 1.2 second Sun 8 minutes Sirius 8 years Andromeda Galaxy 2.5 million years How can we know what the universe was like in the past? • Light travels at a finite speed (300,000 km/s). Destination Light travel time Moon 1.2 second Sun 8 minutes Sirius 8 years Andromeda Galaxy 2.5 million years Thus, we see objects as they were in the past: The farther away we look in distance, the further back we look in time. The meaning of a Light-Year • The distance light can travel in 1 year • About 10 trillion kilometers (10x10^12 km) or (6 trillion miles) How far is a light-year? Distance = Speed x Time 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 How far is a light-year? 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 =9,460,000,000,000 km Example: The distance to the Orion Nebula is about 1500 light years. We see the Orion Nebula as it looked 1500 years ago. Example: This photo shows the Andromeda Galaxy as it looked about 2.5 million years ago. Question: When will we be able to see what it looks like now? • At great distances, we see objects as they were when the universe was much younger. We see a galaxy 7 billions light-years away as it was 7 billion years ago. Light from nearly 14 billion light years away shows the universe as it looked shortly after the Big Bang, before galaxies existed Can we see the entire universe? Question Why can’t we see a galaxy 15 billion light-years away? (Assume the universe is 14 billion years old.) A. Because no galaxies exist at such a great distance. B. Galaxies may exist at that distance, but their light would be too faint for our telescopes to see. C. Because looking 15 billion light-years away means looking to a time before the universe existed. Question Why can’t we see a galaxy 15 billion light-years away? (Assume the universe is 14 billion years old.) A. Because no galaxies exist at such a great distance. B. Galaxies may exist at that distance, but their light would be too faint for our telescopes to see. C. Because looking 15 billion light-years away means looking to a time before the universe existed. What have we learned? • What is our physical place in the universe? – The Earth is a small planet in the solar system. – The solar system is one of many planetary systems in the Milky Way Galaxy – The Milky Way galaxy is one of about 40 galaxies in the Local Group of galaxies. – The Local Group is one a many groups in the Local Supercluster of galaxies • How did we come to be? – Part of the matter in our bodies came from the Big Bang, which produced hydrogen and helium. – All other elements were constructed from H and He in the core of the stars and then recycled into the interstellar space from which new star systems were formed, including our solar system. What have we learned? • How can we know what the universe was like in the past? – When we look to great distances, we are seeing events that happened long ago because light travels at a finite speed. • Can we see the entire universe? – No. The observable portion of the universe is about 14 billion light-years in radius because the universe is about 14 billion years old. How big is Earth compared to our solar system? Let’s reduce the size of the solar system by a factor of 10 billion; the Sun is now the size of a large grapefruit (14 cm diameter). How big is Earth on this scale? A. an atom B. a ball point C. a marble D. a golf ball Let’s reduce the size of the solar system by a factor of 10 billion; the Sun is now the size of a large grapefruit (14 cm diameter). How big is Earth on this scale? A. an atom B. a ball point C. a marble D. a golf ball Our Solar System - Sizes 1 mile = 1.6 km Earth diameter =12,756 km = 1.28 x 104 km Jupiter diameter = 142,984 km = 1.43 x 105 km Sun diameter = 1,292,000 km = 1.292 x 106 km Diameter of Sun ~ 109 times diameter of Earth Diameter of Jupiter ~ 12 times diameter of Earth The scale of the solar system • On a 1-to-10- billion scale: – The Sun is the size of a large grapefruit (14 cm). – Earth is the size of a ball point, 15 meters away. – The Earth is 8 light-minutes away from the Sun How far away are the stars? On our 1-to-10-billion scale, it’s just a few minutes’ walk to Pluto. How far would you have to walk to reach Alpha Centauri (one of the closes start to the solar system)? (Alpha Centauri is 4.3 light-years away) A. 1 mile B. 10 miles C. 100 miles D. the distance across the United States (2500 miles) Answer: D, the distance across the United States How big is the Milky Way Galaxy? The Milky Way has about 100 billion stars. Its diameter is about 100,000 light years On the same 1-to-10- billion scale, how big is the Milky Way? Question Suppose you tried to count the more than 100 billion stars in our galaxy, at a rate of one per second.
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