Unit 3 The Universe Chapter 4 ~ The Formation of the Universe o Section 1 ~ The Scale of the Universe o Section 2 ~ The Formation of the Universe o Section 3 ~ The Future of the Universe Chapter 5 ~ Galaxies and Star Systems o Section 1 ~ Types of Galaxies o Section 2 ~ An Introduction to Stars Chapter 6 ~ Characteristics of Stars o Section 1 ~ Characteristics of Stars o Section 2 ~ The Sun s Energy o Section 3 ~ The Life Cycle of Stars Unit 3 covers the following framework standards: ES 8 and 12. Content was adapted the following: Prentice Hall (Ed.). (2009). The Science Explorer: Astronomy. Boston, MA: Pearson. 28
Chapter 4 The Universe Section 4.1 The Scale of the Universe Terms: Measurement Universe Scientific Notation Astronomers define the universe as all of space and everything in space. The universe is enormous, almost beyond imagination. Astronomers study objects as close as the moon and as far away as quasars. They study incredibly large objects, such as galaxies that are millions of light-years across. They also study the behavior of tiny particles, such as the atoms within stars. Since the numbers astronomers use are often very large or very small, they frequently use scientific notation to describe sizes and distances in the universe. Scientific notation uses powers of ten to write very large or very small numbers in shorter form. Each number is written as a number between 1 and 10 multiplied by a power of 10. For example: 1,200 is written as 1.2 x 103. One lightyear is about 9,500,000,000,000,000 meters. Since there are 15 digits after the first digit, in scientific notation this number is written as 9.5 x 1015 meters. Girl Height < 2 x 100 m Earth Diameter 1.3 x 107 m Sun Diameter 1.4 x 109 m 100 104 108 Cat s Eye Nebula Diameter 3 x 1016 m Andromeda Galaxy Diameter 2 x 1021 m 1016 1020 Virgo Supercluster Diameter 9 x 1023 m 1024 The Immensity of Space The structures in the universe vary greatly in scale. To understand the scale of these structures, imagine that you are going on a journey through the universe. Refer to the image above as a guide. Start at the left with something familiar a sitting girl. She is about 1.5 meters tall. Now shift to the right and change the scale by 10,000,000 or 107. You re now close to the diameter of Earth, 1.28 x 107 meters. As you move from left to right across the image, the scale increases. The diameter of the sun is about 100 times that of Earth. Beyond the solar system sizes of observable objects become much larger. For example, within our galaxy, the beautiful Cat s Eye Nebula is about 3 x 1016 29
meters across. Beyond our galaxy are billions of other galaxies, many of which contain billions of stars. For example, the nearby spiral galaxy Andromeda is about 2 x 10 21 meters across. The Milky Way is part of a cluster of fifty or so galaxies called the Local Group. The Local Group is part of the Virgo Supercluster, which contains hundreds of galaxies. Summary: Astronomers study incredibly large and incredibly small objects that make up the universe. The universe is all of space and everything in space. Since the numbers astronomers use are often very large or very small, they frequently use scientific notation to describe sizes and distances in the universe. Scientific notation is a way of writing large numbers in a short way. Scientific notation uses powers of 10. Each number is written as a number times 10 and a power of 10. For example, consider the number 1,200. Using scientific notation, that number is written like this: 1.2 x 10 3. 30
Section 4.1 The Formation of the Universe Terms: Big Bang Theory Nuclear Fusion Hubble s Law Cosmic Background Radiation The Big Bang Astronomers theorize that the universe began about 13.7 billion years ago. At that time, the part of the universe we can now see was no larger than the period at the end of this sentence. This tiny universe was incredibly hot and dense. The universe then exploded in what astronomers call the big bang. According to the big bang theory, the universe formed in an instant, billions of years ago, from an enormous explosion. The explosion hurled matter in all directions. After only a few seconds, protons, neutrons, and electrons could form. After a few minutes, those subatomic particles came together to create hydrogen. Energy in the universe was great enough to initiate the fusion of lighter elements to form heavier ones. This process, called nuclear fusion, fused hydrogen nuclei into helium nuclei. The first neutral atoms that included electrons did not form until about 380,000 years later. As the universe expanded, it gradually cooled and became less dense. The matter in the early universe was not smoothly distributed across space. Dense clumps of matter held close together by gravity were spread around. Eventually, these clumps formed countless trillions of stars and billions of galaxies. Since the big bang, the size of the universe has been increasing rapidly. The universe is billions of times larger now that it was early in its history. Moving Galaxies An American astronomer, Edwin Hubble, discovered important evidence that helped astronomers develop the big bang theory. In the 1920s, Hubble studied the spectrums of many galaxies at various distances from Earth. By examining a galaxy s spectrum, Hubble could tell how fast the galaxy is moving and whether it is moving toward our galaxy or away from it. In other words, the direction and speed of galaxies could be studied. Hubble discovered that, with the exception of a few nearby galaxies, all galaxies are moving away from us and from each other. Hubble found that there is a relationship between the distance to a galaxy and its speed. Hubble s law states that the farther away a galaxy is, the faster it is moving away from us. Hubble s law strongly supports the big bang theory. 31
To understand how the galaxies are moving, think of raisin bread dough that is rising. If you could shrink yourself to sit on a raisin, you could see all the other raisings moving away from you. The farther a raisin was from you, the faster it would move away, because there would be more bread dough to expand between you and the raisin. No matter which raisin you sat on, all the other raisins would seem to be moving away from you. You could tell that the bread dough was expanding by watching the other raisins. The universe is like the bread dough. Like the raisins in the dough, the galaxies in the universe are moving away from each other. In the universe, it is space that is expanding, like the dough between the raisins. Cosmic Background Radiation After the origin of the Big Bang hypothesis, many astronomers still thought the universe was static. This would mean that the space between objects would have no heat at all, so the temperature would measure 0 K. However, in 1965 two American physicists, Arno Penzias and Robert Wilson, accidentally detected faint radiation, or heat, on their radio telescope. This mysterious glow was coming from all directions in space. Scientists later concluded that this glow, now called cosmic background radiation, is the left over thermal energy from the big bang. Measuring at about 3 K (Figure below), this tiny amount of heat is left over from the Big Bang. Summary: According to the big bang theory, the universe was at first very hot and very small. It was no larger than a period at the end of a sentence. The universe then exploded and expanded outward. That explosion is called the big bang. Since the big bang, the universe has been expanding growing in size. No one can know what came before the Big Bang because there is no remaining evidence. Astronomers estimate that the universe is about 13.7 years old by carefully measuring the speed that galaxies move apart. Edwin Hubble discovered that all galaxies are moving away from each other, supporting the Big Bang. The tiny bit of cosmic background radiation in the universe is thermal energy remaining from the Big Bang. 32
Section 4.3 The Future of the Universe Terms: Dark Matter Dark Energy What will happen to the universe in the future? One possibility is that the universe will continue to expand. Another possibility is that the force of gravity will begin to pull the galaxies back together, resulting in a reverse big bang, or big crunch. Recent discoveries lead many astronomers to conclude that the universe will likely expand forever. Until fairly recently, astronomers assumed that the universe consisted solely of the matter they could observe directly. However, the American astronomer Vera Rubin disproved this idea. Rubin made detailed observation of the motion of spiral galaxies. She discovered that the matter the astronomers can see, such as stars and nebulas, makes up as little as ten percent of the mass in galaxies. The remaining mass exists in the form of dark matter. Dark matter is matter that does not give off any electromagnetic radiation. Dark matter cannot be seen directly. However, its presence can be inferred by observing the effect of its gravity on visible objects, such as stars, or on light. Astronomers don t know much about dark matter what it is made of or all the places where it is found. Yet, astronomers estimate that about eighty percent of the universe s mass is made of dark matter. In the late 1990s, astronomers observed that the expansion of the universe appears to be accelerating. That is, galaxies seem to moving apart at a faster rate now than in the past. This observation was puzzling as no known force could account for it. Astronomers infer that a mysterious new force, which they call dark energy, is causing the expansion of the universe. Current estimates indicate that most of the universe is made of dark energy and dark matter. Summary: New observations lead astronomers to conclude that the universe will likely expand forever. Most of the universe is made up of dark matter and dark energy. Dark matter is matter that does not give off electromagnetic radiation. Because visible light is part of electromagnetic radiation, dark matter cannot be seen. Galaxies seem to be moving apart faster than they used to move. Astronomers think that the force moving them is called dark energy. 33