stretched out and appear more red. When an object is moving toward an observer, the object is moving in the same direction the light is being emitted from, and the wavelengths of light become squished up and appear more blue. This is something that we call redshift or blueshift. In either case, the colors that an object gives off shifts toward one end of the spectrum or the other. Redshift doesn t mean that an object will turn red when it moves, but instead that the color of that object will move toward the red end of the spectrum. For example, if an object, say a star in space, is emitting white light, but it is moving away from us, its light will become redshifted and may appear yellow or orange. However, if the star is moving really fast, it could actually appear red. The amount of shifting that takes place will depend on the speed of the object moving. If an object is moving slowly, it will be slightly shifted. If an object is moving very fast, the light it is emitting will be shifted a lot. The diagram to the right shows what these spectral lines may look like. The diagram represents hydrogen light coming from two distant stars in our own galaxy. The normal spectra or non-moving spectra is shown in the middle image (Spectra of Hydrogen). To the left is Image credit: Cassie Grether, 2018. the blueshifted spectral lines, while to the right is the redshifted spectral lines. From this analysis it can be observed that the blueshifted star is a star moving toward our solar system while the star on the right is a star moving away from our solar system. How does all of this relate to the initial question of this section: Is the universe getting bigger or smaller? When we use this spectral evidence we can see that everything in space is moving. Everything. Planets, stars, entire galaxies, everything is moving. When we look outward beyond our galaxy we see that nearly all galaxies in the visable universe are redshifted. All of them are moving away from us, and what s more, they are all moving away at a proportional speed to their distance from us. The universe is getting bigger. 8 Unit 1 Astronomy Chapter 2 - The Big Bang Section 2.1: The Expanding Universe When Edwin Hubble looked out into the velvety night sky he thought he would see a random assortment of blueshifted and redshifted galaxies. To his surprise, that is not what he found. Not only was every single galaxy moving, but nearly each and every one of them were redshifted. Think for a moment what that means. Redshift means that objects are moving away. So what Hubble discovered was that almost all galaxies are moving away from us, and each other. A photograph of this can be seen in the image at the top of the next page. In this image each spec of
liht does not represent stars, but instead each blip of light is a galaxy out side of our own Milky Way Galaxy. Each little point of light holds billions of stars. Not only did Hubble notice that the galaxies were redshifted, but the further a galaxy was away from us, the faster it was moving. If the galaxies are redshifted, and that means they are moving away from one another and spreading out, what do you think this means about the size of the Universe? Edwin Hubble combined his measurements of the distances to galaxies with other astronomers measurements of redshift. From this data, he noticed a relationship, which is now called Hubble s Law: the farther away a galaxy is, the faster it is moving away from us. What could this mean about the universe? It means that the universe is expanding, the Universe is getting bigger. The figure to the right shows a simplified diagram of the expansion of the universe. The idea is that all matter was once contained at one point in space and everything is expanding, moving away from that same point. One way to picture this is to imagine a balloon covered with tiny dots to represent the galaxies. When you blow the balloon up, the dots slowly move away from each other because the rubber stretches as the balloon expands, moving the dots apart. An inflating balloon is only a rough analogy to the expanding Universe for several reasons. One important reason is that the surface of a balloon has only two dimensions, while space has three dimensions. But space itself is stretching out between galaxies like the rubber stretches when a balloon is inflated. This stretching of space, which increases the distance between galaxies, is what causes the expansion of the universe (all existing matter and space considered as a whole; the cosmos). In this diagram of the expansion of the universe over time, the distance between galaxies gets bigger over time, although the size of each galaxy stays the same. One other difference between Earth Science 9
the universe and a balloon involves the actual size of the galaxies. On the balloon, the dots will become larger in size as you inflate it. In the Universe, the galaxies stay the same size due to gravitational forces within the galaxy, and possibly other forces. Section 2.2: The Formation of the Universe Before Hubble, most astronomers thought that the universe didn t change. ey thought that how the universe is now is how it was, and how it will be. We know now it is expanding, but if the universe is expanding, what does that say about where it was in the past? If the universe is expanding, the next logical thought is that in the past it had to have been smaller. How Did the Universe Form? The Big Bang Theory The Big Bang Theory is the most widely accepted theory of how the Universe was formed. Scientific theory isn t the same as the word theory used in everyday speech. You may hear someone say, I have a theory about that. What they really mean is they have an idea, a thought, or at most a hypothesis. Theories in science are treated as fact. They are conclusions drawn from evidence like facts, data, and Big Bang Timeline. Nasa.gov Timeline visualizing the moments a er the Big observations. To be Bang to 13.7 billion years. a supported theory, the concept must be backed up by many, many evidence and have experiments and tests that help provide more evidence for its validation or correctness. If a theory can be shown to be false it ceases to be a theory and a new theory must be derived that better fits the new evidence. According to the Big Bang theory, the universe began about 13 to 14 billion years ago. Everything that is now, ever was, or ever will be in existence was squeezed into a very small point. Nothing existed outside of this point- not space, not even time. Then for whatever reason currently not understood by scientific means, this point expanded rapidly. All of space-time was born from this moment as energy spewed forth from this violent and hot birth. Energy was so intense that small particles of atoms began to fuse into existence. Eventually this matter formed into the lightest and smallest elements we still have today: hydrogen and helium. What came before the Big Bang? Well, science is based on evidence, and as it stands currently, 10 Unit 1 Astronomy
we have no evidence of what was before the beginning. Currently we have no idea what came before the Big Bang so we just can t say. After the Big Bang NASA.gov: This image is an artists rendition showing what it might have been like to see the first stars in the Universe lighting up the darkness. In the first few moments after the Big Bang, the universe was unimaginably hot and dense. As the universe expanded, it became less dense and began to cool. However, in these initial stages of the Big Bang it was not yet cool enough for electrons to attach to the hydrogen and helium nuclei (middle of an atom where neutrons and protons reside). This took a cooling of 380,000 years Image of first stars in the Universe ;NASA.gov: This image is an artists rendition showing what it might have been like to see the first stars in the Universe lighting up the darkness. or so.slowly matter began clumping together under the force of gravity. The larger the clumps became, the stronger their force of gravity. The stronger their force, the more matter these clumps could draw in. During this time space was dark. But it was still expanding constantly stretching the distance of space. Eventually, these giant nebulae created of hydrogen and helium began to glow hot under the pressure of gravity. Four hundred billion years from the initial expansion of energy from the Big Bang, the lights begin to go on- the stars were being born and the Universe was alight. Eventually this became countless trillions of stars making up billions of galaxies, and other structures that now form most of the visible mass of the universe. Section 2.3: What Evidence Points to the Universe Aging? Visual showing nuclear fusion in star; NASA These first supermassive stars provide us with another piece of evidence that the universe is aging. Stars by their very nature, use hydrogen as a basic fuel. They go through a process called fusion. Fusion occurs when two or more elements combine in an energetic way that causes their nuclei to become one. It is not the same as chemical bonding. Fusion takes one thing andmerges it into a brand new thing. For example, under the tremendous pressures and heat of the core of a star, hydrogen collides with multiple hydrogen atoms. If they do this just right, they will merge their nuclei and become helium. The image to the left shows a visual example of nuclear fusion inside a star s core. Earth Science 11
The hydrogen in stars is being changed by fusion into helium and bigger elements. As time goes on the hydrogen continues to be used up and turned into helium. When the first elements formed after the Big Bang about 92% were hydrogen, with the rest being 8% helium and traces of lithium. Today we have about 74% hydrogen and 24% helium as well as the 90 other naturally occurring elements. This is some evidence that the Universe is aging and changing because the elements are becoming more abundant. By abundant I mean that there are more kinds of elements than ever. Periodic Table of Elements By calculating how much hydrogen the Universe started with, and the rate that stars use it, we can determine when stars began fusing. We can also use this rate to estimate how much longer fusion will continue. Right now you live in the middle of the Universe s life. Hundreds of billions of stars in hundreds of billions of galaxies are using up hydrogen and turning it into heavier elements as the universe continues to age. Like all things, the Universe has a lifetime and eventually it will expire. How Do We Know What Ancient Stars Were Like? Most or all of the first stars of the Universe are probably dead and gone by now. So how do scientists observe or gather evidence about them at all? How could they possibility know anything 12 Unit 1 Astronomy
from the past? Here s a funny thing about light. Light travels. It moves through space-time just like you and me, and very similarly to sound. Let s start with sound because it is something that is a bit easier to understand. Sound travels through matter like air and wood. If you knock on wood the sound waves will travel from one end to the other like ripples in a pond. When someone knocks on your door the sound takes time to travel to your ear in another room. Now sound travels pretty quickly, but you can test this out. Go outside with a friend and get some space between you- the length of a soccer field is plenty of space. Yell to your friend and have them raise their hand high in the air the moment they hear your voice. As you yell, what you will see is a delay in your friend s response to raise their hand. This is because the sound as it travels as a wave across the field takes time to move. Light is no exception. Light travels in a wave as well. The cool thing about light, though, is it can travel through matter like air as well as through the empty vacuum of space. Each star gives off light. Light traveling from our own star takes about 8 minutes to reach the Earth. The stars you see in the night sky are also giving off light, but their light originates much farther than our own star. The farther the star the more space the light has to travel the longer it takes for the light to reach us. Alright, so I hopefully you are still with me because this is where it gets funky. If the light traveling from the Sun takes eight minutes to reach the Earth, we are not seeing the Sun as it is now, but instead how it was eight minutes ago, This is because the light it gave off took eight minutes to reach us. So in essence we are seeing the Sun eight minutes ago in its past. Now let s take this a step further. If we are looking at a star that is 200,000 light-years away we are not seeing this star as it is now, we are seeing the star as it was 200,000 years ago. To put that in perspective, if there were aliens that lived in a far off galaxy 65 million light-years away, and they had this amazing technology that allowed them to have perfect resolution here on Earth, they would not see us. Because the light shining off of our planet would take 65 million years to reach them, instead of seeing us, they would see into our past. Instead of taking a picture of New York City they would be taking a picture of the dinosaurs. So, how do we know what some really old stars are like? Well, here is an amazing thing. The further we look into space, the more distance we are looking into. The farther we look out into space the farther back in time we can see. In fact, when you look at the night sky and see those stars twinkling in the velvety darkness of night, some of those stars you see are not really there. Some of those stars have used all of their fuel and their light has gone Approximate Light Distance Time out long ago. We still see them Signal Travel Times because their light is still taking its 1 foot 1.0 nanoseconds journey through space even though 1 meter 3.3 nanoseconds the source, its star, is now dark. Geostationary satellite to Earth 119 milliseconds In the table below you can see Earth s equator 134 milliseconds the approximate travel times for light at several different distances. From Moon to Earth 1.3 seconds Compare the travel times for light Sun to Earth 8.3 minutes in familiar distances like a foot to Sun to the nearest star 4.2 years the time it takes light to travel from Across the Milky Way Galaxy 100,000 years our nearest galactic neighbor the Andromeda Galaxy to Earth 2.5 million years Andromeda Galaxy. Earth Science 13
MORE EVIDENCE OF THE BIG BANG After the publication of the Big Bang, many astronomers still thought the Universe was static (or not changing). However, the Big Bang Theory made a prediction that was different from what was expected in a static universe. It predicted that there should be some heat energy left over from the Big Bang. Meaning that with the continued expansion of the Universe, the temperature should be very low today, only a few K (Kelvin) above absolute zero (Kelvin is an absolute temperature scale starting at a zero which is the temperature in which all matter stops moving including electrons). In 1964, two researchers at Bell Laboratories built a microwave receiver for telecommunications research. Using it, they discovered microwave radiation in all parts of the sky. The radiation had a temperature of 3 K (Figure below). This is evidence that was predicted to exist by the theory helped add more validity to the already strong Big Bang Theory. This left over radiation is called CMB or cosmic background radiation. Another evidence that helps support the expanding Universe is theredshift factor. In the last chapter we described how redshift is the tendency for waves like that of light to become shifted or changed when in motion. When an object is redshifted it means its motion is moving away from an observer. When we point the Hubble telescope and others to the night sky twe see that almost all galaxies are redshifted, and the farther galaxy is away, the more its redshift. This shows that the Universe is expanding since every other point is moving away from all other points. Take a look at the photo. Galaxies should give off white light. (Note: in the photo white galaxies may appear blueish due to your screen display.) Try to find yellow, orange and even red blips in this photo. Those blips are galaxies that are moving so fast that they have become redshifted. Now not all redshifted galaxies are actually red. Remember, if they aren t moving too fast they will just appear slightly off of Photo of distant galaxies; NASA: Photo taken by Hubble Space telescope of distant white galaxies. making them appear yellow or orange. 14 Unit 1 Astronomy This image shows the cosmic background radiation in the Universe as observed from Earth and space probes.
Section 2.4: A SIDE NOTE Earlir I stated that nearly all galaxies are moving or expanding away from us (well, more like the space between us is expanding or stretching apart). There is one galaxy that is not moving away from the Milky Way, however. It is instead blueshifted. Because we are so close to one another, as far as gravity is concerned, the Milky Way and the Andromeda galaxies are on a collision course. The figure to the right shows an artists rendition of what the night sky would look like when the Andromeda Galaxy becomes ever nearer to our own home. To see what might happen in the future regarding our two galaxies, see this video put out by NASA: https://www.youtube.com/watch?v=2wei8wbjkkk THINGS TO REMEMBER ABOUT THE BIG BANG The universe contains all the matter and energy that exists now, that existed in the past, and that will exist in the future. The universe also includes all of space and time. According to the Big Bang theory, the universe was squeezed into a very small volume and then began expanding explosively about 13 to 14 billion years ago. Redshift is a shift of element lines toward the red end of the spectrum. Redshift occurs when the source of light is moving away from the observer. Light from almost every galaxy is redshifted. The farther away a galaxy is, the more its light is redshifted, and the faster it is moving away from us. The redshift of most galaxies is evidence that the universe is expanding. Cosmic background radiation is left over energy from the Big Bang. Chapter 3 - A Star s Life & The Nebular Hypothesis Section 3.1: Stars, an Introduction Milky Way photo in Bryce Canyon, Utah; This photo of the Milky Way was taken in Bryce Canyon, Utah. (Photography by David Wicks, 2010) When you look at the sky on a clear night, you can see hundreds of stars (or thousands, if you re away from city lights). Each year light pollution decreases the amount of stars that we can see in the sky. Thirty years ago when I was a child I would spend summer nights lying awake staring up at the Milky Way out on my parent s deck. The picture to the left is a photo from Bryce Canyon, Utah. This is the view I remember from my childhood that is now gone from most cities and towns. Away from the city lights, you can still see this magnificent sight. Earth Science 15