How Old is the Universe? How Do We Know It s So Old?

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1 How Old is the Universe? Until recently, astronomers estimated that the Big Bang occurred between 12 and 14 billion years ago. To put this in perspective, our own Solar System is thought to be 4.5 billion years old. Humans have existed for only a few million years. A) DRAW THESE EVENTS & TODAY ON THE TIMELINE BELOW HINT: 1 inch can equal 1 billion years, and1 billion = 1,000 million My Space Timeline How Do We Know It s So Old? Astronomers estimate the age of the universe in two ways: by looking for the oldest stars; and by measuring the rate the universe is expanding and extrapolating (calculating) back to the Big Bang ABOUT STARS: How old a star gets, called the life cycle of a star, depends on its mass. High mass stars are much brighter than low mass stars, so they rapidly burn through their supply of hydrogen fuel just like a brighter light bulb uses up more electricity. A star like our Sun has enough fuel in its core to burn at its current brightness for about 9,000,000,000 years (9 billion). A star that is twice as massive as our Sun will burn through its fuel supply in only 800,000,000 years (800 million). A 10 solar mass star (a star that is 10 times more massive than the Sun) burns nearly a thousand times brighter and has only a 20,000,000 year (20 million) fuel supply. But a star that is half as massive as the Sun burns slowly enough for its fuel to last more than 20,000,000,000 years (20 billion). 1

2 The Stars Astronomers Study Astronomers can say how old the universe is at the least by studying how large, and how old, stars are especially stars in clusters. This image shows a close up of a Globular Cluster (see large box on right) called Messier Object 15 which is in the Pegasus Constellation of our Milky Way Galaxy. Globular clusters are a dense collection of roughly a million stars. We live on the outskirts of our galaxy, so the next nearest star to our Sun is Proxima Centauri. However, if we lived near the center of a globular cluster there would be several hundred thousand stars closer to us than Proxima Centauri! Since all of the stars in a globular cluster formed at roughly the same time, they can serve as cosmic clocks. When scientists look at where one cluster is in space compared to another, they get a pretty good idea of the age of all the different galaxies. Make Your Own Expanding Universe! 1 st use a marker to make 10 dots on a deflated balloon; number each dot then measure & record how far away they are from Dot #1 2 nd blow 3-4 normal breaths into the balloon to inflate it half way & bend down the end under a paperclip so no air escapes; measure & record how far away each dot is from Dot #1 now 3 rd repeat this process after you double the amount of air in the balloon then answer the Observation Statements on the next page 2

3 B. OBSERVATION STATEMENTS (Fill in the blanks) B.1 After expanding the balloon once, the Dots #2-9 moved Dot #1 B.2 After expanding the balloon again, the Dots #2-9 moved Dot #1 B.3 Each time the size of all the dots themselves. We can use this balloon activity to make inferences from observed facts just like astronomers do about how the universe expands it s what detectives do to solve a crime by retracing the path of a bullet! B.4 Fill in the blanks in the table below. Observation The dots on a balloon move from Dot #1 when the balloon expands The dots on a balloon become when the balloon expands Inference Since Dots #2-9 appear to move from Dot #1, they must have been before the balloon expanded Since Dots #2-9 appear to get as the balloon expands, they must have been before the balloon expanded WHAT DO THESE INFERENCES MEAN? If you imagine the balloon is the whole universe, and each dot is a galaxy in the universe, you can see why scientists think the universe used to be smaller long ago, and objects in galaxies used to be closer together. This is one of the main reasons scientists created the scientific theory called the Big Bang. The table on the next page shows other things scientists have observed in the last century, and what they think this information shows about the age of the universe. 3

4 Important Astronomy Discoveries about the Age of the Universe Scientists have been looking carefully at the distant universe for nearly 100 years, using ever more powerful telescopes, like the Hubble. This table shows what they found, and what it may mean. Observation Almost all galaxies are red-shifted (i.e. their electro-magnetic light waves get longer & move towards the red end of the spectrum the farther away they are). The most distant galaxies exhibit the greatest red-shift. The Hubble Constant shows that all objects far off in space appear to be moving away from Earth at kilometers per second for every kiloparsec (3250 light years of space). The Cosmic Background Explorer (NASA s Explorer 66 or COBE space satellite) found that the temperature of intergalactic space was not zero. Inference Almost all galaxies are moving away from the Milky Way. The most distant galaxies are moving away the fastest. The Universe has been expanding for 8 to 15 billion years. The universe has not yet cooled from the rapid Big Bang expansion. RED-SHIFT THE HUBBLE CONSTANT NASA The lines or waves This table shows how in the spectrum of the red-shift & speed of our Sun are on the distant galaxies (on the left & the shifted, vertical axis, the left line) longer electro- increase the more magnetic waves of distant those far away a supercluster of galaxies are from our COBE & WMAP are 2 distant galaxies galaxy (on the horizontal satellites that measure are on the right. axis, the bottom line) space temperatures & show the universe is still cooling down

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