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Chapter 1

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John Lester

Astronomy Textbook Notes Chapter 1 Our Place In The Universe Solar System – consists of Sun and all the objects that orbit it: the planets and their moons, and countless smaller objects including their moons and countless smaller including rocky asteroids and icy comets. Cosmic Address: Earth  Solar System  Milky Way Galaxy  Local Group  Local Super cluster. Milky Way Galaxy – huge, disk-shaped collection of stars.  Containing a few more than 100 billion stars.  Our galaxy is one of about 40 galaxies in the local group.  The local group is part of the Local Super clusters – which is one small part of the universe.  Located halfway from the galactic center to the edge of the galactic disk. Galaxy – great island of stars in space, containing a few hundred million to trillion or more stars.  Galaxies appear to be arranged in giant chains and sheets with super voids between them. Super clusters – clusters of galaxy clusters.  Our local group is located in the outskirts of the local super cluster. - The universe in other words, is the sum total of all matter and energy, encompassing the super clusters and voids and everything within them. Observable Universe – the portion of the entire universe that can be seen from Earth (tiny portion of the entire universe). The Big Bang and the Expanding Universe: - The entire universe is expanding – meaning that the average distances between galaxies are increasing with time.  This implies that the galaxies were once close together in the past and that if we go back far enough, we will reach a point when this expansion began = big bang.  Occurred 14 billion years ago.  Universe began to expand since big bang but smaller forces of gravity has drawn matter together.  While the universe as a whole continues to expand, individual galaxies and galaxy clusters do not expand.  Milky way galaxy probably formed within a few billion years after the Big Bang. - Gravity drives the collapse of clouds of gas and dust to form stars and planets. Stars go through life cycles. A star is born when gravity compresses the material in a Astronomy Textbook Notes Chapter 1 Our Place In The Universe cloud to the point where the center becomes dense and hot enough to generate energy by nuclear fusion (process in which lightweight atomic nuclei smash together and stick or fuse, to make heavier nuclei). - A star dies, and put much of its content back in space (some dies in massive explosions called supernovae). This matter mixes in with other matter in the galaxy, eventually becoming part of a new cloud. - Planet is considered a planet if:  It orbits a star (but its not a star or moon).  Large enough for its own gravity to make it round  Has cleared most other objects from its orbital path. o If a planet meets the first two criteria’s ONLY it is considered a dwarf planet.  Asteroids and Comets make up a class called small solar system bodies. Cosmic Origins: 1) Birth of the Universe: began with the hot and dense Big Bang. 2) Galaxies as cosmic recycling plants: early universe contained only two chemical elements: hydrogen and helium. 3) Life Cycles of Stars: Many generations of stars have lived and died in the Milky Way. 4) Earth and Life: Elements were manufactured by stars – some through nuclear fusion that makes stars shine, and others through the nuclear reactions accompanying the explosions that end stellar lives. By the time our solar system formed 4.5 billion years ago, earlier generations of stars had converted 2% of the galaxy’s original hydrogen and helium into heavier elements. We are technically “star stuff” (Carl Sagan). The cloud that gave birth to our solar system was made up of 98% hydrogen and helium and 2% other elements. How old is the solar system? - Age is approx. 5 bill years. = 5x10^9 - 50,000,000 human lifetimes - Importance: very slow changes alter the Earth and solar system dramatically over deep time. --> To contrast to our times which is shallow times. --> There was a time before the solar system (not been here forever). This changes our perspectives. Time-Distance Connection: - Light travels extremely fast (300,000 kilometers per second).  Can circle the Earth 8 time in one second.  It would take 8 minutes for light from Earth and the Sun. - One light-year is the distance light can travel in 1 year (10 trillion kilometers, 6 trillion miles). Astronomy Textbook Notes Chapter 1 Our Place In The Universe  Unit of distance, not time. - Because of this, the farther away we look in distance, the further back we look into time.  The light we see from the far side of the galaxy must have left on its journey to us 100,000 years before the light from the near side.  Looking far into space means looking back into time. - The farther away we look into space, the further back in time. The age of the universe therefore puts a limit on the size of the observable universe – the portion of the entire universe that we could observe in principle. We have no hope of seeing or studying anything beyond our observable universe. How “big” is the solar system?  Words like “big” do not have enough information. We just need numbers (km).  Memorize the hows and what’s; not numbers.  What would be different if the numbers was different?  Earth’s radius: 6,378 km or diameter = 2 X radius = 12, 756 km.  What do these numbers tell us? What would change if radius were different? The Scale of the Universe: - The voyage model shows the Sun and the planets, and the distances between them at one ten-billionth of their actual sizes and distances.  On a scale of 1 to 10,000,000,000 the sun is about the size of a grapefruit. Planets are much smaller, with Earth the size of a ball point and Jupiter the size of a marble on the scale. The distances between planets are huge to compare to their sizes, with Earth orbiting 15 meters from the sun on the scale.  Shows the planets in a straight path, so we’d need to draw the planet’s orbit around the model sun to show the full extent of the planetary system.  Orbits would require an area measuring more than a kilometer on the side – an area equivalent to the space of 300 football fields.  Moon lies only 4 mm away from the Earth in the voyage model.  You can walk roughly 600 meters distance from the Sun to Pluto in a few minutes.  To walk to the nearest star (Alpha Centauri), you would need to walk to California (4400 km away on the 1-10 billion scale).  Using the scale on which the Milky Way galaxy is the size of football field, the distance to the nearest star would be about 4 mm. There are so many stars in our galaxy that it would take thousands of years the count them.  Counting all 100 billion stars would take you 3000 years… (no breaks). How big is the universe? Astronomy Textbook Notes Chapter 1 Our Place In The Universe  The milky way is only 1 of roughly 100 billion galaxies in the observable universe. It would take thousands of years to count all the galaxies in the world (just like stars).  100 billion stars per galaxy = 10,000,000,000,000,000,000,000 stars in all the universe (roughly). o Compare to the beach  counting all the grains on all the beaches of the world is comparable to number of stars in the observable universe. - Comparing solar system with galaxy: solar system = 1 single star systems, and galaxy = 100 billion star systems. Which is the nearest star? Not Polaris (north star) -
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