Astronomy Lecture Notes Elaboration 1-2 PDF

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elaboration of chapter 1 and chapter 2 lecture notes ...

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Astronomy Homework #1

Lecture Notes (Chapter 1) Page 1: Middle Left and Right, Bottom Right •

What is our place in the universe?: Our galaxy has a cosmic address which covers a very large expanse of both space, time and nearly every object within the galaxy. Earth is a planet in our solar system which consists of asteroids or comets and even more planets, stars like the sun, and the moon. The solar system, however, is within our Galaxy which is the Milky Way and it is one of the two largest galaxies from more than 70 in the local group in the local supercluster. Earth orbits the Sun out of more than 100 billion stars in our galaxies.

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Solar (Star) System: The solar system is a single star system that consists of planets, their moons and the sun including comets and asteroids. The solar system belongs to the galaxy and it is located about halfway from the galactic center to the edge of the galactic disk. There are 7 more planets along Earth, making the solar system have 8 now that Pluto is considered a dwarf planet. The planets orbit the sun and planets, stars, moons and the sun seem to revolve around the Earth. The sun is way bigger than the planets that orbit it, and it outweighs all of the planets combined in mass. When viewed to scale, what seems to be the most intriguing is the emptiness in the solar system.

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Star (Astro 1402): Stars are large make up of gas that produces heat and light and a diskshape collection of these stars makes up the Milky Way. The sun which the Earth orbits is a star as well as the stars we see in the night sky and a billion more stars in the galaxy. Stars are orbited by planets, asteroids, and comets. Stars makes up star systems like the solar system and the nearest star system is the Alpha Centauri which is a three-star system about 4.4 light years away. If one tried counting all the stars in the galaxy, it would take thousands of years.

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Planet: Planets orbit stars and that makes them shine through the reflection of the star’s light that they orbit. Planets vary immensely in size, for example, the Great Red Spot on

Jupiter could swallow all of Earth. When an IAU vote defined “planets” as something that made Pluto and Eris not fit into the category, it established that a planet has to be large enough for its own gravity to make it round and has a clear orbital path to orbit a star. This made Pluto be considered a dwarf planet and not a part of the (now) 8 planets in the solar system that orbit the sun, and although dwarf planets fit two of the planet descriptions, they have not cleared its path to orbit the sun. However, there is now technology that is capable of finding planets which orbit stars near. •

Medium and Large Moons: For starters, moon is like a satellite which orbits a planet. For the Earth’s moon, it takes about 1 second for the light from the moon to reach Earth. It is the only other world that humans have stepped on. Moon sizes have been categorized as small, medium or large. Medium and Large moons are spherical since they have the selfgravity to be spherical formed and have a considerable amount of ice in addition to metals and rocks. They have been proved to show a high level of past and present volcanism and tectonics. Medium and Large moons were formed through the Jovian planet’s disks when they were younger and orbit and have the same orbit direction as planet rotation

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Galaxy (Astro 1402): A galaxy is defined as “a great island of stars in space being held together by gravity and orbiting a common center.” Although some galaxies may be isolated, most of them are found in groups. Moreover, the galaxy we are in is called the Milky Way and it has been declared one of the biggest galaxies out of more than 70 in the “local group” which is the group the Milky Way belongs in. Galaxies are collections of billions of stars and the Milky Way contains more than 100 billion star systems. Galaxies and solar systems should not be mistaken although they may be since our galaxy is approximately 100 million times larger than the solar system.

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Looking back in time: Light takes more than 1 second from the Moon to the Earth and approximately 8 minutes from the Sun to the Earth. The stars that we see in the nights sky are actually very far away which means the light that is produced takes years to reach us, and that is the reason why their distances are measured in light-years. “The farther away we look in distance, the further back we look in time” so if we are seeing a star in the sky, we are actually seeing the light produced from the star the way it was looked however

many years (light-years) it took to travel to Earth. For example, if it took a star 5 light years to reach Earth, then the star we are seeing is actually how it looked 5 years ago. It is the same with galaxies. •

Definition: Light-Year: Light-years are used to measure the distance of how much light travels. One light-year is the measure of the distance light can travel in one year which is about 6 trillion miles. Light-years is mostly used to measure distance in stars and galaxies. Many confuse light-year as a measure of time, but it is just distance. Light of course travels at the speed of light and that is approximately 300,000 km per second. The equation for a light-year would be distance equals speed multiplied by time.

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At great distances, we see objects as they were when the universe was much younger: What is meant by “At great distances, we see objects as they were when the universe was much younger” is that what we see in the sky is not how it looks in the galaxy in the present. Since objects in the galaxy like stars and planets are measured in light-years, the objects appear in the sky as they did however many years ago it took for the light to reach us. Just as previously stated, if a star’s light took 2 light years to reach Earth, when we are seeing it in the night sky, is how it looked 2 years prior.

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Youngest galaxy: The youngest galaxy is the galaxy that looks the farthest.

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The scale of the solar system: There is a model made specifically to show the solar system shrunken to scale. That is called the Voyage Scale model and it makes the distances between the planets and the stars along with other distances between objects in the solar system at one ten-billionth of their actual distances and sizes. In the Voyage Scale Model, the sun may be the size of a grapefruit while the Earth is just the size of a ballpoint in a pen which orbits the sun at a distance of 15 meters. The model also shows some key facts about the solar system. Shown to scale, the solar system’s emptiness is the most striking feature.

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How big is the universe? Our cosmic address shows in diagrams the structure of the universe. This helps as a visual to put a number on the size of different structures in the

universe. The milky way is one of about 100 billion galaxies in the universe. There is no actual number to how many stars are in the universe, but if it is assumed that there are about 100 billion stars per galaxy, then to get the number of stars in the universe, that number would have to be multiplied by 100 billion. To be put into perspective, counting the stars in the universe would be like counting every grain of sand on every island. There are more than 70 galaxies, and those galaxies are within the universe. If one believes counting more than 100 billion stars in the galaxy would be time consuming, imagine the universe. Page 6: Middle Right, Bottom Left and Right •

How did we come to be? Birth of universe: Our universe began in a hot, dense state which is defined as the Big Bang. The universe expanded with time, and it still does on smaller scales with gravity pulling matter together in order to produce more galaxies. The Big Bang is the name given to the birth of the universe, when expansion began about 14 billion years ago. It began as a small piece of the universe then in time it became what it is now, still expanding. Most galaxies formed within a few billion years ago after the universe’s birth initiated in the Big Bang.

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How did we come to be? Galaxies as Comic Recycling Plants: In the early stages of the universe, there was only two chemical elements which were hydrogen and helium. The rest of the elements were created by stars and recycled from one stellar generation in galaxies including the Milky Way. A star’s birth consists of gravity compressing the material in a cloud to the point where the center becomes dense and hot to generate energy by nuclear fusion to make a heavier nucleus after atomic nuclei fuse together. When a star dies, it exhausts its usable fuel into space releasing new elements, and during its lifetime it produces energy and new elements through the process of nuclear fusion. Galaxies serve as “cosmic recycling plants” for stars’ released energy after its lifetime.

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How did we come to be? Life cycle of stars: There has been many generations of stars in the Milky Way that have lived and died. They begin as clouds of gas and dust which allow planets to form in surrounding disks. Then, when stars die by exploding, they scatter their elements produced into space. Nuclear fusion allows stars to shine by the energy produced which manufactures elements that are heavier than the two original (hydrogen and helium). Basically, the material that humans and Earth are made up of was

produced by stars that died after their lifetime before the sun came along. Astronomer Carl Sagan said that “we are ‘Star stuff.’” Page 7: Top Right and Bottom Right •

How do our lifetimes compare to the age of the universe? The lifetime of the universe can be compressed into one year to be put into perspective so that each month equals a little more than 1 billion years. This is called the cosmic calendar which naturally starts January 1st representing 14 billion years ago when the Big Bang first occurred and December 31st at the stroke of midnight is the present. In the cosmic calendar created by Carl Sagan, the milky way was formed around February. Many generations of stars enriched the galaxy with the “Star stuff” after dying. The solar system and Earth probably formed until early September or 4 and a half billion years ago and by late September, there was life on Earth. Animals became prominent mid- December and dinosaurs appeared the day after Christmas. In the cosmic calendar, Dinosaurs disappearing only occurred yesterday. Finally, on December 31st, human ancestors began to walk upright. Compared to the universe’s lifetime, the entire history of civilizations of humans falls into the last half-minute. Our species is the youngest of infants.

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How is Earth moving in our solar system? Earth rotates and orbits around the sun which are the two basic motions. It rotates in its axis counterclockwise, the axis being the imaginary line which connects the North and South pole. Since Earth rotates from west to East, the Sun appears to rise in the east and sets in the west. Its average orbital distance is about 150 million kilometers. As the Earth rotates, it orbits the Sun taking one year to orbit.

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Earth orbits the sun once every year…: It takes Earth a year to complete its orbit around the sun. Earth’s orbital path around the sun defines the ecliptic plane and its axis tilt is perpendicular to the ecliptic plane at 23.5 degrees and it is at an average distance of 1 atomic unit which is 150 million kilometers from the sun. The Earth also orbits the sun in the same direction it does its axis which is counterclockwise viewed above the North pole as a result of the way the planet was created. Since planets were born in a spinning disk of gas around the sun as it was being formed, the planets like Earth orbit and rotate in the same direction as its original spinning disk.

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Our Sun moves randomly relative to the other stars in the local solar neighborhood…: The solar system moves relative to nearby stars in our local solar neighborhood. The stars that make up the solar neighborhood move randomly relative to one another. The sun moves relative to stars nearby at about 70,000 kilometers per hour which is fast in the solar system. Stars move very fast, but it is not seen from Earth because they are so far away that it is unnoticeable to the naked eye unless being watched for thousands of years. That is also why constellation patterns seem to remain fixed, but in 10,000 years they will look different from the way we see them. An example would be leaves floating down a stream. Their motions seem to be random relative to one another just like the stars, but they are still being carried through the same current in the same direction. The solar system is located about 27,000 light-years from the galactic center and it competes one orbit of the galaxy in about 230 million years.

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More detailed study of the Milky Way: Most of the mass of the galaxy is located outside the visible disk which occupies the galactic halo that surrounds and encompasses the disk. The mass, however, is invisible to telescopes. So, information about this mass is very limited, but it is referred to as “dark matter” since there is not much light. Other studies of galaxies indicate that they are mostly made of dark matter. Observations imply that dark matter outweighs ordinary matter that makes up planets and stars significantly, which makes the dark matter the significant source of gravity. This gravity has led to the formation of other galaxies, clusters and superclusters.

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Visible portion of a galaxy…: There has been studies of galactic rotation that shows that although most visible stars lie in the disk and central bulge, most of the mass actually lies in a halo that surrounds and encompasses the disk. This is called dark matter because the mass does not emit light that has been detected.

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Hubble discovered that…: Hubble made discoveries that consisted of two facts, one is that virtually every galaxy outside the Local group is moving away from us and that the more distant the galaxy is, the faster is will appear to be racing away. The explanation for this is that the entire universe is expanding. The reasoning for this is that distant galaxies move away from us faster because they are the ones carried along with the expansion, like raisin cake as an example. So, the faster expansion’s rate is, the quicker the galaxies reach their current position and the younger the universe must be.

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How do galaxies move within the universe?:As previously stated, galaxies are carried along with the universe’s expansion and the billions of galaxies in the universe more relative to another. Within the Local Group, some galaxies move towards us, some move away, and various smaller galaxies orbit the Milky Way. The speeds are astronomical.

Lecture Notes (Chapter 2) Page 1: Middle right, Bottom right 

Constellations: Patterns in the sky help locate a region of the sky with well-defined borders. There are about 88 constellations in the sky.

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The Celestial Sphere: the stars in a constellation appear to be close to one another but in reality, they are far apart because they are at different distances from Earth. This is because we lack the perception of depth when looking into space due to the fact that stars are so far away, however the ancient Greeks mistook this as imagining the constellations and stars as laying on a great celestial sphere surrounding Earth. The definition for the celestial sphere is the imaginary sphere on which the objects in the sky appear to reside when observed from Earth and it allows to map the sky as seen from the Earth.

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The Milky Way: The Milky Way is a band of light which circles all around the celestial sphere that passes through more than a dozen constellations. The Milky Way band of light bears an important relationship to the Milky Way Galaxy. The Milky Way galaxy is shaped as a thin pancake with a bulge in the center and the view we have of the universe from our location is a little more than halfway out from the center of the pancake. The stars and interstellar clouds within the Milky Way in the night sky are what make a band of light that that make a full circle around the sky.

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The Milky Way = Zone of Avoidance: The dark lanes that run down the center of the Milky Way contain the densest clouds, making our view of stars kept from being seen behind them. In most directions, these clouds would keep us from seeing more than a few thousand light-years into our galaxy’s disk. Because of this, a big amount of our galaxy

remained concealed and we could not see it until few decades ago, when there was new technology created in order to allow us to see past the clouds by watching various forms of light that are hidden from our eyes like radio waves, infrared light, and X rays. 

The Local Sky: The local sky is the sky viewed from a specific location on Earth or another planet, things in the local shy are pinpointed by the coordinates of altitude and azimuth. The sky from your point of view appears to be a hemisphere or dome, and it rises from the fact that we only see half of the celestial sphere since the ground covers the bottom half. This boundary between the Earth and sky is the horizon with the point directly overhead is the zenith with an altitude of 90 degrees. Lastly, the meridian is an imaginary half-circle stretching from the horizon due south through the zenith to the horizon due north.

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We measure the sky using angles: Objects in the sky are described through angular sizes or separations without actually knowing the distance, the farther an object is the smaller the angular size. The angular size of an object is the angle measure that was formed by extending imaginary lines outward from our eyes to span an object or the space between two objects. Angular size does not necessarily say an object’s true size because the angular size also depends on distance. For example, the sun is about 400 times as large in diameter as the moon, but it has the same angular size in our sky because it is also about 400 times as far away. The angular distance between objects in the sky is the angle that’s between us, each degree is subdivided into 60 arcminutes and each arcminute is divided into 60 arcseconds.

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Why do stars rise and set?: The universe seems to be circling around us and the stars move slowly across the sky from east to west. Although our ancestors took this appearance at face value, which means that we lie in the center of the universe that rotates around us each day, we now know that that information is backwards because Earth is what rotates and not the universe around us. We can imagine the celestial sphere rotating around the Earth to picture the movement of the sky, and from this perspective we can see how the universe seems to rotate around us and every object on the sphere

appears to make a circle around the Earth each day. The motion may look more complex in the local sky because of the boundary the horizon makes. Page 4: Top left, Middle Left, Bottom Right 

Our view from Earth (Texas): When looking out of the galactic plane through the solar system, there is a clear view to the distant universe. When looking in any direction into the galactic plane through the solar system, one could see the stars and interstellar clouds that make up the galaxy through the nig...