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Chapter 1

Chapter 1 and Chapter 2.docx

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Department
Astronomy
Course
AS101
Professor
Shohini Ghose
Semester
Fall

Description
Chapter 1 Field of view: The area visible in an image. Usually given as the diameter of the region. Solar system: The sun and its planets, asteroids, comets and so on Planet: A non-luminous body in orbit around a star, large enough to be spherical and to have cleared its orbital zone of other objects Star: A globe of gas held together by its own gravity and supported by the internal pressure of its hot gases, which generate energy by nuclear fusion Astronomical Unit (AU): Average distance from earth to the sun; 1.5 X 10 km, or 93.3 miles Light Year: Unit of distance equal to the distance light travels in one year -Starlight is recorded by brigtness and this is how they are displayed in astronomical diagrams. The size of a star image in a photograph tells you not how big the star really is but only how bright it looks Galaxy: A large system of stars, star clusters, gas, dust, and nebula orbiting a common centre of mass Milky Way: The hazy band of light that circles our sky, produced by the glow of our galaxy. Milky Way Galaxy: The spiral galaxy containing our sun, visible in the night sky as the Milky Way. Spiral arms: Long spiral pattern of bright stars, star clusters gas, and dust. Spiral arms extend from the centre to the edge of the disk of spiral galaxies. Supercluster: A cluster of galaxy clusters Chapter 2: User's Guide to the Sky: Patterns and Cycles 2.1 Stars Constellations -constellations named within western culture originated in ancient civilizations of Mesopotamia, Babylon, Egypt, and Greece beginning as much as 5000 yrs. ago- 48 are still in use -Constellations- One of the stellar patterns identified by name, usually of mythological gods, people, animals, or objects. Also, the region of the sky containing the star pattern. -constellation boundaries are approximate, not definite -1928- International Astronomical Union (IAU) established a total of 88 official constellations with clearly defined permanent boundaries that make up the whole sky together -constellations are no longer a group of stars, but a section of the sky- a viewing direction- and any star within the region belongs to that one constellation -Asterism- a named grouping of stars that is not one of the recognized constellations- "Big Dipper" -most constellations/asterisms are made up of stars that aren't physically associated with one another, except that they lie in approximately the same direction as viewed from Earth The Names of Stars -names of constellations are Latin or Greek, most individual star names derive from Ancient Arabic -another way to name stars is to assign Greek Letters to the bright stars in a constellation in approximate order of brightness -brightest= alpha, second brightest= beta, and so on -add the possessive form of the constellation, brightest in Canis Major= alpha Canis Majoris Brightness of Stars -Astronomers measure the brightness of stars using the magnitude scale -six brightness groups- first magnitude is the brightest, sixth magnitude is faintest -Greek Astronomer Hipparchus (190-120 BCE) made the first star catalog (used the system) -140 CE Egyptian-Greek astronomer Claddius Ptolemy used magnitude system in his catalog -Star brightnesses expressed in this system are known as apparent visual magnitudes (M )- A mevsure of the brightness of a star as seen by human eyes on Earth -Scientifically accurate- refer to the flux: A measure of the flow of energy out of a surface. Usually applied to light. Used because brightness is subjective. -precise modern measurements of the brightness of stars are still connected to observations of apparent visual magnitude that originated many years ago Limitations of apparent visual magnitude system: Some stars are so bright the magnitude scale must extend to negative numbers. With a telescope you can find stars much fainter than the limit for your unaided eyes. Thus the magnitude system has also been extended to numbers larger than sixth magnitude to include faint stars. Although some stars emit large amounts of infrared or ultraviolet lights, those types of radiation are invisible to human eyes. The subscript "V" in m is v reminder that you are counting only light that is visible. An apparent magnitude tells only how bright the star is as seen from Earth but doesn't tell anything about a star's true power output because the star's distance is not included. 2.2 The Sky and Its Motions The Celestial Sphere -Ancient astronomers believed that the sky was a great sphere surrounding Earth with the stars stuck on the inside like thumbtacks in the ceiling. The celestial sphere is an example of a scientific model. 1. Sky objects appear to rotate westward around Earth each day, but that is a consequence of the eastward rotation of Earth. This produces day and night. 2. What you can see of the sky depends on where you are on Earth. For example, Australians see many constellations and asterisms invisible from North America, but they never see the Big Dipper. 3. Astronomers measure distances across the sky as angles expressed in units of degrees and subdivisions of degrees called arc minutes and arc seconds. Scientific Models A scientific model is a carefully devised mental conception of how something works, a framework that helps scientists think about some aspect of nature. A scientific model does not have to be true to be useful. Because scientific models are not meant to be totally correct, you must always remember the assumptions on which they are based. The Sky Around Us 1. The eastward rotation of Earth causes the sun, moon, and stars to move westward in the sky as if the celestial sphere were rotating westward around Earth. From any location on Earth you see only half of the celestial sphere, the half above the horizon. The zenith marks the top of the sky above your head, and the nadir marks the bottom of the sky directly under your feet. The apparent pivot points are the north celestial pole and the south celestial pole. Halfway between the celestial poles lies the celestial equator. 2. What you see in the sky depends on your latitude. As you walk southward, the celestial pole moves toward the horizon, and you can see further into the southern sky. The angular distance from the horizon to the north celestial pole always equals your latitude-- the basis for celestial navigation. 3. When you think of the celestial sphere, you can measure distances on the sky as angular distances in degrees, minutes of arc, and seconds of arc. An arc minute is 1/60th of a degree, and an arc second is 1/60th of a minute of arc. Then the angular diameter of an object is the angular distance from one edge to the other. 3a. Circumpolar constellations are those that never rise or set. From a high latitude such as Norway, you would have more circumpolar constellations, and from Quito, Ecuador, located on Earth's equator, you would have no circumpolar constellations at all. Precession -In addition to the daily motion of the sky, Earth's rotation adds a second motion to the sky that can be detected only over centuries. You can say either that Earth's axis is tipped 23.5 degrees from vertical or that Earth's equator is tipped 23.5 degrees relative to its orbit; the two statements are equivalent. -Precession: The slow change in orientation of the Earth's axis of rotation. One cycle takes nearly 26, 000 years. -Earth has a slig
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