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Chapter 1

# AS101 Chapter 1: Astronomy Notes: Chapter 1

Department
Astronomy
Course Code
AS101
Professor
Ioannis John Haranas
Chapter
1

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Astronomy Study Notes
Part 1: Exploring the Sky
The Scale of the Cosmos: Space and Time
-Scientiﬁc Notation: The system of recording very large or very small numbers by
using powers of 10
-Field of View: The area visible in an image, often given as the diameter of the region
-Each view is made from a distance that is some power of 10 times farther away,
until the distance becomes so large that we jump with higher increments. Every
time we move 10 times away, our FoV encompasses an area 10*10 larger than the
previous square
-Distances are ﬁrst expressed in metres until they become so large that a metre
becomes too small as a unit
-Earth is approximately 13000 km in diameter
-The rotation of Earth on its axis each 24 hours carries us eastward, and as we
cross the sunset line into darkness we say that the Sun has set
-The moon has a diameter about one-fourth that of Earth, and orbits the Earth at an
average distance of 380000 km
-Astronomical Unit (AU): Average distance from Earth to the Sun (1.5*10^8 km)
-Using this unit, we can say the average distance from Venus to the Sun is 0.7 AU
-The 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 gasses, which generate energy by nuclear fusion
-The Sun: Our star, whose diameter is about 110 times larger than Earth’s and its light
takes about 8 minutes to reach earth. The Sun is a fairly typical star, a bit larger than
average, and it is located in a fairly normal neighbourhood in the universe
-The stars around us are separated by about 11000 AU
-Light-Year (LY): Unit of distance equal to the distance light travels in one year
(equivalent to about 10^13 km or 63000 AU)
-The space between stars is ﬁlled with thin clouds of gas that eventually create stars
-Galaxy: A large system of stars, star clusters, gas dust, and nebulae orbiting a
common centre of mass
-Milky Way: The hazy band of light that circles our sky, produced by our galaxy's glow
-Milky Way Galaxy: The spiral galaxy containing our sun and is visible in the night sky
-Our galaxy contains over 100 billion stars, is over 80000 LY in diameter, and, like
many others, has spiral arms
-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
-Our galaxy is part of a cluster of a few dozen galaxies called the local group
-Galaxies are commonly grouped together in clusters
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-Supercluster: A cluster of galaxy clusters
-Superclusters are linked to form long ﬁlaments and walls which outline voids that
seem nearly empty of galaxies. These ﬁlaments and walls appear to be the largest
structures in the universe
-The Cosmic Calendar: A concept that puts the time scale of the universe into our
yearly calendar where each month is a little in excess of 1 billion years
-Midnight January 1st: The Big Bang happens
-March 1st: Milky Way Galaxy begins to coalesce
-August 15th: Solar System begins to form
-September 30th: Primitive life on Earth emerges
-December 15th: Complex living structures form
-December 25th-30th: Dinosaur Era
-Last 30 Seconds: When all of recorded history occurred
-11 Seconds Ago: Pyramids were built
-1 Second Ago: Copernicus theorized that Earth orbits the Sun
-0.04 Seconds Ago: You were born (if you are 18)
User’s Guide to the Sky: Patterns and Cycles
-Constellations:
-Constellation: One of the stellar patterns identiﬁed by name, usually of mythological
gods, people, animals, objects; also the region of sky containing that star pattern
-Asterism: A named grouping of stars that is not one of the recognized
constellations (ie. Big Dipper is an asterism a part of the constellation Ursa Major)
-The Names of Stars:
-The names of the constellations are in Latin or Greek
-Most Individual star names derive from ancient Arabic, much altered over centuries
-Another way to identify stars is to assign Greek letters to the bright stars in a
constellation in approximate order of brightness
-The brightest star is usually designated alpha (α)
-The second brightest beta (β)
-The third brightest gamma (γ)
-And so on
-A Greek-letter star name also includes the possessive form of the constellation
name which gives clue to the relative location and brightness of the star
-The Brightness of Stars:
-Magnitude Scale: The astronomical brightness scale; the larger the number, the
fainter the star
-The ancient astronomers divided the stars into 6 brightness groups, the brightest
were called ﬁrst-magnitude stars and the scale continued down to sixth-
magnitude stars
-Apparent Visual Magnitude (mv): A measure of the brightness of a star as seen by
human eyes on Earth
-Flux: A measure of the ﬂow of energy out of a surface, usually applied to light
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