| Stars and Galaxies: Lecture Notes: Class 1 |
| Introduction: Scale of the Cosmos Scale: Size, Distance Metric System - check appendix
Scientific notation - to avoid writing big numbers
Astronomical Unit (AU) - average distance from Earth to Sun; used to measure distances within the Solar System:
Light year (ly) = the distance light travels in a year (in a vacuum); used to measure distances within the galaxy:
There are vast distances between the stars; next closest star to sun is over 4 light years away (Proxima Centauri, 4.2 ly). Galaxies - collections of billions of stars, gas and dust. They range in size from 1500 to 300,000 or more light years in diameter. The Milky Way Galaxy (the one we live in) has about 300 billion stars, and is about 75,000 light years in diameter. The next closest galaxy is around 2 million light years away. Milky Way is one galaxy in the Local Group, a cluster of about two dozen galaxies in a region 6 million light years in diameter. Chapter 1: The Sky Constellations: Originally groups of stars designated by ancient cultures, named after characters in myths and lore. The commonly known Western culture constellations are based on those created about 5000 years ago. Constellation names are Latin. Today there are 88 constellations. These are more than just star pictures; they are regions with clearly defined boundaries (like states in the US). An asterism is a group of stars that is part of a constellation or several constellations: the big dipper asterism is part of the constellation Ursa Major; the summer triangle asterism contains stars from three constellations, Lyra, Cygnus and Aquila. Stars in constellations and asterisms are NOT physically related to each other, as they are at vastly different distances from each other. They just appear to be near to each other from our view point. The brightest stars in the sky have proper names in Arabic (Betelgeuse, Rigel, Sirius). Stars have designations as well, with is the possesive from of the constellation the star is in (remember that a constellation is a region of sky) and a Greek letter, assigned in order of brightness. For example, the brightest star in the constellation Orion would be alpha Orionis; next brightes would be beta Orionis, and so on. The magnitude scale is used to describe a star's brightness. In 150 BC Hipparchus developed star classes, rating the stars from brightest (1st class) to dimmest (6th class). The modern magnitude scale is based upon this system, with a first magnitude star being about 2 1/2 times as bright as a second magnitude star. The scale extends into negative numbers (very bright) and into the 20's (very faint, only visible with telescope). The lower the number, the brighter the object. Negative numbers indicate extreme brightness. Apparent magnitude (m) is how bright a star or other object looks to us. This can depend not only on how bright the star really is, but how far away it is as well. Absolute magnitude (M) is a measure of the inherent brightness of a celestial object. This scale is defined as the apparent magnitude a star would have if it were seen from a standard distance of 32.6 light-years (10 parsecs). Sky and Motion Celestial sphere -- to be continued... Web Resources: Unit Converter: Convert any kind of units (distance,
temperature, astronomical).
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