The Night Sky

Big Questions: What do we see when we look at the night sky with the naked eye? How are stars named? Why is the apparent magnitude of stars only a relative measurement? What is it relative to? What are constellations? How has human culture influenced how we name and label objects in the sky?

The night sky… Most of what we see when we look at the night sky are stars - spheres of gas that give off light energy Also visible (at least some of the time) o Planets o The Moon o Comets o Meteors (briefly visible) o Nebulae o Star clusters o Part of our galaxy (the Milky Way) o Other galaxies

Star Names There are multiple systems that have been used to name stars from ancient times to today Brighter stars tend to have many names… Vega, the fourth brightest star in the sky, has over 40 different names! Today, star names are assigned by the International Astronomical Union. You can’t pay to “name a star” (at least, not officially)

Common names: The brightest stars in the sky were named thousands of years ago Most common star names that we use today come from ancient Arabic, because those records survived the Dark Ages and were translated into Latin Modern NameArabic NameMeaning AldebaranAd-Dabaran"Follower" of the Pleiades BetelgeuseYad al-Jauza'Hand of Orion, or armpit of the giant RigelAr-RijlThe foot DubheAd-DubbThe bear

Scientific names: In the 1600s, an attempt was made to bring order to star names. Stars were named using a Greek letter (  ) in order of their relative brightness within a given constellation. Alpha is the brightest, Beta is second brightest, etc. The Greek letter is followed by the Latin possessive form of the constellation Betelgeuse Rigel In the Constellation Orion: Betelgeuse =  Orionis Rigel =  Orionis

Star Catalogs: Star catalogs have also been created, listing stars according to various criteria (position in its constellation, distance from Earth, coordinates in the sky, etc.) So the star Vega is also known as o alpha Lyrae o 3 Lyrae o BD +38°32’38” o HD o SAO o And MORE!

Projection Effect The stars in a constellation appear to be projected onto the flat surface of the sky In truth, the stars in the same constellation are often located at very different distances from Earth m/watch?v=pXDyzSkxFvs m/watch?v=pXDyzSkxFvs

The Magnitude Scale First introduced by Hipparchus ( B.C.E.) o Brightest stars --1 st magnitude stars o Faintest stars seen with unaided eye-- 6 th magnitude o So … smaller numbers = brighter This measures Apparent Visual Magnitudes based on what we can see from Earth with the naked eye

Today’s Apparent Magnitude Scale As our understanding of the universe has evolved, the magnitude scale has been expanded o negative numbers for extremely bright objects o larger numbers for dim objects not visible to the naked eye

Apparent Magnitude vs. Luminosity Apparent Magnitude – how bright it looks from Earth o Betelgeuse has an apparent magnitude of 0.45 o The sun has an apparent magnitude of -26 Luminosity – how much light it actually gives off o Betelgeuse is actually about 10,000 times more luminous than the sun o Distance makes the difference!

Constellations In ancient times, constellations only referred to the brightest stars that appeared to form images, and were often associated with mythological figures. Babylonian, Egyptian, and Greek astronomers named many of the constellations we know today 48 ancient constellations are still in use

Naming constellations: Culture influenced the patterns and mythology of the constellations o The Greeks saw Orion the Hunter, while the Lakota tribe saw the Great Hand, a symbol of peace between the gods and man o Romans and Iroquois both saw a bear in Ursa Major, but told different stories about it o Most of the names and myths we know today have Greco-Roman origins because those cultures kept many written records

Modern Constellations In 1922 the International Astronomical Association (IAU) e stablished a total of 88 official constellations with clear boundaries Today’s constellations represent an area of the sky – not just a group of stars Each celestial object is part of only one constellation

Asterisms: Recognizable patterns in the stars o The Big Dipper is an asterism within the Big Bear, which itself is an asterism within the constellation Ursa Major (that entire section of sky) o What most people call “constellations” are technically asterisms Note the yellow border of the constellation, with the bear and big dipper asterisms inside

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Big Questions: What do we see when we look at the night sky with the naked eye? How are stars named? Why is the apparent magnitude of stars only a relative measurement? What is it relative to? What are constellations? How has human culture influenced how we name and label objects in the sky?