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4.1 – 4.3 What is space like? • No air – vacuum • No air friction – force that resists motion • Mostly black – absence of light • Extreme temperatures Galactic Shapes • spiral galaxies –include the Milky Way (our galaxy) –all have a central nucleus • have long curved arms –contain a lot of gas and dust –barred spirals • the spiral arms are attached to a straight “bar” that runs through the center of the galaxy • Spiral • Barred Spiral Barred Spiral Galaxy • elliptical galaxies –resemble an egg or a football –not as structured • irregular galaxies –groups of stars that have no defined shapes –the Large Magellanic Cloud and Small Magellanic Cloud »the two closest galaxies to ours • Elliptical galaxy • Irregular galaxy • Celestial Clouds • nebulae –large clouds of gas and dust floating in space –They are most common in spiral and irregular galaxies –horse head nebula is one of the most widely known nebulae Helix Nebula • Horsehead nebula Constellations »Group of stars outlining an imaginary picture »most have been identified since ancient times »Constellations were once used to navigate by travelers because they appear to revolve around the North Star »They are also used to refer to 88 regions of the sky (celestial sphere) •North Star –Polaris is the scientific name for the north star –It is located directly above the Earth’s “axis of rotation” »The axis is the Imaginary line that runs from the north pole to the south pole on which the Earth spins »The word constellation has two current meanings -The oldest refers to a pattern of stars -The reference used by most current astronomers refers to a section of sky. -We will look at some of the patterns or arrangements of stars. »The constellations seen in the Northern Hemisphere are divided into groups determined by the season in which people can see them. So we call them: »Spring constellations »Autumn constellations • 2 groups in the northern hemisphere – Spring Constellations – Autumnal Constellation Constellations of the Northern Hemisphere –Ursa Major •The Great Bear •Contains the Big Dipper –Most familiar of all constellations • Ursa Minor –The Little Bear –More commonly called the Little Dipper –Polaris is at the point of Ursa Minor • Queen Cassiopeia –Named after a mythical queen of Ethiopia –Looks like a big “M” or “W” • King Cepheus –Looks like a house with a long pointed roof – Andromeda • Mythological daughter of Cassiopeia and Cepheus • Andromeda is also called “The Lady in Chains” • Her head is part of the Great Square star formation • Contains the Andromeda Galaxy – Whale • Monster that was to devour Andromeda • This constellation is often located beneath the horizon