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Stars and Galaxies Earth Science 11 Chapter 21 Constellations Constellation: A groups of stars that appear to form patterns in the sky. 88 different constellations can be seen from the Northern and Southern hemispheres. Best known is the Big Dipper: Part of a larger constellation called Ursa Major, or the Big Bear Constellations can be used for: Navigational aids Find other constellations including POLARIS and “North Star” Constellations Circumpolar constellations: Never set below the horizon In the north appear to rotate around the north star How many you see depends on your latitude EX. URSA Major, URSA Minor and Cassiopeia (northern) The apparent movement of these circumpolar constellations is due to the earth’s rotation. Earth rotates from WEST to EAST Therefore stars, the moon, and the sun all RISE in the EAST and SET in the WEST Seasonal Changes in Constellations Circumpolar constellations change in the sky with the change in seasons Fall: Big dipper is near northern horizon Cassiopeia is nearly straight overhead Spring: Big dipper is high overhead Cassiopeia is near northern horizon Seasonal Changes in Constellations Each star is moving on its own, sometimes at very high velocities. In time the constellations we are familiar with today will no longer be there Each star will have moved and new constellations will be made Some constellations only appear at certain times of the year. The most famous winter constellation is Orion the Hunter Orion contains the red supergiant Betelgeuse AND the blue supergiant Rigel Distances to Stars The closest star to the earth is the SUN Distance is measured 2 ways: 1. Astronomical unit (AU): The average distance between earth and the sun is 150 million km Sun – 1AU Jupiter = 4AU (at closest encounter) Pluto = 38AU (at closest encounter) How far is the nearest star? If the earth was a dot 1 cm away from the sun then using the same scale the next star would be 2.5 km away (Alpha Centauri) In km Alpha Centauri is 40 trillion km away. Using km is not practical in astronomy because the scale is too small. 2. Light year (LY) Distance light travels in 1 year (9.5 trillion km in 1 year) Ex. Earth to: Moon = 2 sec Sun = 8 mins Alpha Centaury = 4.3 LY North Star = 680 LY Physical Properties of Stars Our sun is classified as a Yellow Dwarf with a diameter of 1,380,000 km Elements in Stars Most stars are 70% Hydrogen and 28% Helium 1-2% of a star’s mass may be heavier elements such as oxygen, carbon, nitrogen, calcium, sodium The spectrum radiated by a star depends on both it’s composition and it’s temperature No two stars have exactly the same spectrum. A star’s spectrum is like its fingerprint Star Brightness 1. Luminosity = true brightness of a star Depends only upon the size and temperature of a star If two stars had the same temperature, the larger star would be more luminous than a cool star Star Brightness 2. Apparent magnitude or brightness: How bright a star appears on earth. Dependent on star’s luminosity and distance from us The brightest stars are “first-magnitude” stars The faintest stars that can be seen with the unaided eye are “sixth magnitude” Each star’s magnitude differs from the next be a factor of 2.5 Ex. First magnitude star is 2.5 brighter than a second magnitude star Some stars are even brighter than first magnitude stars (1.0) Ex. Sirius the brightest star in our sky, has an apparent magnitude of -1.43 Ex. A 100 watt light bulb is much brighter than a flashlight bulb The 100 watt bulb has GREATER luminosity However, if held up close, the flashlight bulb would look brighter Flash light has a greater apparent magnitude Star Brightness 3. Absolute Magnitude: How bright the star would be at 32.6 light years Used to express the luminosity of stars as if all stars were the same distance from Earth Ex. Sun = 4.8 (average); Rigel = 6.4 (very bright) Giants, Supergiants, & Dwarfs Giant: Larger in diameter than the Sun Luminosity: 10-1000 times the Sun Stars more luminous than giants are called supergiants Supergiant: Mass: 8-12 times the Sun Luminosity: 10,000-1,000,000 times the Sun Red supergiants are the largest of all the stars Dwarf: Less luminous Absolute magnitude (brightness) no more than 1 Most are red, orange, yellow or white White dwarfs are very faint, small, and dense (same size as earth but 100,000 times more dense Origin of a Star According to the Proto-star Theory, stars form wherever dense clouds of gas and dust exist These are called stellar nurseries and have a average diameter of 25 light years HUGE clouds of gas and dust occur (nebulae) in parts of space between the stars Contain as much material as the stars themselves These clouds are about 99% gas (hydrogen) Remaining 1% is a mixture of very fine particles of silicon carbide, graphite, diamonds, nitrogen and other elements It is believed this gas and dust comes from remains of exploding stars and supernovas Sometimes these great clouds of dust and gas start to come together under their own gravity Nebulae: Areas in space where such cloud formations can be found Most are invisible When these clouds are lit up by a star we can see them and they are immense Nebulae that are not near stars may show up as dark patches in space. Formation of a Red Giant When a star has used up its “stable” fuel, the force of fusion no longer balances with the force of gravity and the star loses its stability 1. when this occurs the core contracts in upon itself and becomes very hot causing the outer layers of the star to expand away from the core. 2. now this radiation and heat starts fusion in the star’s outer layers causing even greater expansion 3. as the expansion continues the star becomes a red-giant or SUPERGIANT Formation of White Dwarfs Finally we come to a stage in the stars life where most of the fuel for fusion is used up 1. the temperature and pressure of the core can no longer support the weight of its outer layers 2. the Giant then collapses 3. with most of its fuel gone the white dwarf cannot maintain its high temperature and in a billion years it will eventually glow fainter until it becomes cold and dark Supernovas When fusion has stopped it leaves an iron core As the star cools this core collapses in upon itself With this collapse, the pressure and temperature within the core rises dramatically The iron core starts to fuse into even heavier elements Now the core wants to collapse even further In this rush to collapse the star EXPLODES so violently that half its mass is blown out into space This explosion has a very intense flare and bright light we call a Super nova For just a few weeks or months this one star can outshine an entire galaxy Supernovas The best recorded supernova was recorded by the Chinese in the year 1054. This brilliant star faded after a year and its outer shell was changed into a great expanding cloud of gas we now know as the crab nebulla. Neutron Stars & Black Holes Supernovas eject half of their mass during the explosion. So what happens to the other half ? The mass that remains is what astronomers call a neutron star In the core of a supernova the forces are so great that every atom’s electrons are crushed into its nucleus. The collapsed electrons combine with the protons to form neutrons A neutron star is only about 10 km in diameter and trillions of times more dense than the sun. Neutron Stars & Black Holes What would happen if an even more massive star would explode into a supernova leaving behind a core that is even more dense than a neutron star? Such gravitational forces would be so great that not even light could escape We call these black holes We cannot see these Black Holes Must determine their location by the effect they have on other objects nearby By the energy (X-rays) given off by the matter that is falling into them What are Galaxies? With the Hubble Telescope we now recognize that space has BILLIONS of Galaxies and each galaxy has BILLIONS of Stars The galaxy to which our Sun belongs is the Milky Way galaxy Our sun is one of 100 Billion stars in the Milky Way The diameter of the Milky way is about 140,000 light years The Milky Way belongs to a small cluster of 17 galaxies called the Local Group Types of Galaxies 1. Spiral Galaxies These have a lens shape, and a bright nucleus made of millions of stars Stars are arranged in spiral arms around the nucleus Each arm contains millions of stars. Ex. Milky Way 2. Elliptical Galaxies Range from spherical to lens shaped Most stars are close to the center and they have no arms 3. Irregular Galaxies These are smaller and fainter, their stars are spread unevenely Origin of the Universe Big-Bang Hypothesis: (No not the TV show) The whole universe was originally packed into one dense sphere of hydrogen About 15 Billion years ago this mass of hydrogen exploded forming a gigantic expanding cloud Some parts of the cloud moved faster than others, but all parts moved outward, away from the center and are still doing so today Eventually the clouds cooled and condensed into galaxies Billions of galaxies were formed (all moving outward) What is the support for this theory? 1. DOPPLER Shift 2. Background Radiation