Chapter 21 Earth Science 11

Transcription

1 Chapter 21 Earth Science 11

2 Constellations Constellation: A group 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 the North Star 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.

3 Seasonal Changes in Constellations Circumpolar constellations change in the sky with the change in seasons Fall: Spring: Big Dipper is near the northern horizon Cassiopeia is nearly straight overhead Big Dipper is high overhead Cassiopeia is near the northern horizon 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.

4 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= 1 AU Jupiter = 4 AU (at closest encounter) Pluto = 38 AU (at closest encounter) How far is the nearest star? If the earth was a dot 1 cm from the sun then using that same scale the next star would be 2.5 km away (Alpha Centauri) In normal scale Alpha Centauri is 40 trillion (4.0 x ) km away Using Kilometers as a measurement is not very practical because it 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 minutes Alpha Centauri = 4.3 LY Betelgeuse, the red supergiant in Orion = 490 LY North Star = 680 LY

5 Physical Properties of Stars Our sun is classified as a Yellow Dwarf with a diameter of 1,380,000 Km Sun Some Stars Density 1.4 x of water air to 1 ton/teaspoon Diameter 108x Earth s smaller than Earth to 2,000 x Sun s Mass 300,000 x 1/100 to 50x Sun s Earth s mass Color yellow red to blue-white Temperatur e 5,500 C (surface) 3,000 to 30,000 C

6 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 its composition and its temperature No two stars have exactly the same spectrum. A star s spectrum is like its fingerprint.

7 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 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 by 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 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 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)

8 Giants, Supergiants, and Dwarfs Giant: Larger in diameter than the Sun Luminosity: times the Sun Stars more luminous than giants are called supergiants Ex. Aldebaran and Arcturus (Red Giants) HUGE and luminous Supergiant: Mass: 8-12 times the Sun Luminosity: 10,000 1,000,000 times the Sun Red supergiants are the largest of all stars Ex. Rigel (blue-white); Canopus (white-yellow), Antares and Betelgeuse (red) 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 time more dense)

9 Variable Stars Variable stars: Most stars shine with a steady brightness; however, some stars vary in brightness over regular periods or cycles These cycles can vary from 1 to 50 days Most have periods of about 5 days. 1. Pulsating stars: Variable stars that change diameter (size) as they change brightness Contraction = star becomes hotter and more bright Expansion = star become cooler and less bright Ex. Cepheids in the constellation Cepheus (a.k.a. Cepheid Variables) 2. Eclipsing Binary: When TWO stars are orbiting each other and one star is dimmer than the other When the dim star moves in front, the apparent magnitude (brightness) drops The effect is like a pulsating star

10 Pulsars Pulsar: an object that gives off powerful bursts of radio waves and light waves in a regular period of time Originally found using a Radio Telescope (picks up low luminosity) Ex. In the middle of the Crab Nebula is a pulsar that produces these radio/light wave bursts every second or less Hundreds of Pulsars are now known and believed to be neutron stars formed in the aftermath of a supernova explosion The fastest pulsar found pulsates 642 times per second.

11 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 an 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 the remains of exploding stars and supernovas Sometimes these great clouds of dust and gas start to come together under their own gravity Other times it takes an explosion of a nearby star to send a shockwave through the nebulae and kick-start the process 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 a dark patch in space Known as a dark nebula like the Horsehead Nebula also in Orion The brightest nebula is the Great Nebula in Orion

12 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 This expansion enlarges the star s surface area The star again radiates more light and appears brighter 2. Now this radiation and heat starts fusion in the star s outer layers causing even greater expansion The core is composed mostly of helium formed from the original hydrogen fusion process 3. As the expansion continues the star becomes a red-giant or SUPERGIANT

13 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 The nuclei of its atoms are squeezed tightly together and this can form what we call a White Dwarf (no larger than our Earth) 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 Occasionally a white dwarf can flare up again due to bombardment of material from another star. This is called a NOVA. Our Sun is expected to have this fate. It will eventually collapse in upon itself, after flaring into a red giant, and become a white dwarf

14 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 SUPERnova For just a few weeks or months this one star can outshine an entire galaxy Ex. The best recorded supernova was recorded by the Chinese in the year 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 Nebula The Crab Nebula can be found in the constellation Taurus the Bull. Ex. The most famous SuperNova occurred in the Large Magellanic Cloud and was visible by the naked eye in This explosion was used to, and is still used, to define the study of supernovas and help predict when and where they may occur. Scientist predicted that supernovas produce a particle called neutrinos Hours before light became visible from this explosion instruments detected the neutrinos here on earth

15 Neutron Stars and 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 ten kilometers in diameter and trillions of times more dense than the sun 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 Ex. Cygnus is a constellation that contains a star called Cygnus X-1. This star is orbiting something we cannot see Very powerful X-Rays are being emitted from this star (not normal) Astronomers now feel that this is the first tangible proof of a Black Hole

16 What are Galaxies? Astronomers used to look at the night sky and see many stars and fuzzy images of what they thought were nebulae With the Hubble Space 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 Every star seen with the naked eye belongs in the Milky Way The diameter of the Milky Way is about 140,000 light years At its thickest point, in the middle, it is about 20,000 light years thick Our Sun, and us, are approximately 23,000 light years from the galaxy s center The Milky Way belongs to a small cluster of 17 galaxies called the Local Group The nearest neighbors in the Local Group, the two Magellanic Clouds, are in the Southern Hemisphere. These two galaxies can be seen without a telescope Another neighbor, Andromeda Galaxy, is faintly visible to the unaided eye in the Northern Hemisphere. The Andromeda Galaxy is larger than the Milky Way and is about two million light years away.

17 Types of Galaxies 1. Spiral Galaxies These have a central lens shape, bright nucleus made of millions of stars Around the nucleus is a flat disk of stars arranged in spiral arms Each Spiral Arm usually come out from opposite sides of the nucleus The arms trail behind the galaxy as it rotates Each arm contains millions of stars Although clouds of dust and gas can be found within the spiral arms almost NO dust or gas occur between the arms Ex. The Milky Way and Andromeda Galaxies 2. Elliptical Galaxies These range from nearly spherical to lens shaped Most stars are close to the center They have no arms and almost no gas and dust clouds 3. Irregular Galaxies These are smaller and fainter Their stars are spread unevenly Ex. The two Magellanic Clouds are in this class

18 Quasars Short for Quasi-stellar radio sources Quasars appear as very faint objects because they are very far away Most luminous objects in the universe Steadily and continuously produce both light and radio waves at very high rates Larger than any known star Scientist think that a quasar may actually be an entire galaxy in an early stage of development. Example: One quasar called PKS is about 12 BILLION lightyears away At this distance no known star can be seen. Therefore, to be seen at that range, it must be as bright as 100 trillion suns and be billions of times more massive

19 Origin of the Universe Big-Bang Hypothesis : The whole universe was originally packed into one dense sphere of hydrogen Not much bigger than the sun is today 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 Red Shift (moving away) / Blue Shift (moving closer) 2. Background Radiation In 1964 two physicists (A. Penzias and R. Wilson) discovered microwave radiation coming from all directions in space This background radiation is thought to be the echo of the Big Bang