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Homework: Pages 442-3 and ALL USING COMPLETE SENTENCES Due Tuesday 11 April 2017 BRING TEXTBOOK TO CLASS TOMORROW 20 points assignment – don’t forget! Sun Test 2017 April 04 Results. I. Measuring the Stars A. PARSEC – PARallax arc SECond The distance an object is from Earth if it appears to move only 1 arc second (angle) as Earth orbits to the opposite side of the Sun. B. Closest star to our Sun is Centauri, which is part of Proxima the Alpha Centauri complex of 3 stars orbiting each other in the Southern sky – we cannot see it from our hemisphere. 1. It is 1.3 Parsecs (1.3 pc) from Earth to Proxima Centauri (4.3 ly) yet we see it fine. What is there between us and it, 4.3 ly away? Not much, otherwise the light would be blocked out – so space is “space.” - The 2nd closest star from our Sun, at only 1.8 pc (6.0 ly) away Moves 10 arc seconds per year! Only a few stars move even 1 arc second per year; yet, over the course of your lifetime, it will barely move one degree. Alpha Centauri will move 3 ½ arc seconds per year. II. Stellar Motion – Stars move in two directions that we observe from Earth: Transverse component is the star’s motion across (V) the sky. Radial component is the star’s motion along our sight – directly towards or away, from us. Proper Motion – Annual movement of a star across the sky (transverse) compared to it’s relative position to the Sun. Magnitude scale originally created by 2nd Century Greek Astronomer Hipparchus. Ranked naked eye brightness into 6 group, with “1” being the brightest and “6” the dimmest. Modified today as: No longer limited to whole numbers. It is “apparent” magnitude only. Goes beyond “1-6” range. 5 magnitude change is exactly a factor of 100 apparent brightness difference. ABSOLUTE MAGNITUDE – The apparent magnitude of all objects if they were all viewed at a distance of 10 parsecs (pc) from the observer. Absolute magnitude is equivalent to luminosity. Temperatures of Stars – Remember, using color (EM wavelength) we can determine temperature using Wiens law. l = 0.29 cm/K Blackbody curve is so constant, Astronomers only need to measure apparent brightness using at few as two frequency measurements. Table at right “V” is measured using “visible” light range (490-590 nm) and “B” blue line sees only “blue” light from 380-480 nm. Star “A”is Rigel, where it is very hot, (30,000 K) so more blue light than yellow. Star (c) is like Betelguese, having more red than blue, being a cooler star at 3000 Kelvin. Originally based upon Hydrogen spectral line intensities, thinking that some stars had more hydrogen than others, with “A” type stars having the most, “B” the second most all the way to “P” having the least. More recent understanding of Hspectra revealed the concentration misunderstanding; yet, the Letter classification remains, with only OBAFGKM letters remaining as stars were slightly reclassified. Classification is based upon STELLAR SURFACE TEMPERATURE. TABLE 17.2 Stellar Spectral Classes SPECTRAL CLASS SURFACE TEMPERATURE (K) PROMINENT ABSORPTION LINES FAMILIAR EXAMPLES O 30,000 Ionized helium strong; multiply ionized heavy elements; hydrogen faint B 20,000 Neutral helium moderate; singly ionized heavy elements; hydrogen moderate Rigel (B8) A 10,000 Neutral helium very faint; singly ionized heavy elements; hydrogen strong Vega (A0), Sirius (A1) F 7,000 Singly ionized heavy elements; neutral metals; hydrogen moderate Canopus (F0) G 6,000 Singly ionized heavy elements; neutral metals; hydrogen relatively faint Sun (G2), Alpha Centauri (G2) K 4,000 Singly ionized heavy elements; neutral metals strong; hydrogen faint Arcturus (K2), Aldebaran (K5) M 3,000 Neutral atoms strong; molecules moderate; hydrogen very faint Betelgeuse (M2), Barnard's Star (M5) Stellar Sizes http://www.youtube.com/watch?v=wHiHFXtE0js Hydrostatic (Main-Sequence) Equilibrium: The simple model of any main sequence star is of a dense gas/fluid in a state of hydrostatic equilibrium. The inward acting force, gravity, is balanced by outward acting forces of gas pressure and the radiation pressure. BINARY STARS: Two stars the orbit each other. Detected by observing both transverse motion and radial motion (via red/blue shift of spectra) as they orbit each other. STELLAR LIFETIMES The rapid rate of nuclear burning deep inside a star releases vast amounts of energy per unit time. How long can the fire continue to burn? We can estimate a star's lifetime simply by dividing the amount of fuel available (the mass of the star) by the rate at which the fuel is being consumed (the star's luminosity): Star lifetime is proportional to: mass luminosity Luminosity = 4psR2T4 or, if you use Solar units where L is measured in solar luminosities, then we can say: Luminosity = R2T4 where the average temperature of the photosphere is 5800 K, so we adjust temperatures accordingly. What is the luminosity of a star having 3 times the radius of the Sun and a temperature of 10,000 K? If solar units are used: Luminosity = R2T4 1. Temperature is 10,000 K/5,800 K Sun = 1.724 times hotter than Sun photosphere. So, we put 1.724 in place of “T”. 2. L = (R)2 (1.724)4 and since Radius is 3 times more, then: 3. L = (3)2(1.724)4 = 9(8.83) = 79.5 times more Luminous than our Sun. Review & Discussion: 1. Parallax is used to measure the distances in stars by measuring the arc angle an object appears to move as the Earth orbits the Sun. This angle can then be converted to Parsecs. 2. A parsec (Pc) is the distance an object is if it appears to move one arc second as the Earth moves 2 AU’s from one side of the Sun to the other. A parsec is 3.26 light years long. 3. Two ways in which a star’s real motion translates into motion observable from Earth are Transverse (sideways motion) and Radial (toward or away) which allow us to determine actual motion. 6. White dwarfs are much smaller than Red Giants, but much hotter (8,500 K White – 4,000 Red). Dwarfs are more dense than Red giants and white dwarfs have a higher absolute magnitude than red giants. 7. Absolute magnitude (brightness) is the inherent brightness of an object, or how much light (photons) it gives off per unit area; whereas, “Apparent” magnitude is the brightness an object “appears” to be when viewed at a certain distance. 8. Stellar temperature is measured by using Wien’s law and preparing a blackbody curve using maximum wavelengths from two filtered telescopes. HOMEWORK: Read Chapter 18 and perform pages 462-463: Review (1-4, 7-9) complete sentences, Concept. Mult. choice (1-4, 8-9) complete sentences and Problem #8 (Show work) Use complete sentences and show work Due Tuesday the 12th of April. Problem #3 Pg. 463 Given the average density of interstellar matter stated in Section 18.1 [10,000 atoms/cubic meter], calculate how large a volume of space would have to be compressed to make a cubic meter of gas equal in density to air on Earth (1.2 kg/m3). Each cubic meter will contain 1,000,000 atoms/m3, so multiply that by 1.7 x10-27 kg/atom to get 1.7 x 10-21 kg/m3 to get the density of interstellar matter. HOMEWORK: Pages 514-15 Review (1,3-6) and Concept M/C (1-5,7-9) Due Friday 15 April 2016 EMISSION NEBULA A cloud of high temperature gas in which the atoms are energized with the light and solar winds from nearby stars or protostars , thus emitting radiation . Act like fluorescent light bulbs as the gases fluoresce as electrons run through the tube. DARK NEBULA Dark nebulae are immense clouds of gas that are in front of a light source (star cluster) that block out a zone of light. Read chapters 17, 18 and 20. Study your homework and class assignments for the test. Page 514 Review and Discussion 1. Stars don’t “live” forever due to the fact that they eventually run out of Hydrogen (then Helium) to fuel the nuclear fusion that makes a star a star. Cooler stars live longer since they “burn” their fuel much slower than hotter stars. 3. The Sun is predicted to burn Hydrogen in it’s core for another 3-5 billion years. 4. When Hydrogen is depleted from the core, the Helium will outweigh it and H fusion will continue in the outer shell and the star will become a Red Giant. 5. What makes an ordinary star become a red giant is when the mass of helium outweighs the mass of hydrogen “burning” in the inner core, causing the helium to collapse inward, which forces the hydrogen to the outer core, where it continues fusion. Since the H is now in the outer core, the diameter is much bigger and, although the amount of energy is still the same, it is spread over a greater surface area, creating a cooler “Red Giant” star. 6. The Sun will reach a diameter of about 0.6 AU when it enters the red-giant branch. Concept Self test: M/C page 515 1. A star will evolve “off the main sequence” when it uses up C.) Most of the hydrogen in the core. 2. On the main sequence, massive stars b.) burn their hydrogen fuel more rapidly than the Sun. 3. Compared to other stars on the H-R diagram, red-giant stars are so named because they are a.) cooler. 4. When the Sun is on the red-giant branch, it will be found at the b.) upper-right part of the H-R diagram. 5. After the core of a Sun-like star starts to fuse helium on the horizontal branch, the core becomes a.) hotter. 7. A white dwarf is supported by the pressure of tightly packed a.) electrons 8. When the Sun leaves the main sequence “Red Giant on the H-R Diagram,” it will become b.) brighter 9. A star like the Sun will end up as a b.) white dwarf. 10. Compared to the Sun, stars plotted near the bottom left of the H-R diagram are much d.) denser. http://lifeng.lamost.org/courses/astrotoday/CHAISS ON/AT317/HTML/AT31707.HTM HR Diagram – discussion and diagrams 1 Parsec Neutron Stars A neutron star is about 20 km in diameter and has the mass of about 1.4 times that of our Sun. On Earth one teaspoonful of a neutron star would weigh a billion tons! Because of its small size and high density, a neutron star possesses a surface gravitational field about 2 x 1011 times that of Earth. Neutron stars can also have magnetic fields a million times stronger than the strongest magnetic fields produced on Earth. Neutron stars are one of the possible ends for a star. They result from massive stars which have mass greater than 4 to 8 times that of our Sun. After these stars have finished burning their nuclear fuel, they undergo a supernova explosion. This explosion blows off the outer layers of a star into a beautiful supernova remnant. The central region of the star collapses under gravity. It collapses so much that protons and electrons combine to form neutrons. Hence the name "neutron star"