PHYS 1403 Stars and Galaxies Topics for Today s Class 1. Measurements a) Measurements b) SI units c) Conversions Review from High School Math d) Scientific Notation e) Uncertainty 2. Chapter 1: Hear and Now a) Scale of Universe b) Astronomical Unit c) Light Year 3. Chapter 2: The Sky a) Constellations b) Magnitude and Brightness of Stars Measurements A tool to use the laws of mathematics to solve problems and to be able to distinguish between logical and illogical arguments. The most fundamental measurements are that of; Length Mass Time L M T System of Units System International (SI) A modern form of metric system Conversion from MKS to FPS http://www.tpub.com/doephysics/classicalphysics6.htm Phys1411 Goderya 1
Scientific Notation Astronomical numbers are very large or very small so Astronomers use Scientific Notation Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu). Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu). Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu). Significant Numbers Significant Numbers Getstartedinsicence.weebly.com slideplayer.com Phys1411 Goderya 2
Significant Numbers Uncertainty Scales of Size and Time Astronomy deals with objects on a vast range of size scales and time scales. Most of these size and time scales are way beyond our every-day experience. Humans, the Earth, and even the solar system are tiny and unimportant on cosmic scales. A Campus Scene A City View 16 x 16 m 1 mile x 1 mile Phys1411 Goderya 3
The Landscape of Pennsylvania The Earth 100 miles x 100 miles Diameter of the Earth: 12,756 km Earth and Moon Earth Orbiting Around the Sun Distance Earth Moon: 384,000 km Distance Sun Earth = 150,000,000 km Earth Orbiting Around the Sun (2) The Solar System In order to avoid large numbers beyond our imagination, we introduce new units: 1 Astronomical Unit (AU) = Distance Sun Earth = 150 million km Approx. 100 AU Phys1411 Goderya 4
(Almost) Empty Space Around Our Solar System The Solar Neighborhood Approx. 10,000 AU Approx. 17 light years The Solar Neighborhood (2) The Extended Solar Neighborhood Approx. 17 light years New distance scale: 1 light year (ly) = Distance traveled by light in 1 year = 63,000 AU = 10 13 km = 10,000,000,000,000 km (= 1 + 13 zeros) = 10 trillion km Nearest star to the Sun: Proxima Centauri, at a distance of 4.2 light years Approx. 1,700 light years The Milky Way Galaxy The Local Group of Galaxies Diameter of the Milky Way: ~ 75,000 ly Distance to the nearest large galaxies: several million light years Phys1411 Goderya 5
The Universe on Very Large Scales Clusters of galaxies are grouped into superclusters. Superclusters form filaments and walls around voids. What are Constellations? In ancient times Constellations = brightest stars that appeared to form groups Represented great heroes and mythological figures Position in the sky told stories handed down from generation to generation over thousands of years Constellations An Ancient Heritage Constellations Easily Recognizable Constellations and Their Brightest Stars Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions. Phys1411 Goderya 6
Constellations of the Zodiac Constellations There are 88 Constellations 12 of these hold special significance because the Sun passes through them in the course of a year. They are called Zodiacal constellations and are also used by Astrologers. Apparent Motion of The Celestial Sphere Apparent Motion of The Celestial Sphere Star Names in a Constellation Stars are named by a Greek letter () according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Betelgeuse Orion Betelgeuse = Orionis Rigel = Orionis Order of Greek Letters Rigel Phys1411 Goderya 7
Consider a Real Nigh Sky Photograph It is clear that not all stars are the same brightness. How do you measure the brightness of a Star? www.startribune.com Measuring the Brightness of Stars Greek astronomer Hipparchus (160-127 BC) invented a number system to measure brightness of stars based on their appearance of size Brightest stars: ~1st magnitude Faintest stars (unaided eye): 6th magnitude This scale is subjective and does not have a quantitative basis Gcseastronomy.co.uk Modern Definition In1856 Norman Pogson proposed that the eye perception of light is logarithmic so five magnitude difference corresponds to 100 2.512, consequently 1 st magnitude star is 2.5 times brighter than 2 nd magnitude star and the 3 rd magnitude star is 2.5 x 2.5 = 6.25 times brighter than 1 st magnitude star. Intensity (Flux) and Magnitude Difference This table is one way to remember the relationship between brightness and magnitude. Larger the magnitude number, fainter the brightness of star Gcseastronomy.co.uk Equation wise Apparent Magnitude (m v ): Brightness of the star irrespective of its distance from us Apparent magnitude versus intensity (flux) m = apparent magnitude I = intensity A = Star A B = Star B Intensity versus apparent magnitude I B ma mb 2.5Log I A Examples Two stars differ by 3 magnitude. What is the intensity ratio? I A 3 (2.512) 16 I B Sirius is 24.2 time more intense than Polaris. What is the magnitude difference? m m 2.5Log 24.2 2.51.38 3.5 A B I A m (2.512) B m A I B Phys1411 Goderya 8
The Modern Magnitude Scale The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects): Example Sirius (brightest star in the sky): m v = -1.42 Full moon: m v = -12.5 Sun: m v = -26.5 Acknowledgment The slides in this lecture is for Tarleton: PHYS1411/PHYS1403 class use only Images and text material have been borrowed from various sources with appropriate citations in the slides, including PowerPoint slides from Seeds/Backman text that has been adopted for class. Phys1411 Goderya 9