PHYS 1411 ntro to stronomy Topics for Today s Class Ch 2 1. Constellations a) ncient and Modern Definitions b) Zodiacal Constellations c) sterisms d) Circumpolar Constellations e) ppearance of Constellations and Distance 2. stronomical Names of Stars a) Greek Letters 3. The Sky and its Motion a) Celestial Sphere 4. rightness and Magnitude a) History b) Relationship between Magnitude and rightness c) Measuring rightness d) Examples What are Constellations? n 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 n ncient Heritage Constellations Constellations of the Zodiac Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions. Phys1411 Goderya 1
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 strologers. sterisms sterism is a pattern of stars that may be part of a constellation and have common historical name. They may have more than one bright stars. sterism are not Universal, they vary according the culture and civilization. The view of the sterism is dependent on the time of the year. Here are some common sterism used in the western culture. ig Dipper and Small Dipper Northern Cross Square of Pegasus Winter Circle and Triangle Phys1411 Goderya 2
Sickle Circumpolar Constellations circumpolar constellations is one that is visibly through out the year. Which constellations are circumpolar depends on the location of the observer on Earth. Circumpolar constellations for Stephenville are different from that of Chicago or Network. Ursa Minor and Cassiopeia are examples, while Ursa Major is almost circumpolar for Stephenville. Projection of Stars in the Sky stronomical Star Names Named by a Greek letter according to their relative brightness within constellation etelgeuse = Orionis; Rigel = Orionis Cengage Learning 2016 Cengage Learning 2016 Order of Greek Letters stronomical and Common Names in ig Dipper Wikipedia: CC Y-S 3.0 Phys1411 Goderya 3
Easily Recognizable Constellations and Their rightest Stars The Sky and Celestial Motions The sky above seems like a great blue dome in the daytime and a sparkling ceiling at night The Early astronomers represented the sky as a sphere and called it celestial sphere abbreviated as CS We will study the CS in more detail, but here is an example of how it looks and how we draw it. Celestial Sphere Drawing Half CS Observer Observers Horizon Wikipedia: CC Y-S 3.0 CC Y-S 3.0 pparent Motion of The Celestial Sphere pparent Motion of The Celestial Sphere Phys1411 Goderya 4
Consider a Real Nigh Sky Photograph t is clear that not all stars are the same brightness. How do you measure the brightness of a Star? www.startribune.com Measuring the rightness of Stars Greek astronomer Hipparchus (160-127 C) invented a number system to measure brightness of stars based on their appearance of size rightest 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 n1856 Norman Pogson proposed that the eye s 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. ntensity (Flux) and Magnitude Difference This table is one way to remember the relationship between brightness and magnitude. rightness Ratio = (2.51) Magnitude Difference Larger the magnitude number, fainter the brightness of star Gcseastronomy.co.uk Equation wise pparent Magnitude (m v ): rightness of the star irrespective of its distance from us pparent magnitude versus intensity (flux) m = apparent magnitude = intensity = Star = Star v = visual wavelength ntensity versus apparent magnitude Examples Two stars differ by 3 magnitude. What is the intensity ratio? 3 (2.512) 16 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 m m 2.5Log m (2.512) m Phys1411 Goderya 5
The Scale of pparent Visual Magnitudes Example The magnitude scale system can be extended Cengage Learning 2016 cknowledgment The slides in this lecture is for Tarleton: PHYS1411/PHYS1403 class use only mages and text material have been borrowed from various sources with appropriate citations in the slides, including PowerPoint slides from Seeds/ackman text that has been adopted for class. Phys1411 Goderya 6