Astronomy 1102 Exam #1 Chapters 1,2,5,6 & 16 Chapter 1 Degrees- basic unit of angle measurement, designated by the symbol -a full circle is divided into 360 and a right angle measures 90. arc minutes-one-sixtieth of a degree (1/60), designated by the symbol. - also called minutes of arc. arc seconds- one-sixtieth of an arc minute, or 1/3600 of a degree, designated by the symbol. 1 = 60 (arcmin) = 3600 (arcsec) 1 (arcmin) = 60 (arcsec) Small-angle formula: D= ad/ 206,265 D=linear size of object d= distance to the object a=angular size of the object, in arcsec Example: On December 11, 2006, Jupiter was 944 million kilometers from Earth and had an angular diameter of 31.2 arcsec. From this info, calculate the actual diameter of Jupiter in kilometers. D=ad/206,265 D= 31.2 x 944,000,000 km / 206,265 = 143,000 km Exponent- a number placed above and after another number to denote the power to which the latter is to be raised, as 2 in 10². -It helps astronomers to avoid confusing terms such as a million billion billion -They are used in the powers-of-ten notion. Examples: ten million: 10^5 Sixty thousand: 6 x 10^4 Four one-thousandths: 4 x 10^-3 Thirty-eight billion: 3.8 x 10^10 Light year- (abbreviated as ly) the distance that light travels in one year. - unit of distance, NOT time. = 3.00 x 10^5 km/s = 1.86 x 10^5 mi/s 1 ly= 9.46 x 10^12km = 63,240 AU (roughly 6 trillion miles) (AU = astronomical unit = average distance between the Earth and the Sun) 1 AU= 1.496 x 10^8km = 92.96 million miles Parsec-unit of distance; 3.26 light years.
- abbreviated pc - 1 pc = 3.09 x 10^12km = 3.26 ly - the distance at which 1 AU perpendicular to the observer s line of sight subtends an angle of 1 arcsec. kilo parsec- abbreviated kpc - 1 kpc = 1000 pc = 10^3 pc - 3260 light years mega parsec- abbreviated Mpc - 1 Mpc= 1,000,000 pc = 10^6 pc Chapter 2 Constellations- a configuration of stars in the same region of the sky. -Origin: Latin for set with stars - names after mythical figures or other distinctive objects How are they useful to astronomers? How many stars are not part of a constellation? Chapter 5 How long does it take light to travel from the Sun to Earth, a distance of 1.50 x 10^8 km? 500 s Wavelength- distance between two successive wave crests. Nanometer- nm, 1nm = 10^-9m Wavelengths listed from longest to shortest Radio- longest-wavelength electromagnetic radiation -wavelengths of a few centimeters or more Microwave- short-wavelength radio waves -1mm-10cm Infrared- electromagnetic radiation of wavelength longer than visible light but shorter than radio waves -700nm-1mm Visible- electromagnetic radiation detectable by the human eye Ultraviolet- electromagnetic radiation of wavelengths shorts than those of visible light but longer than those of xray -400nm-10nm x-ray- electromagnetic radiation whose wavelength is between that of ultraviolet and gamma rays -wavelengths shorter than 10nm (10-0.01nm) Gamma ray- the most energetic form of electromagnetic radiation -shortest wavelength -highest frequency -highest photon energy
Frequency and wavelength of an electromagnetic wave v = c/λ v=frequency of an electromagnetic wave (in Hz) c= speed of light = 3 x 10^8 m/s λ= wavelength of the wave (in meters)
Blackbody- a hypothetical perfect radiator that absorbs and re-emits all radiation falling upon it. -In what ways is it black? At room temperature, 300K, it would appear very black. -If you were to shine a light beam on a perfect blackbody, what would happen to the light? The light would be absorbed. Blackbody radiation-light emitted from a blackbody Wien s law for a blackbody λmax = 0.0029 K m / T λmax= wavelength of maximum emission of the object (in meters) T= temperature of the object (in Kelvins) -The higher the temperature (T) of a blackbody, the shorter its wavelength of max. emission. (the hotter it is the bluer it is) Stefan-Boltzman law for a blackbody F=σT^4 F= energy flux, in joules per square meter of surface per second σ = a constant = 5.67 x 10^-8 W m^-2 K^-4 T= object s temperature, in Kelvins -the flux from a blackbody is proportional to the fourth power of the object s temperature (the hotter it is, the brighter/ more luminous it is) -If you double the Kelvin temperature of a hot piece of steel, how much more energy will it radiate per second? 2^4 = 16 times more -Bellatrix has a surface temperature of 21,500K, what is its wavelength of maximum emission in nanometers? What color is this star? 135 nm -Antares emit s a wavelength of 853nm. What is the surface temp of Antares? What color is this star? 3400 k -Photon-a discrete unit of electromagnetic energy How is the energy of a photon related to its wavelength? The greater the wavelength the lower the energy of a photon associated with that wavelength. What kind of photons carry the most energy? - violent photons. λ= 400nm What kinds of photons carry the least energy? - red photons. λ= 700nm
Energy of a photon (in terms of wavelength) E= hc / λ E= energy of a photon h=planck s constant c= speed of light λ= wavelength of light Energy of a photon (in terms of frequency) E=hv E= energy of a photon h= Planck s constant v= frequency of light Spectral lines- in a spectrum, an absorption or emission feature that is at a particular wavelength Spectral analysis- identification of chemical substances from the patterns of lines in their spectra. Kirchoff s laws Law 1: a hot opaque body, such as a perfect blackbody, or a hot, dense gas produces a continuous spectrum- a complete rainbow of colors without any spectral lines Law 2: a hot, transparent gas produces an emission line spectrum- a series of bright spectral lines against a dark background Law 3: a cool, transparent gas in front of a source of a continuous spectrum produces an absorption line spectrum- a series of dark spectral lines among the colors of the continuous spectrum. Furthermore, the dark lines in the absorption spectrum of a particular gas occur at exactly the same wavelengths as the bright lines in the emission spectrum of that same gas. Spectroscopy- systematic study of spectra and spectral lines Chapter 6 Optical window- the range of visible wavelengths to which Earth s atmosphere is transparent. Radio window- range of radio wavelengths to which Earth s atmosphere is transparent Why aren t there x-ray windows or an ultraviolet window? -atmosphere is opaque Why is it necessary to keep an infrared telescope at a cool temperature? -the infrared blackbody radiation from the telescope itself would outshine the infrared
radiation from astronomical objects. Chapter 16 The Sun Distance from earth: 1AU = 149,598,000 km Light travel time to earth: 8.32 minutes Mean angular diameter: 32 arcmin Radius: 696,000 km Mass: 1.9891 x 10^30 kg Composition: 74% hydrogen 25% helium 1% other elements Mean temperature: Surface: 5800K Center: 1.5 x 10^7 K Luminosity: 3.90 x 10^26 W Kelvin-Helmholtz contraction- the contraction of a gaseous body, such as a star or nebula, during which gravitational energy is transformed into thermal energy. Why is it ruled out as a source of the present-day Sun s energy? -the Sun would have had to be much larger in the relatively recent past but geological and fossil record show that the Earth is older than 25 million years, which disputes Helmholtz s calculations. Why can t coal be the source of the sun s energy? -This process can t continue for a long enough time to explain the age of Earth Hydrogen fusion-the thermonuclear conversion of hydrogen into helium. -Why do thermonuclear reactions occur only in the Sun s core, and not in its outer regions? Bc only the core is hot enough Hydrostatic equilibrium- the balance between the weight of a layer in a star and the pressure that supports it. Thermal equilibrium-the combining of nuclei under conditions of high temperature in a process that releases substantial energy. Conduction- the transfer of heat by passing energy from atom to atom Convection- the transfer of heat by moving currents of fluid or gas containing that energy. Radiative diffusion- random migration of photons from a star s center to its surface Neutrino- a subatomic particle with no electric charge and very little mass, yet one that is important in many nuclear reactions. -Why is it useful to study neutrinos coming from the sun? study the neutrino flux
and conditions of the sun s core -Why are neutrino detectors placed deep underground? Bc most neutrinos past through Earth as if it isn t there. -What is the solar neutrino problem? Discrepancy between theory and observation -3 layers that make up the sun s atmosphere Photosphere Chromosphere Corona Granules-a convective cell in the solar photosphere. super granules- a large convective feature in the solar atmosphere, usually outlined by spicules. Spicules- a narrow jet of rising gas in the atmospheres chromoshpere. Stellar Luminosities and Brightnesses The luminosity of a star is really important -For the Sun, luminosity told us rate of nuclear fusion - We estimated how long the Sun s fuel could last For the stars we can work it out: - b = brightness (W/m2) - L = luminosity (W) - d = distance (m) - This is the inverse square law - brightness varies inversely with distance squared b =L/4πd 2 The Brightness of the Sun and Sirius The Sun - Luminosity = 1L = 3.86x1026W - Distance = 150 million km = 1.5x1011m Sirius - Luminosity = 26.1L = 1.01x1028W - Distance = 8.61 light years = 8.15x1016m Measuring brightness of stars is called photometry b = L/4πd 2 = 3.86x1026 / 4π (1.5x1011)2 = 1370W /m2 b = L/4πd 2 = 1.01x1028 / 4π (8.15x1016)2 = 1.21x10 7W /m2