Astronomy Week 8 Notes
Astronomy Week 8 Notes EESC1150
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This 5 page Class Notes was uploaded by Erin Bleck on Tuesday July 19, 2016. The Class Notes belongs to EESC1150 at Boston College taught by Dr. Thomas Kuchar in Summer 2016. Since its upload, it has received 28 views. For similar materials see Astronomy in Earth and Environmental Sciences at Boston College.
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Date Created: 07/19/16
Notes March 15, 2016 - Photons (light-waves) are emitted from an atom when an electron moves from a higher energy level to a lower energy level o Energy = h x v - Photons can also be absorbed by an atom when an electron moves from a lower energy level to a higher energy level o Energy = h x v o SAME ENERGY LEVEL DIFFERENT (ex. 2 to 3 or 3 to 2) SAME PHOTON Identifying Atoms by Their Light - Spectroscopy – the study of light emitted or absorbed by an object at various wavelengths to determine its composition and physical state (e.g., temperature) - How a spectroscope works o A narrow slit focuses the light o A grating or a prism splits the light into its component colors - Emission spectrum of hydrogen (photons are emitting light, so electrons moving down) o Light at only some wavelengths o Blue, purple, green, and red = Hydrogen Emission Spectrum o Unique to hydrogen (like a barcode) o The red line is called “H alpha” - Different atom, different spectrum o Every element has its own spectrum o Note the differences between hydrogen and helium Helium = purple, blue, green, yellow, salmon, red - Absorption spectra (photons are absorbing energy, so electrons moving up) o Black lines where emission spectra have color; everywhere else has color - Summary of spectra o Continuous spectrum – the source emits light that is continuous and all colors are present o Emission-line spectrum – a hot, thin gas will emit characteristic frequencies of light o Absorption line spectrum – a cooler gas will absorb light from a hotter source behind it at the characteristic frequencies of that gas o Kirchoff’s Laws (1859) Continuous – a hot solid object produces light with a continuous spectrum Emission – a hot, tenuous gas produces light with spectral lines at discrete wavelengths (i.e. specific colors) which depend on the energy levels of the atoms in the gas Absorption – a hot solid object surrounded by a cool tenuous gas (i.e. cooler than the hot object) produces an almost continuous spectrum with gaps at discrete wavelengths depending on the energy levels of the atoms in the gas - Solar spectrum – absorption lines all the time because of atmosphere above its surface (satisfies Kirchoff’s third law) - Comet spectrum – illuminated by the heat radiation from the Sun Notes March 17, 2016 Anything that has heat (energy) emits light - Anything that has heat (energy) emits light o Brightness of an object (i.e. amount of energy it gives off) is determined by its size and temperature o Astronomy (and Physics) uses its own temperature scale to measure heat - Temperature scales o (coldest) absolute zero liquid nitrogen water freezes water boils light bulb filament Sun’s surface Sun’s core (hottest) o Fahrenheit o Celsius o Kelvin At absolute zero – there’s no thermal energy in the atoms (the atoms do not move) o Temperature – the measure of the motion of atoms - Determining luminosity: Stefan-Boltzmann Law o Luminosity is the total energy (of all light over all wavelengths) emitted by an object in each second Measured in Watts o Luminosity of a hot body rises rapidly with temperature o Luminosity (L) of an object depends on its surface area (A) and its temperature (T) o L = O x T^4 x A o Depends strongly on T Double T energy increases 16-fold! 2^4 = 16 o BIG and HOT objects have GREATER LUMINOSITY than small and cool objects o For people: L = (5.67x10^-8) x (310)^4 x A L = 785 Watts Assuming surface area of the body is about 1.5m^2 Larger people emit more, smaller people emit less Measure of metabolism (remember, takes more energy for a smaller person to use oxygen vs. larger person [elephant on LSD]) - S-B applied to a lightbulb o Filament has T = 2000K L ~90 Watts Area of a filament is about 1cm^2 = 10^-4m^2 o If the bulb is 2500K (25% increase in T) L ~220 Watts – almost 2.5 times as much energy! - H-R Diagram o 1 Solar Luminosity = 3.839 x 10^26 Watts o Stars can range from 0.0001 to 10,000 times the Sun’s brightness o Note: X-axis goes from hot to cold (left to right) for star’s surface T - White Light o Isaac Newton showed: White light could be split into component colors with a prism And then recombined into white light o As we change colors, we don’t get all the same numbers from them Red – 70 Orange – 78 Yellow – 83 Green – 85 Blue – 81 Violet – 78 o The colors are not all represented equally o Blackbody Curve – a sign of thermal radiation (a hot glowing object) - Blackbodies o A blackbody – a theoretical object that emits radiation whose spectrum has a specific shape that depends only on the temperature of the object The Sun, an electric stovetop element, or a piece of charcoal approximate a blackbody Produce a continuous spectrum (the whole rainbow of colors) A tenuous gas cannot be (only emission lines!) We do not see a continuous spectrum; only emission spectrum - Blackbody curve o Shows an object’s energy output versus wavelength with a characteristic peak o The wavelength at which the peak of this curve occurs tells us about the object’s temperature and color (cooler) red orange yellow green blue (hotter) Cool objects emit light that peaks at long wavelengths (red) Hot objects emit light that peaks at short wavelengths (blue) - Wien’s Law o Simply: the higher the temperature of an object, the shorter the wavelength of the peak for the light emitted by the object o Wavelength = ((2.9x10^g)K-nm)/T o Relates the temperature of an object to the peak wavelength in its blackbody curve - What color is our 5800K Sun? o The Sun emits light over all wavelengths of the electromagnetic spectrum; just not at visible wavelengths o However, the Sun emits most intensely in the green/yellow part of the spectrum Because its blackbody curve peaks in the green/yellow range o A star, like the Sun, which peaks in the middle of the visible part of the spectrum appears yellowish-white - What if the Sun became hotter? o The peak would move to the left (towards the blue range of the spectrum) and would look bluer - What if the Sun became cooler? o The peak would move to the right (towards the red range of the spectrum) and would look redder - The blackbody spectrum – how it works o As temperature increases (size of the stars are equal): The peak wavelength (dominant color) becomes bluer (higher energy; shorter wavelength) The spectrum becomes brighter at all wavelengths Although the blue star appears blue, it emits more yellow and red light than the yellow and red stars; the yellow star emits more red light than the red star - Apply Wien’s Law to people o T = 310K o Wavelength = 9300nm Infrared o Wavelength = ((2.9x10^g)K-nm)/T - Apply Wien’s Law to a lightbulb o Filament has T = 2000K Wavelength – 1450 nm (infrared) Most of the energy is in the infrared L = 90 Watts of mostly thermal energy o If the bulb is 2500K (25% increase in T) Wavelength = 1160 nm (infrared) L = 220 Watts 2.5x as much energy, but still emitting the most energy in infrared - Note: Wien’s Law only applies to light emitted by an object (not reflected!) o A red delicious apple (red has wavelength = 700nm) is not 4000K but room temperature (~300K) It reflects light
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