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Astronomical filters ASTR320 Monday October 17, 2016 Astronomical filters • It is often the case that we want to understand the “color” of an astronomical object, measured in magnitudes • We can divide up the spectrum of light produced by an object into distinct bandpasses by placing a filter in the optical path of the telescope Examples: The electromagnetic spectrum Earth’s atmosphere is transparent to visible light and some microwaves and radio waves. Color indices • In astronomy we define the colors of stars quantitatively, using color indices • Suppose we measure fluxes in two different filters: • We can make a color index by subtracting: • We generally write the color index as letters that denote filters, e.g.: • Note that the distance cancels, so the color is the same for absolute and apparent magnitudes. Color indices • By convention, pick cA-cB based on Vega (an A0V type star) so that for Vega: • By convention, generally write colors with the shorter wavelength passband first: – B-V, U-B, J-K • So that smaller numbers are always “bluer” and larger numbers are “redder” – A-B<0 means “bluer than Vega” – A-B>0 means “redder than Vega” What color indices measure • Most usefully, temperature • Stars behave largely like blackbodies – A hot, opaque object produces a continuous blackbody spectrum of light characterized by its temperature Blackbody radiation A blackbody is an object that absorbs all light. • Absorbs at all wavelengths • Characterized by its temperature It is also the perfect radiator: • Emits at all wavelengths (continuous spectrum) • Total energy emitted depends on temperature • Peak wavelength also depends on temperature Blackbody curves Stefan-Boltzmann Law Energy emitted per second per area by a blackbody with Temperature (T): E = sT Where s is Boltzmann's constant. 4 Wien’s law Wavelength of maximum emission is inversely related to temperature max 2,900,000 nm max T wavelengt h of maximum emission T temperature (in Kelvins) The color of a star is related to its temperature Betelgeuse: a reddish star (cooler). Rigel: a bluish star (hotter). Rigel: • Teff = 11,000K • B-V = -0.03 Betelgeuse: • Teff = 3,500K • B-V = 1.85 Stellar spectra in order from the hottest (top) to coolest (bottom). Astronomical filters • Measurements of astronomical objects are made by different telescopes and instruments all over the world • When imaging astronomical objects, it is common to place one of several different filters (conceptually, a colored piece of glass) in the optical system to measure the color of an object quantitatively • Once enough astronomers began to make modern measurements of astrophysical sources, it became clear that a standardized system was needed so the measurements could be compared and confirmed Astronomical filter systems We design photometric systems to maximize information that can be gleaned from extremely low resolution spectroscopy, i.e., photometry. The Johnson-Morgan UBV System • • • • The UBV system was originally designed by Johnson and Morgan (1953) to understand stars (particularly hot stars). The Johnson-Morgan V band is meant to simulate and perpetuate measurements historically made by the human eye, to which it approximately matches. The Johnson-Morgan B band approximates the blue sensitivity of the original photographic emulsions to typical stars. The B-V color provided a measure of the temperature of (hotter) stars. Johnson and Morgan realized that much more information was possible by adding a third filter in the ultraviolet. The UBVRI system • In the 1960s, Johnson (and later others) extended the UBV system to the red and infrared, with R,I,J,K,L,M,N.... bands. • In the optical, then, we have the UBVRI broadband system. • It was found that the UBV system did not work well for very cool stars, like K and M spectral types, and these very red stars were easier to study at redder wavelengths. So the V, R and I bands are often used to study these kinds of stars. SDSS ugriz system • The Sloan Digital Sky Survey used a filter set based on the Thuan-Gunn system • Used in the SDSS survey, and is now commonly used by most astronomers in order to compare to the huge dataset of SDSS Other filter systems • Stromgren: stellar classification, absolute magnitudes, surface gravities • Washington/DDO: metal abundances • Tuned narrowband filters: measure specific emission line features in galaxies or nebulae, redshifts of galaxies/quasars