Photometry and Spectroscopy

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Photometry and Spectroscopy. Astronomy 315 Professor Lee Carkner Lecture 7. Quiz #1. Next Monday (March 26) Covers lectures 1-9 About 16 multiple choice (~50% weight) About 4 short answer/problems (~50% weight) Equations and constants provided But unlabeled - PowerPoint PPT Presentation

Transcript of Photometry and Spectroscopy

  • Photometry and Spectroscopy

    Astronomy 315Professor Lee CarknerLecture 7

  • Quiz #1Next Monday (March 26)Covers lectures 1-9About 16 multiple choice (~50% weight)About 4 short answer/problems (~50% weight)Equations and constants providedBut unlabeledYou must bring pencil and calculator!No cell phones/PDAsObserving list 1 due this Friday

  • Studying for Quiz #1StudylecturesexerciseshomeworkreadingsCan you:Identify the key concepts of the class?Write a paragraph explaining key concepts?Solve math problems from exercises and book without help?Study guide posted on web page

  • Studying Stars

    Stars are too small to see structure

    Spectra are studied through spectroscopy

  • SpectroscopyIf we take a spectrum of a star, what does it look like? What do the lines and their strength tell us?Strength of line depends on: The temperature being such that the transitions can occur

  • Spectral SignaturesAn atoms electrons can be in a number of states from 1 (the ground state) to removed from the atom completely

    At higher temperature they are in higher states

    Ionized atoms are represented with roman numerals (e.g. Ca II, calcium with 2 electrons missing)

  • Hydrogen Transitions

  • Spectral Lines in StarsMost stars have very similar compositions

    The spectrum we take only covers a certain energy range Three reasons: Temperature so high that electrons only produce higher energy transitions

  • The Balmer SeriesAll stars are made primarily of hydrogen, but many stars have weak H lines

    In what stars do we see Balmer lines?Not in cool stars (electrons all in ground state)

    Only in medium hot stars are the Balmer lines strong

  • Spectral Types

    The spectral types are (from high to low temperature): Each spectral type is divided into 10 sub classes 0 - 9 (from high to low T)

  • Temperature Dependence of Stellar Spectral Lines

  • Stellar Spectra -- Image

  • Spectral Typing

    Spectral type gives us temperatureO and B stars T ~ A and F stars T ~ G, K and M stars T ~ Spectral typing is accurate to about 2-3 subcategories or a few hundred degrees

  • The Spectral TypesStars were first classified by strength of the H Balmer line Eventually it was determined that this sequence did not reveal anything of astrophysical significance

  • PhotometryWe want to get an accurate quantitative measure of brightnessOur system is composed of two things:

  • Magnitude

    The magnitude scales is logarithmic and is related to the flux by:m2 m1 = 2.5 log10 (f1/f2)where the flux is defined as the amount of energy received from the star per unit area per unit time (watts/m2/s)

  • Notes on MagnitudeMagnitude scale runs backwards Scale is semi logarithmicA star that is n less magnitudes has 2.5n times the flux 5 magnitude difference is factor of 100 difference in flux Magnitude is sometimes indicated with an m

  • Magnitudes of Selected ObjectsSun: Moon: Venus: Sirius: Faintest star you can see: Faintest star with small telescope: Large telescope and CCD camera: Hubble Space Telescope:

  • Filters

    Use a set of standard filters, such as the UBVRI scale

    e.g. V = 500-600 nm, B = 400-480 nmWe report the magnitudes with the letter of the filter

  • Standard UBVRI Passbands

  • Color Index

    The color index gives an estimate of the temperatureExample B-V:

    Negative B-V means smaller B magnitude, which means more blue light, indicating a hot star

  • Star NamesOnly the brightest stars in the sky have proper names e.g., Rigil Kentaurus from Rijl al-Qanturis meaning Foot of the CentaurBright stars also have a Bayer designation

    Alpha (a) Centauri, Beta (b) Centauri, Gamma (g) Centauri, etc.

  • Next TimeRead Chapter 17.1-17.6