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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.