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 PresentationTRANSCRIPT
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Photometry and Spectroscopy
Astronomy 315Professor Lee
CarknerLecture 7
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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 You must bring pencil and calculator!
No cell phones/PDAs Observing list 1 due this Friday
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Studying for Quiz #1 Study
lectures exercises homework readings
Can 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
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Studying Stars
Stars are too small to see structure
Spectra are studied through spectroscopy
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Spectroscopy If 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
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Spectral Signatures An atoms electron’s 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)
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Hydrogen Transitions
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Spectral Lines in Stars Most 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
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The Balmer Series All 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
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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)
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Temperature Dependence of Stellar Spectral Lines
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Stellar Spectra -- Image
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Spectral Typing
Spectral type gives us temperature O 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
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The Spectral Types Stars were first classified by strength of
the H Balmer line
Eventually it was determined that this sequence did not reveal anything of astrophysical significance
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Photometry We want to get an accurate
quantitative measure of brightness Our system is composed of two
things:
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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)
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Notes on Magnitude Magnitude scale runs backwards
Scale is semi logarithmic
A 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”
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Magnitudes of Selected Objects
Sun: Moon: Venus: Sirius: Faintest star you can see: Faintest star with small telescope: Large telescope and CCD camera: Hubble Space Telescope:
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Filters
Use a set of standard filters, such as the UBVRI scale
e.g. V = 500-600 nm, B = 400-480 nm We report the magnitudes with the letter of
the filter
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Standard UBVRI Passbands
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Color Index
The color index gives an estimate of the temperature
Example B-V:
Negative B-V means smaller B magnitude, which means more blue light, indicating a hot star
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Star Names Only the brightest stars in the sky have proper names
e.g., Rigil Kentaurus from Rijl al-Qanturis meaning
“Foot of the Centaur” Bright stars also have a Bayer designation
Alpha () Centauri, Beta () Centauri, Gamma () Centauri, etc.
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Next Time Read Chapter 17.1-17.6