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
Photometry and Spectroscopy
Astronomy 315Professor Lee
CarknerLecture 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 You must bring pencil and calculator!
No cell phones/PDAs Observing list 1 due this Friday
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
Studying Stars
Stars are too small to see structure
Spectra are studied through spectroscopy
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
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)
Hydrogen Transitions
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
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
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 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
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
Photometry We want to get an accurate
quantitative measure of brightness Our 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 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”
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:
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
Standard UBVRI Passbands
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
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.
Next Time Read Chapter 17.1-17.6