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Photometry and Spectroscopy Astronomy 315 Professor Lee Carkner Lecture 7

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

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Page 1: Photometry and Spectroscopy

Photometry and Spectroscopy

Astronomy 315Professor Lee

CarknerLecture 7

Page 2: Photometry and Spectroscopy

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

Page 3: Photometry and Spectroscopy

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

Page 4: Photometry and Spectroscopy

Studying Stars

Stars are too small to see structure

Spectra are studied through spectroscopy

Page 5: Photometry and 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

Page 6: Photometry and Spectroscopy

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)

Page 7: Photometry and Spectroscopy

Hydrogen Transitions

Page 8: Photometry and Spectroscopy

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

Page 9: Photometry and Spectroscopy

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

Page 10: Photometry and Spectroscopy

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)

Page 11: Photometry and Spectroscopy

Temperature Dependence of Stellar Spectral Lines

Page 12: Photometry and Spectroscopy

Stellar Spectra -- Image

Page 13: Photometry and Spectroscopy

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

Page 14: Photometry and Spectroscopy

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

Page 15: Photometry and Spectroscopy

Photometry We want to get an accurate

quantitative measure of brightness Our system is composed of two

things:

Page 16: Photometry and Spectroscopy

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)

Page 17: Photometry and Spectroscopy

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”

Page 18: Photometry and Spectroscopy

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:

Page 19: Photometry and Spectroscopy

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

Page 20: Photometry and Spectroscopy

Standard UBVRI Passbands

Page 21: Photometry and Spectroscopy

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

Page 22: Photometry and Spectroscopy

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.

Page 23: Photometry and Spectroscopy

Next Time Read Chapter 17.1-17.6